Replication of DNA and Chromosomes

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Slide 1

Nature of the Genetic Material

Property 1 - it must contain, in a stable form, information encoding the organism’s structure, function, development and reproduction

Property 2 - it must replicate accurately so progeny cells have the same genetic makeup

Property 3 - it must be capable of some variation (mutation) to permit evolution

Slide 2

Historical Perspective

1928 - Griffith discovered a “transforming principle” in heat killed bacteria (Property 1)

1944 - Avery demonstrated that the transforming principle is sensitive to DNase

1952 - Hershey & Chase used 32P and 35S labelling, of bacteriophage T2 DNA and protein respectively, to show only the DNA enters the host cell and can be passed to progeny phage (consistent with Property 2)

Slide 3

Replication of DNA and Chromosomes

Speed of DNA replication: 3,000 nucleotides/min in human 30,000 nucleotides/min in E.coli

Accuracy of DNA replication: Very precise (1 error/1,000,000,000 nt)

Slide 4

Meselson and Stahl (1958)

Slide 5

Slide 6

Taylor and co-workers (1957)

3H-labelled chromosomes

after one further replication

in unlabelled media

Slide 7

Bi-directional replication in E. coli

Slide 8

Only One Replication Origin in E. coli

Slide 9

Multiple Origins in Eukaryotes

Eukaryotes replicate their DNA only in S-phase

Eukaryotes have larger chromosomes

Replication speed 2,600 npm.

Largest Drosophila chromosome is 6.5 x 107 nucl., but it can replicate in 3-4 min. From a single origin, bidirectional replication would take 8.5 days. ==> The chromosome must have some 7,000 origins of replication.

Slide 10

A replicating Drosophila chromosome

Slide 11

Origins initiate replication at different times.

Slide 12

Two DNA polymerases are involved in eukaryotic replication

DNA polymerase d has no primase activity and is thought to be the polymerase that synthesizes the leading strand.