PSYC20008 Lecture Notes - Lecture 11: Dna Gyrase, Semiconservative Replication, Deoxyribonucleotide
14 Jun 2018
School
Department
Course
Professor
Lecture 2 - Wednesday 27 July 2016
BIOL10005 - GENETICS & THE EVOLUTION OF LIFE
LECTURE 2
STRUCTURE & REPLICATION OF DNA
CHAPTER 10 - GENES, CHROMOSOMES AND DNA
DNA SYNTHESIS: AN OVERVIEW
•DNA content in a cell is doubled by separating the two strands of the helix and sung each stand
as a template to synthesise a new complementary strand.
•This is called semiconservative replication, which generates two double helices for distribution to
the 2 gametes during mitosis.
•A linear single stranded DNA chain has a 5’ and a 3’ end, giving the polarity that is referred to as
5’ to 3’. Two single stranded DNA strands can come together and form a double stranded DNA
helix by complementary base pairing. The strands of a double stranded DNA molecule run
antiparallel to each other (one is 3’ to 5’ and the other is 5’ to 3’).
•DNA synthesis sis the sequential addition of nucleotides from deoxyribonucleoside triphosphates
(dNTPs), of which there are 4 types that correspond to the 4 nucleotides;
•dATP
•dCTP
•dGTP
•dTTP
•Synthesis of DNA is always in a 5’ to 3’ direction relative to the strand being synthesised.
•Deoxyribonucleotides are added to the 3’ end of a DNA strand by the formation of an ester
linkage between the 5’ phosphate of the incoming dNTP and the 3’ hydroxyl (OH) at the end of
the strand, forming a phosphodiester bond (hence the name).
•This extends the sugar phosphate backbone and leaves the 3’ OH group of the newly added
deoxyribonucleotide free for the addition of the next deoxyribonucleotide.
•Importantly, nucleotides are incorporated into the growing chain according to the rules of base
pairing, so that the base added to the new strand is complementary to the base of the template; A
opposite T and C opposite G.
DNA REPLICATION IN PROKARYOTES
•Circular chromosome of a prokaryote is compressed into the nucleoid using RNA, in direct
contact with the cytosol.
INITIATION OF REPLICATION
•Bacterial chromosomes have a single origin of replication, where unwinding takes place. It is
initiated by a cut or nick of a bond made by DNA gyrase between adjacent nucleotides in one
strand. As the DNA unwinds, a replication fork is generated at each side of the origin, and new
strand assembly takes place at these forks as the double helix continues to unwind.
•Each replication fork has a leading strand growing toward the fork and a lagging strand growing
away from the fork, which proceed in opposite directions and eventually meet at the terminus,
where synthesis stops, which has a specific sequence for the binding of terminator proteins.\
ENZYMES NEEDED FOR REPLICATION
•Separation of the duplex is done by ATP driven enzymes.
•DNA helicases peel apart the base paired strands by breaking H bonds.
•Single strand binding proteins keep the strands apart and stabilise them.
•DNA gyrase makes a cut in one strand of the DNA.
•DNA polymerases add dNTPs to add nucleotides to the 3’ OH group.
•DNA polymerase III edits any incorrect additions of base pairs.
