Speaker A
In this video, we're going to talk about transcription and translation.
This video explains transcription and translation processes in protein synthesis, detailing steps, key components, and their biological roles.
Key Takeaways
- Transcription and translation are essential steps in protein synthesis.
- RNA polymerase reads the DNA template strand to create mRNA.
- mRNA codons correspond to specific amino acids via tRNA anticodons during translation.
- The ribosome facilitates peptide bond formation and protein assembly.
- Protein synthesis ends at stop codons, followed by protein folding in the Golgi apparatus.
Summary
- Transcription converts DNA into messenger RNA (mRNA) through initiation, elongation, and termination steps.
- RNA polymerase binds to the promoter region (TATA box) and synthesizes mRNA from the template strand.
- mRNA processing includes capping, poly-A tail addition, and RNA splicing to remove introns and retain exons.
- Translation uses mRNA codons to assemble proteins at the ribosome with the help of transfer RNA (tRNA).
- The ribosome has three sites (E, P, A) where tRNA molecules bind and amino acids form peptide bonds.
- Translation initiation starts at the start codon AUG, and elongation continues until a stop codon is reached.
- Stop codons (UAA, UAG, UGA) trigger release factors that disassemble the ribosome and end protein synthesis.
- Proteins are processed and folded in the Golgi body to achieve their functional shape.
- The video includes a practice problem on transcribing DNA to mRNA sequences.
- The explanation covers fundamental molecular biology concepts relevant to gene expression and protein synthesis.
Full Transcript — Download SRT & Markdown
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And here is just a basic overview of these two processes.
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Transcription is the process by which DNA gets converted into mRNA, also known as messenger RNA.
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And translation is the process of converting the information stored in messenger RNA and using it to build protein.
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Now, if you recall, which organelle is used to make proteins?
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What would you say?
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The answer is the ribosome. The ribosome is the site at which proteins are manufactured in the cell.
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So we're going to talk about that shortly, but let's go over transcription first before we talk about translation.
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Transcription occurs in three steps: initiation, elongation, and termination.
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Now, during initiation, RNA polymerase binds to the promoter region of DNA.
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Now, what is the promoter region?
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The promoter region is basically a short sequence of DNA.
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In eukaryotic DNA, it's T A T A A A, also known as the TATA box.
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Now, this sequence is located 25 nucleotides upstream of the site where transcription begins.
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The next thing that RNA polymerase does is it causes the two DNA strands to separate.
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And during elongation, it begins to add nucleotides to the growing mRNA strand that we see here.
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Thus, RNA polymerase synthesizes mRNA starting from the five prime end
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going to the three prime end.
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However, RNA polymerase, it reads the DNA strand
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in the three to the five prime direction.
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Now, there's two strands that you need to be familiar with.
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The first one, the one that's used to synthesize mRNA, is called the template strand
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or the antisense strand.
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So that's the one in which RNA polymerase is active upon.
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The other one, which is not used, this is called the non-template strand,
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also known as the sense strand.
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Some textbooks will call it the coding strand
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because its sequence matches up with RNA.
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Except the fact that uracil is found in RNA, but thymine is found in DNA.
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Now, during the last step of transcription, which is the termination step,
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the RNA polymerase molecule, the mRNA strand, they all separate from the DNA template strand.
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Now, in this step, the poly-A polymerase enzyme, it caps the three end of the mRNA strand.
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And this is known as the poly-A tail.
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Now, also, during the beginning of transcription,
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the five end is also capped.
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And the reason for this is to protect the mRNA strand from being degraded by certain enzymes.
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At the end of the termination step during transcription, DNA has been used to create a pre-messenger RNA strand.
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Now, this particular strand has something known as introns and exons.
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Introns are basically longer sequences of nucleotides that do not code for anything.
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So these, they must be removed in a process known as RNA splicing.
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Now, the exons, those are shorter sequences of nucleotides,
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and they're going to be used to synthesize proteins.
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And so the exons, they remain, but the introns, they must be removed.
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And so now we have a completed messenger RNA strand.
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Now, let's work on a practice problem.
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Let's say if you're given a sequence of nucleotides on a DNA strand.
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And you're asked to write the corresponding sequence on an mRNA strand.
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What would it be?
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Feel free to pause the video and try it.
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So if we're reading the DNA strand from the three to five direction,
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we're going to have to write the corresponding mRNA sequence in the five to three direction.
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So what letter corresponds to G?
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It's important to know that G always corresponds to C.
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And vice versa, C corresponds to G.
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Now what letter corresponds to A?
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A usually corresponds to T, but there's no T in RNA.
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Instead, A is going to correspond to U for uracil.
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But T in DNA corresponds to A in RNA.
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And so if we continue, everything else is going to be U, C, A, U, A, U, G, C.
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And so this is the mRNA strand that corresponds to the nucleotide sequence in DNA listed above.
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Now, let's talk about translation,
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which is the process of taking the information stored on an mRNA strand and using it to construct a protein.
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So once the mRNA strand is synthesized in the nucleus, it leaves the nucleus and enters the cytosol,
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where it interacts with a free ribosome or one that is attached to the rough ER.
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Now, within the ribosome, it's going to interact with a tRNA molecule or a transfer RNA molecule.
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Now, let's focus on the sequence of nucleotides on the mRNA strand.
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Notice that they're separated in sets of three.
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Each set of three nucleotides represents a codon,
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which matches up with another three nucleotides on a tRNA molecule, known as an anticodon.
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And each codon matches up with a specific amino acid.
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And so that's how the information stored in the mRNA strand can be used to construct a specific protein.
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We're going to talk more about this later in this video.
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Translation, like transcription, occurs in three steps: initiation, elongation, and termination.
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So here in this picture, we have a ribosome, which is composed of two subunits,
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the small subunit and the large subunit.
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And the ribosome has three active sites: the E site, the P site, and the A site.
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Now, during initiation, we have the start codon AUG,
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which corresponds to the anticodon UAC.
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Now, that particular tRNA molecule has the methionine amino acid attached to it.
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And this tRNA molecule, it enters the ribosome at the P site, also known as the peptidyl site,
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where the peptide bonds are formed.
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Now, during the second step of translation, that is during elongation,
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another tRNA molecule enters the A site.
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So I'm just going to draw it here.
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And during that process, a covalent bond will form between the two amino acids that we see here,
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highlighted by the red circles.
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Now, as the process continues, the tRNA molecule in the P site
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will move to the E site.
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And as it does so, it's going to lose an amino acid.
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And so what's going to happen is we're going to have a growing chain of amino acids
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that will leave or extend out of the ribosome.
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So make sure you understand that the tRNA molecules, they enter the A site,
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and then they exit from the E site.
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And in the process, the amino acids are being joined together.
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So over time, this polypeptide chain
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termination step begins when a stop codon is read.
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Now, there are three stop codons that you need to be familiar with.
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The first one is UAA,
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and then the second one is UAG,
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and the third one is UGA.
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Now, these stop codons, also known as nonsense codons,
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they don't code for any specific tRNA molecule.
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Instead, when these are read at the A site, they cause a release factor to enter the A site,
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which causes the small and the large ribosomal subunits to basically disassemble.
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So this is where translation ends,
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and the protein, it leaves the ribosome, where it eventually goes to the Golgi body
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for further processing and modification.
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At the Golgi body, the proteins undergo folding where they form a specific shape to perform a specific function.
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And that's basically it for this video. Hopefully, it gave you a good overview of transcription and translation.
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So that's all I got. Thanks for watching.
Topics:transcriptiontranslationprotein synthesisDNAmRNARNA polymeraseribosometRNAcodonbiology
