Lecture 13 - Genetic Code and Mutation

What are the 4 basic rules of translation

1. translation of mRNA has a precise starting point so the reading frame is translated correctly

2. Each codon consists of 3 nucleotides and the codons do NOT overlap

3. AUG = start codon. UAA, UAG, UGA = stop codon

4. More than one codon can specify an amino acid SO the genetic code is described as being degenerate

Wobble

When some anticodons can fit into more than one codon (there are less than 64 anticodons)

How do you identify the correct reading frame?

Identified through a lack of stop codons

Nonsense mutations

A sense codon has been altered to a STOP codon

Misense mutations

chagnes a triplet so that it specifies a different amino acid. Insertions or deletions can disrupt the reading frame and result in a nonfunctional protein

Point mutations

A nucleotide change from one base to another

• Usually happens with mistakes in DNA replication (NOT caught by 3’ → 5’ exonuclease proofreading activity of DNA polymerase")

What are positional effects of point mutations

Mutations that occur on specific codon positions

What are two silent mutations

1. Synonymous

2. Nonsynonymous

Synonymous substitutions

Nucleotide changes that are silent and usually harmless. Natural selection should NOT affect the presence of these mutations

Nonsynonymous substitutions

Nucleotide changes (random mutations) that result in amino acid change and are often harmful. Natural selection SHOULD eliminate these mutations

What are three expectations when comparing two genes that originate from a common ancestral gene have duplicated and diverged?

1. Many more differences (generated by mutations) at silent sites than at amino acid changing sites

2. more mutations among introns and other non coding regions than in exons

3. Amino acid changing mutations should appear to have occurred less frequently than silent ones

Describe mutatiosn in the germ line

Occurs in gametes or cells that produce gametes. PASSED on to the progeny

Describe mutations in the somatic line

Organism is a mosaic of BOTH normal and mutant cells. NOT passed on to the progeny. Some spontaneous mutations cause cancer