We will continue this by looking at briefly examples of the various types.

1) Point mutations

original DNA sequence:

                   5'CAT CAT CAT CAT
                   3'GTA GTA GTA GTA

aa sequence:     His   His   his   his

Substitution

 5'CAT   CAT   TAT   CAT
 3'GTA   GTA   ATA   GTA
    his       his       tyr        his

Addition

 5'CAT     CAT     GCA     TCA     T
 3'GTA     GCA     CGT     AGT     A
    His         his         ala         ser     ser

Deletion

 5'CAT     CAC     ATC     AT
 3'GTA     GTG      TAG     TA
    His        his           ile        ile

These all classify as point mutations - mutations in a nucleotide pair.  These are also missense mutations as they result in the incorporation of a different amino acid.  Additions and deletions lead to frameshift mutations - change in the reading frame of the mRNA.

Reverse mutations

These mutations can all be classified as forward mutations.  A Reverse or back mutations can take two forms: a true reverse mutation or reversion which produces the original DNA sequence or a suppressor mutation which is a second foward mutation resulting in wild type or almost wild type phenotype (or restore normal function) in the double mutants. E.g. a second foward mutation such as an addition followed by a deletion. This restores the reading frame.

 (Insert G)    G (delete T)     T

         CAT     CAT     GCA     TCA     CAT     CAT
         GTA     GTA     CGT     AGT      GTA     GTA

 - also called an  intragenic suppressor.  If the altered amino acids were not critical to function of the protein (e.g. DNA binding, protein folding) then the resulting protein may have normal functionality.

However, suppression can occur at a number of levels.  For example, the second mutation can be within a tRNA gene so that the mutant codon can be correctly coded for.  This is seen in the tRNA suppression of nonsense mutations.

E.g.
 normal sequence in DNA          mutant sequence

 5'TCG     TCG     TCG               TCG     TAG     TCG
 3'AGC     AGC     AGC             AGC     ATC     AGC

mRNA                                                      (Transversion)
 3'UCG     UCG     UCG             UCG     UAG     UCG
    Ser         ser         ser                 Ser      amber
                                                                  Nonsense codon

The amber termination codon causes early termination of transcription and a non-functional protein is produced.

A second mutation in a tRNA gene may produce a tRNA with an anticodon 3'AUC which could mobilise serine (or other aa) and this would be a nonsense suppressor, specifically an amber suppressor, and the normal protein function could be restored.

E.g. in E. coli there is a tRNA gene carrying AUC the anticodon that will suppress UAG amber mutations by inserting tyrosine in the polypeptide chain.  This is another example of suppression.

Suppressor mutations can be distinguished by reversions in genetic crosses as a reversion is at the same site in the genome as the original mutation while suppressor is at a different site in the genome.  If a suppressor mutation is crossed with a wild type in a haploid organism, then the original forward mutation can reappear in the progeny.

Fig
 

This is an example where the original and suppressor mutations are on a different chromosome (in trans), but even if the mutations are in cis, they can be separated by recombination in a subsequent cross and the original mutation will reappear in the progeny.

In a true reversion, though, the DNA is restored to the original wild type sequence and the original mutation cannot reappear during a subsequent crossing (except by repetition of the mutation).

2) Chromosomal mutations

 There are methods of staining chromosomes so that some regions take up more stain than others.  This is referred to as chromosome banding - differential staining along the longitudinal axis - e.g. C-banding with Giemsa stain, Q-banding with fluorescent dues that bind to nucleoprotein complexes.  This allows effects of chromosomal mutations to be observed.

         duplication

         inversion

         deletion

         translocation

         reciprocal translocation
 

3. Genomic Mutations

e.g. Polyploidy

diploidy. Triploidy, tetraploidy etc

modern cultivars of wheat are hexaploid

there is also allopolyploidy - a polyploid condition resulting from the combination of genetically distinct chromosome sets of different species or varieties.

Aneuploidy - chromosome number of an individual is not an exact multiple of the typical haploid set for the species e.g. 2n+1 would be hyperdiploid and trisomic for a particular chromosome.  Down's syndrome in humans is trisomy 21.

(in fact all that is necessary for Down's syndrome is for cells to have 3 copies of a specific region of the long arm of chromosome 21.)

Other aneuploid types:

hypodiploid: e.g. 2n-1, i.e. monosomic for one chromosome

there are thus possible a variety of aneuploids:  2n±1, 2, 3 etc

In haploid organisms:
         n+1 is hyperhaploid and disomic for one chromosome.

         n-1 is hypohaploidand nullisomic for one chromosome.  This is usually lethal as with many aneuploid states

In diploid organisms autoploidy and aneuploidy result from breakdown of the normal cell division process.  If chromosomes duplicate but fail to separate then polyploidy can result.

- If, in meiosis, one chromosome pair fails to separate, aneuploidy results leading to one hyperhaploid and one hypohaploid daughter cell.

- If these hyperhaploid and hypohaploid gametes fuse with normal gametes then the progeny are hyperdiploid and hypodiploid, respectively.
 

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