Central Dogma - Online Tutor, Practice Problems & Exam Prep

The central dogma of molecular biology describes the unidirectional flow of genetic information from DNA to RNA through transcription, and from RNA to protein via translation. Transcription, facilitated by RNA polymerase, synthesizes RNA from a DNA template, while translation occurs in the cytoplasm, utilizing ribosomes and transfer RNA (tRNA) to decode mRNA codons into amino acids, forming proteins. This process is essential for all living organisms, highlighting the significance of understanding transcription and translation in molecular biology.

Central Dogma of Molecular Biology

In this video, we're going to do a quick refresher on the central dogma of molecular biology. Recall from your previous biology courses that the central dogma of molecular biology casually refers to both the processes of transcription and translation. Recall that the process of transcription is a process that builds an RNA molecule by using a DNA molecule as the coding template and recall that the process of translation is the process that builds a protein molecule by using the encoded messages of RNA. As we move forward in separate videos, we will briefly refresh your memory on the process of transcription as well as the process of translation. What you may not have known about the central dogma of molecular biology is that it only casually refers to both the processes of transcription and translation. But it directly refers to the unidirectional flow or, in other words, the one-directional flow of biochemical information from DNA to protein. In other words, it is possible for DNA to be replicated, and it's also possible for RNA to be reverse transcribed back into DNA. But once the transfer of nucleic acid information goes to protein, this is an irreversible process or a one-directional or unidirectional process.

If we take a look at our image down below, we can better visualize this. Notice on the left-hand side over here, we're showing you a DNA molecule, and we know that DNA can be replicated. Thus, this circular arrow going from DNA back to DNA is just representing the process of DNA replication. In addition to the DNA being replicated, we know that the DNA can be used as a coding template to build an RNA molecule. This arrow that goes forward here from DNA to RNA is the process of transcription. Also, this RNA could be used as a template to build a corresponding DNA molecule, and so that would be the reverse arrow going backward from RNA to DNA. This reverse arrow is going to be reverse transcription because it's the opposite of transcription. And of course, we know that the RNA molecule can encode a message for building a protein, and this is the process of translation.

The central dogma of molecular biology only casually refers to transcription and translation. However, it directly refers to this unidirectional flow of information here, this one-directional flow of information. Notice we only have one arrow going from left to right from RNA to protein. Once the information in nucleic acids has been transferred to a protein, this is an irreversible process, and the protein is not going to be used as a template for building a corresponding RNA; that does not happen. This unidirectional flow of information is what the central dogma of molecular biology directly refers to, and this applies to all living organisms.

This here concludes our brief refresher on the central dogma of molecular biology, and as we move forward, once again, we'll get to talk a little bit more about transcription and translation to refresh your memories from your biology courses. I'll see you all in that video.

Transcription

So later in our course, we're going to talk in a lot of details about transcription. But in this video, we're just going to do a quick review. So recall that transcription uses an enzyme known as RNA polymerase. And in our previous videos, we talked about how words that end in -ase tend to be enzymes. And RNA polymerase is an enzyme, and it's an enzyme that polymerizes or builds RNA. So the function of this enzyme can kind of be read backwards, an enzyme that builds RNA. And so RNA polymerase can build many types of RNA, including messenger RNA or mRNA. And the RNA molecules that are built by RNA polymerase are built in a specific direction. And that direction is from the 5' to the 3' end of the molecule. And it does that by aligning free RNA nucleotides to a DNA template, as we already know. And so, RNA molecules, recall, have the same exact sequence as the DNA coding strand, with the exception that all of the thymines in the DNA are replaced with uracils in the RNA.

And let's take a look at our example below. And quick secret, this diagram here is not nearly as complicated as it looks. And so we've got a DNA molecule that stretches across the diagram, and the DNA molecule has 2 strands. It's got a dark-colored purple strand and a light-colored purple strand, and these strands go in opposite directions. They are antiparallel. And in the back, notice that we have this big pink thing that's kind of hard to miss. So has the ability to unwind the DNA, and we can see that here. When it unwinds the DNA, it separates the DNA coding strand from the DNA template strand. And the DNA template strand, which is down below here, is literally used as a template to build the RNA. And so free RNA nucleotides are aligned to the template and added to the 3' end of the mRNA molecule. And so the RNA is always going to grow from the DNA as being rewound back into its original state.

