DNA Mutation & Repair

Types of Mutations

• Mutations can vary in cause and effect 

• Substitution: change in one or more bases  

• Deletion: removal of nucleotides 

• Insertion: addition of extra nucleotides 

• Most likely to have a neutral or a negative effect than a positive effect 

Common Causes of Mutation: Replication

• Can happen naturally during Replication when mismatch is not recognized 

• 1 in 100,000 bases incorrectly added; 2/3,  

• DNA Polymerase 3, 99% of the time recognizes a mistake and fixes it 

• Mismatch repair catches mistakes 99.9% in post replication repair 

Common Causes of Mutation: Mutagenic Agents

• Dose makes the poison

• 1/3 are caused by exposure to mutagenic agents  

• Coal burning plants (iodine radiation) 

• UV rays 

• Cigarettes  

Point Mutations & Protein 

• Point Mutations: chemical changes in just one pair of a gene 

• The change of a single nucleotide in a DNA template strand can lead to the production of an abnormal protein 

Mismatch Repair

Mismatch Repair runs along DNA strands and feels for oddities 

• Recognizes mistake and removes it 

• DNA Polymerase 1 fixes mistakes by filling in gaps 

• Ligase seals everything in 

• Repair attempts can sometimes introduce mutations or perpetuate them 

Silent mutations

have no effect on the amino acid produced by a codon because of redundancy in the genetic code

Missense mutations

still code for an amino acid, but not necessarily the right amino acid 

Nonsense mutations

change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein

Insertions & Deletions 

• Insertions and deletions are additions or losses of nucleotide pairs in a gene 

• These mutations have a disastrous effect on the resulting protein more often than substitutions do 

• Insertion or deletion of nucleotides may alter the reading frame, producing a frameshift mutation (change in the reading frame effects all downstream codons, confuses ribosomes) 

Frame Shift Mutation

Mutation caused by a number of nucleotides not divisible by 3 (insertion or deletion of 1 or 2 is worse than a high number that is evenly divisible by 3)