Mutations - Online Tutor, Practice Problems & Exam Prep

Mutations are permanent changes in an organism's DNA sequence, impacting RNA and protein synthesis. They can be classified as point mutations, which include silent, missense, and nonsense mutations, or frameshift mutations, which involve insertions or deletions that alter the reading frame. Silent mutations do not affect the amino acid sequence, while missense mutations change one amino acid, and nonsense mutations introduce a premature stop codon. Understanding these mutations is crucial for grasping genetic diversity and evolution.

Mutations

This video, we're going to begin our lesson on mutations. And so mutations are defined as permanent changes in the DNA sequence of an organism. And so these mutations, these permanent changes in the DNA sequence are very, very important. And that is because mutations in the DNA, these changes in the DNA, can actually impact the RNA via transcription. And, of course, if it can impact the RNA via transcription, that means that these mutations can also impact proteins via translation. And so these changes in the DNA ultimately can have a widespread impact on the DNA, ultimately the RNA, and then ultimately the proteins. Now these mutations, they can either be harmful, meaning that they would reduce the chance of the organism's survival. They could also be beneficial, meaning that they improve the chances of the organism's survival, or they could also be neutral, meaning that they do not have an impact on the organism's survival. And so in terms of their impact and result, mutations can vary in their impact and result, and it depends on the specific scenario and the specific mutation. Now mutations can actually occur either naturally, through natural processes that occur within the cell, or the mutations can actually be induced by environmental factors or chemical factors that are called mutagens, which are chemical agents that can induce mutations. Now mutations themselves are going to be largely responsible for the tremendous diversity that is found among living organisms. And so the reason that there are so many different types of living species is because of the accumulation of mutations and other features throughout very, very long periods of time. And so this here concludes our brief introduction to mutations. And in our next lesson video, we'll be able to talk about different types of mutations. So I'll see you all in that video.

Types of Mutations

In this video, we're going to be talking about the different types of mutations. And so it turns out that there are several different types of mutations, including the following mutations that we're going to talk about down below in our image. Now first recall from our last lesson video that a mutation is a permanent change in the DNA sequence of an organism. However, a mutation, a change in the DNA, can consequently lead to changes in the RNA, which can lead to changes in the protein or the amino acid sequence. And so here in this image, we're only showing you the normal sequence for the mRNA, the messenger RNA, and the normal sequence for the polypeptide. And so this would be the sequences of the mRNA and the polypeptide under normal conditions when there is no mutation. And so down below in our image, what we're showing you are mutations that occur to this normal sequence. And so we've got the mutations grouped as either being point mutations, also known as substitutions, over here on the left-hand side of our image, our point mutations. And then we've got the other set of mutations that are known as frameshift mutations, and those are over here on the right-hand side of our image. So we're going to start off with the point mutations, which are also known as substitutions. And so the point mutations or the substitutions, these are going to be involving the change of just one single nucleotide, in the DNA. And so they're going to be a substitution of one nucleotide for another nucleotide.

Now there are three different types of point mutations that we're going to be talking about. The first type of point mutation that you should know is called the silent mutation. And the reason that it's called the silent mutation is because if you're just looking at the amino acid sequence or the polypeptide chain, you wouldn't be able to tell that there was a mutation. And that's because silent mutations have no effect on the amino acid sequence. But there is still a change of one nucleotide in the DNA and in the RNA. But that one change in the DNA and the RNA, that one nucleotide change in the DNA and the RNA has no effect, no impact on the amino acid sequence. And so what you'll notice is looking at the silent mutation, the polypeptide sequence is exactly as it is in the normal situation. There's no change in the polypeptide mutated one down below, it has a codon of GGC, but the normal one is GGA. And so the A nucleotide is being swapped for the C nucleotide, and that is a mutation. But because this mutation does not change the amino acid sequence, it's a silent mutation. Now the next type of point mutation that we're going to talk about is called a missense mutation. Now the missense mutation is a point mutation, so there's going to be a change of just one nucleotide. But this time, the change of one nucleotide actually will change one amino acid. And so the change of just one amino acid is characterized by a missense mutation. And so what you'll notice here in this image is that there is one nucleotide being changed. The codon up above says CUA, and so the U is being swapped for a C, and normally there is a Leucine or a Leu amino acid at this position, but in the missense mutation down below, it leads to a proline amino acid. So there is a change of the amino acid here. And so again, changing just one amino acid is characteristic of the missense mutation. Now the third and final type of point mutation that we're going to talk about is called the nonsense mutation. And the nonsense mutation is characterized by having the introduction of a premature stop codon. And so there is just going to be the change of just one nucleotide and that change of one nucleotide changes the codon to a stop codon and it will prematurely cut the amino acid chain short. And so notice that this amino acid chain is only just going to have one amino acid in it because it is being cut short by this mutation that creates a premature stop codon. And so you can see the change here. Instead of having GGA in the codon, the first G of the codon is changed to a U, down below in the mutation, and that UGA codon is a stop codon, and so that's the introduction of a premature stop codon is characterized by a nonsense mutation.

