Four requirements for DNA to be genetic materialMust carry informationCracking the genetic codeMust replicateDNA replicationMust allow for information to changeMutationMust govern the expression of the phenotypeGene function

DNA ReplicationProcess of duplication of the entire genome prior to cell division

Biological significanceextreme accuracy of DNA replication is necessary in order to preserve the integrity of the genome in successive generations In eukaryotes , replication only occurs during the S phase of the cell cycle. Replication rate in eukaryotes is slower resulting in a higher fidelity/accuracy of replication in eukaryotes

The mechanism of DNA replicationArthur Kornberg et alInitiationProteins bind to DNA and open up double helixPrepare DNA for complementary base pairingElongationProteins connect the correct sequences of nucleotides into a continuous new strand of DNATerminationProteins release the replication complex

Basic rules of replicationSemi-conservativeStarts at the originCan be uni or bidirectionalSemi-discontinuous Synthesis always in the 5-3 direction RNA primers required

DNA replication Of the 3 possible models, replication isA) Semi-conservative Meselson-Stahl experiments

B) Starts at originInitiator proteins identify specific base sequences on DNA called sites of origin

Prokaryotes single origin site E.g E.coli - oriCEukaryotes multiple sites of origin (replicator)E.g. yeast - ARS (autonomously replicating sequences)

ProkaryotesEukaryotes

C) Uni or bidirectional

Replication forks move in one or opposite directions

D) Semi-discontinuous replicationAnti parallel strands replicated simultaneouslyLeading strand synthesis continuously in 5 3Lagging strand synthesis in fragments in 5-3

Semi-discontinuous replicationNew strand synthesis always in the 5-3 direction

Where does energy for addition of nucleotide come from?What happens if a base mismatch occurs?DNA polymerase has 3 5 exonuclease activity in order to correct errorsFrom cleavage of high energy phosphate of incoming triphosphate

Why does DNA replication only occur in the 5 to 3 direction?

Basic rules of replicationSemi-conservativeStarts at the originCan be uni or bidirectionalSemi-discontinuous Synthesis always in the 5-3 direction RNA primers required

TopoisomerasesHelicases PrimaseSingle strand binding proteinsDNA polymeraseTethering proteinDNA ligase

- Prevents torsion by DNA breaks- separates 2 strands - RNA primer synthesis - prevent reannealing of single strands- synthesis of new strand- stabilises polymerase - seals nick via phosphodiester linkage

Core proteins at the replication fork

Core proteins at the replication forkNature (2003) vol 421,pp431-435Figure in Big Alberts too

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