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A mutation is the change in DNA sequence of an organism. These changes can occur in individual nucleotides or in large sections of chromosomes. While many people associate mutations with disease or dysfunction, not all mutations are harmful. Some are neutral, and in rare cases, they may even be beneficial. Mutations are also the ultimate source of genetic variation, which is essential for evolution.
Mutations can affect different types of genes and processes. Some mutations interfere with homeostasis, such as those that change how a protein functions in metabolism, immunity, or cell signaling. Others affect developmental genes, which can disrupt growth, patterning, or cell differentiation. The impact of a mutation often depends on both where it occurs and what kind of change it causes.
Where Mutations Happen
Where Mutations Happen
There are two really important questions regarding the location of mutations:
What part of the DNA did the mutation occur in?
What type of cell did the mutation happen in?
To answer the first question, we have a variety of types of mutations:
Coding region mutations can change the amino acid sequence of a protein, which may affect the protein's shape or function.
Promoter mutations and enhancer mutations affect the ability to begin transcription. This can affect the amount of gene product produced as well as changing when or where a gene is made.
Intergenic regions are stretches of DNA between genes. These areas often have no known function, so mutations here usually have no effect. However, they can cause thing such as the creation of new enhancers.
For the second question, there are two types of mutations.
Germ-line mutations occur in the cells that give rise to gametes. These mutations can be passed on to offspring.
Somatic mutations happen in body cells and cannot be passed to the next generation. They will affect the individual, but nots its kids.
Types of Mutations
Types of Mutations
There are several different ways that the DNA sequence can be altered. Some mutations involve a change to a single nucleotide, while others involve the addition or removal of larger sections of DNA.
Point mutations affect a single nucleotide. These are the most basic type of mutation and can occur in several ways.
Substitution mutations happen when one base is replaced with another. Depending on the context, this can lead to different outcomes:
A silent mutation does not change the amino acid due to redundancy in the genetic code.
A missense mutation changes one amino acid to another, which may affect protein structure or function.
A nonsense mutation changes a codon into a stop codon, causing translation to end early and usually producing a shortened, nonfunctional protein.
Insertions and deletions ("indels") occur when one or more nucleotides are added to or removed from the DNA sequence. These can either disrupt the reading frame or leave it intact, depending on how many nucleotides are involved.
An in-frame mutation adds or removes a number of nucleotides that is a multiple of three. This preserves the grouping of codons, so only certain amino acids are affected while the rest of the protein remains mostly unchanged.
A frameshift mutation adds or removes a number of nucleotides that is not a multiple of three. This shifts the reading frame of the gene, which usually changes every amino acid that comes after the mutation.
As a good example of the difference between these, let's use the sentence:
THE CAT ATE THE RAT
Each word represents a codon. Now imagine there was an insertion of three bases:
THE CAT ATE THE BIG RAT
The five original codons remain unchanged, there is just one new one. This is an in-frame mutation.
On the other hand, let's look at the deletion of one base:
THE CAT ATE THE RAT
THE CAT TET HER AT
Everything after the mutation has changed. This is a frameshift.
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