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After learning about DNA, have you ever wondered,
how can the DNA actually result in a trait?
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Let's take an example - like eye color.
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Yes, your DNA has the genetic information
that codes for the color of your eyes.
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Your eye color is based on a pigment that
is inside the eyes.
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But, in order to have that pigment, you have
genes, which are portions of DNA, that can
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code for proteins which help make that pigment.
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So what we’re going to talk about is how
your DNA can lead to the making of a protein.
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This process is called protein synthesis.
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Synthesis essentially means to “make something”
so protein synthesis means to make protein.
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And you may wonder, “What’s the big deal
about proteins?”
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Well you may not realize this but proteins
are kind of a big deal.
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They do all kinds of things.
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Proteins are involved in transport, in structure,
in acting as enzymes that make all kinds of
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materials, in protecting the body…and so
much more.
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You've got to make proteins - it’s essential
for you to live.
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And what is so COOL is that you are making
proteins right now as you sit and watch this
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video.
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It’s happening in your cells.
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They’re making proteins.
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So back to your DNA and its role in all of
this.
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All of your cells have DNA---well a few exceptions---and
that DNA is in the nucleus.
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Some DNA is noncoding DNA.
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Some DNA makes up genes that are not activated.
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More about that in our gene regulation video.
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But we’re going to talk about genes that
are coding for active proteins.
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So how are we going to get the information
from these genes out of the nucleus so that
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the cell can start producing the proteins
that it needs to make?
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Well let us introduce you to the amazing work
of RNA.
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We have a video comparing and contrasting
RNA---we’ll be brief here in saying that
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RNA is a nucleic acid like DNA.
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But it has a few differences.
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Its role in protein synthesis also is HUGE.
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Before we get into the process, please note
our typical disclaimer: we tend to simplify
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topics while staying as accurate as we can---but
as always, we hope you will have the desire
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to explore this complex, amazing process later
on to learn all about the extra information
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that we don’t have the ability to include
in this short video.
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In protein synthesis, we can look at two major
steps.
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One is transcription and the other is translation.
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Transcription has a C in it, and translation
has an L in it.
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I remember that C comes before L in the alphabet,
which helps me remember that transcription
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comes first.
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I like a lot of alphabet mnemonics.
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Now, transcription is when we’re going to
transcribe the DNA into a message.
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In your cells, the DNA is in the nucleus,
so therefore, we’re doing transcription
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in the nucleus.
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In the step of transcription, an enzyme called
RNA polymerase will connect complementary
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RNA bases to the DNA.
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These RNA bases are bonded together to form
a single stranded mRNA.
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The m in mRNA stands for messenger. Messenger RNA consists of a message made of RNA that has been based
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on the DNA.
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We do want to mention that this mRNA is not
usually ready to go right away.
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There's usually a significant amount of mRNA editing that occurs--- we highly encourage you to
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do some reading about that because it’s
not only fascinating---it’s critical for
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the process to work correctly.
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So what's something great about being mRNA?
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Well in eukaryotes, you get out of the nucleus.
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The mRNA can go out of the nucleus into the cytoplasm where it’s going to attach to
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a ribosome.
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Ribosomes make protein.
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The ribosome is made of rRNA, and that’s
an easy one to remember because the “r”
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stands for ribosomal RNA.
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The ribosome is going to build our protein
in the next step called translation!
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You know, you can find a lot of great clips
and animations on translation that are just
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fantastic.
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We’re just going to break down some basics of what's happening.
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In the cytoplasm, if you look at this, you have all these tRNA molecules available.
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tRNA stands for transfer RNA.
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They carry an amino acid on them.
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An amino acid is the monomer for a protein;
it's a building block for protein.
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Since we’re making proteins, we’re going
to need those amino acids to build it.
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If you have a bunch of amino acids together,
you can build a protein.
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So it’s the tRNA that is going to bring
those amino acids together to make that.
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But wait, how does the tRNA know which amino
acids to bring?
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That’s why the mRNA, the message, is so
important because it’s going to direct which
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tRNAs come in and therefore which amino acids
are transferred.
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All of these tRNAs are looking for complementary bases.
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When they find the complementary bases on the mRNA, they transfer their amino acid
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When tRNA is bringing in the amino acids,
it reads the bases---represented by these
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letters here on the mRNA--- in threes.
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So it doesn’t read one letter at a time;
it reads it in triplets.
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That’s called a codon.
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So, for example, in this mRNA, the tRNA would
read the codon AUG.
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One of these tRNAs contains a complementary
anticodon---which in this case is UAC.
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All tRNAs that have the anticodon UAC will
be carrying an amino acid called methionine.
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A tRNA with the UAC anticodon comes in to
pair with the complementary AUG codon
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on the mRNA.
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It transfers the amino acid it carries, methionine.
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The tRNA will eventually leave, but it will
leave behind its amino acid.
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That’s the first amino acid before looking at the next codon.
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Before we do the next codon to carry this on--- if you’re wondering---how did you
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know that the tRNA that went with the AUG codon
would be carrying an amino acid called methionine?
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Well, for that, you will find a codon chart
helpful!
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You can learn to use a codon chart to determine
which amino acid each mRNA codon will code
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for.
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Isn’t is so fascinating that scientists
have been able to determine which amino acid
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corresponds with these codons?
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I used to have a codon chart poster and just
marvel at that.
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You can see on a codon chart that the AUG
codon on the mRNA codes for methionine.
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AUG is also considered a start codon as methionine
is typically going to be your first amino
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acid in proteins.
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There are many types of amino acids in the
codon chart, but there are even more possible
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codon combinations.
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That means there can be more than one codon
that code for the same amino acid.
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For example, according to the mRNA codon chart,
all of these mRNA codons here code for the same
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amino acid: leucine.
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That means their complementary tRNAs all carry
the same amino acid: leucine.
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Ok, so going back to the mRNA---let’s try
the next codon on this mRNA.
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CCA.
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On the codon chart, you can see that codes
for the amino acid proline.
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The complementary tRNA has the anticodon GGU and look, there’s the proline that we knew
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it would be carrying.
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The tRNA will transfer that amino acid and
eventually leave where it can go pick up another
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amino acid.
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These amino acids are held together by a peptide
bond.
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And it will keep on growing.
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Typically at the very end of the mRNA, there
is a stop codon.
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Stop codons do not code for an amino acid, but when the ribosome reaches it, it indicates
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that the protein building is finished.
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So the result of translation is that you built
a chain of amino acids that were brought in
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certain sequences based on the coding of
the mRNA.
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But remember that mRNA was complementary to
the DNA.
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So the DNA ultimately was the director of
the entire protein building, of course, it
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couldn’t have done it without some serious
help from mRNA, rRNA, and tRNA.
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Protein folding and modification may occur
and the protein may need to be transported---this
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can all vary based on the protein's structure and function.
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Another fascinating topic for another Amoeba
Sisters video.
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Well that’s it for the Amoeba Sisters and
we remind you to stay curious!
