Transcription 1

Transcription 1

– [Voiceover] The central
dogma of molecular biology tells us that DNA makes RNA makes protein. And so, just to clarify, the information that’s
in DNA is transmitted, or copied, onto RNA, in a process called transcription, and then the information that’s in the RNA is used to make the
corresponding polypeptide in a process, translation. And so, just to recap, E3 nucleotides on the DNA are going to code for one amino acid, and so on the RNA each of the nucleotides, as well, code for one amino acid, and then in translation those amino acids are put together to form the polypeptide which
then makes the protein. Right now, we’re going to
focus on transcription. Before we talk about transcription, we know a bit about DNA, but what is RNA? RNA stands for ribonucleic acid, and it’s very similar to DNA, so let’s talk about some
of the similarities. They’re both nucleic acids. Both DNA and RNA are nucleic acids. That means that they’re made up of units called nucleotides. Each nucleotide has a sugar, a phosphate group, and a nitrogen base. So, let’s draw a nucleotide. Here’s our backbone, so that would be the sugar
and the phosphate group. That would be the backbone part. And then let’s draw a nucleotide. I’m just going to draw
a yellow nucleotide, which happens to represent thiamine, so that would be our nitrogen base. So, that’s how DNA and RNA are similar. What about some of the differences between DNA and RNA? I’m just going to make some room here. By looking at it, the first thing that’s obvious is DNA is double-stranded, and RNA is single-stranded, usually. There are some exceptions to this. There are some viruses that
have double-stranded DNA. I’m sorry, double-stranded RNA. But, in general, RNA is single-stranded. Another difference between DNA and RNA is that the sugar in DNA is deoxyribose, and the sugar in RNA is ribose, And they’re pretty similar, but let’s just take a quick look at what deoxyribose and ribose look like. So, then molecule on the
left, right over here, this is deoxyribose, and the molecule on the right is ribose, and they’re extremely
similar to each other. The only difference between them is that in this spot right over here deoxyribose has just a hydrogen and ribose has an OH, an oxygen and a hydrogen, so deoxyribose tells us that it’s missing an oxygen, it’s deoxy, without an oxygen that would normally be found in ribose. That’s the only difference between deoxyribose and ribose. Let’s go back to talking
about the differences between DNA and RNA. Another difference between the two is that in DNA, one of the nitrogen bases is thiamine, and it’s one of the pyrimidines, and in RNA wherever we would normally have a thiamine, instead we have a uracil, and uracil is also very
similar to thiamine, it’s also a pyrimidine, and we’re going to take a quick look at these two molecules to
see what they look like. So, on the left, we have
a thiamine molecule, and on the right we have a uracil, and the only difference
between these two molecules is that in this spot over here, there’s a methyl group in thiamine and in uracil there’s only a hydrogen, so you can see they’re extremely similar to each other. There’s one more difference
I want to mention. DNA is more stable than RNA. And, we want this, because DNA is actually what stores
genetic information in a permanent way, so we want it to be a
very, very stable molecule that can hold onto the information for long amounts of time. RNA, on the other hand, its job is just to transfer
the genetic information from the DNA to the ribosome
to make our proteins, and so it’s not as important for RNA to be as stable as DNA, and RNA is constantly made in the cell and degraded and then
recycled all the time, whereas DNA kind of just stays put. So, now that we spoke about
some of the differences between DNA and RNA, let’s focus a little bit more on RNA. So, here we have our RNA molecule, and then we’re also going to just leave our DNA for now. There are actually a number of different types of RNA. The first RNA we’re going to talk about is mRNA, and that M
stands for “messenger”. Messenger RNA is the RNA that’s made during transcription, so if we were to transcribe this DNA, we would make a piece of mRNA, and the job of mRNA is to
then go to the ribosome, and the mRNA is going to kind of run through the ribosome something that looks like this, and then the ribosome
is going to put together the corresponding polypeptide chain. Hence, the name messenger RNA, it takes the information
that’s stored in the DNA and then acts like a messenger by bringing that
information to the ribosome so that the ribosome can put together the corresponding polypeptide. I want to just digress for a bit and talk about some of the differences in mRNA that you’d find
between eukaryotic cells and prokaryotic cells. So, in eukaryotic cells, mRNA is usually monocistronic, and that means that one piece of mRNA is generally going to code
for one polypeptide chain. Prokaryotic mRNA can be polycistronic, that means that you can
have one piece of mRNA that’s coding for a few polypeptides, so you might have one part that codes for one polypeptide, and then maybe the next
part codes for another one, maybe it codes for a third polypeptide, and a fourth polypeptide, and usually when you have a piece of polycistronic mRNA, the polypeptides that
are being coded for are related in function in some way. Another difference I want to mention is where mRNA is made. In eukaryotic cells, mRNA is made in the nucleus. And for that matter, transcription also happens in the nucleus. I’m just going to abbreviate it like that. In prokaryotic cells, well, they don’t have a nucleus, so mRNA is made in the cytoplasm. And, well, transcription also happens in the cytoplasm so I’m just going to write “also transcription”, just going to abbreviate it, as well. Interestingly, in prokaryotic cells, translation can actually happen at the same time as transcription, so you can have a piece of mRNA being made and while it’s being made, it starts to attach to the ribosome and be translated, and the reason that that can happen is that they’re happening
in the same place. In eukaryotic cells, that can’t happen. The mRNA is made in the nucleus, and then it has to leave the nucleus and go to the cytoplasm, where the ribosomes
will then translate it. The last difference I want to mention is that in prokaryotic cells the primary transcript, I’m going to explain
what that is in a moment, the primary transcript is mRNA. The primary transcript refers to the RNA that’s made during transcription, so in prokaryotic cells, the RNA that’s made, that’s mRNA. That’s ready to go to the
ribosome and be translated. Not so in eukaryotic cells. In eukaryotic cells, the primary transcript must be processed. After it’s processed,
well, it becomes mRNA. So only after it’s processed
is it messenger RNA and can it go to the
ribosome and be translated. The next type of RNA I want to talk about is rRNA, and that R stands for ribosomal RNA. Ribosomal RNA is found in the ribosome. The ribosome is made up of two components, made up of rRNA and it’s also made up
of other polypeptides. The interesting thing about ribosomal RNA is that even though it’s a nucleic acid, it actually acts like an enzyme, and this is kind of odd because usually proteins act as enzymes, and nucleic acids do not act as enzymes, so this is just an interesting
fact to know about rRNA, and in fact rRNA is known as a ribozyme. A ribozyme is an RNA that
has enzymatic function. In fact, it’s the enzymatic activity of the rRNA in the ribosome that puts together the polypeptide. The last type of RNA I
want to talk to you about is tRNA. That T stands for transfer. Transfer RNA has this 3-D structure, and it looks a little bit like a clover. The job of transfer RNA is to carry amino acids, and then to bring these
amino acids to the ribosome, so that the ribosome can put together a polypeptide chain. Let’s just recap. We spoke about messenger RNA, we spoke about ribosomal RNA, and we spoke about transfer RNA. I just want to mention the enzymes that put together RNA in eukaryotic cells. In eukaryotic cells, mRNA is put together by RNA polymerase II. Ribosomal RNA is put
together by RNA polymerase I, and transfer RNA is put together by RNA polymerase III.

