Chapter 4

Mutation and Genetic Variation

Evolution is impossible without mutation because mutation is the source of genetic variation.

I) DNA – information for life

A) Building an organism

1) Structural genes – genes for cell membrane proteins, cell structures, non-enzymatic polypeptides.

2) Enzymatic genes – produce polypeptides that modify other substances. e.g., Ldh

3) Regulation:

a) Spatially – cell-cell interactions. Information is needed in some tissues but not others. e.g., Adh

b) Temporally – some information needed all the time some only during specific stages. e.g., hemoglobin

B) Structure of DNA

1) 2 basic macromolecules

a) Pentose sugar

b) nitrogenous base

2) assembled into nucleosides

3) joined by high-energy phosphodiester bonds in

5’ => 3’ direction

4) double helix – two complementary, anti-parallel strands held together as a spiral helix by weak hydrogen bond. Naturally formed, lowest energy state.

C) Information content of DNA

1) Genes – complex basic unit of information

a) sequence in DNA (i.e., part of chromosome) that performs a specific function

b) linear sequence of nucleotides

c) Locus – physical location on chromosome where gene sequence can be found

d) Allele – alternate sequence of a gene. Still performs same basic function but MAY be modified in how (i.e., eye color)

2) Protein coding genes

a) exons, introns, and flanking regulatory regions

b) highly processed

c) triplet codon specifies amino acid

3) Non-protein coding genes

a) same as #2 above only no exons and introns

b) Regulatory

c) Ribosomal RNA genes

d) Transfer RNA genes

e) Junk DNA

D) Packaging of DNA

1) Chromosomes – 1 long, continuous double stranded molecular of DNA wrapped around histones (proteins) forming nucleosomes which are further wrapped into solenoids which are further wrapped into chromosomes

2) Homologues – most Eukaryotes have (at least) two copies of each chromosome

a) nearly identical DNA sequence

b) genes in same linear order most of the time

II) Mutations – any alteration to the sequence of DNA

A) Somatic versus gametic

1) Somatic mutation may or may not affect the fitness of an organism. Either way are not evolutionary significant because they are not passed onto the next generation

2) Gametic mutations may or may not effect the fitness of an organism, but are evolutionary significant.

B) Point mutations – changes to single nucleotides

1) Transitions – purine ó purine or

pyramidine ó pyramidine

2)

Transversions – purine ó pyramidine

3)
Synonymous –DO NOT change amino acid sequence (silent mutations).

4)

Non-synonymous –DO change amino acid sequence.

5)

Frame shift – insertions or deletions of one or a few nucleotides in a coding sequence. Usually very detrimental.

6)

Nonsense –change an amino acid codon one to a stop codon.

C) Chromosomal mutations – mutations that happen to large segments of the DNA

1)

Inversions – segments of chromosome are inverted in orientation

a) pericentric involved centromere

b) paracentric outside of centromere (as above). Very detrimental in heterozygotes. Reduced fertility.

c) both generally suppress recombination

2) Translocation – movement of a piece of DNA from one chromosome to another.

a) results in unbalanced gametes in heterozygotes. Reduced fertility

3) Fission or fusion – braking or joining of chromosomes or chromosomal segments

a) generally same effect as above.

4) Duplications and Deletions

a) transposable elements

b) polyploidy – allopolyploidy and autopolyploidy

c) unequal crossing over

i.

duplications usually detrimental in heterozygotes (downs syndrome) causing unbalanced genotypes. Deletions almost always detrimental.

III) Relationship between genotype and phenotype

A)

Dominance – phenotype of heterozygote is identical to one of the homozygotes

B) Additive – phenotype of heterozygote is intermediate between homozygotes

C)

Co-Dominant – special case where phenotype of heterozygote is the same as BOTH homozygotes.