DNA replication and the cell cycle Flashcards
telomere
highly repetitive DNA that allows the end of the chromosome to be replicated- also protects it from degradation
centromeres
repetitive dna which forms the spindle attachment site in mitosis
origin of replication
special sequence where duplication of DNA begins; each chromosome will have many origins
features of the chromosome that reflects their need to replicate and partition
telomere
centromere
origin of replication
DNA pol
vs
RNA pol
- dna polymerase is far more accurate - due to it having a longer lifespan
- DNA polymerase has a proof reading mechanism- it can detect if it has incorporated the wrong base–> CAN GO BACK AND REMOVE FALSE NUCLEOTIDES AND THIS IS DONE BY EXONUCLEASE
does RNA polymerase have a proof reading mechanism
no
evidence of semi conservative dna: Meselson and Stahl
- they proved this by growing bacteria for many generation in DNA that contained a heavy isotope of nitrogen called N15- which became incorporated in the DNA- DNA became more dense
- they then grew it for a number of generations in normal nitrogen
- all dna became less dense
- proved half dna was conserved
what is needed for dna syntehsis
DNA polymerase and Mg2+
dNTPs (deoxynucleotides)
single stranded template of DNA
-primers 3” OH group- short single strand of DNA with a hydroxyl group
dna synthesis occurs
5’ to 3’ because DNA polymerase acts on the 3’-OH of the existing strand for adding free nucleotides
what provides energy for polymerisation
when nucleotide joins the growing DNA polynucleotide- 2 phosphates are lost–> leaving one to join
complementary base pairs are joined via
h bonds between single stranded dna
dna polymerase requires
a primer with a 3’OH residue to extend from
bacterial genomes are..
small, compact, circular
- no histones
- associated with Mg2+ and polyamides
eukaryotic genomes
large
arranged as a liner chromosomes
histones
what determines the speed of replication
sliding clamp- around 50bp per s in eukaryotes. In bacteria around 1000bps
what loads the sliding clamp on the DNA
clamp loader
-sliding clamp encircles the double stranded dna and ensures high productivity
enzymes at the replication fork
helicase ss binding protein primate DNA polymerase sliding clamp nucleases dna ligase
helicase
unwinds double stranded DNA
ss binding protein
stabilises ssDNA
primase
synthesises RNA primer
dna polymerase
synthesis of new dna strand
dna polymerase
proof read
siding clamp
keep dna polymerase on DNA
nucleases
trim the okazaki fragments
dna ligase
join okazaki fragments
okazaki fragments
Okazaki fragments are short, newly synthesized DNA fragments that are formed on the lagging template strand during DNA replication. They are complementary to the lagging template strand, together forming short double-stranded DNA sections
single stranded binding proteins
prevents base-pairing until DNA polymerase arrives- however does mot mask the nucleotide – otherwise single stranded dna may base pair with itself and become folded
which protein detects incorrect base pairing in newly synthesises DNA
mismatch repair protein MutS- detects incorrect base pairing in newly synthesised DNA
mismatch protein mechanisms
erro in newly made strand
- mistmatch proofreading proteins attach-nut s to strand and MutL to the Mut S.
- dna is scanned and detects nick in new dna
- this area is removed
- this area becomes single stranded and the correct nucleotides will join
- repaired dna
DNA synthesis is …
bidirectional from sites of origin- there may be a number of sites of origin–> forms replication bubbles
what forms replication bubbles
the bidirectional nature of dna synthesis
s phase
dna replication
m phase
mitosis
G1 and G2
where checking mechanism occur and events that occur between replication and division
where is the site of dna replication
replication fork
events at replication fork
- both strands copied at replication fork
- 5’ to 3’
- leading strand will have continuous synthesis (only needs to be initiated one)
- lagging strand will have discontinuous synthesis (multiple rounds of initiation and synthesis
leading strand
continous synthesis- only needs to be initiated once
lagging strand
discontinue synthesis- multiple rounds f imitation and synthesis
however bacterial dna has…
one origin of replication
what loads histones onto newly synthesised DNA
histone chaperons –> to condense the new dna into chromosomes
helcase unwinding of dna causes
supercoiling ahead of the replication fork which needs to be unwound by TOPOISOMERASE- by breaking and reforming phosphodiester bonds
which enzymes undoes supercoiling before the replication fork
topoisomerase- breaking and reforming phosphodiester bonds
why do leading and algid strand occur
due to the antiparallel nature of dna–> synthesis is occurring in the opposite direction on each stand. Lagging strand is more complicated because it is synthesises 3’ to 5’ meaning a diff type of dna polymerase is used
why is lagging strand more complicated
synthesised from 3’ to 5’- meaning a diff dna polymerase is used
what is the lagging strand also referred to
OKAZAKI FRAGMENTS
synthesis of the lagging strand
- primase synthesises short RNA oligonucleotides (primers) copied from dna
- dna polymerase elongates RNA primers with new dna
- nucleases remove RNA at 5’ end of the neighbouring fragment and dna polymerase fills in the gap
- dna ligase connects adjacent okazaki fragments
how many basepairs per second in bacteria
1000bps
how many BP per second in eukaryotes
50bps
