3-1 Flashcards
DNA replication needs to be
Accurate and fast
Complementary bases suggest that each strand of a double helix could serve as
A template
Three proposed models of DNA replication
Conservative replication
Dispersive replication
Semiconservative replication
Conservative replication
Two original strands stay together in one molecule (double helix)
Dispersive replication
Each strand gets broken down, copied, and then reassembled
Semi-conservative
Each strand serves as a template, is copied, and each strand stays intact but two strands do not come back together
All DNA takes places in a
Semiconservative manner
Meselson and Stahl’s experiment
Creates an intermediate band of DNA between N14 and N15 after one round of replication to rule out conservative replication
Creates a intermediate band and a light band after a second round of replication to rule out dispersive replication
Agrees with semi-conservative replication
Major steps in DNA replication
Initiation
Unwinding
Elongation
Termination
Initiation
Prokaryotes have circular DNA with an OriC
OriC is recognized by initiator proteins, slightly unwinding DNA at the Ori
Unwinding
Stretch of DNA unwinds
Helicase and other single stranded binding proteins attach to single stranded DNA
Unwinding
Dna replication requires
Single stranded DNA
Unwinding
DNA Helicase needs
Single stranded DNA to do its job
Unwinding
DNA helicase
Breaks hydrogen bonds
Unwinding
Single stranded binding proteins
Protect single stranded DNA
Prevents secondary DNA structures
Sequence independent
Form tetramers
Unwinding
DNA gyrase
Topoisomerase (type II)
Relieves torsion by cutting DNA (dsDNA break)
Removes a twist via ATP
Reseals broken DNA ends
Elongation
DNA polymerase
Synthesizes DNA strands
Elongation
DNA polymerase can only attach nucleotides to a pre-existing
3’ OH group
Elongation
Synthesized using
dNTP
Elongation
DNA and RNA synthesis occurs in the
5’–>3’ direction
Elongation
DNA polymerase 1,3 used in
Replication
Elongation
Primers synthesis
Primase (a RNA polymerase) makes RNA primers
Joins with helicase at replication fork
Elongation
DNA polymerase III
Synthesizes DNA from 5’ to3’ direction
Attaches new nucleotides to 3’ OH sugar phosphate backbone
Has 3’ to 5’ exonuclease activity (corrects errors)
Elongation
Leading strand
Being synthesized towards the fork
One continuous DNA strand
5’ to 3’
Elongation
Lagging strand
DNA synthesized away from fork
Discontinuous DNA strands called Okazaki fragment
5’ to 3’
Elongation
DNA polymerase 1
Synthesizes DNA strand in 5’ to 3’ direction
Also has 3’ to 5’ and 5’ to 3’ exonuclease activity
Removes RNA primers and replaces them with DNA
Only on lagging strand
Elongation
DNA ligase
After replacement by DNA poly 1, a gap still remains which is filled by DNA ligase
Joins Okazaki Fragments together
Termination
DNA replication ends when two replication forks meet
Additionally, some organisms contain specific termination sequence (Ter) that stops DNA replication
How is DNA replication accuracy achieved
Low error in base pair selection by DNA polymerase
Proofreading by DNA polymerase
Mismatch repair occurs soon after DNA replication
Mismatch repair system
Abnormalities in DNA secondary structures recognized by mismatch repair system
Gets rid of nucleotides on new DNA strand
