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
   the helix.
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

  3’ end
                      5’ end
    strands are antiparallel


                                                  F1             F2

 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


                                              DNA ligase
                     RNA primers

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;
cancer cells.

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