Source: Video and Images Created by Amanda Soderlind
Welcome to this lesson today on protein synthesis transcription. Today, we are going to be discussing the process of transcription and its role in protein synthesis. The path from genes to proteins involves two steps, transcription and translation. Transcription is the first of those two steps.
In transcription, a single strand of RNA is assembled using DNA as a template. It's going to pair using those base pair rules. And this process of transcription occurs in the nucleus of the cell.
Basically, as I mentioned, we're assembling RNA from a DNA template. We're going to talk real briefly just about the differences between DNA and RNA. RNA stands for ribonucleic acid, and DNA stands for deoxyribonucleic acid.
Both of these types of molecules contain genetic information, but RNA, genetic information that it contains, is specific to protein building. So it helps to build proteins within the cell. RNA is also single stranded, while DNA is double stranded. DNA has our double helix; it's double stranded. Whereas, RNA would just be a single strand.
RNA has ribose sugar in its nucleotide. And DNA has a deoxyribose sugar in its nucleotide. Then also in the nucleotide, the nitrogen bases of RNA are adenine, uracil, cytosine, and guanine. Whereas in DNA, our nitrogen bases in the nucleotides are adenine, thymine, cytosine, and guanine.
As I mentioned, when we're doing this process of transcription-- we're using the DNA as a template to make RNA-- it pairs using the base pair rules. So if we have a strand of DNA, for example, and let's say the base pairs that we have on it are adenine, thymine, cytosine, and guanine. So these are our nucleotides bases for DNA.
If we're making an RNA, using this DNA as a template, it's going to pair with uracil. So adenine pairs uracil. Adenine of the DNA pairs with uracil of the RNA. Because RNA does not contain thymine. So adenine pairs with uracil.
Then thymine would pair with adenine, cytosine with guanine, and guanine with cytosine. So this is how the base pairing works when RNA is being built off a DNA template.
We have three different types of RNA. Genes are transcribed into these three types of RNA. Messenger RNA, called mRNA, carries protein-building instructions and is the type of RNA that's actually transcribed into a protein. rRNA is ribosomal RNA. And ribosomal RNA combines with proteins to form a ribosome, which plays a role in protein synthesis.
Real quick, structure of a ribosome is something like this. Excuse my drawing skills. But basically, it's composed of a large subunit and a small subunit.
These subunits are produced in the nucleolus and then sent out into the cytoplasm. And then in the cytoplasm, they join together and help produce proteins in the second step of protein synthesis, which is called translation. So this is just a brief intro to the structure of a ribosome.
Then tRNA stands for transfer RNA. Transfer RNA, basically, picks up amino acids and pairs with mRNA to build polypeptide chains. In transcription-- in this process of transcription-- we're focusing on messenger RNA, because ribosomal RNA and transfer RNA play more of a role in transcription-- oh, I'm sorry-- in translation and don't really play a role in transcription.
In transcription, we're focusing on messenger RNA. So let's take a look at how the process of transcription works. We'll use this diagram here.
Basically, we have a gene region on a strand of DNA. We're looking at a specific gene region on a strand of DNA. Basically, genes are transcribed into proteins. So when we're looking at this, we're looking at a specific gene region and not a whole entire strand of DNA. But we're focusing on a specific gene region that will code for certain types of proteins.
OK, so this is a gene region on a strand of DNA. Basically, what's going to happen is an enzyme, called RNA polymerase-- this pink dotted part around here is representing RNA polymerase-- is going to unwind the DNA. RNA polymerase is unwinding the DNA, separating it, and then it's allowing messenger RNA-- so in the orange here, this is our forming messenger RNA. The DNA has been unwound, and then messenger RNA is forming, using this strand of DNA as a template like we talked about.
This strand of DNA is being used as a template to form a strand of messenger RNA. And then, basically, this RNA polymerase will just move down this gene region as that messenger RNA is built. And then, when it's totally finished, the DNA will wind back up into what we started with up here.
If we actually look at this section right here and we zoom in, that's what we have right here. This diagram, here, is basically just a zoomed in picture of what we would be looking at here.
As I mentioned, the DNA is used as a template to form messenger RNA, which will then be used in the next stage of protein synthesis to produce proteins. This is our strand of messenger RNA. This is our strand of DNA.
We have something in messenger RNA called a codon. Codons are sets of three nucleotides in messenger RNA that are used to build proteins. And codons code for amino acids. There are a total of 64 different types of codons that make up the genetic code and provide these protein making instructions.
Start codons mark the first amino acid of a polypeptide chain. AUG is an example of a start code. So if we had adenine, uracil, guanine together in this chunk of three nucleotides, that would signal the start of a polypeptide chain.
Then a stop code marks the end of a polypeptide chain. And stop codons are UAA, UAG, and UGA. It marks the end of a polypeptide chain. That's when that polypeptide chain would be finished being made. So start and stop codons.
And then in between, we have these other codons that code for specific amino acids. And then, as those amino acids are being built, they're linked together. And then they form polypeptide chains.
Basically, we're using the DNA as a template for this mRNA to be built. And then depending on the nucleotides that line up in this mRNA, it forms these codons, which then code for specific amino acids, which then form polypeptide chains. Once the nucleotides are joined, the messenger RNA is released from the DNA. The DNA rewinds back together.
But at this point, the messenger RNA is not completely finished. So introns are parts of this messenger RNA that don't code for proteins. Basically, they're just snipped out, and axons are the parts of the messenger RNA that do code for proteins. Once the introns are snipped out, the axons are spliced together. And then, the messenger RNA can head into the cytoplasm for the next stage of protein synthesis, which is translation.
This lesson has been an overview on the process of transcription in protein synthesis.