Lecture 21 - DNA Replication Flashcards
How can DNA replication begin ?
DNA is semi-conervative - in order for replication 2 strands must first be separated
Each strand used as a template to determine order of nucelotides that will be synthesised in complementary strands
Makes 2 double helix molecules, in each there is one old strnd and one new strand
What direction does DNA synthesis occur in?
DNA synthesis ALWAYa occurs in a 5’ -> 3’ direction by the formation of phosphodiester bonds.
Template strand and prime strnd are anti-parralel
How is a new strand of DNA produced during replication?
The new strand is produced by extension of the prime strnd. Done by polymerisation of new nucleotide ontoi 3’ hydroxyl end.]
Chemical rxn here is a nucleophilic attack on the phosphate of the new DNTP
This binds the nucleotide into place int primary strna and release pyrophaisphete
What is meant by a nucleophilic attack ?
The transfer of electrons to form a diester bond.
Why is the anti-parallel nature of DNA a problem for DNA replication?
DNA synthesis can only proceed in a 5 to 3 prime direction but on the other lagging strand, DNTP is being added to 5 prime. This is wrong.
How does DNA replication solve the problem of DNTP trying to attach to the 5’ end of a lagging DNA strand?
Okozaki fragments allow DNA polymerase to bind to the prime side on the lagging strand
What are E.coli and human replication forks?
Their key components are highly conserved
Whta is thw first step of DNA replication
Formation of the replcation fork where the paired DNA strands are separated by DNA helicase
What is the second step in DNA replication ?
Binding of primers to 3’ hydroxyl end.
]Acts as a template for DNA polymerase to bind to and DNA synthesis of new strand.
What does the antiparalle orientation of parental strnas mean ?
Means a unidirectional orientation of new DNA synthesis (5’ - 3’) means that both new strands cannot be synthesised continuously
What is the difference between the leading and lagging strand ?
Leading Strand - CONTINUOS 5 to 3 prime
Lagging strand - discontinuous also 5 to 3 prime direction
What are Okazaki fragments ?
RNA primers which binf to DNA by DNA primase and act as a template for DNA polymerase
They are short fragments of DNA
WHat happens in third step of DNA replication ?
Primers allow DNA polymerase to bind and start synthesis as a new strnad.
DNA primase makes these primers (RNA DNA hybrids complementary to template) in order to allow DNA polymerase to bind and extend by adding nucleotides onto the 3’ hydroxyl end. This happens on the leading strand, lagging strand is different.
How do the following synthesise a continuous DNA strand from lagging strand and Okazaki fragments ?
DNA polymerase - extends RNA primer - requires a Primer-Template junction
Ribonuclease H - removes RNA primer
DNA Polymerase - extends across gap
DNA ligase seals the nick
How does DNA Helicase use ATP ?
Uses ATP to separate parental DNA strands at the Replication Fork and move Replication Fork forward.
Breaks H bonds between strands
ATP -> ADP + Pi