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DNA Replication
Discovering the structure of DNA
• DNA = Deoxyribose nucleic acid
• Made out of sugars (deoxyribose), phosphates
and nitrogen bases
Discovering the structure of DNA
• Structure was discovered in 1953 by James
Watson and Francis Crick
Others contributed to the discovery of DNA
Rosalind Franklin: DNA image through X-ray technology
Erwin Chargaff found that number of bases were approximately equal to their bonding partner
A = T & C = G
Cell division – why DNA replicates!
• Cells divide
Growth, Repair, Replacement
• Before cells divide they have to double cell
structures, organelles and their genetic
information
DNA replication
DNA REPLICATION
the process by which DNA makes a copy of itself (cell division)
SEMI-CONSERVATIVE - half of the old strand is saved
DNA replication
Replication: 1st step
• Unwind DNA– helicase enzyme
• unwinds part of DNA helix• stabilized by single-stranded binding proteins
single-stranded binding proteins replication fork
helicase
DNA replication
DNA replication
DNAPolymerase III
Replication: 2nd step
But…We’re missing something!What?
Where’s theENERGYfor the bonding!
Build daughter DNA strand add new
complementary bases DNA polymerase III
energy
ATPGTPTTPCTP
Energy of ReplicationWhere does energy for bonding usually come from?
ADPAMPGMPTMPCMPmodified nucleotide
energy
We comewith our ownenergy!
And weleave behind anucleotide!
YourememberATP!Are there other waysto get energyout of it?
Are thereother energynucleotides?You bet!
DNA replication
DNA replication
Original DNA strands
DNA replication
Newly assembled DNA strands
DNA replication
Semi-conservative replication
Figure 11.8 Schematic representation of DNA Polymerase IIIFigure 11.8 Schematic representation of DNA Polymerase III
Structure resembles a human right hand
Template DNA thread through the palm;
Thumb and fingers wrapped around the DNA
DNA polymerases• DNA polymerase III
– 1000 bases/second!– main DNA builder
• DNA polymerase I– 20 bases/second– editing, repair & primer removal
DNA polymerase III enzyme
Arthur Kornberg1959
Roger Kornberg2006
Editing & proofreading DNA
• 1000 bases/second = lots of typos!
• DNA polymerase I – proofreads & corrects typos
– repairs mismatched bases
– removes abnormal bases• repairs damage
throughout life
– reduces error rate from 1 in 10,000 to 1 in 100 million bases
Replication fork
3’
5’
3’
5’
5’
3’
3’ 5’
helicase
direction of replication
SSB = single-stranded binding proteins
primase
DNA polymerase III
DNA polymerase III
DNA polymerase I
ligase
Okazaki fragments
leading strand
lagging strand
SSB
The next part of the process
• Use a strand of DNA to make RNA
• RNA is then used to make proteins– happens at ribosomes– proteins control everything about the
organism
DNA --> RNA --> Protein
Proteins are the building blocks of the organism (traits)
Transcription - process where RNA is made from DNA
Translation - process where proteins are made from RNA
