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
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