And so over here on the far right, notice that this is a representation of the DNA molecule and the RNA molecule on the left. And so the dark-colored purple strand goes from 5' to 3' from left to right, and so this is a representation of that strand. The light-colored strand goes from 5' to 3' from right to left, and so that's represented here. And the RNA molecule goes from 5' to 3' from left to right. And so we're given the DNA sequence of the coding strand, and we have 2 questions. What's the sequence of the template strand, and what is the sequence of the RNA strand? And so recall our base pairing rules. A's always pair with T's, and G's always pair with C's. And so, when we apply that here, G's pair with C's, A's pair with T's, and C's pair with G's again. And so, this is the sequence of our template strand. Now, what's the sequence of our RNA strand? Is it going to be the same as the coding strand or the same as the template strand? And recall, we've already said that the RNA molecule has the same sequence as the coding strand, with the exception that thymines are replaced with uracils. And so that means that our RNA strand down here is going to have the same sequence as our coding strand. So we're going to have GGA, except we're going to replace thiamines with uracils, and C.

And transcription is a fundamental process that occurs in every single organism, so it's good to touch up on the basics. And in our next video, we're going to do a recap on translation. So I'll see you guys in that video.

Translation

So you guys already know that translation is completed by ribosomes. But recall that there are also specialized RNA molecules called transfer RNAs or tRNAs that are also involved. And you've learned that ribosomes create proteins by reading the mRNA strand in 3 nucleotide chunks called codons. And these codons are interpreted with a genetic code. And down below, we have a genetic code shown by this table. A genetic code. And recall that tRNAs pair with amino acids. Acids. And each of the tRNAs contains an anticodon that is complementary to the codons of the mRNA. And so down in our example below, we're going to use the genetic code to fill in the blanks throughout our diagram. And so notice in the back, we have this big teal structure that we know as the ribosome and ribosomes have a large and a small ribosomal subunit. If this were a eukaryotic ribosome, it would be an 80S ribosome with a 60S large subunit and a 40S small subunit. But if this were a prokaryotic ribosome, it would be a 70S ribosome with a 50S large subunit and a 30S small subunit. So, in addition to the ribosome, we also have an mRNA molecule in purple. And the mRNA molecule has a 5' and a 3' end and it also has a sequence. And the first three letters of this sequence correspond to our codon, which is AUG. And up above, we have our tRNA molecule, and we also have 2 more tRNA molecules shown in these positions. And notice that the tRNA molecules are associated with and pair with amino acids that are shown up above. And in addition to pairing with amino acids, tRNAs also have an anticodon, which are shown at the bottom of these tRNAs. And the anticodons, recall, pair and, are complementary to the codons of the mRNA. And so recall that the genetic code here uses the codons of the mRNA, not the anticodons of the tRNAs. And so recall that the genetic code, the first letter of the codon limits you to a specific row. The second letter of the codon limits you to a specific column. And the 3rd letter of the codon limits you to a specific position within the block. And so our first codon is AUG. So A limits us to this row, U limits us to this column and to this block, and G limits us to this exact position within the block, which corresponds to a methionine amino acid. And we can see that that's true, that here we have a methionine amino acid, and that corresponds to an AUG codon. And so notice that our ribosome is moving in this direction. So it's moving from the 5' to the 3' end of the mRNA. And so it's moving in this direction and then here comes our tRNA that's making its way into this position. And notice that our next codon on the mRNA is UGG. And so for the anticodon, U's recall, are pretty much T's, so they pair with A's. So we can put an A here for the anticodon. G's again pair with C's. So our anticodon is going to be ACC. Now, again, when we use the genetic code, be sure that you're using the codons of the mRNA, not the anticodons of the tRNA. So we're going to use UGG, not ACC. Now, when we use UGG, we get UGG and that's a tryptophan, and we can see that that is true, that here we have a tryptophan, and the tryptophan corresponds with a UGG codon. So, now the ribosome, is going to link the methionine and tryptophan amino acids to start creating a protein. And now, it can the ribosome keep moving, so now these tRNAs start to shift into the empty slots and eventually, they'll shift out of the ribosome completely. So now here comes our next tRNA making its way to the ribosome. And notice that our next codon is UUC. And so again, U's are pretty much T's and they pair with A's. So here we have AAG, because these are the codons. And so now, this is going to make its way in here. And again, recall that the genetic code uses the codons of the mRNA, not the anticodons of the tRNA. So we're going to use UUC, and notice that UUC gets us phenylalanine. So up here, we can put PHE for phenylalanine. And then again, this will also be connected to the chain of amino acids that continues to grow as tRNAs continue to feed in. And so, again, what'll happen is this N over here will end up being the N-terminal end of the protein, whereas the last, amino acid that comes in will end up being the C-terminal end of the protein. And recall that translation is a process that all living organisms do and the genetic code is pretty much universal and applies to all living organisms with some minor exceptions here and there. Now, in our next video, we're going to directly compare transcription and translation. So I'll see you guys in that video.

