Evolution and Genetic Engineering - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 698

14.1.2.1. Germ-Line Mutations and Somatic Mutations
In multicellular organisms with dedicated reproductive cells, mutations can be catego-
rized into two broad categories. Germ-line mutations are those that occur in the reproductive
cells and can be passed on to the next generation via the reproduction process.
Somatic mutations involve the cells outside the reproductive system and usually are not
transmitted to the next generation, unless the organism is reproducing asexually.

14.1.2.2. Lethal, Non-Lethal, and Neutral Mutations
Lethal mutations are harmful mutations or damage to DNA that leads to the death of the
mutant organism. These mutations do not have any long-term effects on the populations.
Non-lethal mutations may be harmful, beneficial, or neutral mutations. These mutations
accumulate within the gene pool and increase the genetic variations. These mutations are
the basis of evolution through natural selection.
Neutral mutations are defined as the mutations that neither have any impact on the func-
tion or performance of the product of a gene nor do they influence the fitness of the organism.
These mutations can accumulate over time due to genetic drift.

14.1.2.3. Point Mutations
Small changes in the DNA molecule are called point mutations and involve a single
base or a single base pair. These type of mutations may either have no effect or may be
weakly beneficial, but majority of them (about 70%) are harmful. Due to this damaging
effect of these mutations, cells and organisms have evolved mechanisms to repair or
remove mutations.
Point mutations are caused either by mistakes in the DNA replication process or by chem-
icals (called mutagens). There are three causes of point mutations: (1) transitions; (2) inser-
tions; and (3) deletions.

14.1.2.3.1. TRANSITIONS OR TRANSVERSIONS
Exchange or substitution of a purine for purine (adenine for guanine and vice versa) or
pyrimidine for a pyrimidine (cytosine for thymine and vice versa) are the most common tran-
sition mutations. It can be caused by a base mismatching during replication or by chemicals
like nitrous acid and base analogs like 5-bromo-2-deoxyuridine. For example:

5→ AACGC T AGATC →3 5→ AACGC G AGATC →3

3← TTGCG A TCTAG ←5 3← TTGCG C TCTAG ←5

where on the sixth base pair position, the base on the upper strand has its T substituted by G,
whereas the lower strand has its base A substituted by base C. This only occurs on one single
point.
Point mutations occurring within a protein-coding region of a gene can have three
different results: silent mutations; missense mutations; and nonsense mutations.
Silent mutationsd The mutated genetic code still codes for the same amino acid. For
example: mRNA codon UUU (codes for phenylalanine) is coded by DNA codon AAA. If
third “A” is exchanged for “G,” resulting mRNA codon UUC will still code for phenylala-
nine. As shown in Table 14.2 , in multiple instances, the change in the third base of a codon