DNA and RNA Basics
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DNA is constructed of:
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Phosphate Group
Pentose monosaccharide (Ribose / Deoxyribose)
Nitrogen Containing Base
Bases are:
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Adenine, Guanine, Cytosine, Thymine/Uracil
C-G, A-T/U is the law of base pairings.
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DNA is a two-chain anti-parallel molecule arranged in a
double-helix.
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RNA is a single-chain molecule.
DNA Replication
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Aims:
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Must be able to outline the theories of DNA
replication.
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Should be able to describe the process of DNA
replication.
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Could be able to interpret the experimental data
on DNA replication.
DNA Replication - Theories
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The complete GENOME (Total quantity of DNA in a
diploid cell) must be replicated, without error, every time
a cell divides.
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IN THEORY there are TWO main ways of replicating
DNA:
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Conservative Method:
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Diagram:
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Semi-Conservative Method:
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Diagram:
DNA Replication - Experiment
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DNA replicates semi-conservatively.
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Evidence – Messelson and Stahl:
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Grew bacteria on medium containing nucleotides labelled with
heavy Nitrogen (15N).
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After many generations a sample of bacteria removed, DNA
extracted and identified by centrifusion.
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Remaining bacteria transferred to medium containing light Nitrogen
(14N).
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After one cell division DNA of bacteria was identified.
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Experiment repeated for two divisions:
DNA Replication - Results
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Diagrams:
DNA Replication - Mechanism
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DNA replication catalysed by enzyme DNA Polymerase.
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Enzyme reads nucleotide and makes ca complementary strand using
free nucleotide triphosphates (ATP, GTP, CTP, TTP).
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Nucleotide triphosphate provide building blocks for DNA synthesis and
energy for 3’-5’ sugar phosphate linkage.
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Diagram:
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DNA polymerase reads genetic code in 3’-5’ direction – Synthesises in
5’-3’ direction.
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There is only ONE type of DNA polymerase – 3’-5’ strand replicated
CONTINUOUSLY = Leading Strand.
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5’-3’ strand replicated DISCONTINUOUSLY = Lagging Strand.
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Lagging Strand replicated in short lengths – Joined together by enzyme
DNA Ligase.
DNA Replication - Details
1.
DNA unwound by DNA Helicase (Energy requiring step [ATP]).
2.
DNA Polymerase reads 3’-5’ strand continuously using Nucleotide
Triphosphates (provide building blocks and energy).
3.
5’-3’ strand is determined by the base sequence in the 3’-5’ strand
due to law of base pairing.
4.
RNA primer added to Lagging Strand. Acts as a binding site for DNA
Polymerase.
5.
Sequences of DNA joined together by DNA Ligase.
6.
DNA Helicase rewinds DNA into double helix.
7.
Point where DNA is unwound known as Replication Fork - Diagram:
8.
Replication Forks may occur in many places along the length of DNA
– Sites of DNA replication known as Replication Bubbles - Diagram:
Activity
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Answer the questions on the sheets
provided.
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Answer the questions from pages 203/4 in
the Biozone books.