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Lesson Video: Mutations Biology

In this video, we will learn how to describe mutations, and explain the impacts mutations can have on organisms.

13:13

Video Transcript

In this video, we’ll learn what mutations are and how they can have negative, neutral, or even sometimes beneficial effects on organisms. And we’ll also take a look at how mutations have sparked the diversity of life on Earth.

Mutations affect the structure and function of DNA. So let’s begin by reviewing DNA structure. If you line up your DNA molecules from one cell from end to end, they’d be about two meters long. And that means that the average length of one of your DNA molecules is longer than two centimeters, which is really long for a molecule. These long molecules are made up of tiny little units called nucleotides. Each molecule is made up of three parts, including a sugar, a phosphate, and one of four types of bases.

Nucleotides bond together from base to base, forming base pairs. The base pairs make up the width of a DNA molecule, but the long length of a DNA molecule that goes far beyond the screen is made up of millions and millions of nucleotides bonded from sugar to phosphate group. The sequence of the four bases along our DNA molecules make up our genetic code. The bases include A or adenine, T or thymine, C or cytosine, and G for guanine. There are only two kinds of base pairs because the base adenine only bonds with thymine and the base cytosine only bonds with guanine. DNA molecules twist into the familiar structure called the double helix.

The structure of DNA with its lengths of genetic code leads to its function, which is to provide instructions for organizing living systems, in other words, organisms. Notice the similarity between the words organism and organizing. Their ancient word root means to do. And that’s appropriate because organisms have to stay pretty busy doing things to stay alive. But the instructions in DNA are not as straightforward as those that you would find in some product that you have to put together yourself or even the blueprints of a house. Instead, DNA codes for protein molecules, which are made out of amino acids. And DNA also has start and stop switches that control when and where these proteins are made.

Our bodies contain tens of thousands, if not more, types of proteins, and they have many different important functions. Sections of DNA, called genes, contain the instructions for how to build these proteins. The genetic code in a gene is copied in a process of transcription that produces a molecule called mRNA ⁠— and the m stands for messenger because it’s a messenger that delivers the genetic code ⁠— to little protein factories called ribosomes that use the instructions from the DNA to build the proteins. This process is basically the same in all organisms, but not all DNA is sectioned off into genes. Parts of DNA that don’t code for protein are called noncoding DNA.

Scientists are still working out the functions of noncoding DNA. But one function it does have is it includes some of the start and stop switches that turn genes on and off. Next, let’s see what a mutation in this gene would look like. Mutations are changes in the genetic code, and it can be something as simple as replacing one base for another. Changing one base out of six billion may seem insignificant, and sometimes it is. But other times it can have a big effect on an organism. Things that can cause mutations are called mutagens, and they can be chemical, biological, or they can be some kind of radiation, like sunlight or other types. And oftentimes, mutations come about just because a mistake was made during DNA replication and the wrong base was added.

Next, let’s look at a few different types of mutations and the effects they might have. Here’s another example of a gene, except instead of using the A, T, C, and G letters that represent the DNA bases, this gene can use any letters. And it’s arranged into three-letter words because genetic code is read in three-letter chunks. So while this isn’t a real gene, it will help us to understand how changes in code can alter the meaning of a message. The gene says that the hat was big for the fat cat, and while that’s pretty silly, we can understand what it means. But let’s say there is a mutation and the C is substituted for with a B. Now we have a FAT BAT with a BIG HAT.

And next, we could have a mutation that switches places of the A and the F in the word FAT. So it becomes AFT. Now our BAT is not FAT, but it’s on the AFT or rear end of a boat. Now a mutation might come along that inserts a base, forcing the rest of the bases to shift over one space. Now our sentence says THE HAT WPA SBI GFO RTH EAF TBA, and the drawing is just an old-fashioned hat and some meaningless scribbles. The same major changes occur if a base is deleted. The reading frame of each word changes, and we call these frame shift mutations. Now our sentence reads GTH EHA WPA SBI GFO RTH EAF TBA and we’re left with only meaningless scribbles.

