Chapter 14 Genetics

Point mutation / base substitution

A change of one base pair to another in a DNA molecule

Missense mutation

A change of one nucleotide of a triplet within a protein-coding portion of a gene may result in the creation of a new triplet that codes for a different amino acid in the protein product

Nonsense mutation

A second possible outcome is that the triplet will be changed into a stop codon, resulting in the termination of translation of the protein

Silent mutation

the point mutation alters a codon but does not result in a change in the amino acid at the position in the protein

Neutral mutations

do not affect gene products or gene expression. Most silent mutations, which do not change the amino acid sequence of the encoded protein, can also be considered neutral mutations.

Transition

If a pyrimidine replaces a pyrimidine or a purine replaces a purine

Transversion

A purine replaces a pyrimidine or vice versa

Frameshift mutation

Another type of change is the insertion or deletion of one or more nucleotides at any point within the gene. causes all of the subsequent three-letter codons to be changed.

A loss of function mutation

one that reduces or eliminates the function of the gene product

Null mutations

Mutations that result in complete loss of function

Recessive mutation

results in a wild-type phenotype when present in a diploid organism and the other allele is wild type

Dominant mutation

results in a mutant phenotype in a diploid organism, even when the wild-type allele is also present.

Dominant negative mutation

results in a mutant phenotype in a diploid organism, even when the wild-type allele is also present.

Haploinsufficiency

occurs when one allele is inactivated by mutation, leaving the individual with only one functional copy of a gene.

Gain of function mutation

or a gene product with enhanced, negative, or new functions.

Suppressor mutation

is a second mutation that either reverts or relieves the effects of a previous mutation. A suppressor mutation can occur within the same gene that suffered the first mutation (intragenic mutation) or elsewhere in the genome (intergenic mutation).

Somatic mutations

occurring in any cell in the body except germ cells, whereas germ-line mutations occur only in germ cells.

Autosomal mutations

mutations within genes located on the autosomes

X/Y linked mutations those within genes located on the X or Y chromosome, respectively.

Are mutations arising in somatic cells transmitted to future generations?

No.

Homogametic sex

The sex that produces gametes with identical sex-chromosome content; in mammals, the female is homogametic.

hemizygous

Having a gene present in a single dose in an otherwise diploid cell. Usually applied to genes on the X chromosome in heterogametic males.

Spontaneous mutations

changes in the nucleotide sequence of genes that appear to occur naturally

Induced mutations

Result from the influence of extraneous factors and can be a result of either natural or artificial agents

Mutation rate

defined as the likelihood that a gene will undergo a mutation in a single generation or in a forming a single gamete

Mutation hot spots

DNA sequences that appear to highly susceptible mutation

Single-nucleotide polymorphisms (SNPs)

A variation in one nucleotide pair in DNA, as detected during genomic analysis

Tautomers

Bases can take several forms

Replication slippage

One strand of the DNA template loops oit and becomes displaced during replication or when DNA polymerase slips or stutters during replication

Tautomeric shifts

change the covalent structure of the molecule allowing hydrogen bonding with non-complementary bases and hence may lead to permanent base-pair changes and mutations

Depurination

the loss of one of the nitrogenous bases in an intact double -helical DNA molecule

Deamination

an amino group in cytosine or adenine is converted to a keto group

Mutagens

Any agent that causes an increase in the rate of mutation.

base analogs

Purine or pyrimidine bases that differ structurally from one normally used in biological systems but whose chemical behavior is the same. For example, 5-bromouracil, which “looks like” thymidine, substitutes for it, and after incorporation into a DNA molecule, can lead to mutations.

Polygenic

caused by variations in several genes

Monogenic

caused by variations in several genes

DNA repair systems

essential to the maintenance of the genetic integrity of organisms and, as such, to the survival of organisms on Earth.

DNA polymerase III

The major DNA synthesizing enzyme in bacteria

Mismatch repair (MMR)

A form of excision repair of DNA in which the repair mechanism is able to distinguish between the strand with the error and the strand that is correct.

Endonuclease enzyme

creates a nick in the backbone of the newly synthesized DNA strand, either or to the mismatch.

Exonuclease

unwinds and degrades the nicked DNA strand, until the region of the mismatch is reached

Ames test

uses any of a dozen strains of Salmonella Typhimurium selected for their increased sensitivity to mutagens and their ability to reveal the presence of specific types of mutations 

Carcinogens

ancer-causing agents

Transposable elements (TEs)

known as “jumping genes,” are DNA sequences that can move or transpose within and between chromosomes, inserting themselves into various locations within the genome.