Lecture 17 Flashcards
DNA rep
where?
general area, then structural name
begins at replication origin
opening at this origin cases 2 replication forks
dna rep initiation
what recognizes rep origin
origin recognition complex, ORC, recos the origin recognition complex.
ORC recruits DNA helicase = unwinds the DNA
S CDK activates dna helices and triggers initiation
DNA rep elongation
at each fork, 2 newly synth DNA strands are of opp polarities
DNA rep happens by complementary base pairings of nuceltodies (AT CG)
2 strands are anti para,
DNA nucleotides
used for dna synth
5C sugar, 1 or 3 phosphate groups added to sugar
diff bases added to C1 (guanine, adenine, urosil, cytosine, thymine
DNA polymerase takes nucleotides and makes phosphodiester bonds bn adj nucleotides in 5’ to 3’ direction, adds on the 3’end of a DNA strand.
DNA pool can’t initiate DNA synth from scratch,
needs RNA primers,
primers produced by RNA polymerase = primase
all old pieces of RNA are removed and replaced with the right DNA nuceltodies
differences in the strands
Once rep is intitiated, DNA rep should happens at both forks and bidrectinally
The strands are anti para and the poly can only synth 5 to 3’
That means one strand is synth continuously bc its direction of its synth mataches the direction of the replication fork. in which the poly can work. = leading strand = continous manner
Other strand is synth in lagged manner, has a problem bc direction of the fork doesn’t match direction of synth (5-3). Solution; we make small strands and stitch them together, small strands = okazaki fragments.
other proteins involved, after helicase….
keep from winding back up
The single stranded binding protiens
As soon as the 2 strands unwind, the ssbp keep strand seperated
- prevent unwinding, prevent reformation of H bonds, once seperated by helicase, (once ATP has been hydrolyzed and E has been invested, they stay seperated until they are replicated.
- since single stranded DNA are vunerable, protects them from digestion
other protein , during poly
a sliding DNA clamp inc efficiency of DNA polymerization
dna poly has low processivity, adds several nucleotides then detaches
keeps In place and allows it to synth long stretched of DNA = binds to clamp
clamp surr dna molecule and template, int w poly to make it stick to DNA molec, prevent from falling off.
other protein, last step, after poly
DNA ligase joins Okazaki fragments, makes phosphodiester bonds bn 2 adj nucleotides
first, RNA is removed, dna ply fills in gaps, then ligase binds.
coordination of dna synth on both strands
2 strands have to be synth together, coordinated
lagging strand makes a place so dna poly making Okazaki frag is in contact with poly of leading strand, now synth is synced.
types of dna poly
dna poly 3 = dna rep in gen
poly 1 = rep for filling gaps after removing ran in bn Okazaki fragments
the end replication problem of eukaryotic.
at end of rep,
continuous has no problem, keeps synth until no more template,
for lagging, there is a removed ran gap that has to be replaced.
here, there is no upstream 3’ OH to be used by DNA poly to replace the RNA
+
if DNA rep happens several times with this gap, and the cell divides without filling gap, the chromosomes continuously get shorter and shorter the more it is replicated
in somatic cell = we leave as it
in germ cell we can’t have this
soln = use an enzyme that extends the end of the chromosome
add a lot fo more chromosomes = room for DNA poly to make a new fragment - fill gap
telomere
protective cap at the end of the chromosme = repetitive DNA sequence,s
———helps prevent loss of imp genetic info when cells divide.
Telomerase = an enzyme that contains an RNA template within it = complementary to the telomere repeat sequence.
Telomerase binds to the template strand
+
extends the template adding more telomere repeats
now enough room for an Okazaki frag to be made on the lagging strands.
DNA poly can now fill in the gap.
telomerase detaches
