Flashcards in chapter 5 (part2) Deck (61):
What starts DNA replication?
initiator proteins that pry open the bases at rhe replication origin
What is the replication origin?
where replication begins, they attract initiation proteins that initiate replication
How long is the replication origin in simple cells (bacteria and yeast)?
several hundred nucleotides long
Where is the replication origin?
an area rich in A-T, because they have only two hydrogen bonds
How large is E. coli DNA?
4.6*10^6 base pairs long
How fast is replication in E. Coli?
500-1000 nucleotides per second
beginning at a single replication origin and is bidirectional
where is the only point at which E. Coli can control replication?
after that replication begins in a bidirectional manner until finished
How does replication begin?
initiator proteins wrap around DNA forming a DNA-protein complex. This attracts DNA helicase to the adjacent strand
What is the job of the helicase loader?
it loads DNA onto the replication fork and keeps DNA helicase in an inactive form until replication begins
What part of replication is regulated in E.Coli?
its controlled through the initiator proteins.
replication only begins when there are enough nutrients to complete an entire round of replication
What is the refractory period?
delay in methylation on newly synthesized andenine. further replication is blocked until these andenine are methylated
How many replication forks are in E.Coli?
two with one replication origin
How fast is replication in eukaryotic chromosomes?
50 nucleotides per second . 1/10th the rate of bacteria. Because of complexity of genome
How many nucleotide pairs are in Eukaryotes?
150 million base pairs
how many replication origins does eukaryotes have?
multiple replication origins. where replication starts for faster replication
What are replication units?
replication origins that are activated all at one time when replication begins
how many replication units are activated at once?
new replication origins are replicated during replication
how far are replication origins within replication units?
30,000-25,000 base pairs apart
How many replication fork are there at an replication origin?
2 replication begins in both directions until they collide with collide with another replication fork or end of chromosome
When does bacteria replicate their DNA?
Continually, they start a new round before the old is done.
when does eukaryotic replication occur?
only during DNA synthesis phase (S phase)
How long is S phase in mammals?
How long is S phase in simpler eukaryotes?
how is S phase coordinated in mammalians?
replication clusters are coordinated in a sequence at different times during S Phase
What determines the when Chromatin is replicated in S phase?
highly condensed chromatin (heterochromatin) is replicated late in S phase
uncondensed chromatin is (euchromatin) is replicated early in S phase
Why doesnt plasmid DNA replicate?
because they do not contain replication origins
What are autonomously replicating origin?
yeast replication origins that have been introduces into bacterial plasmids so they can replicate
What happens if you remove to many replication origins?
the chromosome might be deleted because the replication is to slow during S phase
Why do eukaryotes carry extra replication origins?
if some fail then the chromosome will be replicated in a timely manner
What is found at each replication origin?
binding site for the origin replication complex (ORC)
stretch of DNA that is rich A-T, so can easily unwind
binding site for proteins that help the ORC
How does the cell ensure that the chromosome is copied once and only once?
Activation or deactivation of the ORC once bound to the origin
When does the ORC dissociates from the origin?
only briefly after replication
and helicase and helicase loading proteins (Cdc6, Cdt1)
What is the prereplicative complex?
helicase loading proteins (Cdc6, Cdt1)
when is the prereplicative complex assembled?
at the beginning of the G1 phase
What triggers G1 to S phase?
activation of the protein kinases (Cdks)
leads to dissociaton of helicase losding proteins, activation of helicase, loading and unwinding of remaining replication proteins
What does the protein kinases trigger (Cdks)?
simultaneously prevents all assembl of prereplicative complexes until M phase resets the entire cycle
What is replication more dependent on in humans?
The condensed or uncondensed structure of chromatin not replication origins. thats why they are not well defined
What does eukaryotic cells posses multiple copies of?
genes that encode for histones, because they are assembled behind the replication fork
How many gene sets are there for histones?
20 repeated gene sets
When are histones assembled?
mainly through S phase
Why is there a feedback loop between the synthesis of histones and DNA synthesis?
because the amount of histones made must match exactly the amount of DNA made.
where are the histones assembled?
Directly behind the replication fork
How ar the nucleosomes broken up as the replication fork passes through the nucleosomes?
1 H3-H4 tetramers- remains associated with new or old DNA
2 H2A-H2B dimers-released from DNA
How are histones reassembled onto newly synthesized DNA?
Newly synthesized H3-H4 is synthesized to fill in the gaps
H2A-H2B dimers half or old and half are new are added at random to new and old DNA
What are histone chaperones?
they bind to histones and release them at the right time
What are histone chaperones also called?
Chromatin assembly factors
What is PCNA?
A eukaryotic sliding clamp that is left behind the replication fork and direct histone chaperones to do their task.
How long is PCNA left on DNA?
after the histone chaperones finish their task
What is inherited from parent to the daughter cells?
the histone modifications of the H3-H4 tetramers
What problem is encountered at the end of a linear chromosome?
it has no place for DNA primase to lay down a RNA primer.
What is the sequence that is embedded at the end of each chromosome in humans?
Telomere DNA sequence -GGGTTA
What is telomerase?
The enzyme that replicates the end of telomeres
What does telomerase use to synthesize the of a chromosome?
What does the enzymatic portion of telomerase use?
it resembles reverse transcriptase
What happens after telomerase extends telomeres?
DNA polymerase comes in and synthesize the complementary strand from the sequence that telomerase laid down
What is the process of making telomeres aided by?
nucleases that chews back on the 5` end.
it makes sure that the 3` end is longer than the 5` end
What is a T-loop?
the 3` end folds over
this ensures that he chromosome is not mistaken for degradative enzymes mistaking them for broken DNA in need of repair
What happens to telomere sequences in yeast?
because yeast cells divide indefinitely, they have a mechanism that keeps their telomere sequence within a certain range unlike human
What happens in telomere sequence in human cells?
replicative cell senescence - telomerase activity is turned down so 100-200 telomere sequences are lost every time the cell divides. daughter cells will inherit defective telomeres and cease dividing
What cells do not undergo replicative cell senescence?
cells that replenish through out life