Discuss the difference between adaptation to environmental mutations and heritable changes due to mutations

List several different types of mutations and discuss how mutations arise in cells

The genetic material can be changed through mutations, which are changes in the nucleotide sequences of genes

Changes that are not repaired are called mutations

Early in this century, some geneticists supported the theory that environmental conditions could cause all heritable changes in the genetic material (adaptations)

A mutation is a change in a DNA base-pair or region of chromosome, of which there are many causes

A somatic mutation affects the individual in which it happens and is not passed on to the succeeding generation

Germ-line mutations may be transmitted by the gametes to the next generation, producing an individual with mutations in both the somatic and germ-line cells

Germ-line mutations are heritable

A chromosomal mutation or aberration is a change in the structure or number of chromosomes

A gene mutation is a change in the DNA sequence of a particular gene

Mutations can occur spontaneously or be induced by a mutagen, which is a chemical or physical agent that increases the frequency of mutational events

Spontaneous mutations occur naturally

A point-mutation is a base-pair substitution mutation, in which one base pair is replaced by another base pair (AT to GC)

Point mutations include:

Transition mutations, in which the change is from one purine-pyrimidine base pair to the other purine-pyrimidine base pair (AT to GC, GC to AT, TA to CG, and CG to TA)

Transitions can be caused by oxidative deamination.

Transversion mutations involve a change from a purine-pyrimidine base pair to a pyrimidine-purine base pair (AT to TA, GC to CG, AT to CG, and GC to TA)

the consequences of this change tend to be more severe than those of transitions.

Transversions can be caused by ionizing radiation.

Mutations can be defined according to their effects on amino acid sequences in proteins and include:

Missense mutations, a gene mutation in which a change in the DNA causes a change in an mRNA codon so that a different amino acid is inserted into a polypeptide during biosynthesis

In sickle-cell anemia: a single nucleotide base-pair change in codon 6 of the -hemoglobin gene leads to an amino acid substitution in the -hemoglobin chain

Nonsense mutations, in which an mRNA codon is changed from an amino acid into a stop codon (UAG, UAA, or UGA)

A Nonsense Mutation and its Effect on TranslationA Nonsense Mutation and its Effect on Translation

Types of Base-Pair Substitution MutationsTypes of Base-Pair Substitution Mutations

Neutral mutations, which produce no detectable change in the function of the protein translated from the message

It is a subset of missense mutations where the new codon codes for a different amino acid which is chemically equivalent to the original one i.e Lys to Arg (protein function is not affected)

Silent mutations are mutations in which a base pair change transforms a codon into another codon for the same amino acid, with no detectable change in the resulting protein

Frameshift mutations, which result when the reading frame of a gene is shifted due to the addition or deletion of one or two more base pairs in a gene

It results in a non-functional protein

Types of Base-Pair Substitution MutationsTypes of Base-Pair Substitution Mutations

Point mutations are of two classes: Forward mutations Reverse mutations

Forward mutations cause the genotype to change from wild-type to mutant

A reversion is a mutational event that changes a mutant phenotype back to wild-type

The effects of a mutation may be diminished or abolished by a suppressor mutation (secondary or second site mutation)

A suppressor mutation is a mutation at a different site from an original mutation that does not result in a reversal of the original mutation but instead masks or compensates for the effects of the initial mutation

There are two major classes of suppressor mutations:

Intragenic suppressors occur within the same mutated gene

mRNA codon aaWT 5’CGT3’ CGU Arg

3’GCA5’ Mut 5’AGT3’ AGU Ser

3’TCA5’Intragenic suppressor

5’AGA3’ AGA Arg 3’TCT5’

Intergenic suppressors occur in different genes

Mechanism of action of an intergenic nonsense suppressor Mechanism of action of an intergenic nonsense suppressor mutation that results from mutation of a tRNA genemutation that results from mutation of a tRNA gene

Mutation rates and mutation frequencies are terms describing the quantitative measure of the occurrence of mutations

Mutation rate presents the probability of a particular kind of mutation as a function of time

i.e. number per nucleotide pair per generation or per gene per generation ex: spontaneous mutation rate in Eukaryote is 10-4 to 10-6 per gene/generation

Mutation frequency is the number of

occurrences of a particular kind of

mutation expressed as the proportion of

cells or individuals in a population i.e. the

number per 100,000 organisms or

number per 1 million gametes

Mutations arise in DNA spontaneously as a result of natural cellular processes

Looping-out during the DNA replication can lead to deletion of bases on the newly synthesized strand or on the template strand.

This will lead to shifted frame on the DNA.

Most spontanous errors are corrected by cellular repair systems

Spontaneous generation of addition and Spontaneous generation of addition and deletion mutants by DNA looping-out deletion mutants by DNA looping-out

errors during replicationerrors during replication

Mutagenic chemicals or radiation can

create induced mutations in DNA

Mutations in the genetic code Can be induced in somatic cells by:

› Chemicals: cancerogen› Radiation: X-ray, UV› Some viruses

Heredity - 5%