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hi everyone this is miss romani and for
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today's lesson we're going to learn
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about genetic mutations
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the concept of genetic mutations is
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actually one that has been popularized
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quite a bit in comic books and in
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mainstream media
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however the way the mutations are
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represented is not usually realistic
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the idea that changes in our dna can
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give us superpowers is appealing
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but and i'm sorry if that disappoints
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some of you
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it is entirely unrealistic and so
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mutations are basically changes to the
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dna code
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they are not the only type of genetic
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abnormalities or
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not the only cause of genetic
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abnormalities changes in the number
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or in the structure of our chromosomes
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something that we call chromosomal
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abnormalities the topic of what happens
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when you have extra chromosomes to a few
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chromosomes or
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pieces missing of chromosomes and so on
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that was covered in the grade 11 course
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today's lesson will focus only on
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genetic mutations
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so let's begin by taking a look at the
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locations of mutations and where they
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can happen
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so first let's take a look at where they
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can happen within the body
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and there are two main places where
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mutations can happen within the body
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the first type are in the somatic cells
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and so we call
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those somatic mutations somatic
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mutations are mutations that happen
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in our body cells so you might think
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well isn't
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all our cells our body cells well as far
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as biology is concerned
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we differentiate body cells which are
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somatic cells which are basically
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most of the cells in your body from the
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cells that are found within your sex
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organs the cells that are found in your
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egg producing ovaries or your sperm
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producing testes
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so somatic mutations are basically the
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ones that happen in any of your cells in
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your body that are just going to
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undergo mitosis and are not the result
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of meiosis
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if you have a somatic mutation it'll
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have
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an effect on a part of your body that is
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affected by
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the mutation the cells that have the
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genetic mutation
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are going to be the progeny of those
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cells with a mutation
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somatic mutations can happen in the womb
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but
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most often they happen once you are
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already born and throughout your lives
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and this is something that we contend
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with
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constantly and oftentimes these types of
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mutations just lead to
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cancers really if anything at all
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the other type of mutation which is the
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one that is most concerning
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in terms of lifelong effects and more
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severe effects are what we call
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germline mutations and these are the
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mutations that occur in our sex organs
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are egg producing ovaries or sperm
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producing testes
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or if they occur on the embryo really
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early on
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in development so maybe in the zygote or
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you know one of the earlier stages of
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embryonic development
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and with germline mutation the entire
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organism
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is affected by the genetic mutation
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because
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if the mutation was on the sperm or the
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egg
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then every single piece of dna that
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the organism has will have that genetic
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mutation
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and so germline mutations are more
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dangerous because they don't just affect
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a little tiny part of your body will
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affect your entire body
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and oftentimes these are the types of
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mutations that lead to genetic diseases
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or genetic disorders germline mutations
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are also the reason why whenever you go
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get an x-ray taken
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they will make sure that they'll cover
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your genital area with a lead
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bib or a lead blanket because any
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mutations that can occur
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in your eggs or sperm cells or your egg
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and sperm producing organs
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could have possibly lifelong and serious
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effect
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on your offspring from the in the future
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so we've categorized the different
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effects that genetic mutations can have
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depending on which types of cells they
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occur
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in but how about we take a look at
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what are the different effects of
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genetic mutations depending on where
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they occur within the dna molecule
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itself
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and so that is the difference between
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mutations that occur
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in sections of our dna that we call
