Understanding Genome Mutations, Aneuploidy, and Mutagenic Agents
Genome Mutations
Genome mutations affect the number of chromosomes characteristic of a species. These mutations appear to be related to abnormal segregation of chromosomes during meiotic division. We distinguish two main types: aneuploidy and euploidy.
Aneuploidy
Aneuploidy consists of a change in the number of chromosomes, either by gain or loss of one or more chromosomes. There are several types:
- Nullisomy: Missing a pair of homologous chromosomes (2n-2 chromosomes).
- Monosomy: Missing a single chromosome (2n-1 chromosome).
- Trisomy: There is an extra chromosome (2n +1 chromosome).
Aneuploidy in Humans
Some diseases in humans are caused by aneuploidy:
Aneuploidy involving Autosomes
- Down Syndrome: Trisomy of chromosome 21 (47 chromosomes).
Aneuploidy involving Sex Chromosomes
- Turner Syndrome: Monosomy X (44 autosomes + X). Affected individuals are women with stunted growth and lack of sexual organ development.
- Triple X Syndrome: Trisomy X (44 autosomes + XXX). Affected individuals are women with little breast and external genitalia development.
- Klinefelter Syndrome: Two X chromosomes and one Y chromosome (44 autosomes + XXY). Affected individuals are men with mental retardation, sterile genitalia, and underdeveloped sexual organs.
- Double Y Syndrome: One X chromosome and two Y chromosomes (44 autosomes + XYY). Affected individuals are men who may be somewhat taller than average and more likely to show violent behavior.
Mutagenic Agents
Mutagenic agents significantly increase the mutation frequency in living organisms. They act by altering or damaging the composition and structure of DNA. We can distinguish between physical and chemical mutagens.
Physical Mutagens
There are two types of radiation that can be mutagenic: non-ionizing radiation and ionizing radiation.
- Non-ionizing Radiation: Ultraviolet (UV) radiation causes the formation of thymine dimers and gene mutations.
- Ionizing Radiation: X-rays and gamma rays are more energetic than UV radiation and, therefore, more dangerous. They cause breakage of chromosomes and gene mutations.
Chemical Mutagens
Chemicals that react with DNA cause three main types of changes:
- Changes in the nitrogenous bases
- Base substitution of similar bases
- Intercalation of molecules
Mutation and Evolution
Evolution is the process of transformation from one species into another through a series of variations that have occurred, generation after generation, over millions of years. Darwin proposed his theory of natural selection.
According to Darwin and Wallace, evolution is produced by three factors:
- High Birth Rate: All species have a high reproductive potential. They produce more offspring than the ecosystem can support, and not all reach adulthood and, therefore, fail to reproduce.
- Variability of the Offspring: Among the descendants of the same set of parents, morphological variations can be seen, and such variations can be transmitted to offspring.
- Natural Selection: Since more individuals are born than can survive, and since different individuals of a species are different, competition for resources ensures that only the fittest survive and transmit their characteristics from generation to generation. Therefore, natural selection involves the survival of organisms best adapted to their environment, which are the ones that leave more descendants.
Small changes accumulate, generation after generation, changing species over vast periods of time.