Introduction to Mutations: Videos & Practice Problems

Mutations are permanent changes in an organism's DNA sequence, which can be classified as spontaneous or induced. Spontaneous mutations occur naturally, while induced mutations result from external factors like mutagens. Types of mutations include point mutations—silent, missense, and nonsense—and frameshift mutations, which involve insertions or deletions that alter the reading frame. Understanding these concepts is crucial for grasping genetic variation and its implications in biology.

Introduction to Mutations

In this video, we're going to begin our introduction to mutations. Mutations can be defined as permanent changes in the DNA sequence of an organism. Depending on the type of mutation and the location of the mutation in the DNA sequence, the mutations can either be harmful, beneficial, or neutral in terms of their impact and result on the cell. Moving forward in our course, we're going to discuss several different types of mutations, and so I'll see you all in our next 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, are 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 3 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 that 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 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 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 gonna 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 gonna 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 2 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, the codon reading frame. And so what you can see here is here we have an inserted nucleotide, 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 frames. 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 gonna 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 C U C becomes C A C , indicating a frameshift mutation. 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.

Answer: (B)

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.

Answer: (D)

Spontaneous vs. Induced Mutations

In this video, we're going to differentiate between spontaneous mutations and induced mutations. And so mutations can occur in one of 2 ways. The first is spontaneously, and the second is through induction. And so spontaneous mutations are going to be random, naturally occurring mutations that occur through normal biological processes. Now, induced mutations, on the other hand, are controlled, deliberate mutations that are caused by an external source such as for example a mutagen, which is really just any chemical that causes mutations. And so spontaneous mutations are really naturally occurring mutations whereas induced mutations are going to be controlled and deliberate mutations that are going to be caused by an external source, which is usually introduced by a scientist.

And so, if we take a look at this image down below, we can better distinguish between spontaneous and induced mutations. And so, notice right here in the middle what we have is a normal bacterial cell. And the normal bacterial cell is going to have a bacterial chromosome. Now, notice down below what we have is a mutant bacteria. And this mutant bacterial cell has a mutation in its bacterial chromosome. And so, you can see we're labeling this as the mutation. And once again, the mutation is a permanent change in the DNA sequence. Now, this mutation that's in this mutant bacteria can arise in one of 2 ways. It could arise through a spontaneous mutation, like what we see over here on the left, which is going to be like a random naturally occurring mutation that occurs through normal biological processes. Or, this mutation could be generated by an induced mutation. And the induced mutation once again is going to be a controlled and deliberate mutation caused by an external source. And so, notice that here we have our scientist adding a mutagen, a chemical, to induce the mutation that is seen here.

And so, mutations can be either spontaneous or induced. And so this here concludes our brief lesson on spontaneous versus induced mutations, and we'll be able to get some practice applying these concepts and continue to learn more as we move forward in our course. So I'll see you all in our next video.

______ mutations occur randomly & ______ mutations are deliberate & occur due to an external source:

• A. Point ; Spontaneous.
• B. Spontaneous ; Point.
• C. Induced ; Mutagens.
• D. Induced ; Spontaneous.
• E. Spontaneous ; Induced.

Answer: (E)

Here’s what students ask on this topic:

What are the different types of point mutations?

Point mutations, also known as substitutions, involve the change of a single nucleotide in the DNA sequence. There are three main types of point mutations:

1. Silent Mutations: These mutations change a nucleotide without altering the amino acid sequence of the protein. For example, a codon change from GGA to GGC still codes for the same amino acid, glycine.

2. Missense Mutations: These mutations result in a change in one amino acid in the protein. For instance, a codon change from CUA (leucine) to CCA (proline) alters the protein's amino acid sequence.

3. Nonsense Mutations: These mutations introduce a premature stop codon, truncating the protein. For example, a codon change from GGA to UGA results in an early stop codon, terminating protein synthesis prematurely.

What is the difference between spontaneous and induced mutations?

Spontaneous mutations occur naturally and randomly during normal biological processes, such as DNA replication errors. They are not influenced by external factors. In contrast, induced mutations are caused by external agents, known as mutagens, which can be chemicals, radiation, or other environmental factors. Scientists often use mutagens to deliberately induce mutations for research purposes. For example, exposing bacteria to a chemical mutagen can cause specific changes in their DNA sequence, leading to induced mutations. Understanding the distinction between these two types of mutations is crucial for studying genetic variation and mutation mechanisms.

How do frameshift mutations 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 entire downstream amino acid sequence, potentially leading to significant changes in the protein. For example, inserting an extra nucleotide can change the reading frame from CUA-GGC to CUG-AGC, altering the resulting amino acids.

In contrast, point mutations, or substitutions, involve the change of a single nucleotide without affecting the reading frame. Point mutations can be silent, missense, or nonsense, depending on their impact on the amino acid sequence. While point mutations affect only one amino acid, frameshift mutations can alter many amino acids, often resulting in nonfunctional proteins.

What are the potential effects of mutations on an organism?

Mutations can have various effects on an organism, depending on their type and location in the DNA sequence. These effects can be:

1. Harmful: Mutations can disrupt normal protein function, leading to diseases or developmental issues. For example, a missense mutation in the hemoglobin gene can cause sickle cell anemia.

2. Beneficial: Some mutations provide advantageous traits that enhance an organism's survival or reproduction. For instance, a mutation that increases antibiotic resistance in bacteria can be beneficial in environments with antibiotics.

3. Neutral: Many mutations have no significant impact on the organism. Silent mutations, which do not change the amino acid sequence, are often neutral. Additionally, some mutations occur in non-coding regions of DNA and do not affect protein function.

What is a nonsense mutation and how does it affect protein synthesis?

A nonsense mutation is a type of point mutation where a single nucleotide change converts a codon into a premature stop codon. This results in the early termination of protein synthesis, producing a truncated and usually nonfunctional protein. For example, if the codon GGA (coding for glycine) is mutated to UGA, it becomes a stop codon, halting translation prematurely. The resulting protein is shorter than normal and often lacks essential functional domains, leading to loss of function. Nonsense mutations can have severe consequences, such as causing genetic disorders or diseases due to the production of incomplete proteins.