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What you need to know:
How is DNA replication preserve the sequence of bases?
What is semi-conservative replication?
How is DNA different from RNA?
DNA Replication
Theories of DNA replication
Conservative Original DNA double helix acts as a template for producing a new templateThe original double helix
is conserved
Destructive
Nucleotides from the original strands end up in all daughter strands
The original double helix is destroyed
Semi Conservative
... the strands are separated and each daughter double helix inherits one original strand
The original DNA strands are conserved, but...
adenine guanine cytosine
tyrosine
How do we know which theory is correct?
Note: all bases contain nitrogen
The Meselson-Stahl Experiment
Bacteria grown on a source of heavy nitrogen (15N) produces heavy DNA
Bacteria grown on a source of light nitrogen (14N) produces light DNA
We can measure the weight of the DNA in a cell by extracting it and then centrifuging it in a caesium chloride gradient
DNA sample
CsCl gradient
Low CsCl conc.
High CsCl conc.
Centrifuge: spin very fast
When centrifuged...... molecules migrate until they reach a layer of the same density
Bacteria fed 14N Bacteria fed 15N Bacteria fed 15N then one generation on 14N
Intermediate band implies that DNA replication is
not conservative
Light band
Heavy band
Bacteria fed 15N then two generations on 14N Further generations on 14N
Light band gets bigger Intermediate band remains the same size Heavy band does not reappear
DNA replication is semi conservative
The two original strands act as templates for the synthesis of two new strands
AC TT AG
AC TT AG
AC TT AG
AC TT AG
CG
Free nucleotides pair with complementary bases
Template strandPairing of complementary bases preserves the sequence of bases when DNA replicates
AC T
T AG
AC T
T AG
AC T
T AG
AC T
T AG
C
G
The enzyme Helicase:
- unwinds the double helix
- breaks the hydrogen bonds between complementary bases
- separates the two strands
The Details:
AC T
T AG
AC T
T AG
AC
TA
CT
C
TA
G
TA
G
G
Free nucleotides pair up with complementary bases, and are held in place, by hydrogen bonding
Helicase
AC T
T AG
AC
TA
CT
AC
T
TA
G
TA
G
TA
G
DNA Polymerase creates two new strands of DNA by creating covalent bond between adjacent nucleotides
TG
TA
GT
A
G
AC
TC
AC T
T AG
AC
TA
CT
AC
T
TA
G
TA
G
TA
G
DNA Polymerase
TG
TA
GT
A
G
AC
TC
New DNA strand New covalent bonds
RNA – Ribose Nucleic Acid
Phosphate
Ribose (Pentose sugar)
Base
• cytosine (C)• guanine (G)• adenine (A)• uracil (U)
RNA is single stranded
Function of RNA – to form a copy of a gene which can be used by ribosomes as a template for synthesising a polypeptide
Each gene is a template for the production of one polypeptide
The copying of a gene from DNA to RNA is called transcription
The ‘reading’ of a RNA copy of a gene and synthesis of a polypeptide chain by the ribosome is called
translation
Homework:
Outline the process of DNA transcription
Explain what a codon is