Point Mutations in DNA: Types, Diseases & Examples

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  • 0:01 What Are Point Mutations?
  • 1:10 Types of Point Mutations
  • 2:30 Diseases & Frameshift…
  • 3:21 Lesson Summary
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Lesson Transcript
Instructor: Angela Lynn Swafford

Lynn has a BS and MS in biology and has taught many college biology courses.

Did you know that very small changes in DNA sequences called point mutations can have huge impacts on the body? Learn about different types of point mutations and a few of the diseases they can cause in this lesson.

What Are Point Mutations?

A mutation is a permanent change in a DNA sequence. DNA is the unit of heredity of all organisms, so this means that mutations can often be passed on to offspring. Mutations don't have to be bad. Some are beneficial, while others are neutral and have no effect.

There are many different categories of mutations, but in this lesson, we will focus on point mutations, or those resulting from a change in one or a few nucleotides at a single location in a DNA sequence. Nucleotides are the repeating units of a DNA sequence. There are four nucleotides, each with a different nitrogenous base: thymine (T), adenine (A), guanine (G), cytosine (C).

Genes are sequences of DNA that code for proteins, and the order of these bases determines the order of amino acids in a protein. Not all mutations are in these coding sequences but if they are, then the result can be that different or non-functioning proteins will be made.

Types of Point Mutations

There are two types of point mutations: base substitutions and frameshift mutations.

Base substitution occurs when one base is switched out with another base

In this example, the C was replaced by an A, changing the second codon sequence.

Frameshift mutation occurs when one base is added or removed.

There are two types of frameshift mutations. An insertion is a mutation in which one or few nitrogenous bases are added to a DNA sequence. This results in a sequence that is a few bases longer than it should be.

In this example, a T was added after the first A, shifting all the other bases to the right and affecting all the codon sequences.

A deletion is a mutation in which one or a few bases are removed, resulting in a shorter-than-normal sequence.

Here, you can see that the first G was removed, creating a sequence that is too short and that affects both codons.

Insertions and deletions are called frameshift mutations because they do not just affect one codon, a three-base sequence that codes for one amino acid, like in base substitutions. Instead, frameshift mutations affect all the codons that occur after the point mutation. This changes how a DNA sequence is read or the reading frame.

Diseases Caused by Point Mutations

Many diseases are caused by point mutations. Let's look at a few of them.

Base Substitution: Sickle Cell Anemia

One base substitution in a certain location in the gene for hemoglobin causes red blood cells to become abnormally-shaped and clog blood vessels. If someone has two copies of this mutation (people get one from their moms and one from their dads), then they will have the disease sickle cell anemia. However, if someone only has one copy of this mutation, he or she does not have the disease and actually benefits by being protected against malaria.

Insertion: Tay Sachs Disease

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