DNA Replication
Self Replication
Every time a cell divides, DNA must be copied so each new daughter cell contains the
full set (diploid) of chromosomes
Each molecule of DNA replicates during interphase, before the cell divides
Results in each chromosome having an identical copy of itself
These are at first joined at the centromere as two sister chromatids
Semi-Conservative Replication
Double helix unwinds a bit at a time, catalysed by a gyrase enzyme
Unzipping – hydrogen bonds between bases break, catalysed by DNA helicase enzyme
Results in two separate strands with exposed bases
Free phosphorylated nucleotides in the nucleoplasm bond to the exposed bases
following base-pairing rules
DNA polymerase enzymes catalyse the addition of nucleotide bases to the single strands
of DNA, using each strand as a template
The leading strand is synthesised continuously while the lagging strand is in fragments
that are later joined, catalysed by a ligase enzyme
Produces two DNA molecules which are identical to each other and the parent molecule
Each molecule contains one old and one new strand, so is semi-conservative
Mutations
Errors can occur in DNA replication and the wring nucleotide can be inserted
This occurs inn I in 108 base pairs (or 1 in 100,000,000)
This can change the genetic code and is an example of a point mutation
During replication, enzymes proof-read and edit out such errors, reducing the likelihood
of mutation
Substitution – mutated new strand has the wrong nucleotide for a base
Insertion – mutated new strand has an extra nucleotide
Deletion – mutated new strand has a missing nucleotide
Self Replication
Every time a cell divides, DNA must be copied so each new daughter cell contains the
full set (diploid) of chromosomes
Each molecule of DNA replicates during interphase, before the cell divides
Results in each chromosome having an identical copy of itself
These are at first joined at the centromere as two sister chromatids
Semi-Conservative Replication
Double helix unwinds a bit at a time, catalysed by a gyrase enzyme
Unzipping – hydrogen bonds between bases break, catalysed by DNA helicase enzyme
Results in two separate strands with exposed bases
Free phosphorylated nucleotides in the nucleoplasm bond to the exposed bases
following base-pairing rules
DNA polymerase enzymes catalyse the addition of nucleotide bases to the single strands
of DNA, using each strand as a template
The leading strand is synthesised continuously while the lagging strand is in fragments
that are later joined, catalysed by a ligase enzyme
Produces two DNA molecules which are identical to each other and the parent molecule
Each molecule contains one old and one new strand, so is semi-conservative
Mutations
Errors can occur in DNA replication and the wring nucleotide can be inserted
This occurs inn I in 108 base pairs (or 1 in 100,000,000)
This can change the genetic code and is an example of a point mutation
During replication, enzymes proof-read and edit out such errors, reducing the likelihood
of mutation
Substitution – mutated new strand has the wrong nucleotide for a base
Insertion – mutated new strand has an extra nucleotide
Deletion – mutated new strand has a missing nucleotide
