Physical Structure and
Replication of DNA
Nature of the Genetic Material
Property 1 - it must contain, in a stable
form, information encoding the organism’s
structure, function, development and
reproduction
 Property 2 - it must replicate accurately so
progeny cells have the same genetic makeup
 Property 3 - it must be capable of some
variation (mutation) to permit evolution

DNA double helix
1. Sugar-phosphate backbone
2. Base-pair "rungs" of ladder
3. Nucleotides attached to S-P molecules
4. Strands antiparallel (run in opposite directions, 5'-->3')
5. Each base-pair "rung" has a purine (A or G) and pyrimidine
(C or T)
6. Strands held together by hydrogen bonds between
nucleotides
7. Chemical structures of nucleotides discourage "incorrect"
pairing
8. G-C pair has 3 hydrogen bonds, A-T only 2-->former is
stronger

How DNA Replicates
DNA Replication
1. Semiconservative = replication results in two DNA
molecules each with two strands, one original and one
new.
2. Sequence of events
a) Helix unwinds
b) Both strands replicate simultaneously, during
unwinding process
c) "Leading" strand replicates continuously from 3' end
of existing strand, with newest end of forming strand
facing into replication fork
d) "Lagging" strand replicates by a series of fragments
placed end-to-end, with newest ends of fragments facing
away from fork; fragments later "ligated"
e) During replication, 2 polymerases "proofread" for
mismatched bases
Replication of DNA and
Chromosomes

Speed of DNA replication:
3,000 nucleotides/min in human
30,000 nucleotides/min in E.coli

Accuracy of DNA replication:
Very precise (1 error/1,000,000,000 nt)
Process of DNA Replication

Hydrogen bonds between base pairs are broken
and the two sides of the ladder unzip
Semi-conservative replication: the
original molecule is no longer present
but each new molecule will have
one original strand

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Polymerases (enzymes) link free-floating
nucleotides to their matching base on the
parent strand

There are estimated to be 3 billion bases in
a human DNA strand

Mistakes are rare but do happen

Proofreader enzymes fix most mistakes
DNA has a direction.
It consists of two antiparallel strands with
distinguishable 5′ and 3′ ends
(the numbers refer to the number of the
carbon in the ribose sugar).
→ base
 Carbon-3 → downstream PO4
 Carbon-5 → upstream PO4
 Carbon-1
The original strand of DNA is read in the 3’ to 5’
direction
The new strand is
assembled in the 5’ to 3’ direction
If both sides were synthesized at once, you'd need
two different DNA polymerases:
one for 5′ → 3′;
and one for 3′ → 5′.
However, experimentally, we find that all DNA
polymerases synthesize the new strand 5′ → 3′
The original strand of DNA is read in the 3’ to 5’ direction,
The new strand is assembled in the 5’ to 3’ direction
Since DNA synthesis only occurs in the 5′ and 3′,
so DNA polymerases must move in antiparallel
directions to synthesise the two daughter helices.
Because the synthesis of DNA only occurs in one
direction, different processes must occur on the
two strands.
These two strands are termed the leading and lagging
strands.
The leading strand is synthesised continuously 5′→3′.
However, the other, 'lagging' strand is still
synthesised 5′→3′ but in discrete chunks called
Okazaki fragments, from the replication fork back
towards the origin.
Gene Recombinations

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The process of cutting out damaged or foreign
segments of DNA and repairing them
Restriction enzymes: The scissors that cut
certain segments out of the DNA strand
DNA Ligase: The glue that restores the
damaged segment
Resources
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From DNA to Protein
DNA and Protein Synthesis in the Cell
Genetics Handbook
Animations at Virtual Cell Biology Classroom
Say it with DNA Protein synthesis tutorial