How Point Mutations, Insertions, and Deletions Affect DNA
- 0:05 Point Mutations
- 1:17 Base Substitutions
- 2:17 Transitions and Transversions
- 4:04 Insertions and Deletions
- 5:12 Lesson Summary
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.
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 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.
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.
Insertions and Deletions
Alright, so we've covered transitions and transversions. Both are types of base substitutions. Base substitutions, remember, are just one kind of point mutation. The other kinds are insertions and deletions. Rather than just replacing a nucleotide base, insertions and deletions involve adding or subtracting a pair of nucleotide bases. An insertion is a point mutation in which one or more base pairs is added to a DNA sequence. Let's see what an insertion would look like. We'll pick a random site in the sequence, right here, between the two thymines. This will be the site of insertion. An insertion mutation would add a base right in between the Ts, making the entire sequence longer by one base. Now the DNA reads a slightly different sequence, and it's longer than before. On the other hand, a deletion would be just the opposite. A deletion is a point mutation in which one or more base pairs is removed from a DNA sequence. If a deletion mutation happened to our strand, then we would lose one base and the DNA strand would end up one base shorter.
We haven't yet discussed how the point mutations in DNA can affect the function of a protein. So before we get into all that, let's make sure we understand the basics. All mutations are changes in the nucleotide sequence of DNA, which occur by way of random mistakes. While some mutations involve large sections of DNA, point mutations are changes in the genetic sequence that occur at a specific point along the DNA strand. Some point mutations are base substitutions, in which a single nitrogenous base is replaced by a different one. When a purine is replaced by another purine, or a pyrimidine by another pyrimidine, we call it a transition. When a purine is replaced by a pyrimidine, or vice-versa, we call it a transversion. Point mutations also include insertions and deletions. An insertion is when base pairs are added to a DNA sequence, and a deletion is when base pairs are removed from a DNA sequence.
Chapters in Biology 101: Intro to Biology
- 1. Science Basics (6 lessons)
- 2. Review of Inorganic Chemistry For Biologists (14 lessons)
- 3. Introduction to Organic Chemistry (8 lessons)
- 4. Nucleic Acids: DNA and RNA (4 lessons)
- 5. Enzymatic Biochemistry (4 lessons)
- 6. Cell Biology (14 lessons)
- 7. DNA Replication: Processes and Steps (5 lessons)
- 8. The Transcription and Translation Process (10 lessons)
- 9. Genetic Mutations (4 lessons)
- 10. Metabolic Biochemistry (9 lessons)
- 11. Cell Division (13 lessons)
- 12. Plant Biology (12 lessons)
- 13. Plant Reproduction and Growth (10 lessons)
- 14. Physiology I: The Circulatory, Respiratory, Digestive,... (12 lessons)
- 15. Physiology II: The Nervous, Immune, and Endocrine Systems (13 lessons)
- 16. Animal Reproduction and Development (12 lessons)
- 17. Genetics: Principles of Heredity (10 lessons)
- 18. Principles of Ecology (18 lessons)
- 19. Principles of Evolution (9 lessons)
- 20. The Origin and History of Life On Earth (4 lessons)
- 21. Phylogeny and the Classification of Organisms (7 lessons)
- 22. Social Biology (6 lessons)
- 23. Basic Molecular Biology Laboratory Techniques (13 lessons)
- 24. Analyzing Scientific Data (3 lessons)
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