This is part of the PowerPoint that covers section 1.2 in the Cambridge Pre-U Biology course. The full PowerPoint covers the following topics: Prokaryotic cell structure, asexual reproduction, introduction to antibiotics, bacterial pathogens. It can be purchased from here: https://sellfy.com/p/nktT/#

1. MAKING SENSE OF
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• For an organism to grow it will need to make more cells.
• For example you started out as a single cell called a
zygote. This cell divided over and over and over again to
make the trillions of cells that now make up you!
• However, its not all about growth because even when
you have stopped growing you need to make more cells
to replace the thousands that you lose every second of
your life!
• New cells can only come from pre-existing cells so it is a
crucial process to understand in biology.
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3. Copyright © 2016 Henry Exham
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2013, Eva Bianconi of the University of Bologna in
Italy estimated that there are 37.2 trillion cells in
an average human body!
That’s a lot of cell division considering you started
off life as one cell.

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You replace your entire stomach lining every 5 days.
About 2 million red blood cells are produced every
second in your body.
Men produce about 1500 sperm every second

5. MAKING SENSE OFMAKING SENSE OF
DNA Replication
Mitosis
Meiosis
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a) Outline the semi-conservative replication of DNA.
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• When cells divide, you don’t
want to put half the DNA into
one of the cells and the other
half into the other.
• If this was the case then the
DNA would get less and less
each time the cell divides.
• Therefore just before a cell
splits it copies all of it’s DNA
first so that it has two full
copies! 7

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• The DNA is normally a bit of a jumbled mess called
chromatin so before a cell divides this DNA becomes
much more organised and easier to move around
the cell for division.
• It is organised into structures called chromosomes.
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• To form a chromosome, the
DNA molecule winds around
proteins called histones to
form nucleosomes.
• They then supercoil to form
the dense chromosome
structure.
• This then replicates before
the cell divides to make a
doubled chromosome
consisting of two
chromatids.
Histones

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This is the human
chromosome number 6
Before the cell divides it will
need to duplicate to
become a double
chromosome
To do this all the DNA must
be copied exactly. This is
called DNA replication and
the process is explained
over the next few slides.

11. Copyright © 2016 Henry Exham
• By now you should know the basic
structure of DNA. But here are the key
points to remember:
– A polymer of nucleotides.
– Joined by phosphodiester bonds.
– Double stranded molecule.
– Strands run antiparallel to each other.
– The bases pair up by complimentary
base pairing (A-T, C-G).
– They are joined together by hydrogen
bonds.
– The strands run from a 3 prime to 5
prime direction.
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12. Copyright © 2016 Henry Exham
• The first stage of DNA
replication is to ‘unwind’ and
‘unzip’ the two strands of
DNA.
• This is done by an enzyme
called DNA helicase.
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A
C
T
T
G
A
A
C
T
T
G
A
5’
5’3’
3’

13. Copyright © 2016 Henry Exham
• These strands then form the
templates for the new strands.
• Free nucleotides come in and
match up by complimentary
base pairing.
• However, it works differently
on the two strands because
the new DNA can only be laid
down in a 5’ to 3’ direction
(referring to the new strand).
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"DNA replication split" by I, Madprime. Licensed under CC BY-SA 3.0 via Wikimedia Commons -
https://commons.wikimedia.org/wiki/File:DNA_replication_split.svg#/media/File:DNA_replicati
on_split.svg

14. Copyright © 2016 Henry Exham
This is carried out by
DNA polymerase III
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The strand that
starts 3’ is called the
leading strand.
This means that on
this strand the new
DNA can be formed
in the 5’ to 3’
direction that it
needs to
By Genomics Education Programme - DNA replication, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=50542885
This is called a
replication fork

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16. This PowerPoint is protected under copyright.
It is designed for educational use. Either personal study or to be
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