Transcription vs. Translation

In this video, we're going to quickly compare transcription and translation. Transcription is in the left column in light blue, whereas translation is in the right column in light pink. So for the product formed, we know that transcription builds RNA molecules, whereas translation builds proteins.

Now, for the macromolecule change, we know that transcription uses DNA nucleic acids to build RNA nucleic acids, but we're still under the nucleic acid macromolecule category, and for that reason, we'll go ahead and put no here. Now, for translation, on the other hand, we're going from RNA nucleic acids to amino acids and proteins. So there's definitely a change in the macromolecule category.

Now, for the major enzyme or structure, we know that transcription uses RNA polymerase, an enzyme that polymerizes or builds RNA. For translation, on the other hand, we know that we use ribosomes.

Now, for the location of transcription, it's going to occur wherever the DNA is because the DNA acts as a template to build the RNA. And in eukaryotic cells, this is going to be in the nucleus. Now for translation, it occurs in the cytoplasm of the cell.

Now, for the direction: in transcription, we're focused on the 5' to 3' direction of RNA synthesis. However, in translation, the protein that's being built is being built from the N-terminal end to the C-terminal end of the protein.

And so this concludes our comparison of transcription and translation, and I'll see you guys in the practice videos.

What is the central dogma of molecular biology directly referring to?

• A. Unidirectional Translation
• B. Multidirectional Translation
• C. Unidirectional Transcription
• D. Multidirectional Transcription & Translation

Consider a DNA template strand of the following sequence: 5'-A C T G C C A G G A A T-3'.

A) What is the sequence of the corresponding DNA coding strand? Include directionality.

DNA Template Strand: 5'-A C T G C C A G G A A T-3'.

DNA Coding Strand:

B) What is the sequence of the corresponding mRNA strand? Include directionality.

mRNA Strand:

So here's a 2-part practice problem asking us to consider a DNA template strand of the following sequence from 5' to 3'. Part A asks, what is the sequence of the corresponding DNA coding strand and include directionality? Below, we have the DNA template strand rewritten exactly as it's shown above. Recall that in our previous videos when we first recapped transcription, we started with a coding strand and we derived a template strand. In this practice problem, we're going in the opposite direction. It doesn't matter what direction you're going in; that all depends on the question being asked. Because regardless of which DNA strand you're trying to derive, you're always going to use the base pairing rules to derive the DNA strands. So, for this practice problem, we're going to need to use the base pairing rules, and recall that A's always pair with T's, and C's always pair with G's. Applying that here, we can get our coding strand sequence. A's pair with T's, C's with G's, T's with A's, G's with C's, C's with G's, A's with T's, G's with C's, A's with T's, and T's with A's. Recall that the strands in the DNA molecule go in opposite directions, so they're antiparallel. That means that this must be the 5' end, whereas over here must be the 3' end. The answer to part A is in this highlighted region here. Now part B asks, what is the sequence of the corresponding mRNA strand and include directionality? Recall that the mRNA strand sequence is exactly the same as the coding strand sequence with the exception that all of the T's are replaced with U's. So, we can go ahead and circle all of the T's in our DNA coding strand, and we can rewrite the same sequence, replacing the T's with U's, to get our mRNA strand sequence. Doing that here, we'll get UGACGGUCCUUA, and the directionality stays exactly the same. So this is the 5' end, and over here is the 3' end. The answer to part B is in this highlighted region here. So that concludes this practice problem, and I'll see you guys in the next one.

Consider a DNA coding strand with the following sequence: 3'-C T T C A T A G C T C G-5'.

Use the genetic code to determine the corresponding amino acid sequence of the translated protein.

