Lesson Video: The Genetic Code Biology

In this video, we will learn how to describe the nature of the genetic code and recall how information is transferred from DNA to protein.

14:52

Video Transcript

In this video, we’ll learn about the genetic code. We’ll first cover how information can be transferred from DNA to mRNA to protein using transcription and translation. Then, we’ll discuss the genetic code and how codons can be used to derive a sequence of amino acids from a sequence of nucleotides. Finally, we’ll see what it means when we say that the genetic code is nonoverlapping, degenerate, and universal. So, let’s get cracking and start this video.

DNA, or deoxyribonucleic acid, is often called the blueprint of life. It provides instructions for building the hair on top of our heads and the toes on our feet. DNA is inside the nucleus of nearly every cell. It’s a double-stranded molecule that’s arranged as a helix. And spanning this DNA are genes that make up the proteins that we need, like the collagen in our skin, for example.

This gene has a specific sequence of nucleotides or bases that provide the instructions for building these specific proteins. Let’s go over the steps needed to turn this gene into a protein. There’s actually two steps involved called transcription and translation. We have a lot of genes, tens of thousands in fact. So, if we wanted to make the protein for collagen and none of these other genes, how is that done?

So, here’s one of our cells with the DNA in blue, and here’s the gene for collagen shown in pink. The first thing that happens is that the cell receives a signal to make collagen. Then, the collagen gene is transcribed and a copy is made. This copy is called mRNA. This process of copying a segment of DNA into mRNA is called transcription. Here, a section of DNA or the collagen gene in this example is transcribed or copied into single-stranded mRNA.

Let’s look a little bit more closely at how this works. Here, the DNA to be transcribed is unwound and opened by an enzyme called RNA polymerase. This enzyme then synthesizes mRNA, shown here in pink, using one strand of the DNA as a template. Once transcription is complete, RNA polymerase detaches from the DNA.

Let’s look a little more closely at the sequence of DNA and mRNA here. Here’s the single strand of DNA that’s being used as the template for transcription shown in blue and the transcript or mRNA, which will be shown in pink. Here, the complementary nucleotides are added to the transcript. So, G or guanine base-pairs with C or cytosine, and T or thymine base-pairs with A or adenine. The same complementary base- pairing rules for DNA apply to mRNA with one difference. Adenine pairs with U or uracil in mRNA. So, uracil takes the place of thymine in mRNA. So, instead of Ts in DNA, we have Us in mRNA.

Now that transcription is complete and we have our mRNA, let’s see what happens next. In the next step, the sequence of nucleotides is converted or translated into amino acids to form the protein. This process is called translation. This forms a polypeptide, which is a chain of amino acids, shown here as these colored circles, that are joined together by peptide bonds. There are 20 different common amino acids. So, this pink circle can represent the amino acid alanine, while this blue circle can represent the amino acid leucine.

Because of the attractive and repulsive chemical properties of each of these amino acids, the polypeptide chain can then fold into the unique shape of the protein. This unique shape contributes to the specific function of the protein.

So, how is it possible to go from the sequence of nucleotides in mRNA to amino acids in the polypeptide chain? The answer to this is in the genetic code. So, here we have a sequence of mRNA that needs to be translated into a polypeptide. To decode this, we need to look at these nucleotides in groups of three. This is called a codon. A codon is a sequence of three nucleotides that code for an amino acid. So, the codon AUU can be translated into the amino acid isoleucine, while the codon CAC codes for the amino acid histidine. GGA codes for glycine, and UGC codes for cysteine.

You might notice that the genetic code is nonoverlapping. This means that the nucleotides present in one codon are not present in the adjacent codon. So, after the first codon AUU, the next codon is not UUC and is not UCA. These two codons would overlap with the first codon. And since the genetic code is nonoverlapping, the codon following AUU is CAC.

So, as we have seen, the sequence in these codons correspond to specific amino acids. To see what each of these sequences can be translated into, we can refer to a diagram called the codon wheel. Here, on the left, you can see a codon wheel. This can be used to translate any codon into its corresponding amino acid. To do this, we need to start from the five prime end of the mRNA molecule, which is indicated here. And we have to work our way to the three prime end, which is shown here.

So, for the codon AGG, we start from the center of the codon wheel and circle the A here. Then we move outwards and circle the G here. And then finally, we circle the final G as indicated here. So, the codon AGG corresponds to the amino acid arginine. Let’s try another one and look at the codon for CAU. So, the C is indicated here, the A is indicated here, and the U is here. CAU corresponds to the amino acid histidine.

