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The week before the 1st day of my first year
of teaching…wow, over a decade ago…was
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an exciting time.
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It was also stressful because I’m a bit
of a perfectionist---but a bit of a messy,
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unable to decorate my classroom kind of perfectionist---actually
maybe I’m not a perfectionist.
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I wanted to make my classroom inviting and
exciting and…I really wanted my students
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to walk in and think, “This is AMAZING.”
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I could never make it look like what I imagined
up.
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But, the most amazing thing did happen in
the week to follow.
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I knew a colleague who had a friend who had
a son who was going to college-confusing-
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but anyway that son could not take his pet
snake to college with him.
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Some rule about dorms and snakes.
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My colleague asked, “Would you like it?
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They said they’d give you the cage and everything;
it could be a classroom pet.”
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I didn’t even have to think about it.
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Of course.
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So Spike, a Texas rat snake, became a classroom
pet, an amazing, popular one at that.
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He was friendly---well, for a snake… he’d
let you hold him and not bite.
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That year, I was the only one in the science
wing with a pet snake and I’m pretty sure
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there were some people that wondered what
was the point.
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Well, I’m big on a relevance so anytime
I would teach a biology concept, I found some
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way to work in Spike.
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Predation?
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Well, let’s talk about Spike’s appetite
for rats.
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Mitosis?
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Let’s talk about why Spike even needs to
make more cells.
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One day in tutorials, a student asked me,
“Since Spike’s parents were bred in captivity,
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did you ever see Spike’s parents?
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Do you think Spike’s parents looked like
him?”
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You can’t ask a question like that and not
expect an answer!
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It was a GREAT question because we were getting
close to our heredity unit.
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Heredity is about how traits are passed down
from parent to offspring.
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We’ve made a playlist of our videos that
focus on heredity including reproduction,
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how to track inheritance in pedigrees, how
to solve genetic problems in Punnett squares,
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and understanding different Mendelian and
non-Mendelian inheritance.
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But you really can’t delve into those and
study heredity without understanding DNA,
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chromosomes, genes, and traits- and that’s
what this introductory video is going to focus
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on.
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So back to the student question.
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Spike has traits.
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The patterns on his body, his size---these
are all traits.
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These traits are coded for in his DNA.
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Some of the traits he inherited can be influenced
by the environment.
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For example, if Spike had not had the nourishment
he needed, that could affect his size.
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Spike’s DNA---the whole DNA code---is actually
found in nearly all of his body cells.
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DNA is not just one big code buried deep down
in an organism like some treasure.
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Spike’s DNA is in the nuclei of nearly all
of his body cells.
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He inherited his DNA from his mother and father.
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I can’t know for sure what Spike’s parents
looked like, but I do know that Spike inherited
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his DNA from them.
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One fun fact: many snake species can reproduce
asexually.
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Had that been the case for Spike, he would
have inherited all of his DNA from only one
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parent.
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But it would still be DNA coding for traits.
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Just like Spike, your DNA codes for your traits,
and your cells can’t function without it.
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DNA determines how tall you are, what color
your eyes are, what color your hair is, or
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even if you’re at risk for certain diseases.
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Also like Spike, your ENTIRE DNA code is in
most of your body cells.
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That’s why in those crime solving shows,
which we may or may not absolutely love, a
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criminal can sometimes be caught by just leaving
a cell from a hair follicle behind.
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DNA has a beautiful structure and that structure
will help you understand how inheritance works.
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DNA stands for deoxyribonucleic acid.
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It’s a type of nucleic acid.
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If you remember from our biomolecules video,
nucleic acids are a type of biomolecule.
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Nucleic acids are made up of building blocks
called nucleotides.
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Nucleotides have 3 parts.
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One of them is a sugar called deoxyribose.
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One of them is a phosphate.
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We sometimes say DNA has a sugar-phosphate
backbone.
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But the most important part of the nucleotide
is the base, because the sequence of the bases
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actually code for traits.
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So as far as the bases go, there are four
types of bases in DNA.
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A lot of times they’ll just use the letters
A, T, C, G.
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The a is for adenine.
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The T is thymine.
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The C is for cytosine, and the G is for guanine.
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These bases actually pair in a specific way,
and there is a popular mnemonic that can help
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you remember which of them pair together:
apples in the tree; that tells you that A
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for apples, T for trees, because the bases
A and T go together.
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The other verse is: car in the garage; that
can help you remember that the base C always
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goes with the base G. Regardless of whether
we’re talking about Spike, or a plant, or
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a protist, or a human like you…these are
the DNA bases in living organisms.
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But the amount of DNA bases overall---and
the sequence of those bases we mentioned----will
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vary among different species, and also, among
different individuals.
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But…it’s likely to infer that Spike has
a sequence of DNA bases that is more similar
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to his parents than he would to, say, a rattlesnake.
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DNA has two strands so there are nucleotides
running up one side and there’s nucleotides
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running up the other side.
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The bases are what pair in the middle.
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The bases are held together by hydrogen bonds.
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The DNA is also twisted in something we call
a double-helix shape.
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Portions of DNA make up genes.
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We can say, for example, that this part of
the DNA here makes up a gene.
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Genes can code for proteins.
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Proteins can have a huge role in expressing
a trait.
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For example, let’s consider your own eye
color.
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Human eye color is a pretty complex trait
that is actually determined by many genes.
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The genes can code for proteins involved in
producing the eye color pigment.
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But proteins coded for by genes play a wide
variety of roles besides just your eye color.
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Proteins are involved in transport, in structure,
in acting as enzymes that can make all kinds
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of materials, in protecting the body…and
so much more.
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We do want to mention that not all genes are
used to make protein.
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And there are parts of DNA that are noncoding.
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And even though nearly all of your body cells
have your entire DNA code---your body cells
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may only use certain portions of those genes.
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Genes can be turned on or turned off by a
variety of mechanisms.
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We call that gene regulation; check out our
video on that.
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Now you have a lot of DNA.
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When it is compacted, it can be organized
into a unit called a chromosome.
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Very helpful when you’re trying to make
more cells and need to get the DNA into those
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new cells.
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Chromosomes in your body involve DNA wrapped
around protein structures.
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Humans have 46 chromosomes.
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That means nearly every body cell in your
body has 46 chromosomes.
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Human sperm and egg cells, on the other hand,
each contain 23 chromosomes.
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So you received 23 chromosomes from your mother
and 23 chromosomes from your father to give
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you your 46 chromosomes.
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Your genetic code.
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So let’s do a recap of the big picture:
here’s a single chromosome.
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You see genes on this chromosome.
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These genes consist of portions of DNA.
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DNA is made up of nucleotides, and it’s
these bases here---the sequence of them---that
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makes the difference in coding traits.
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Phew.
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Understanding this foundation is essential
for understanding heredity---whether you’re
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talking about you----or Spike.
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So this may bring up more questions now like
where do dominant and recessive traits come
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in?
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What about alleles and how do you arrange
those around a Punnett square?
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You can explore those concepts and more in
our heredity playlist.
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Well that’s it for the Amoeba Sisters and
we remind you to stay curious!
