REPLICATION OF DNA
1. Origins of replication-specific nucleotide sequences along the
DNA molecules to which certain proteins (DNA B) can attach
and begin replication; hundreds per eukaryotic chromosome.
2. Replication forks- the Y-shaped places where DNA is being
unzipped by helicases; replication is bidirectional; two
replication forks form at each origin and move in both
3. Helicases-enzymes that break the H bonds linking the
complementary bases and unzip the two sides of
4. Topoisomerases (swivelases) breaks one side of the helix
ahead of where helicase is unzipping it and allows it to swivel
and untwist to relieve the strain.
5. Single strand binding proteins (SSB)-attach to the backside of
each of the unzipped strands and hold them
apart and keep them from kinking.
REPLICATION OF DNA
6. RNA primase-enzyme which lays down a short piece of RNA
primer to provide a 3’ end for DNA polymerase III to start from.
Neither of the DNA polymerases can start from “scratch” they
can only add nucleotides to an existing 3’ end.
7. DNA polymerase III-actually a complex of several enzymes;
it is fast but can only attach new nucleotides to the 3’ end of
an existing strand; also can not fill in the last 3-5 nucleotides
in a gap
8. DNA polymerase I-much slower removes the RNA primer
nucleotides and replaces them with DNA nucleotides
attaching them to the 3’ of the last Okazaki fragment, it is also
used in repair
9. Leading Strand-side of the new DNA which has a 3’ end to
which DNA polymerase III can attach and rapidly add new
nucleotides after only one short RNA primer is added
REPLICATION OF DNA
10. Lagging Strand-side with a 5’ end, thus new RNA primers
must be added every 100-200 nucleotides, so the
DNA pIII can attach DNA nucleotides to the 3’ end
working back toward the origin of replication
11. Okazaki fragments-sections of DNA 100-200 nucleotides
long which are formed on the lagging strand
between primers, after DNA pIII runs into the next
primer it pulls out and DNA pI comes in, removes the
RNA primer and replaces it with DNA but it cannot
make the last bond between the sugar and phosphate
12. DNA Ligase-enzyme that connects the new DNA
segment to the growing DNA strand, by joining the last
sugar and phosphate together
P.A. Levene-a prominent molecular biologist in the 40’s.
He determined the structure of a nucleotide but then
proposed that it was a tetranucleotide(wrong) which
each contained one of each nitrogen base. Many
biologists believed his theory.
Chargaff- % of adenine = % of thymine
% of guanine = % of cytosine
implied that A was always found with T and
C was found with G
Linus Pauling-determined the alpha helix of proteins but
was trying to make a triple helix which obviously did not
Rosalind Franklin was a
graduate student working
for Maurice Wilkins. She
did the best X-ray
diffraction studies but never
got the credit.
5’end 3’ end
strands are antiparallel
All 14N All 15N Mixture All DNA ½ DNA(½ 15N
DNA DNA 15N ½ 15N ½ 14N)
From bacteria From bacteria and ½ 14N ½ 14N DNA
grown on 14N grown on 15N 14N bacteria with bacteria with all
for generations for generations DNA all 15N DNA 15N DNA grown
grown on on 14N for 2
light for one generations
basic controls generation
origin of replication
origin of helicase
Telomerase is an enzyme that adds telomere repeat sequences to
the 3' end of DNA strands. By lengthening this strand DNA
polymerase is able to complete the synthesis of the "incomplete
ends" of the opposite strand.
Telomerase: is a ribonucleoprotein.
Its single snoRNA molecule — called TERC ("TElomere RNA
Component") — provides an AAUCCC (in mammals) template to
guide the insertion of TTAGGG.
Its protein component — called TERT ("TElomere Reverse
Transcriptase") — provides the catalytic action.
Thus telomerase is a reverse transcriptase; synthesizing DNA
from an RNA template.
Telomerase is generally found only in the cells of the germline,
including embryonic stem (ES) cells; unicellular eukaryotes like
Tetrahymena thermophila; some — perhaps all — "adult" stem
cells and "progenitor" cells enabling them to proliferate;