Molecular Biology-DNA Replication Flashcards
replication
cell division involved in duplication of DNA
when does replication occur?
during S phase (synthesis) in the interphase of the cell
parental DNA
the old DNA
daughter DNA
new DNA
conservative replication
the parental ds-DNA would remain as it is while an entirely new ds-genome was created
dispersive theory
both copies of the genomes were composed of scattered pieces of new and old DNA
replication is semiconservative:
after replication, one strand of the new double helix is parental (old) and one strand is the newly synthesized daughter strand
helicase
the enzyme that unwinds the double helix and separates the strands
origin of replication (ORI)
the sequence where helicase begins to unwind, recognized by specific proteins
which protein recognizes ORI in prokaryotes?
DnaA
which protein recognizes ORI in eukaryotes?
there are three proteins: two of which are synthesized in M and G1 phase but rapidly destroyed once the S phase begins; this links DNA replication to the cell cycle, ensuring DNA replication doesn’t initiate during other phases of the cycle
topoisomerases
cut one or both of the ds DNA strands and unwrap the helix to release excess tension created by helicase
single-strand binding proteins (SSBPs)
protect DNA that has been unpackaged in preparation for replication and help keep the strands separated
open complex
separated strands of DNA complex
primase
an RNA polymerase that synthesizes RNA primer
DNA extension requires:
an RNA primer; DNA polymerase can only add onto an existing chain
DNA polymerase (DNA pol)
catalyzes the elongation of the daughter strand using the parental template and elongates the primer by adding dNTP’s to its 3’ end
in the polymerization reaction, what acts as the nucleophile?
3’ hydroxyl group to displace 5’ pyrophosphate from the dNTP to be added
DNA pol is part of a large complex of proteins called:
replisome-components/accessory proteins help DNA polymerase and allow it to polymerize DNA quickly
why do eukaryotes have a larger replisome containing more proteins?
because of complexity of eukaryotic system; requires unwinding of DNA from histone proteins
what is the thermodynamic driving force of the polymerization reaction?
the removal and hydrolysis of pyrophosphate (P2O7) from each dNTP added to the chain
which direction does polymerization occur?
5’ to 3’ (WITHOUT EXCEPTION)
what does DNA pol require?
a template and a primer
in which direction is the template strand read?
3’ to 5’
replication fork
the area where the parental double helix unwinds
leading strands
able to elongate continuously into the widening replication fork
lagging strand
must wait until the replication fork widens before beginning to polymerize, forms fragments
Okazaki fragments
the chunks of DNA that comprise the lagging strand
in which direction does the replication fork grow?
in both directions, away from the origin. Each replication fork contains a leading strand a lagging strand
replication of the leading strand is:
continuous
replication of the lagging strand is:
discontinuous, results in Okazaki fragments
eventually, what happens to RNA primers?
they are replaced by DNA and the fragments are joined by DNA ligase
DNA pol III
super-fast & super-accurate elongation of the leading strand (high processivity), exonuclease activity, no known function in repair
proofreading function
the ability to correct mistakes by 3’ to 5’ exonuclease activity
DNA pol I
starts adding nucleotides at the RNa primer, poor processivity (slow), exonuclease/proofreading, removes RNA primer via exonuclease activity while leaving behind new DNA, important for excision repair
DNA pol II
5’ to 3’ polymerase activity, exonuclease/proofreading, participates in DNA repair pathways, backup for pol III
DNA pol IV and DNA pol V
error prone in polymerase activity, function to stall other polymerase enzymes at replication forks when DNA repair pathways have been activated (important part of prokaryotic checkpoint activity)
how many chromosomes and origins do prokaryote chromosomes have?
only one chromosome with only one origin
theta mechanism/theta replication
describes the replication process of prokaryotes
how many origins do eukaryotic chromosomes have?
several; eventually the replication forks will meet and the daughter strands will be ligated together
what happens if telomeres become too short?
they reach a critical length when the chromosome can no longer replicate. cell can either activate a senescent state or apoptosis
Hayflick limit
the number of times a normal human cell type can divide until telomere length stops cell division
telomerase
enzyme that adds repetitive nucleotide sequences to the ends of chromosomes and lengthens telomeres
telomerase is a ribonucleoprotein complex that contains:
an RNA primer and a reverse transcriptase enzyme; polymerases and then translocates to allow for extension of telomere repeats
reverse transcriptase
read RNA and generate DNA, allows for chromosome exntesion