So this practice problem wants us to consider a DNA coding strand with the sequence from 3' to 5' and to use that information along with the genetic code to determine the corresponding amino acid sequence of the translated protein. Down below, the DNA coding strand is rewritten exactly as shown above. To get the mRNA strand, we need to use our base pairing rules, where C's base pair with G's, and T's with A's. No, that is not right. Recall that the base pairing rules here would only be used if we were given a template strand, but we are given a DNA coding strand. The mRNA strand, remember, uses the template strand and base pairing rules to build the strand. However, the coding strand is exactly the same sequence as the mRNA strand, except all of the thymines are replaced with uracils. We can rewrite the exact same sequence, replacing the T's with U's. When we do that, we get this sequence: UCG. It is exactly the same as the one above, except the T's are replaced with U's. The directionality remains the same, with 3' and 5' ends as labeled. Now we are ready to jump straight into using our genetic code with the assumption that we flipped the directionality from 3' to 5' in accordance with standard practice to 5' to 3'. We rewrite our sequence in the correct orientation to get: GCU CGA UAC UUC. The directionality has been flipped, now showing 5' to 3' but representing the same exact RNA molecule. Our first codon, GCU, translates to the amino acid Alanine. The next codon, CGA, translates to Arginine. The third codon, UAC, gives Tyrosine. The final codon, UUC, results in Phenylalanine. These are the three-letter abbreviations for these amino acids, and technically, this is the answer to our problem. However, recall that the proteins that are built also have directionality. On this end, closer to the 5' end, will be the N-terminal of the protein that has a free amino group. On the opposite end, we have the C-terminal, closer to the 3' end of the mRNA sequence. The final amino acid sequence from N-terminal to C-terminal is Alanine, Arginine, Tyrosine, Phenylalanine, which is highlighted in green below. I will see you guys in the next video.

Here’s what students ask on this topic:

What is the central dogma of molecular biology?

The central dogma of molecular biology describes the unidirectional flow of genetic information within a biological system. It states that genetic information flows from DNA to RNA through the process of transcription, and from RNA to protein via translation. Transcription involves the synthesis of RNA from a DNA template, facilitated by the enzyme RNA polymerase. Translation occurs in the cytoplasm, where ribosomes and transfer RNA (tRNA) decode the mRNA codons into a sequence of amino acids, forming proteins. This process is fundamental to all living organisms and underscores the importance of understanding transcription and translation in molecular biology.

What is the role of RNA polymerase in transcription?

RNA polymerase is the enzyme responsible for synthesizing RNA from a DNA template during transcription. It binds to the DNA at the promoter region and unwinds the DNA strands. RNA polymerase then aligns free RNA nucleotides with their complementary DNA bases on the template strand, synthesizing the RNA molecule in the 5' to 3' direction. The RNA sequence produced is complementary to the DNA template strand and identical to the DNA coding strand, except that uracil (U) replaces thymine (T). This enzyme is crucial for the accurate and efficient production of various types of RNA, including messenger RNA (mRNA), which is essential for protein synthesis.

How does translation differ between prokaryotic and eukaryotic cells?

Translation in prokaryotic and eukaryotic cells differs primarily in the ribosomal structure and the cellular location where it occurs. Prokaryotic cells have 70S ribosomes, composed of a 50S large subunit and a 30S small subunit, while eukaryotic cells have 80S ribosomes, consisting of a 60S large subunit and a 40S small subunit. In prokaryotes, translation can begin while transcription is still ongoing because both processes occur in the cytoplasm. In contrast, eukaryotic translation occurs in the cytoplasm after transcription and RNA processing are completed in the nucleus. These differences reflect the complexity and compartmentalization of eukaryotic cells compared to prokaryotic cells.

What is the significance of the genetic code in translation?

The genetic code is crucial in translation as it dictates how the sequence of nucleotides in mRNA is translated into the sequence of amino acids in a protein. The genetic code consists of codons, which are three-nucleotide sequences on the mRNA. Each codon specifies a particular amino acid or a stop signal for translation. Transfer RNA (tRNA) molecules, each with an anticodon complementary to an mRNA codon, bring the corresponding amino acid to the ribosome. The ribosome then links these amino acids together to form a polypeptide chain. The genetic code is nearly universal, underscoring its fundamental role in the biology of all living organisms.

What are the main differences between transcription and translation?

Transcription and translation are two distinct processes in the central dogma of molecular biology. Transcription occurs in the nucleus (in eukaryotes) and involves the synthesis of RNA from a DNA template, facilitated by RNA polymerase. The product of transcription is an RNA molecule, which can be mRNA, tRNA, or rRNA. Translation, on the other hand, occurs in the cytoplasm and involves the synthesis of proteins from mRNA. Ribosomes and tRNA molecules play key roles in decoding the mRNA codons into a sequence of amino acids, forming a polypeptide chain. While transcription deals with nucleic acids, translation involves a change from nucleic acids to proteins.