How Point Mutations, Insertions, and Deletions Affect DNA

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  • 0:05 Point Mutations
  • 1:17 Base Substitutions
  • 2:17 Transitions and Transversions
  • 4:04 Insertions and Deletions
  • 5:12 Lesson Summary
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Lesson Transcript
Instructor: April Koch

April teaches high school science and holds a master's degree in education.

What are genetic mutations? How do they affect our DNA? This lesson covers the basics of point mutations and provides simple examples of different mutation types.

Point Mutations

Have you ever thought that you could be a mutant? Have you ever wished you were? Maybe if you were a mutant you could sprout wings and fly like a bird, or turn invisible, or shape-shift into different animals! Maybe if you got stung by a radioactive bee, or if you were raised in a wastewater treatment plant, then you could develop amazing superpowers and run around fighting crime all day!

Alright, so we know that's not going to happen to any of us. Radioactive insects and industrial wastewater would probably do more harm to us than good. But there are mutant organisms out there, and they're the result of genetic mutations. Mutations can occur in any organism, from people and other animals to plants, bacteria, fungi, and protists. A mutation is any change in the nucleotide sequence of an organism's DNA. Some mutations are more drastic than others. For example, chromosome mutations involve changes to large sections of DNA or even entire chromosomes. We'll save those mutations for the chapter on genetics. Here, we're going to focus on smaller mutations called point mutations. Point mutations are changes in the genetic sequence that occur at a specific point along the DNA strand.

There are lots of different ways a point mutation can come about. They're caused by random mistakes that occur while a DNA sequence is being built. Point mutations don't usually happen to more than one nucleotide. They generally involve a single nucleotide being added, subtracted, or replaced with a different kind. We can split up point mutations into two basic types: base substitutions and insertions and deletions. We'll look at base substitutions first.

Base Substitutions

Base substitutions are just what they sound like; they're point mutations in which one nitrogenous base is substituted by a different base. Take this DNA sequence, for example: G A T T A C A. Let's say there's a mistake in the DNA strand, which causes the first thymine, or T, to be switched with a guanine, or G. Now the nucleotide sequence is slightly different. It reads G A G T A C A. A single base substitution has occurred.

Transitions and Transversions

Base substitutions are further divided into two types: transitions and transversions. Transitions are base substitutions that swap a purine for another purine or a pyrimidine for another pyrimidine. You may have forgotten the difference between a purine and a pyrimidine, so let me refresh your memory. A purine is a double-ring nitrogenous base, like adenine and guanine. A pyrimidine is a single-ring base, like thymine and cytosine. You can remember the difference by using the phrase 'All gods are pure.' So A and G are purines, and C and T are pyrimidines.

Purines include adenine and guanine.
Purine Types

Thymine and cytosine are pyrimidines.
Pyrimidine Types

Let's get back to transitions and transversions. A transition is when a purine is swapped for a purine or a pyrimidine for a pyrimidine. If a transition happened to our original DNA sequence, it could be that the G is swapped for an A. That would be a purine-to-purine substitution. We would also call it a transition if one of the Ts was swapped for a C or the C to a T.

Now, if you switch a purine with a pyrimidine, or vice-versa, then we call it a transversion. There are more possibilities for transversions than there are for transitions. If we take the nitrogenous base adenine, which is a purine, then it can make a transversion by being replaced with a C or by being replaced with a T. While the A has two ways of making transversions, it only has one way of making a transition. The two words sound really similar, don't they? Just remember that in a transversion, the nitrogenous base switches to a different 'version,' from a purine to pyrimidine, or vice-versa. And to remember transitions, just imagine this graphic and that all the transitions 'sit' at the same level with each other.

Graphic indicating transitions
Transitions sit at the same level

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