Download Topic 7.1

Topic 7.1
7.1.U4 DNA replication is continuous on the leading strand and
discontinuous on the lagging strand
• Distinguish between the lead strand and the
lagging strand.
A: On the leading strand synthesis occurs
continuously while on the lagging strand it
occurs in fragments.
• What are Okazaki fragments and which strand
do they form on?
A: Fragments formed on the lagging
strand.
7.1.U5 DNA replication is carried out
by a complex system of enzymes.
• Summarise the roles of the enzymes of DNA
Replication:
a. DNA gyrase- releases the strain ahead of
DNA helicase.
b. DNA helicase- unwinds the DNA at the
replication fork.
c. DNA primase- creates a RNA primer on the
leading strand and many on the lagging.
1. RNA primer- initiates the activity of
DNA polymerase on the lagging strand.
7.1.U5 DNA replication is carried out
by a complex system of enzymes.
• Enzymes cont:
d. DNA polymerase- responsible for linking
the DNA nucleotides to the 3’ end of the strand.
1. Also can proofread and remove
RNA primers.
e. DNA ligase- connects the gaps between
fragments.
7.1.U5 DNA replication is carried out by a complex system of enzymes.
Summary of the enzymes involved in DNA Replication
DNA Gyrase (aka
topoisomerase) moves in
advance of helicase and
relieves strain and prevents
supercoiling on the
separated strands
DNA Helicase unwinds and separates
the double stranded DNA by breaking
the hydrogen bonds between base
pairs
http://commons.wikimedia.org/wiki/File:DNA_replication_en.svg
7.1.U5 DNA replication is carried out by a complex system of enzymes.
Summary of the enzymes involved in DNA Replication
DNA Ligase joins the Okazaki fragments
together to create a continuous strand
RNA Primase synthesises a short RNA primer
on each template strand to provide an
attachment and initiation point for DNA
polymerase III
DNA Polymerase III adds deoxynucleoside
triphosphates (dNTPs) to the 3' end of the
polynucleotide chain, synthesising in a 5' 3' direction
DNA Polymerase I removes the
RNA primers and replaces them
with DNA
http://www.ib.bioninja.com.au/_Media/dna_replication_med.jpeg
7.1.U3 DNA polymerases can only add nucleotides to the 3’ end
of a primer.
• In which direction does DNA polymerase move
along the template strand? What implication
does this have for the addition of bases on the
growing strand?
A: DNA polymerase always moves in 5’ to 3’
direction.
• Contrast replication in prokaryotes and
eukaryotes.
A: In prokaryotes there is one site of
replication. In eukaryotes there are many.
7.1.U6 Some regions of DNA do not code for proteins but have
other important functions.
• Distinguish between coding and non-coding
regions of DNA.
A: Coding genes code for the production of
polypeptides non coding do not. Some have
functions in regulating of gene expression.
7.1.U6 Some regions of DNA do not code for proteins but have
other important functions.
• Most of the eukaryotic genome is non-coding.
There are two types of repetitive sequences:
moderately repetitive sequences and highly
repetitive sequences otherwise known as satellite
DNA. Give an example of a region of DNA that
contains highly repetitive sequences and outline
the function of that region.
A: The ends of the chromosomes called
telomeres. They protect the ends of the
chromosomes.