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What Causes Cancer? - Page 2

Types of Somatic DNA Mutations

There are many types of somatic mutations. We will only discuss two types to keep very detailed information to a reasonable quantity.

Classified by impact on protein sequence: These are small-scale mutations of the gene

1. Point or Missense mutations are a change in one DNA base pair (a misspelling) that results in the substitution of one amino acid for another in the protein made by a gene. (Most SNPs are point mutations)

Image Courtesy of U.S. National Library of Medicine
Explanation of Image:

a. The top line is half of a strand of DNA (DNA has two strands)

b. Researchers know that C always matches G, T always matches with A and A always matches with T, so they only write the first half of the base pair

c. The first three bases are C-A-T. Notice the line under them to show that this is a condon (three base pairs) for the amino acid Histone.

d. Look at the bottom strand of DNA and find the blue "C". This should be an A.

e. A point mutation (misspelling) has occurred during the cell cycle in this part of the DNA sequence

f. This mistake changes C-A-T to C-C-T, so it does not code for Histone anymore and may result in a malfunctioning protein.

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If you want to see illustrations of the mutations below go to:

Cont. of impact on protein sequence mutations:

2. Frameshift Mutations are caused by the addition or loss of a base pair. This alters the
content of every triplet codon that follows in a reading frame.

3. Splice-site Mutations occur within genes in the noncoding regions (introns) just next to the coding regions (exons) and can have profound effects on the resulting protein, which may lead to disease.

4. Regulatory Mutations occur in the noncoding region and change the rate of protein production. These include mutations to promoter sequences, enhancer sequences, and repressor regions.

5. Deletions occur when one or more base pairs are eliminated from the original DNA sequence

6. Insertions occur when one or more base pairs disrupt the original sequence and are incorporated into the DNA sequence where they otherwise would not be found.

7. Translocations occur when segments of one chromosome break off and fuse to a different chromosome, without any loss of genetic material.

8. Inversions are mutations that arise when two breaks occur in a chromosome and the piece is reinserted in reversed order.


  • Amplifications (gene duplications) leading to multiple copies of all chromosomal regions,

    Deletions of large chromosomal regions, leading to loss of the genes within those regions.

    Chromosomal translocations: interchange of genetic parts from nonhomologous chromosomes.

    Insertions: The insertion of a large DNA sequence into a different chromosome

    Inversions: reversing the orientation of a chromosomal segment.

  • Loss of heterozygosity: loss of one allele, either by a deletion or recombination event, in an organism that previously had two different alleles.

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