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