BIO282 Lecture Notes - Lecture 30: Uracil, Amine, Pyrimidine
Lecture 30 MUTATION
Learning objectives
By the end of the lecture you should:
1. Refresh yourself with mutation theory
2. Understand how mutations occur
3. Understand the difference between spontaneous and induced mutation
4. Understand the how spontaneous mutations are created
5. Understand the categories of mutagens and how they induce mutation
6. Understand the types of mutations created and their impact on the cell
What causes mutations?
There are two main causes:
1. Internal causes:
• Errors in replication of nucleic acid (eg polymerase error)
• Errors during cell division (eg nondisjunction, segregation)
• Errors in repair (eg improperly repaired DNA breaks, recombination)
• “Spontaneous” chemical changes
2. External causes:
• By mutagenic agents including
o physical agents
o chemical agents
o biological agents (next lecture)
• “Induced” chemical changes
Processivity - DNA polymerase III has a sliding clamp
• one mistake for every billion base pairs copied.
• Some polymerases has been engineered to increase fidelity (how accurate they are
at putting the right bases in)
• Processivity (how fast their turn over is, bases per hour)
• Final configuration of the protein is essential to it binding to the DNA in a very
specific shape
• Beta sliding clamp
o Has an interaction with the polymerase and DNA
o It holds onto the DNA and the enzyme and enables the enzyme to stay
attached tightly and have a very specific reaction with the DNA in this
interaction process
• Not all polymerases in all cells have this capability
o i.e. fidelity and processivity differ
o Cloning of a gene desires fidelity over processivity
o Analysis of a particular mutation desires processivity (speed) over fidelity
• Genetic engineering aspects look at enhancing both fidelity and processivity
Extension
• Thiamine is a triphosphate looking to be incorporated to the adenine in the
phosphate backbone
• As a process of incorporation of this base there is a release of 2 phosphates
(diphosphate release)
• Then the DNA polymerase can move onto the next base
Extension (Error)
• Cytosine in the back bone and an attempt of pairing it with Adenine (difficulty
pairing)
• There Is still a release of the diphosphate to drive the reaction
• A polymerase can come along and recognise that the wrong base is present, go
back and excise that base and incorporate into the growing strand the correct base
(proofreading)