And so that's it for the point mutations, silent mutation, missense mutation, and nonsense mutation. So now we're going to shift over to the frameshift mutations. And so as the frameshift mutation kind of implies, the codon reading frame is going to shift or change, And that means that the codons are going to be read in a different reading frame. And so for example, there are two different types of frameshift mutations. The first is going to be an insertion. An insertion mutation is when there is the addition of one or more nucleotides that is going to cause a change in the reading frame of, the codon reading frame. And so what you can see here is here we have an inserted nucleotide, an A nucleotide that has been added to the sequence here. And so the addition of one nucleotide, the insertion of this one nucleotide changes the reading frame of, the codon reading frame of everything that is downstream of the mutation. And so ultimately, frameshift mutations have the potential to cause amino acid changes to all of the amino acids that are downstream. And so you'll notice that the addition of the G here, in this fourth nucleotide shifts over all of the other nucleotides and that changes the reading frame. So it changes all of these corresponding amino acids. And you can see the one amino acid here has just been shifted out since one has been inserted. Now frameshift mutations can also occur as deletions. And so the deletion is going to be the removal of one or more nucleotides. And so the deleted nucleotide here is right here in this gap. And so you can see, previously, it was CUA, but the U is deleted. And so what that means is it changes the codon reading frame. So we have a new codon, which would be CAC instead of CUC. And that changes all of the codon reading frame. And that can also change all of the amino acids downstream. And so what you can see here is that the point mutations are only going to be changing just one nucleotide, substituting one nucleotide for another nucleotide, but there's no change in the codon reading frame. But with the frameshift mutation with an insertion or a deletion, that will change or shift the codon reading frame. And so this here concludes our introduction to the different types of mutations. And we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

Which of the following mutations, occurring just after the start codon in the mRNA is likely to have the most serious effects on the polypeptide product?

• A. Deletion of one codon.
• B. Deletion of one nucleotide.
• C. Insertion of three nucleotides.
• D. Substitution of one nucleotide.

A single base substitution is LEAST likely to be deleterious (dangerous) when the change results in _____.

• A. Replacement of a codon specifying a hydrophilic amino acid with a codon that specifies a hydrophobic amino acid.
• B. Replacement of a codon which codes for an amino acid with a stop codon.
• C. The change of a codon specifying a specific amino acid important for the active site of the protein.
• D. Replacement of a codon specifying an amino acid with a redundant codon specifying the same amino acid.

 A section of DNA has this base sequence: AGCGTTACCGT. A mutation in this DNA strand results in this base 

sequence: AGGCGTTACCGT. What type of mutation does this change represent?

• A. A Frameshift mutation.
• B. A missense mutation.
• C. A nonsense mutation.
• D. A silent mutation.

A nonsense mutation:

• A. Causes an incorrect amino acid to be inserted into a polypeptide chain.
• B. Causes synthesis of a polypeptide chain to be terminated prematurely.
• C. Prevents the start of translation.
• D. Only affects the mRNA code but does not affect the amino acid sequence of the polypeptide.

Here’s what students ask on this topic:

What are the different types of mutations in DNA?

Mutations in DNA can be classified into two main categories: point mutations and frameshift mutations. Point mutations involve a change in a single nucleotide and include silent mutations (no change in amino acid sequence), missense mutations (change in one amino acid), and nonsense mutations (introduction of a premature stop codon). Frameshift mutations involve the insertion or deletion of nucleotides, which shifts the reading frame of the codons, affecting all downstream amino acids. Understanding these mutations is crucial for grasping genetic diversity and evolution.

How do mutations impact protein synthesis?

Mutations impact protein synthesis by altering the DNA sequence, which in turn affects the RNA produced during transcription. This altered RNA can lead to changes in the amino acid sequence of the protein during translation. Depending on the type of mutation, the impact can vary: silent mutations have no effect on the protein, missense mutations change one amino acid, and nonsense mutations introduce a premature stop codon, truncating the protein. Frameshift mutations shift the reading frame, potentially altering all downstream amino acids and significantly affecting the protein's function.

What is a silent mutation and how does it affect the amino acid sequence?

A silent mutation is a type of point mutation where a single nucleotide change in the DNA sequence does not alter the amino acid sequence of the resulting protein. This occurs because the genetic code is degenerate, meaning multiple codons can code for the same amino acid. For example, if the codon GGA (which codes for glycine) is changed to GGC, it still codes for glycine, resulting in no change to the amino acid sequence. Therefore, silent mutations do not affect the protein's function.

What are frameshift mutations and how do they differ from point mutations?

Frameshift mutations involve the insertion or deletion of one or more nucleotides in the DNA sequence, which shifts the reading frame of the codons. This shift alters the grouping of nucleotides into codons, potentially changing all downstream amino acids and significantly affecting the protein's function. In contrast, point mutations involve a change in a single nucleotide and do not shift the reading frame. Point mutations can be silent, missense, or nonsense, each with different impacts on the amino acid sequence and protein function.

What is a nonsense mutation and what is its effect on protein synthesis?

A nonsense mutation is a type of point mutation where a single nucleotide change converts a codon that normally codes for an amino acid into a stop codon. This premature stop codon truncates the protein, leading to a shorter and usually nonfunctional protein. For example, if the codon GGA (which codes for glycine) is changed to UGA, it becomes a stop codon, terminating translation prematurely. Nonsense mutations can have severe effects on protein function and are often associated with genetic diseases.