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coding regions versus sections of our
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dna which we call non-coding region
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and it might surprise a lot of you to
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learn that as a matter of fact
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most of our dna is what we consider to
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be non-coding dna
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some estimates estimate that about 99 of
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our dna
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is actually non-coding with only one
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percent or so of our dna
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coding for genes and coding for proteins
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when scientists first figure that out
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when they finished the
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human genome project they were extremely
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surprised to find that so much of our
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dna
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was actually what they at the time
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called junk dna
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because they didn't know what it did
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they just knew that it did not actually
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code for
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proteins and so therefore vast amounts
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of our dna was not
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containing the instructions for our
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bodies so
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imagine if we go back to the recipe book
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analogy that i gave you before
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and one percent of the recipe book was
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actually recipes and 99 of it was
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what at first appeared to be just
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nonsense that did not contain any
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recipes
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and so when dna mutations happen in what
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we call
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non-coding regions of the cell the
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effects of those genetic mutations are
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going to be
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vastly different than when they occur in
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the genes themselves so they can happen
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in
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the vast areas of the dna
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molecule that are in between the genes
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but even within the genes if you
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remember from our last lesson we have
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both axons and introns
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so sometimes you can have mutations that
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happen within introns within the gene
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itself
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and so those types of mutations are not
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going to be
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as detrimental to the organisms as
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mutations that happen within
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the actual genetic code that codes for
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proteins
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within the exons of genes that are then
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going to
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be interpreted and read and transcribed
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and translated into an
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actual protein and when we take a look
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at the junk dna though just to clarify
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again
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so i don't want you to kind of come out
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of this lesson with an idea that most of
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our dna has
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no purpose whatsoever there's many
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different purposes that we've so far
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found for a lot of our dna and this is
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an area of biology
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and genetics that is yet to be fully
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explored there's a lot that we still
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want to learn about that
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but one of the things that we've learned
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is that first of all some of that
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non-coding dna can be the introns within
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the coding sequence so those
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genes that have those introns within
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them that are not necessarily going to
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then be part of the
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mrna transcript that exits the nucleus
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for translation but then there's other
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sections for example
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one thing that we find is that a lot of
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our dna is composed of these things
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called repetitive elements
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and repetitive elements could be say for
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example the genetic code
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gac gac repeated maybe hundreds and
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hundreds or thousands of times
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and we don't know why that is but you
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know this is an
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area of dna that is very vast within our
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genome
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that just happens to be there for so far
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unknown reasons
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we also know that some of our dna that
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is non-coding
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is actually the remnants of viral
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infections from the past
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so these are types of viruses called
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retroviruses these are viruses that can
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when they infect our cells they insert
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their dna code within our genome itself
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and any time that that has happened
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within the history of humanity and not
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just humans but
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our ancestors within the history of our
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evolution
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if those retroviruses inserted their dna
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within germline cells then their dna
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ends up being part of our genome today
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so a lot of our dna
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is also dna that came from viruses that
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infected
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our ancestors and going
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all the way back to the earliest cell
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we can actually do dna analysis in order
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to determine the level of relatedness
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between species so one of the things
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that we look for sometimes
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is those viral pieces of dna because
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if you find similarities in those
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retrovirus
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then you can determine whether or not
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they are related to each other to a
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certain degree
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some of our dna that is considered to be
