Molecular basis of inheritance part 2 Flashcards
What does helicase do
unwinds dna strands at the replication fork by breaking H-bonds
What does DNA polymerase do (4)
- Synthesizes new DNA strand by adding nucleotides to free 3’-OH grp of pre-existing DNA strand or RNA primer
- Uses parental strand as template
- Makes DNA in 5’ –> 3’ direction (links 5’ phosphate grp of new nucleotide to 3’-OH of growing strand)
- Reads template strand from 3’–> 5’
What does topoisomerase do
Relieves “overwinding” ahead of replication fork
what does primase do (2)
- RNA polymerase that synthesizes an RNA primer complementary to the DNA strand
- Provides a 3’ end for DNA polymerase
What does ligase do (2)
- glues together nicks in DNA sugar-phosphate backbone (forms phosphodiester bond)
- needed after RNA primers are replaced by DNA and for Okazi fragments
What do single-stranded binding proteins do
stabilizes single stranded DNA
Circular double stranded DNA (like in E. coli cell) has how many origins of replication and how many replication bubbles
- 1 origin of replication
- 1 replication bubble
Linear double stranded DNA (eukaryotic cell) has __ origins of replication and ____ replication bubbles
- Many origins of replication (100s-1000s)
- Many replication bubbles
What is the first step of DNA replication
initiation
where does initiation begin
origins of replication, which are short specific sequences of DNA
What 3 proteins bind to the origin of replication during initiation
- helicases (unzipping of DNA)
- single stranded binding proteins (stabilizes ss DNA)
- topoisomerase (relieves strain of unwinding)
in a replication bubble, how many replication forks
2 (Y-shaped region)
What enzyme does elongation involve
DNA polymerase III
What is elongation in DNA replication (4)
- synthesizes new strand of DNA using parental strand as template
- adds nucleotides according to base pairing rules
- links nucleotides in growing DNA strand
- requires free 3’ OH grp
What end does DNA polymerase add nucleotides to
3’end of existing nucleotide => primer needed for synthesis
in what direction does synthesis occur
5’ to 3’ direction
DNA polymerase reads template in what direction
3’ to 5’ direction
template dna containing a 3’ to 5’ strand and a 5’ to 3’ strand means replication is…
bidirectional
in dna leading strand:
DNA polymerase III synthesizes dna in what direction
same direction as dna unwinding= 5’ to 3’ toward replication fork
in what direction is template read in leading strand ?
3’ to 5’
how many rna primers needed in leading strand
1
what is a lagging strand
from discontinuous dna synthesis
in what direction does dna polymerase III synthesize DNA for lagging strands
opposite direction of DNA unwinding (5’ to 3’ away from replication fork)
in what direction is the template read in lagging strands
3’ to 5’
how many rna primers for lagging strands
many
what are the fragments linked by in lagging strands
DNA ligase
what is the problem on lagging strands
new DNA sequence is being built away from the fork (bc being built from 5’ to 3’)
what are RNA primers replaced with on Okazaki fragments
DNA pol I
what is the cause of the end replication problem
removal of rna primer from beginning of lagging strand leaves gap & fee 5’- PO4 grp
Why can’t the gap left by the removal of rna primer on lagging strand be filled by DNA pol
bc there is no free 3’-OH grp, only 5’
what is the consequence of the end-replication prob (gap from rna primer removal can’t be filled by DNA pol) (3)
- each replication results in shorter DNA
- can lead in possible deletion of genes
- factor that limits life span of cells
what comes to solve end-replication problem
telomeres
what are telomeres
repeats of noncoding nucleotide sequences
what is the sequence of nucleotides of telomeres
TTAGGG
AATCCC
what are telomeres added by and when
- added by telomerase enzyme
- added during early development
what happens to telomeres as cell ages
shortens
in what type of cell is telomerase active
germ cells (not somatic cells)
what happens if telomerase turned on in somatic cells
cancer
do telomeres prevent dna shortening
no
why do prokaryotes not have the dna shortening problem
they have circular chromosomes
DNA id a highly accurate process. How does DNA polymerase have a role in this
it proofreads each nucleotide added
when DNA pol is proofreading, what happens if incorrect pairing occurs (2)
- incorrect nucleotide will be removed
- synthesis resumes once error corrected
what happened to errors not caught by DNA pol DURING DNA replication
repaired by mismatch repair
what happened to errors/changes in existing DNA AFTER DNA replication
REPAIRED BY NUCLEOTIDE EXCISION REPAIR
how does nucleotide excision repair work
- a nuclease removes changed nucleotides
- DNA pol fills gap & DNA ligase glues together backbone
what can changes in DNA be caused by
- chemicals, radioactivity, ultraviolet
