General Biology/Genetics/Mutation

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A mutation is a permanent change to an organism's genetic material (DNA or RNA). Mutations are a rare but significant biological process, since they provide the variation on which evolution acts and are also the source of cancer.

An organism's genetic material is made up of polymers (chains) of four different nucleotides, like a recipe book written in a language of only four letters. A mutation event is when the order of the nucleotides in DNA change, usually when the DNA is being copied.

Mutations come in a number of forms:

Point Mutations

Point mutations are all mutations which involve a single nucleotide. These come in the form of substitutions, insertions and deletions:

Substitution

Substitution Mutations: In substitution mutations, a nitrogenous base of a triplet codon of DNA is replaced by another nitrogen base or some derivative of the nitrogen base, changing the codon. The altered codon codes for a different amino acid substitution.The substitution mutations are of two types:

1.Transitions: It is the replacement of one purine in a polynucleotide chain by another purine(A by G or G by A) or one pyrimidine by another pyrimidine(T by C or C by T)

2.Transversions:A base pair substitution involving the substitution of a purine by pyrimidine or pyrimidine by a purine is called transversion.

Larger mutations

Larger mutations which involve more than one nucleotide also include insertions and deletions, but can also include inversions, rearrangement of nucleotides and duplication of entire genes:

Chromosomal mutations

Chromosomal mutations involve changes to entire chromosomes. These mutations are particularly rare:

Effects of mutations

Mutations can have a variety of different effects depending on the type of mutation, the significance of the piece of genetic material affected and whether the cells affected are germ-line cells. Only mutations in germ-line cells can be passed on to children, while mutations elsewhere can cause cell-death or cancer.

Mutations can be classified by their effects:

Silent Mutation

Silent Mutations are DNA mutations that do not result in a change to the amino acid sequence or a protein. They may occur in a non-coding region (outside of a gene or within an intron), or they may occur within an exon in a manner that does not alter the final amino acid chain.

Missense Mutation

Missense mutations are types of point mutations where a single nucleotide is changed to cause substitution of a different amino acid. This in turn can render the resulting protein nonfunctional. Such mutations are responsible for diseases such as Epidermolysis bullosa.

Original notes

• “Rare” change in nucleotide sequence
  • Somatic vs germline
• only those in germline are heritable
  • Point mutation
• Single nucleotide change
  • Change in gene position
• Transposition
• Chromosomal rearrangement
• Mutagenic agents
• Raw material for evolutionary change

Point mutation

• Ionizing radiation
  • UV light induces thymine dimers
• Reparable
• Error during DNA synthesis
• Movement of transposons
  • McClintock
• Chemical mutagens
• May alter
  • Properties of promoter, enhancer
  • Amino acid sequence of polypeptide

Acquisition of genetic variability

• Mutation
• Sex (fusion of genomes)
• Recombination
  • Crossing over
• Reciprocal (may result in gene conversion)
• Unequal (gives rise to gene families)
  • Independent segregation
• Transposition by transposons
• Conjugation in bacteria
  • One way transfer from donor to recipient

Eukaryote genome

• Thousands of transposons
• Millions of transposon derived elements
  • LINES, SINES
• Above may constitute largest portion of genome
• Pseudogenes
• Tandem clusters (rRNA genes; nucleolus)
• Multigene families
• Single-copy genes (one copy per 1n)

Barbara McClintock

• Discovered transposons in perhaps greatest and ultimately most important intellectual endeavors in genetics
  • Maize
  • Worked alone
• Transposons: likely responsible for considerable evolution in eukaryotic genomes
• Likely origin of viruses

References

This text is based on notes very generously donated by Paul Doerder, Ph.D., of the Cleveland State University.

Further reading

Books

• Jones, S. 1993. The Language of the Genes. Harper Collins ISBN 0006552439.

Websites

• w:Mutation
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