Cambridge Pre-U Biology - 1.3 DNA Replication

MAKING SENSE OF

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Copyright © 2016 Henry Exham

• 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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Copyright © 2016 Henry Exham3

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.


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

MAKING SENSE OFMAKING SENSE OF

DNA ReplicationMitosisMeiosis

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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!

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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


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.


• 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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• 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

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5’

5’3’

3’


• 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).

13"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_replication_split.svg

https://commons.wikimedia.org/wiki/File:DNA_replication_split.svg#/media/File:DNA_replication_split.svg


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

https://commons.wikimedia.org/w/index.php?curid=50542885


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