Lesson Video: Genetic and Chromosomal Mutations Biology • Third Year of Secondary School

Video Transcript

In this video, we will learn about changes in DNA that we call mutations. We will see how these mutations can affect the information that DNA carries and learn how to identify the different types of mutations on both genes and chromosomes. We’ll also learn more about spontaneous mutations and induced mutations.

Our body has hundreds of different types of cells, each of which perform very specific functions. Each cell type helps keep our bodies systems running smoothly, keeping us alive, active, and healthy. Each of these cell types depend on our genetic material, DNA, for the instructions needed to perform their functions properly. In this video, we’ll learn about the mutations in DNA that can change these instructions. These mutations can introduce errors in the way these cell types function, which can lead to serious medical problems.

There are two types of mutations, genetic mutations and chromosomal mutations. We’ll discuss both of these types of mutations. But first, let’s review the structure of DNA and how it can be used to provide instructions for cells. Our DNA is stored in the nucleus of our cells as structures called chromosomes. Chromosomes contain highly compacted DNA that’s wrapped around special proteins called histones. If we unravel the DNA, we can see its structure here. DNA is made up of two strands that twist together to form a shape called the double helix. The colored lines we see in between these two strands are called nitrogenous bases. And they come as four different types: guanine or G for short, represented in orange; cytosine, as shown in blue; adenine, as shown in green; and thymine as shown in pink.

If we zoom in on this section of the molecule and untwist the helix, we can see that these bases can pair with each other to form a base pair. Guanine always pairs with its complementary base cytosine, and adenine always pairs with its complementary base thymine. So if we write out a sequence of nucleotides, as shown here, then we can figure out what the complementary strand will be based on these rules. So it’s often enough to just write the sequence of a single strand since the other strand sequence is implied.

So how is this DNA sequence converted into something useful? Well, Throughout DNA are multiple genes that are made up of a specific sequence of these bases. So let’s pretend that this is a segment of a DNA sequence for a gene. This DNA sequence can be decoded to produce a protein with a particular function. To do this, there’s a couple of steps. First, the gene undergoes a process called transcription. This produces a copy of the gene that we call messenger RNA, or mRNA, from DNA. mRNA is single stranded, and it contains uracil in place of thymine which is in DNA. This sequence of mRNA can then be translated to give a sequence of amino acids.

To do this, mRNA is decoded in groups of three nucleotides that we call codons. Each codon codes for a specific amino acid. The codon GUC codes for the amino acid valine. The codon for CGG codes for arginine. And the codon AGU codes for serine. So the sequence of DNA in a gene corresponds to a specific sequence of mRNA, which corresponds to a specific sequence of amino acids. These amino acids can then fold into a unique shape to form a protein with a specific function.

Now let’s talk a little bit about mutations in genes or genetic mutations. There’s actually three types of genetic mutations that we’ll be discussing. There’s the substitution mutation, the insertion mutation, and the deletion mutation. Let’s start with the substitution mutation. These sequences on the left are the same as the sequences on the right. What we’re going to do now is introduce a substitution mutation into the DNA sequence on the left, and we’ll see what happens. So we’ll change this G to an A. This is an example of a substitution mutation because one base was substituted for another. This mutation will then be carried on into the mRNA.

So now this codon reads AUC, whereas before it read GUC. AUC codes for a different amino acid compared to GUC. So instead of the valine amino acid, we now get isoleucine. So this mutation is also carried over to the protein, which may now change its shape relative to the unmutated protein. This might have an impact on the function of the protein. And it may not even function at all. Substitution mutations tend to change single amino acids. So what about the other kinds of mutations? Insertion mutations insert nucleotides into our DNA sequence. So here we can see an extra adenine inserted into the sequence. This is carried over into the mRNA. And now, if we underline all the codons in the mRNA, we can see that these first two codons are the same as the original codons. But this last codon is different.

Insertion mutations actually cause what’s called a frameshift, because the sequence of codons that are being read during translation, called the reading frame, has now changed after the point of this insertion. This might be easier to understand with a simple sentence like “the fox and the boy run.” Each of these words are three letters long, like our codons in mRNA. So when we insert the letter p towards the beginning of the sentence while keeping each word three letters long, we can see that this sentence doesn’t make sense anymore because the reading frame has been altered by inserting this letter, but only after this point of insertion, while the first two words aren’t affected.

So now, if we get back to our example, this altered codon changes from producing serine, as over here, to lysine and also changes all the amino acids from this point forward. This causes our protein to dramatically change from the original protein because most of the amino acids are now different. And so it will have a completely different shape and will be unlikely to function like the original protein. As you may now see, mutations can be harmful if they change the structure of the protein and the protein becomes nonfunctional.

The final type of genetic mutation we’ll cover is the deletion mutation. For this example, I’ve extended our DNA sequence a bit more. In this mutation, a nucleotide is deleted from our DNA sequence, which is therefore also deleted from the mRNA sequence. And now everything moves over. This causes a frameshift, just like our insertion mutation, except this time we’re missing this adenine nucleotide and everything is shifting over to the left. So now, when we look at the codons in mRNA, again, they don’t match but only pass the point of this deletion. Just like the insertion mutation, this causes all the amino acids to change further on from the point of the mutation, which again causes a dramatic change in the protein structure. Deletion mutations can therefore cause proteins to be nonfunctional.

Although this kind of mutation is more likely to be harmful and leave the protein nonfunctional, it’s also possible that it’s a beneficial mutation. Here, the protein may be more effective in its function or may have a new function altogether. In this case, a beneficial mutation may allow an organism to better adapt to his environment.