55 thoughts to “Transcription 1”

  1. It gives way too much background. I think it's reasonable to assume a certain level of knowledge in the people watching the video. I eventually just gave up on this video.

  2. I loved this, other vids dont cover everything so I'm left not understanding the little details but you broke it down perfectly. only thing I wanna know is what do you mean the RNA has to be processed to be mRNA? what occurs

  3. Not what I was looking for, and video was too long for a minimal amount of material covered. I know there is a version of this with Sal doing it somewhere so I'll look for that.

  4. I like how the video is called transcription but gives an overview of the entire central dogma. This is not very useful for advanced studies as a Khan Academy Medicine video should be for.

  5. This video was not very helpful!!!
    She goes off track, and shares what she calls, "fun facts" that are unnecessary.

  6. She told us what RNA stood for, which is fine, for people who have probably never had a biology class.  How did this video even get through??? I need more depth than what this video offered.

  7. This is "khanacademymedicine;" yet, this information is nowhere near comprehensive enough for anything medically related. Huge fail.

  8. For those people who complain about the video being too long or not informative enough : Go read the book or any other material that is required by your course. Use these videos as a complimentary material to what you are suppose to be studying. Too long video? Really? How long will it take you to read the chapter or section in your book? Read and understand – probably hours….. every piece of background information given in this video is a quick review for those who forgot material from gen bio 1.

    I think the video was very helpful and informative – connected the dots that I hadn't connected prior to watching this video. The second video is just as good as the first one.
    Thank you.

  9. whether her explanation may be boring or too long… I don't think it's as some of you commented. I'd rather thank her for her sincerity

  10. isn't transcription the same as production of mRNA, min: 12:05? so why do you say mRNA is made in the nucleus and also transcription?

  11. many people complaining such as such in the comment section just because it's not Sal who's discussing . damn brats 😆😆😆

  12. This video may be what you are looking for(AK Lecture)

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