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Jeremiah 10:12
12 He hath made the earth by
his power, he hath
established the world by
his wisdom, and hath
stretched out the heavens
by his discretion.
©2001 Timothy G. Standish
Replicons
Timothy G. Standish, Ph. D.
©2001 Timothy G. Standish
The Information Catch-22
“With only poor copying fidelity, a primitive
system could carry little genetic information
without L [the mutation rate] becoming
unbearably large, and how a primitive system
could then improve its fidelity and also
evolve into a sexual system with crossover
beggars the imagination.”
Hoyle F. 1987. "Mathematics of Evolution",
Acorn Enterprises: Memphis TN, 1999, p 20
©2001 Timothy G. Standish
The Cell Lifecycle
The cell lifecycle is well defined and can be
divided into four stages:
– Gap 1 (G1) - The growth phase in which most cells
are found most of the time
– Synthesis (S) - During which new DNA is synthesized
– Gap 2 (G2) - No transcription or translation occurs
and final preparations for division are made
– Mitosis - Cell division
Once Synthesis is initiated, the cell is committed
to division
Thus initiation of DNA replication is a carefully
controlled process in cells
©2001 Timothy G. Standish
The Cell Lifecycle
Gap 1 - Doubling
of cell size.
Regular cellular
activities.
Transcription and
translation etc.
Synthesis of DNA Regular cell
activities cease and
a copy of all nuclear
DNA is made
S
G1
G2
M
Gap 2 - Final
preparation for
division
Mitosis - Cell
division
©2001 Timothy G. Standish
Stages of Replication
Replication can be divided into three stages:
1 Initiation - When DNA is initially split into two
strands and polymerization of new DNA is started
2 Elongation - When DNA is polymerized
3 Termination - When the new strands of DNA are
completed and some finishing touches may be put
on the DNA
Both elongation and termination may involve
proof reading of the DNA ensuring that mutations
are not incorporated into newly formed DNA
strands
©2001 Timothy G. Standish
Initiation
Initiation starts at specific DNA sequences
called origins (Ori C = origin in E. coli
chromosomes)
Long linear chromosomes have many origins
First the origin melts (splits into two single
strands of DNA)
Next primers are added
Finally DNA polymerase recognizes the
primers and starts to polymerize DNA 5’ to 3’
away from the primers
©2001 Timothy G. Standish
Initiation - Forming the
Replication Eye Or Bubble
Origin of Replication
5’
3’
3’
5’
Replication eye or
replication bubble
3’
5’
5’
3’
3’
5’
5’
5’
3’
3’
5’
3’
3’
5’
5’
3’
©2001 Timothy G. Standish
Large Linear Chromosomes Have
Many Origins Of Replication
Origins of Replication
5’
3’
3’
5’
5’
3’
3’
5’
5’
3’
3’
5’
5’
3’
3’
5’
5’
3’
3’
5’
©2001 Timothy G. Standish
Bacteria Have Circular
Chromosomes
Cell Wall
Cell Membrane
Chromosome
Origin of
Replication
Cytoplasm
Termination of
Replication
©2001 Timothy G. Standish
Replication
Replication goes
in both directions
Original
Chromosome
Start of replication
Theta (q) structure
Mid replication
End of replication
Two chromosomes
©2001 Timothy G. Standish
Binary Fission Revisited
©2001 Timothy G. Standish
Various Replication Strategies
D-loop
Rolling circle
©2001 Timothy G. Standish
F plasmid
2 Conjugation
F+ bacteria
Mating Bridge
F- bacteria
F
plasmid
©2001 Timothy G. Standish
F plasmid
2 Conjugation
F+ bacteria
Mating Bridge
F- bacteria
F
plasmid
©2001 Timothy G. Standish
F plasmid
2 Conjugation
F+ bacteria
Mating Bridge
F- bacteria
F
plasmid
©2001 Timothy G. Standish
F plasmid
2 Conjugation
F+ bacteria
Mating Bridge
F- bacteria
F
plasmid
©2001 Timothy G. Standish
F plasmid
2 Conjugation
F+ bacteria
Mating Bridge
F- bacteria
F
plasmid
F
plasmid
©2001 Timothy G. Standish
Hfr Recombination
F+ bacteria
F plasmid
Integration
Hfr cell
F- bacteria
Transfer of
genetic material
©2001 Timothy G. Standish
Hfr Recombination
F+ bacteria
F plasmid
Integration
Hfr cell
Interruption
of conjugation
F- bacteria
Crossing over
Transfer of
genetic material
©2001 Timothy G. Standish
Hfr Recombination
F+ bacteria
F plasmid
Integration
Hfr cell
F- bacteria
Recombinant
Bacteria
©2001 Timothy G. Standish
©2001 Timothy G. Standish