Mutations in regions of DNA that don’t code for proteins may have no effects at all, unless that noncoding region is a switch for a gene that turns the gene on or off, or if it’s a frame shift mutation that runs into the gene. The bottom line here is that when you change the code, it might change the instructions. And if the instructions are changed, whatever you’re trying to make might not come out the same. Let’s start to take a look now at how mutations in DNA affect proteins instead of how changes in sentences affect cats.

Although actual genes are much longer, here are a couple copies of the same gene, so in other words, identical sections of DNA that code for the same protein. Let’s add a mutation to one of these genes so that two of the bases switch places. The first step of protein synthesis is to transcribe or copy a molecule of mRNA from the DNA gene, where we should have a CG in our mRNA. Because of the mutation, we now have a GC. The second step of protein synthesis is called translation. And that’s when the genetic code in the mRNA is read three bases at a time to determine the sequence of amino acid monomers that make up the protein.

The sequence of amino acids from the normal gene reads M, T — not the base T, but the amino acid T— C, and then a stop signal, while the sequence from the mutated gene reads amino acid M, T, and then L, which is different, and then a stop signal. The shape of these proteins depends on the sequence of amino acids. So the shapes can come out differently if one of the amino acids has changed. It’s important to note, though, that not all mutations will change the amino acid. Here, ACC codes for amino acid T and so does ACG. So some mutations have bigger effects than others. Now we should probably review why protein shape is important at all.

The reason is that structure leads to function. Think about all the functions that your hand has, and if your hand was like a horse’s hoof, would it have the same functions? Let’s take a look at cell transport proteins as an example. They’re shaped pretty much like a tunnel, and their function is to allow certain things to enter or exit cells. But if their shape is affected by a mutation, that may not be able to happen. An example of this is the mutation that causes the disease cystic fibrosis.

Enzymes are also made out of proteins most often. And they have a special area called an active site, where they catalyze or speed up chemical reactions. If a mutation causes the active site to lose its shape so it can no longer function, then active site won’t be active, and those chemical reactions that it catalyzes won’t happen nearly fast enough. Maple syrup urine disease, which can be fatal, is an example of a disease that’s caused by a mutation that affects the shape of an enzyme’s active site. Not all mutations have to be negative. A few might be positive. An example is our lactase enzyme, which has evolved since the agricultural revolution to allow people with this mutation to digest milk throughout their lifetime instead of just as infants.

Most commonly, though, mutations have no effect on the protein’s function. Sometimes they don’t even affect the protein shape. But even if the shape does change, it may not necessarily cause the function to decline. Another important example is what happens when the shape and function of proteins that regulate the cell cycle are affected by a mutation. The cell cycle is basically the life cycle of a cell, and part of it is called mitosis. Mitosis is the division of the cell so that there are then two cells. And that occurs over and over again at a certain rate. Each cell will divide in two. But certain mutations in proteins that affect the cell cycle can cause this rate of division to increase. And uncontrolled cell division like this is what cancer is. So mutations can also lead to cancerous cells.

So mutations usually have no effect at all on protein function. But sometimes they do cause changes to protein shape that affect function and that can cause genetic diseases or cancer. But on the amazing and spectacular side, mutations are also raw material for evolution. Mutations, which are mistakes in genetic code, can lead to variation in a population such as a flower being yellow instead of white. If the individual with the mutation and the different trait reproduces at a higher rate than the others, it will leave behind more offspring that look like itself than the others.

Without mutations, there could only be one form of life. But since Earth keeps on changing, it would probably go extinct once Earth changed enough. So mutations allow enough variation for populations to adapt as Earth changes. And that way, life keeps going. A mutation is a change to the genetic code of an organism. Mutations that happen within a gene, which is a section of DNA that codes for a protein, can result in proteins that still have same normal function or proteins that have decreased or a total loss of function or occasionally they might even improve the function of a protein.

Proteins with decreased function can lead to problems like genetic disease or cancer. But proteins with improved function increase the variation in a population and can lead to evolution. Types of mutations include when one base is substituted for another and inversions when two bases switch places. Entire sequences of DNA bases can shift if a base is deleted or inserted.

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