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Some Notes for Exam Review

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of 15
DNA was proved to be genetic material with bacteria that had
smooth and rough colonies.
Smooth colonies-> lethal; rough->non lethal
DNA= nucleic acid= has nitrogenous base, joined to pentose sugar
ring, rings attached by phosphates
Purine=G&A--> double ring; pyrimidines C& T → Single ring;;;
purines must bind with pyrimidines
How did they find out DNA was double stranded?
Watson and Crick had 3 observations:
DNA is a polypeptide made of phosphate groups; sugar and bases
X-ray diffraction data of DNA crystals showed that DNA was long
and skinny; had two parts that were parallel
Because of double stranded DNA
Purine G&A; and Pyrimidines C& T
-A basepairs with T (2 H bonds) <--- therefore easier to
-C basepairs with G (3H bonds
-Bases are planar and perpendicular to the axis of the
helix (stacked)
There are 10 basepairs per DNA turn;
One strand of DNA dictates the sequence on the other strand
RIGHT HANDED double helix!!!; with polarity because its anti
G0/G1/M/S phase of cell
Base-Pairing Enables DNA Replication
DNA Synthesis Begins at Replication Origins
New DNA Synthesis Occurs at Replication Forks
The Replication Fork Is Asymmetrical
DNA Polymerase Is Self-correcting
Short Lengths of RNA Act as Primers for DNA Synthesis
Proteins at a Replication Fork Cooperate to Form a
Replication Machine
Telomerase Replicates the Ends of Eukaryotic Chromosomes
DNA synthesis is SEMICONSERVATIVE (meaning that half of
the new strand is actually old DNA)
Need to have replication of origin; identify it and then
separate the two DNA strands for replication to occur
-The origin of replication is bound by a complex of proteins
termed: the origin recognition complex which recruit additional
proteins to designate the start site of replication and allow
replication to occur
Bacteria have circular genomes-->Therefore only one origin of
Eukaryotes have many origins of replication because
linear chromosomes
time (because have to replicate the DNA in less than 8 hours
DNA is read: 3->5
DNA is synthesized 5->3 ALWAYS
Lagging strand is made discontinuously and produces Okazaki
fragments; leading strand is made continuously
Steps of DNA replication;
1. the DNA strands are denatured unwound at origin of replications
(generally A-T rich; by enzymes that unwind the DNA called
2.binding proteins keep the DNA in a single stranded form-- do not
cover bases so replication can occur
3.addition of an RNA primer by Primase(makes RNA primer) –
DNA polymerase can only add to a pre-existent strand, cannot start
4.Polymerization template read 3->5 and new strand is made >3;
Okazaki fragments produced
5.Joining of DNA by Ligase
DNA polymerase has proofreading ability; but polymerization and
proofreading occur at two sites; proofreading ensures correct
nucleotide is being added and if anything goes back and takes it
out before adding another
Repair mechanisms have evolved to ensure good replicated DNA--
DNA glycosylase recognizes unnatural bases and repairs them
Mismatch repair r removes replication errors and sues the old
strand as a template the repair the new one
-recognizes and excises the damaged strand to leave a gap;
2.polymerase fills in the gap;3. Phosphate backbone is joined by
ligase ---> picks the strand with no nicks!!! (the new strand!
As we get older, the more mutations we accumulate and
higher the chances there'll be a deleterious mutation e.g. cancer
DNA sustains insults daily! – DNA always undergoing damage
2 spontaneous damage are:
Depurination (loss of G or A base from DNA)
Damination (loss of NH3 converting C into U)
Or because of UV radiation can have A Thymine Dimer (covalent
bonding of 2 adjacent thymines) ---> may cause a block in
transcription replication