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junk dna actually has a very important
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function as regulatory sequences so
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these are actual
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pieces of dna that may not code for
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proteins but they might code for say
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trna that are involved in translation
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or micro rna that are involved in
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regulating gene expression
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or even some of the proteins that
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are involved as transcription factors or
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something like that
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so these are sequences of dna that may
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not code for genes
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that produce proteins that we use in our
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body but they might code for
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other proteins or rna that is used
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during the regulation of our genetic
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expression
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and then last we have a lot of what are
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called pseudogenes
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these are genes that are not turned on
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anymore
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these are probably the remnants of our
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evolutionary history
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you know genes for traits that we no
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longer carry
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that we no longer express but their
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history is there within our code
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so there's a lot of reasons why we have
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all this junk dna
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and last also we have areas of our
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chromosomes that are filled with
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repeated sequences that are non-coding
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that have actually
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a purpose structurally within the
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chromosome so for example
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the centromere where the two chromatids
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attach
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in the center that's a lot of non-coding
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regions of dna
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and then at the ends of our dna we have
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vast sections that are called telomeres
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telomeres
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are vast sections of non-coding
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sequences
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that are serve a purpose to prevent
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the destruction of our genetic code as
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we
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age and as we reproduce so when
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mutations happen
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on 99 of our dna which is the non-coding
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regions
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they are going to have very little to
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zero effect
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on our cells and our bodies because
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they're not the parts of the dna that
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actually contain our genetic code
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that codes for our proteins so let's
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focus then on what happens when
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mutations happen on that one percent of
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our dna that is coding
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we can actually categorize the mutations
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into
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a couple of different types the first
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type is what we call
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point mutations and point mutations are
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basically mutations that occur
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on a single base pair because the dna
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molecule is double stranded whenever we
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take a look at mutations
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we have to take a look at what happens
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to not just a single letter
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but also its complementary base so it's
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always going to be done in pairs
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and there are different types of point
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mutations
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first one is something called the base
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substitution
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substitutions as the name implies are
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basically when
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one letter is replaced or substituted
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with another
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and so there's two types of base
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substitutions
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the first one are called transversions
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transversions happens when
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appearing is replaced with a pyramidine
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or a pyramidine is replaced with a
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purine so for example in this particular
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example
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you have an initial sequence with a
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thymine
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and the thymine has been replaced with
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an adenine that
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pyrimidine which is thymine is now an
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adenine which is appearing
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and another type of mutation though is
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what we call a transition
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a transition is when a purine is
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replaced with another purine or a
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pyrimidine is replaced with another
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pyrimidine
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so for example you have a guanine that
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is being replaced by an adenine
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guanine and adenine are both purines so
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you're replacing a purine
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with another purine another type of
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point mutation
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are what we called insertions insertions
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as the name implies
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is when we are inserting something
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within the code so this is not
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substituting or replacing
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a base with another base this is
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actually introducing
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another letter within the code itself so
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where the air is pointing right there
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what would happen then if we were to add
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a nucleotide pair right there in the
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middle of that code and so that's an
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insertion it basically makes the dna
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strength a little longer by one base
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pair
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and it can have severe effects on the
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genetic code itself
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the opposite of an insertion is what we
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call the deletion and so let's say
