DNA Replication-Watson and Crick published a second paper which suggested a hypothesis for
the replication of DNA. That each
strand of the DNA molecule could
act as a template for the synthesis
of opposite strand of the DNA
molecule
The question arises-Is DNA
replication conservative, (The old
strand is kept in tact, giving rise to
two new strands), semiconservative (gives rise to 2
strands an old strand and a new
strand) or dispersive (new strands
having both new and old strands)?
Matt Meselson and Frank Stahl
(1958) demonstrated that
replication was semi-conservative
using radioactive nucleotides with
dividing bacteria.
The
bacteria
were given
radioactive
nucleotides
so that all
the DNA
was
radioactive.
Then once
it was all
radioactive,
the bacteria
was given
normal
radioactive
and then allowed to replicate once. The DNA was then run on an agarose gel to see what
banding pattern appeared. If replication was conservative, it is expected that two bands would
appear, one radioactive and one normal. Only one radioactive band appeared instead of a two
bands. This eliminated the conservative theory. The experiment was repeated but this time the
bacteria were able to replicate twice. If replication was dispersive, only one band would be
expected because all the DNA would be the same weight. What appeared though were two
bands. One was radioactive and the other
was not. This eliminated the dispersive
theory. DNA replication is semi-conservative
theory. Because DNA is such a long
molecule, replication must occur at the same
time in many places.There a number of
proteins involved in DNA replication.
In prokaryotic cells, the chromosome is
circular and not linear like eukaryotic cells.
There is also only one origin for replication which attached to the plasma membrane. Replication
of the chromosome occurs in both directions like eukaryotes. Prokaryotes have far fewer DNA
base pairs than eukaryotes. E. coli has about 4.6 million base pairs whereas a human eukaryotic
cell has 3 billion base pairs.
Notes about enzymes
-DNA polymerase can only add to the 3' end
of a nucleotide. This means that synthesis
can only occur from the 5'->3' direction.
-DNA polymerase must always have a
nucleotide in front of it to hang the DNA
nucleotide on.
-Therefore an RNA primer must be laid down
first and then replaced by DNA polymerase.
RNA primase does this.
-Helicases break the hydrogen bonds and
gyrases relieve the stress.
-Ligases suture DNA fragments together in the
replicated molecule.-Nucleotides are always
added on as triphosphates. When the nucleotides
are added then two phosphates are cleaved off
making a pyrophosphate.
1. When it is time for the DNA to replicate, proteins
attach to the places on the DNA molecule called
origins of replication. On a prokaryotic cell there is
one such site, but on a eukaryotic cell there are
hundreds to thousands.
2. Helicases unwind the DNA strand and gyrases
will nick one of the strands to relieve stress.
3. Single-stranded binding proteins
stabilize the unwound parental DNA
strand.4.
Primase will
lay down a
RNA primer
so that the
DNA
polymerase
can start
base pairing
because
DNA
polymerase
must have
replicated in
continuously.
something to bind to the molecule is
both directions. One side is laid down
The other side is laid down in
discontinuous fragments because and
can only
grow from the 5' to the 3' direction.
These
fragments are called Okazaki
fragments.5.
The Okazaki fragments are joined
together by
ligase.6. The RNA nucleotides are eventually replaced by DNA nucleotides when the DNA
polymerase runs into them.
Once the DNA has been replicated, there is one
problem. The usual replication machinery provides no
way to complete the 5’ ends, so repeated rounds of
replication produce shorter DNA molecules. The solution
to the problem is to put on the ends of the DNA,
repetitive sequences of DNA. These sequences are
noncoding sequences. These are called telomeres.
Telomeres only provide junk sequences so they prevent
the erosion of genes are cells replicate.
Note- in gametes, the shortening of telomeres would
cause serious problems in multicellular organisms. If
chromosomes of germ cells became shorter in every cell
cycle, essential genes would eventually be missing from
the gametes they produce.
An enzyme called telomerase catalyzes the lengthening
of telomeres in germ cells Proofreading and Repairing
DNA
•DNA polymerases proofread newly made DNA,
replacing any incorrect nucleotides
•In mismatch repair of DNA, repair enzymes correct errors
in base pairing
•DNA can be damaged by chemicals, radioactive
emissions, X-rays, UV light, and certain molecules (in
cigarette smoke for example)
•In nucleotide excision repair, a nuclease cuts out and
replaces damaged stretches of DNA