Gene and Chromosomal Mutations

What is a mutation? Mutations are changes made to an organism’s genetic material. These changes may be due to errors in replication, errors during transcription, radiation, and many other things. Mutations can occur within a specific gene (small scale), as well as to the chromosome as a whole (large scale).

Are mutations always bad? Favorable mutations present organisms with an advantage over others and ensure their survival. These mutations will accumulate in a population. Less favorable mutations are removed from the gene pool through natural selection. – Organisms with these mutations will not survive.

Mutations A.Small Scale Mutations B.Large Scale Mutations

Small Scale Mutations: Gene Mutations There are three main types of gene mutations according to Mechanism: – Point Mutations – Deletions (Frame shift mutations) – Insertions (Frame shift mutations)

Point Mutations Point mutations occur when a single nucleotide in a gene is exchanged for another nucleotide. For example, an A could be exchanged for a T or a C could be exchanged for a G. These mutations have an effect on protein synthesis. One DNA triplet is altered, therefore one mRNA codon is altered, therefore one amino acid is affected

Examples of Point Mutations Sickle cell anemia Lactose intolerance

Insertions Insertions occur when one or more new nucleotides are inserted into the DNA sequence. When this occurs, there is a shift in the reading sequence of DNA during protein synthesis and a completely different protein strand may be formed.

Deletions Deletions are when one or more nucleotides are removed from the DNA sequence. As with insertions, this causes a shift in the reading sequence of DNA and can produce a completely different protein strand than the original. Deletions are irreversible mutations.

Types of Mutations according to Cause: 1.Spontaneous Mutation 2.Induced Mutation

Spontaneous Mutation The cause of a spontaneous mutation is unknown. Spontaneous mutations result from DNA replication errors – The rate is approximately 1in10 6 replicated genes.

Induced Mutation An induced mutation is brought about by a mutagen. A mutagen is a physical agent or a chemical agent that causes an alteration of the base sequence of the DNA molecule.

Induced Mutations Types of Mutagens : Physical mutagens – heat – ultraviolet radiation Chemical mutagens – nitrous acid – benzpyrenes – aflatoxin – base analogues

Ultraviolet Radiation Ultraviolet radiation causes the formation of thymine dimers. Thymine dimers block DNA replication. Cell excises (cuts out) the thymine dimers using enzymes and repairs the damage. A mutation occurs when the repair is faulty.

Nitrous Acid Nitrous acid converts the adenine molecule so that it will pair with cytosine. When the DNA replicates, the newly synthesized stand will have a cytosine opposite the adenine on the conserved strand (instead of a thymine).

Benzpyrene and Aflatoxin Benzpyrene and Aflatoxin both produce frameshift mutations so that one or more base pairs are inserted or deleted from the DNA when it replicates.

Large Scale Mutations: Chromosomal Mutations There are three major chromosomal mutations that can occur: – Duplications – Inversions – Deletions

Duplication Duplication occurs when a segment of a chromosome is doubled. The segment of DNA that is doubled may contain many genes.

Inversions Inversions occur when a piece of DNA is removed from the chromosome, inverted, and then placed back into the chromosome. When it is re-inserted, the genes are in reverse order. There is no loss of genetic material, however gene interactions may be altered and phenotypes may change.

Deletions Deletions occur when a whole chunk of the chromosome goes missing. Breaks in chromosomes can be caused by radiation, heat, viruses, chemicals, and other environmental causes. Consequences of these mutations will depend on what part of the chromosome has been deleted.

Deletions Examples of disorders caused by deletions: – Prader-Willi Syndrome Deletion of the long arm on chromosome 15 Characterized by compulsive eating