CSB474H1 Lecture Notes - Origin Recognition Complex, Okazaki Fragments, Dna Replication

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Published on 7 Oct 2011
School
UTSG
Department
Cell and Systems Biology
Course
CSB474H1
Professor
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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
break
-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
parallel
G0/G1/M/S phase of cell
DNA REPLICATION MAIN POINTS
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
replication;
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
helicases
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
synthesis
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
spontaneously
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

Document Summary

Dna was proved to be genetic material with bacteria that had smooth and rough colonies. 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. 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. A basepairs with t (2 h bonds) <--- therefore easier to. Bases are planar and perpendicular to the axis of the break helix (stacked) One strand of dna dictates the sequence on the other strand. Short lengths of rna act as primers for dna synthesis. Proteins at a replication fork cooperate to form a. Dna synthesis is semiconservative (meaning that half of the new strand is actually old dna)