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[MBG 2040] - Final Exam Guide - Everything you need to know! (23 pages long)


Department
Molecular Biology and Genetics
Course Code
MBG 2040
Professor
Baozhong Meng
Study Guide
Final

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UofG
MBG 2040
FINAL EXAM
STUDY GUIDE

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Chapter 10- Replication of DNA and Chromosomes (220-249)
BASIC FEATURES OF DNA REPLICATION IN VIVO
DNA replication occurs semi conservatively, is initiated at unique origins and usually proceeds bi-directionally
from each origin of replication
In humans, DNA replication occurs at the rate of about 3000 nucleotides per minute
In bacteria, about 30 000 nucleotides are added to a DNA chain per minute
It is very rare that there will be a mistake in DNA replication- about one mistake per billion nucleotides
incorporated after synthesis and the correction of the mistakes in replication
o Majority of the genes of identical twins are identical, but some will have changed owing to replication
errors and other mutations
Synthesis of DNA involves three steps:
o Chain Initiation
o Chain Extension/ Elongation
o Chain Termination
Semi-Conservative Replication
Watson and Crick deduced the double-helix structure of DNA (complementary base pairing) and recognized that
the base pairing specificity provides basis for DNA replication
o Two complementary strands of the double helix unwind and separate and each strand guides the
synthesis of a new complementary strand
o Sequence of bases in parental strand is used as the template (template strand)
E.g. adenine in the parent strand will serve as template via hydrogen bonding potential for the
incorporation of thymine in the complementary strand
o This mechanisms of replication is called semi-conservative replicationbecause the parental molecule is
half conserved
In conservative replication the parent double helix would be conserved and a new progeny double helix would be
formed
In dispersive replication, segments of both strands of the parent DNA molecule would be conserved and used as
templates for the synthesis of complementary segments which would be joined producing progeny DNA strands
Cairns, Meselson and Stahl showed that DNA replicated semi-conservatively in E. coli cells
o E. coli cells were grown for many generations in a medium in which the heavy isotope 15N was substituted
for light isotope 14N
o Purine and Pyrimidine bases in DNA contain nitrogen, hence the DNA of cells grown with 15N would have a
greater density then the cells grown on medium containing 14N
o Molecules with different densities can be separated by equilibrium density-gradient centrifugation
o Density of most DNAs is approx. the same as density of concentrated solutions of heavy salt such as
cesium chloride (CsCl)
o When 6M of CsCl is centrifuged for long periods of time an equilibrium density gradient is formed
If DNA is present in such a gradient, it will move to a position where the density of the CsCl
solution is equal to its own density
Cells growing in 15N were washed and placed in medium containing 14N
Cells were allowed to grow and after varying amount of time DNA was extracted and analyzed in
CsCl equilibrium density gradients
Results were consistent with the semi-conservative replication
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2
All DNA from cells after one generation of growth in 14N had a density halfway between the
desities of hea ad light DNA—hybrid density
Hybrid density: produces two progeny double helices, both of which have 15N in one strand and
14N in the other strand
After two generations of growth in 14N, half of the DNA was hybrid density and the other half was
light
Conservative replication WOULD NOT produce any DNA molecules with hybrid density
o After one generation of conservative replication of heavy DNA in light medium, half of the DNA still would
be heavy and the other half would be light
Dispersive replication would have caused the DNA to shift from heavy towards light in each generation
o Half hea afte oe geeatio, uate hea afte to geeatios…
ire
Unique Origins of Replication
site of initiation of DNA replication is known as the origin of replication on the circular chromosome of E. coli
Figure 1: E. coli Chromosome Replicates Semi-Conservatively
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