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we remove a pair so for example let's
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remove that
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cytosine to guanine base pair and so now
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the dna strand is a little bit shorter
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by one base pair
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and that is again going to have some
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effects on the genetic code
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and the proteins that it codes for
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non-point mutations
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can also occur these are mutations where
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you have more than one base pair
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affected so you can have
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for example the substitution of more
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than one base pair or
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the in insertion or deletion of
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two or three or more base pairs and so
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the consequences of any of these
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mutations
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the base pair substitutions insertions
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or deletions
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can vary depending on the type of
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mutation that occurs
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and what effect they can have on the
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actual genetic code so
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let's take a look at that next what are
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the effects of those mutations
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on the protein sequence itself
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so for the next few slides we're going
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to take a look at
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several different effects that changes
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in the genetic code can have on the
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protein sequence
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and each of the next few slides are
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going to look very similar to this one
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on the left hand side you have in purple
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the initial non-mutated sequence of a
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strand of dna
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notice that it's double-stranded right
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because dna is double-stranded but then
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you see
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beneath it the mrna the single-stranded
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mrna
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that that dna sequence codes for
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so transcription has happened and we
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have that section of mrna
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and we're going to then show what that
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altered sequence would look like on the
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mrna
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which in turn is then going to be
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translated into a polypeptide
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an amino acid sequence there in blue
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i've also added the genetic code so that
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we can kind of follow along
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to see what the effects of the genetic
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mutations are so let's start with this
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first genetic mutation so you can see
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here
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the mutation has replaced a thymine
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adenine-based pair with a cytosine
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guanine base pair so this is a base
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substitution
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and in the initial sequence the original
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non-mutated sequence
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that particular genetic code would have
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been
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transcribed into an mrna that had a
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codon
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uca remember that the genetic code is
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read in three letters
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and each of the three letters is called
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a codon and so when those
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three letters are then translated
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into an amino acid sequence take a look
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at
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the genetic code table at the bottom
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then
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uca would actually code for the amino
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acid serine
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so serine ser that's its short form
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it's found right here now in the mutated
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version of the codon though
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we no longer have uca we now have cca
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and cca no longer codes for serine it
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actually
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codes for proline so now we have one
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altered amino acid
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so in this type of mutation which we
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call the missense mutation
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a single amino acid has been changed due
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to usually
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a single base pair substitution we
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substitute one letter
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in the initial sequence of the genetic
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code which in turn
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leads to a different codon that is going
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to code for a different amino acid so we
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call these
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missense mutations another thing that
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can happen when you have a base pair
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substitution
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so again here we go we're going to
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substitute a different base pair in this
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particular case the thymine to adenine
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is going to
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be substituted into a cytosine to
00:16:14
guanine but it's a different one
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and in this particular case then the
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codon that is affected
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is the previous codon which is ggu
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now ggu codes for the amino acid glycine
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that's the original code now let's take
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a look at what happens after the
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mutation
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so in this particular case as to the
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mutation the codon
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ggu is now ggc
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let's take a look at what effect that
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has on the
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amino acid that is coded for well ggc
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well it also codes for glycine so
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this particular type of mutation
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actually
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has no effect on the amino acid sequence
00:16:58
and we call that a silent mutation
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same type of mutation that occurred in
00:17:03
the previous example with a base pair
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substitution
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but because in the previous example it
00:17:09
was the first letter
