DNA Synthesis•Eukaryotes

•DNA unwinds at a replication origin •Two replication forks form •Bidirectional synthesis of DNA •Much more DNA; complexed with proteins

•Eukaryotes have much more DNA

•Yeast 250-400 replicons •Mammalian cells up to 25,000

•Solution: Multiple replication origins

•Yeast 4X that of E. coli •Drosophila 100X that of E. coli

DNA Synthesis in Eukaryotes

Pre-initiation replication complex:ORCCdt1Cdc6MCM

Enzyme Function

Prokaryotes Eukaryotes

Helicase

Primase

Polymerase

Proofreading

Sliding clamp

Prevent ss closure

DnaB

DnaG

Pol III

Pol III

b sliding clamp

SSBPs

MCM

Primase s.u. of a-polymerase

Pol d and e

Pol d and e

PCNA (Proliferating Cell Nuclear Antigen)

RPA (Replication Protein A)

Comparison of some replication components

A Problem and a Solution

Telomere Formation

The Damage and Repair of DNA

The molecular basis of point mutations1. Base substitution (mismatches)

transitiontransversion

Two main types of DNA damage:

*point mutations*structural changes

2. Tautomeric shift

3. Base analogs and chemical mutagens

Gene MutationThe molecular basis of point mutations

DNA DamageThe molecular basis of structural damage1. Hydrolytic damage

N-glycosidic bond – very weak and easily broken

Deamination

Deamination

Depurination

DNA DamageThe molecular basis of structural damage

2. Alkylation

DNA DamageThe molecular basis of structural damage3. Oxidation and Radiation