Evolution and Genetic Engineering - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 696

a consequence of genotypic changes are usually irreversible. Similarly, the genotypic changes
or alterations in the genotype (mutations) are permanent and mostly irreversible.
Mutations may involve a single nucleotide base or a single nucleotide base pair (bp). These
small changes usually affect a single gene, its coded protein or the expression of a gene. Other
mutations may involve large sections of DNA. These mutations result in the duplication of
a section of DNA, deletion and loss of a segment of DNA, relocation of a segment of DNA
from one part of the genome to another, or rearrangements of the segments of DNA in rela-
tion to the others.
Some larger mutations also involve duplication or loss of an entire chromosome. In some
cases, especially in plants, a whole set of chromatin or chromosomes are duplicated, resulting
in triploid, tetraploid, or polyploid generations.

14.1.1. What Causes Genetic Mutations?
Mutations can occur due to the internal flaws in DNA replication mechanisms, due to
external/environmental factors or both working together. For example, an environmental
factor (e.g. UV light or DNA-reactive chemicals) can create a small alteration in the compo-
nents of DNA, and internal mechanisms, in an effort to repair it, can create a permanent
change or mutation in the genotype of a cell. Exposure to certain chemicals like mustard
gas, alkylating agents, etc. can cause DNA damage that can result into mutations. Tab l e
14.1 illustrates common causes of mutations.

TABLE 14.1 Common Causes of Mutations

Common causes of mutations Description

Spontaneous

Aberrant recombination Abnormal crossing over may cause deletions, duplications, translocations, and

inversions.

Aberrant segregation Abnormal chromosomal segregation may cause aneuploidy or polyploidy.

Errors in DNA replication A mistake by DNA polymerase may cause a point mutation.

Toxic metabolic products The products of normal metabolic processes may be chemically reactive agents

that can alter the structure of DNA.

Transposable elements Transposable elements can insert themselves into the sequence of a gene.

Depurination On rare occasions, the linkage between purines (i.e. adenine and guanine) and

deoxyribose can spontaneously break. If not repaired, it can lead to mutation.

Deamination Cytosine and 5-methylcytosine can spontaneously deaminate to create uracil or

thymine.

Tautomeric shifts Spontaneous changes in base structure can cause mutations if they occur

immediately prior to DNA replication.

Induced

Chemical agents Chemical substances may cause changes in the structure of DNA.

Physical agents Physical phenomena such as UV light and X rays damage the DNA.