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in the codon that was substituted that
00:17:13
actually led to
00:17:15
a different amino acid being coded for
00:17:17
but because in this particular case it
00:17:19
is the third letter in the codon that is
00:17:21
substituted and oftentimes
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the third letter in the codon does not
00:17:26
really matter
00:17:27
in terms of determining what amino acid
00:17:29
is being coded for because many amino
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acids
00:17:32
are coded for by more than one codon and
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it's usually that third letter
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that might differ between them so in
00:17:39
this particular case glycine
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is coded for by the codon
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gga ggc ggu
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and ggg so any four of those codons
00:17:53
would still code for glycine
00:17:54
so let's take a look at another mutation
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in this particular case
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we have now another base substitution
00:18:00
we've substituted
00:18:01
this cytosine to guanine-based pair for
00:18:04
this adenine to thymine
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in the original codon we have a codon e
00:18:09
ugc which codes for the amino acid
00:18:12
cysteine
00:18:13
right here let's take a look at what
00:18:15
happens after this mutation
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the new codon is uga
00:18:20
that's very similar to ugc but in this
00:18:24
case
00:18:24
changing that third letter in the codon
00:18:27
actually
00:18:28
does not lead to the exact same amino
00:18:30
acid as the previous
00:18:31
code but it actually leads to a stop
00:18:34
codon
00:18:35
and so with a premature stop codon
00:18:38
now the sequence is going to be
00:18:40
terminated when the mrna is read by
00:18:43
ribosomes it's going to read a stop
00:18:45
codon
00:18:46
very early on in the mrna sequence and
00:18:49
is going to terminate translation
00:18:51
and a polypeptide that might have been
00:18:53
maybe a thousand amino acids
00:18:56
long might now just be 10 15
00:18:59
800 but it's going to be shorter and so
00:19:02
whenever that happens we call that a
00:19:03
nonsense mutation
00:19:05
a nonsense mutation is a mutation that
00:19:07
causes one of those three stop codons to
00:19:09
appear
00:19:11
somewhere within the genetic code well
00:19:14
before it is meant to appear
00:19:16
which shortens the length of the
00:19:17
polypeptide
00:19:19
the opposite of that would be what we
00:19:20
call the sense mutation
00:19:22
a sense mutation would be one in which a
00:19:24
stop codon is lost
00:19:26
so this would be a mutation in which say
00:19:29
the original
00:19:30
code might have been uga which coded for
00:19:33
a stop codon
00:19:34
but now let's say for example that
00:19:36
changes to ugc
00:19:38
say and then so now instead of having a
00:19:40
stop codon we add a cysteine amino acid
00:19:43
and then the mrna is going to continue
00:19:45
to be red and adding more amino acids so
00:19:48
you end up with a protein that is longer
00:19:50
than the original protein
00:19:51
and so having a shorter protein or a
00:19:53
longer protein
00:19:54
sequence is going to be detrimental to
00:19:57
the shape of the protein and therefore
00:19:59
the function of the proteins
00:20:00
so let's take a look at what happens
00:20:02
when instead of a base substitution we
00:20:04
have something called an
00:20:05
insertion and so in this case let's add
00:20:09
a base
00:20:10
right here between that guanine to
00:20:12
cytoscene
00:20:13
and guanine to cytosine bases let's add
00:20:16
one base pair right there and so the
00:20:19
original
00:20:20
code would code for the codon ggu
00:20:23
in the original sequence which codes for
00:20:26
the amino acid glycine
00:20:29
and after the insertion we now
00:20:32
have an adenine in between those two
00:20:34
guanines so instead of being ggu the
00:20:37
codon will be read as
00:20:39
gag because now there's an extra letter
00:20:41
in between those two guanines
00:20:43
and because ribosomes read the dna code
00:20:47
three letters at a time regardless of
00:20:49
what it was meant to be initially
00:20:51
it's not going to count the fact that
00:20:53
atomy shouldn't have been there it just
00:20:54
reads
00:20:54
the next three letters so the next three
00:20:56
letters in that mrna sequence
00:20:58
instead of being ggu is now gag
00:21:02
and so gag codes for glutamic acid which
00:21:06
is a completely different amino acid
00:21:09
but then let's take a look at what
00:21:10
happens to the code after that point too
00:21:13
because the next codon in the original
00:21:15
sequence would have been
00:21:16
uca which would code for the amino acid
00:21:19
serine
00:21:20
and in this new code because we added an
00:21:23
adenine
00:21:24
in the previous codon then the uracil
00:21:29
in that codon instead of being the third
00:21:31
letter in that codon it becomes the
00:21:33
first letter on the next codon
00:21:35
and so instead of having uca
00:21:38
we now have uuc for the next codon
00:21:42
and that codes for the amino acid
00:21:44
phenylalanine
00:21:46
and then every single codon after that
00:21:50
is similarly affected
00:21:51
so essentially by adding a base pair in
00:21:54
the middle of a genetic sequence
00:21:56
we've essentially shifted the reading
00:22:00
frame
00:22:01
of the mrna so that the codons are read
00:22:04
off by
00:22:05
one and so we call this frameshift
00:22:07
mutations
00:22:08
and frameshift mutations they
00:22:10
essentially cause
00:22:12
every single amino acid from the point
00:22:15
of the mutation on
00:22:16
to be different than the original code
00:22:19
because of a shifting on the reading
00:22:22
frame of the mrna sequence because
00:22:24
ribosomes read mrna
00:22:26
three letters at a time regardless of
00:22:29
what the initial sequence was supposed
00:22:30
to be
00:22:31
frame shift mutations do not only happen
00:22:33
when you have insertions
00:22:35
that will also happens when you have
00:22:36
deletions same effect
00:22:38
adding a base or removing a base will
00:22:40
essentially
00:22:41
shift the reading frame of the
00:22:44
mrna so that codons are going to be off
00:22:47
by
00:22:48
one or if it's two by two from that
00:22:51
point on
00:22:52
the only time the friendship mutations
00:22:54
are not as bad
00:22:55
is when you have three bases that are
00:22:57
inserted or deleted
00:22:58
because the codons are three bases long
00:23:01
by
00:23:02
inserting three bases or deleting three
00:23:04
bases we're essentially just either
00:23:06
deleting one amino acid or inserting
00:23:08
an extra amino acid which could have a
00:23:09
very bad effect but may not
00:23:11
be that dangerous to the organism as a
00:23:14
frameshift mutation is which
00:23:16
essentially creates a completely
00:23:18
different protein from the point of
00:23:19
mutation on as with the original
00:23:21
intended so
00:23:22
there is no way that this new protein is
00:23:24
going to work
00:23:25
at all like it was intended because it
00:23:28
is essentially a completely different
00:23:30
code from that from the point of the
00:23:31
mutation on
00:23:32
so just very briefly let's just
00:23:34
summarize the different types of
00:23:36
mutations and their effects
00:23:37
substitutions can lead to missense
00:23:39
mutations silent mutations
00:23:41
nonsense or sense mutations but they
00:23:43
never lead to frame shift mutations
00:23:45
whereas insertions in the lesions will
00:23:47
always lead to friendship mutations of
00:23:50
some form
00:23:51
unless it is three bases that are
00:23:53
inserted or three
00:23:54
bases that are deleted but
00:23:57
this friendship mutations themselves can
00:23:59
also lead to nonsense mutations
00:24:01
if i'd stop coding appears too early as
00:24:03
a result of the frame shift
00:24:05
or a sense mutation if a stop codon does
00:24:07
not appear early enough as a result of
00:24:09
the frame shift
00:24:11
so let's take a look now how mutations
00:24:13
can affect the entire organism
00:24:15
there's a variety of different ways by
00:24:17
which mutations can affect an organism
00:24:19
but in general we're going to either see
00:24:21
maybe something like a morphological
00:24:22
change which is an
00:24:23
obvious physical change in
00:24:25
characteristics of the organism
00:24:27
biochemical changes are not as obvious
00:24:30
they're inside the body
00:24:32
but for example in the case of cystic
00:24:34
fibrosis a single base pair substitution
00:24:37
leads to a missense mutation
00:24:39
with a single amino acid change in the
00:24:42
cftr
00:24:43
chloride channel which now produces a
00:24:45
mutant and non-working chloride channel
00:24:47
protein which is the cause
00:24:49
of cystic fibrosis and of course
00:24:52
mutations can sometimes be lethal
00:24:54
if they're completely incompatible with
00:24:55
life the causes of mutations are
00:24:58
basically two-fold
00:24:59
they can either occur spontaneously
00:25:02
during
00:25:03
errors that occur naturally during dna
00:25:05
replication
00:25:06
that either missed by the last step
00:25:09
which is the proofreading step of dna
00:25:11
replication
00:25:12
and therefore accumulate in the cell and
00:25:15
this is one of the main causes of both
00:25:17
aging and cancers just
00:25:19
natural spontaneous errors and dna
00:25:21
replication
00:25:22
but they can also be induced by our
00:25:24
environment so these are what we call
00:25:25
induced mutations and induced mutations
00:25:27
can come from
00:25:28
a variety of different forms or
00:25:30
environmental agents called mutagens and
00:25:32
mutagens can be things like radiation
00:25:34
like uv radiation or x-rays chemicals in
00:25:38
our environment
00:25:39
chemicals in our food or even infectious
00:25:42
agents like viruses
00:25:44
or bacteria that can actually cause
00:25:46
genetic mutations as well
00:25:48
so there's a variety of different things
00:25:49
that can cause mutations
00:25:51
and so that's the lesson for today i
00:25:53
will talk to you later
00:25:55
bye