Now that we’ve covered genetic mutations, let’s start looking at chromosomal mutations. Mutations in DNA can happen in the sequence of bases, while chromosomal mutations can be much larger and affect the whole chromosome. As we had seen earlier, most cells in our body contain structures called chromosomes that are made up of DNA. This X-shaped structure is actually a duplicated chromosome and is what we see after the chromosome has duplicated and it’s preparing for cell division. When the cell isn’t preparing to divide, the chromosome exists as a single unduplicated structure that’s represented here. In reality, the chromosome is actually a very long and thin structure. And it’s not compacted to look like this. But we’re gonna draw it this way to explain chromosomal mutations.

So here’s a chromosome. And on this chromosome, we have a few genes indicated as these blue bands. In reality, chromosomes can contain hundreds to thousands of genes. So you can imagine that if this chromosome itself were mutated, then many genes may be altered, and this can cause serious health issues. So now, we’ll look at three different types of chromosomal mutations.

A chromosomal duplication is when a region of a chromosome is duplicated. You can see this here. This makes the chromosome larger because it now contains extra genetic material. It also has two copies of all the genes that were contained in that region. This can increase the production of those corresponding proteins, which can cause problems for the cell. In a chromosomal deletion, a region of the chromosome is missing. We can see this here. Now this chromosome is missing genes that were in that region. And this can cause problems for the cell.

The last type of chromosomal mutation that we’ll discuss is an inversion. Instead of representing the genes as blue lines, as in the previous diagrams, we’ll represent them here as letters, which will help us see how this kind of mutation works. In an inversion, a chromosome segment breaks off and reattaches in an inverted orientation. So now instead of DEF, it flips around to FED. In this type of mutation, the amount of genetic material doesn’t change. Because of this, these kinds of mutations may not lead to serious health problems, but they can still cause issues, for example, with fertility.

Now that we understand how genetic and chromosomal mutations work, let’s see what some of the causes are. There are two major types of mutations, spontaneous and induced or acquired mutations. In spontaneous mutations, errors in DNA replication can occur that introduce mutations in the sequence of the DNA being copied. If this occurs in gametes, like our egg or sperm cells, then this kind of mutation can be transmitted to every cell of the embryo and therefore the human that develops. In this case, this mutation can then be passed on to future generations. Induced or acquired mutations occur because of exposure to external factors called mutagens, for example, radiation or chemicals. These kinds of mutations can cause single mutations in the DNA sequence or can cause breakage of different sections of chromosomes.

Only the cells that are exposed to these kinds of factors are at risk of having that mutation. These kinds of mutations are generally not passed on to the next generation. As mentioned, mutations can be harmful or even beneficial if a mutagen happens to alter a gene. But they can also be neutral if they happen to occur in an area of DNA that doesn’t contain any genes. This is actually more likely to occur because about 99 percent of all our DNA doesn’t contain genes. Now let’s apply what we’ve learned to a practice question.

The diagram provided shows a simplified outline of the different types of chromosomal mutations that can occur. Which diagram, 1, 2, or 3, demonstrates an inversion mutation?

This question is asking us about chromosomal mutations. So what exactly is a chromosome anyway? Inside most of our cells is our DNA. Our DNA is organized on structures called chromosomes. These are long linear pieces of compacted DNA that contain a number of genes. We’re indicating these genes here as a few colored orange lines, whereas in reality there can be thousands of genes on a single chromosome. These genes provide the instructions our cells need to properly function. A chromosomal mutation is when the structure of this chromosome is somehow changed. This can cause problems for the cell because these mutations might have an impact on the genes contained within the chromosome.

There are several types of chromosomal mutations, duplications, deletions, and inversions. Let’s take a closer look and see how each of these can happen. To draw these, we’ll be labeling different segments of the chromosomes with different letters to help us see what impact these mutations have on the chromosome structure. In a duplication, a part of the chromosome is duplicated or copied. In this example, we now have two segments that contain the letter E. You’ll also notice that this makes the chromosome larger because it now includes extra genetic material. It also has two copies of all the genes that were contained in that region. This can increase production of those proteins, which can cause problems for the cell.

In a chromosomal deletion, a part of the chromosome is now missing. Now, this chromosome is missing genes that were in that region, which can also cause problems for the cell. Inversion mutations, which is what this question is asking us about, involve a segment of a chromosome breaking off then reattaching in the inverse orientation. We can see this here, how the section DEF is inverted to FED.

Now let’s turn our attention to the diagram on the left and see if we can figure out which of the three mutations represents a chromosomal inversion. Let’s make sure that we pay attention to these colored bands on each segment of the chromosomes. We can use these to help determine what happened to each mutated chromosome. In mutation 1, this section of the chromosome is missing. This is an example of a chromosomal deletion. We can also see a chromosomal deletion in mutation 3. In addition, we can also see these two sections duplicated in mutation 3. In mutation 2, we can see that these two bands have been inverted. Therefore, mutation 2 demonstrates an inversion mutation.

Now let’s go over some of the key points that we covered in this video. Mutations are changes in the sequence of DNA. Depending on what part of our DNA is affected, mutations can be harmful, beneficial, or neutral. Genetic mutations include substitutions, insertions, or deletions, while chromosomal mutations include duplications, deletions, or inversions. Spontaneous mutations occur as mistakes in copying DNA, while induced mutations are caused by external factors like radiation or chemicals.