Oh, what’s this? A codon has suddenly been added. I wonder what it corresponds to? Why don’t we clear this codon wheel, and then you can pause the video so you can try to figure it out. The corresponding amino acid for the codon GCG is alanine.

Now, let’s talk about some other features of the genetic code. You may have noticed that more than one codon can code for the same amino acid. For example, histidine can be coded by CAC and CAU. Both of these codons will give the amino acid histidine. This means that the genetic code is degenerate or redundant, meaning that more than one codon can be translated to the same amino acid. The amino acid alanine can actually be coded by four different codons. An exception to this is with the amino acid methionine. Methionine is also sometimes known as a start codon because it initiates translation. Likewise stop codons terminate translation. These codons tell the protein synthesis machinery where to start and end translation.

Now, let’s clean up the screen a bit and talk about another feature of the genetic code. The genetic code is universal to all life on Earth. So, the codon CCG codes for proline in bacteria, in plants, in humans, and so on. This has had a huge impact on biotechnology. For example, suppose we wanted to produce a lot of insulin to treat diabetes in humans. Because of the universal nature of the genetic code, we can insert this insulin gene into another organism, like this bacterium, which can then be transcribed and translated to give the same insulin protein in humans, which can then be harvested and used to treat diabetes. This way, we can make a lot of insulin for a fraction of the cost of extracting it from humans.

Now, let’s look at a practice question to apply what we’ve learned.

A sequence of DNA is transcribed into an RNA sequence. This RNA sequence reads from five prime to three prime GCUUUCACGCAC. Use the codon wheel provided to determine the sequence of amino acids. Arginine, serine, threonine, proline. Serine, leucine, alanine, histidine. Alanine, phenylalanine, threonine, histidine. Serine, leucine, alanine, glutamine. Or alanine, leucine, threonine, glutamine.

This question is asking us about how to translate a sequence of mRNA into the corresponding amino acids. Before we can answer this, let’s clear the answer choices and review some key points.

Let’s say our cell here needs to produce insulin. The gene for insulin is located here in pink in the cell’s DNA. In order for this cell to produce insulin or any other protein, it must go through two processes called transcription and translation. During transcription, the gene for the protein is transcribed or copied to produce what’s called messenger RNA or mRNA. This messenger RNA is a message for the cell that tells it that it needs to make the protein, or insulin in this case.

Transcription is the process of converting a section of double-stranded DNA or our insulin gene shown here in pink into a single-stranded mRNA molecule. Like DNA, the sequence for mRNA is written in the five-prime to three-prime direction and contains four different nucleotides or bases: adenine or A for short, guanine, cytosine, but instead of thymine, which is in DNA, RNA uses uracil or U for short.

After transcription, the sequence in the mRNA molecule can be translated into its corresponding amino acids. This step is called translation, and it forms a polypeptide with each of these colored circles representing a different amino acid. This polypeptide can then fold to form its corresponding protein, or insulin in this example.

Now that we’ve covered transcription and translation, let’s turn our attention to this mRNA sequence in the question and describe how this specific sequence can be translated into amino acids.

An mRNA sequence is translated in groups of three nucleotides called a codon. A codon is a sequence of three nucleotides that code for an amino acid. Codons are always read in a way that they’re not overlapping. So, in this sequence, this is the first codon, this is the second codon, this is the third codon, and here’s the last codon. So, this mRNA sequence has four codons.

Now, in order to translate the sequence of nucleotides into its corresponding amino acid, we need to use a codon wheel, like the one shown on the left. To use the codon wheel, we start from the inside. This corresponds to the five-prime end of the codon. And we work our way outwards to the three-prime end of the codon. So for this codon, we’ll be working from the five-prime to the three-prime end.

So, with the codon GCU, we start with G, then we move over to C, and finally we land on U. So, the codon GCU corresponds to the amino acid alanine. Then, for the next codon UUC, we do the same thing. So, the first nucleotide is U, then U again, and then C. This corresponds to the amino acid phenylalanine. Then, for the codon ACG, this corresponds to threonine. And finally, the codon CAC corresponds to the amino acid histidine. Therefore, the corresponding amino acid sequence for the given mRNA sequence is alanine, phenylalanine, threonine, and histidine.

Now, let’s look at some of the key points that we covered in this video. Information is transferred from DNA to mRNA by transcription, then from mRNA to protein by translation. Groups of three nucleotides, or codons, are translated to their corresponding amino acid. These codons can be decoded using a codon wheel. Finally, the genetic code is nonoverlapping, degenerate, and universal.

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