Reminder: where we started

CAMPBELL : exploring life, chapter 1

The cell cycle

Development of an organism

DNA replicationwhy we are talking about replication

• E. coli bacterium : single chromosome : 4.6 million nucleotide pair

• Human cell : 46 DNA molecules: 6 billion base pairs

Important questions for considering replication of double helix

• Two strands of DNA : high affinity >> created by various interactions present– Therefore………….– A mechanism is required to open up the strands

and get access to the bases

Unwind and over winding

Must be highly accurate

• Proof reading mechanism is required

Must be very rapid: Enzyme action

Components of replication

Replication at many sites: origin of replication at many sites in Eukaroytes

New strand of DNA elongates

Three replication bubbles by Transmission electron microscopy

Catalysis of elongation

• DNA polymerases• The rate of elongation is about 500

nucleotides per second in bacteria and 50 per second in human cells

Elongation of new DNA strand

Antiparallel elongation

• Double helix• Strands are antiparallel• 5’---3’, 3’---5’• This antiparallel nature also should be reproduced in the

newly formed double helix• This is achieved by adding nucleotides only to the 3’ free

end of a growing DNA strand, never to the 5’ end by DNA polymerase.

• Thus a new strand only elongate from 5’ to 3’direction.• LEADING strand…

Incorporation of a nucleotide into a DNA strand : elongation step

What happened to the other template strand?

• Formation of LAGGING strand?• 5’—3’ direction is mandatory for DNA POLIII to

work• DNA polIII works opposite to the replication fork• Small fragments of DNA are added, OKAZAKI

fragments (1000-2000 nucleotide long in E.coli), 100-200 in eukaryotes

• DNA ligase joins the sugar-phosphate bond of these fragments

DNA replication: leading and lagging strand

Leading strand (DNA pol III)

Lagging strands

Priming DNA synthesis: synthesis of lagging strand

Summary of DNA replication

Proofreading and repairing

Replicating the ends of DNA molecule