AQA AS Biology Unit 2: DNA

Notes on DNA for AQA AS Biology Unit 2 

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  • Created by: LucySPG
  • Created on: 25-05-13 12:42
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Every living organism contains DNA.
DNA stands for deoxyribonucleic acid and is the chemical that determines inherited
characteristics and it contains vast amounts of information in the form of the genetic code.
Nucleotide Structure
DNA is made up of just three basic components that combine to form a nucleotide.
Individual nucleotides of DNA are made up of:
- A sugar called deoxyribose
- A phosphate group
- An organic base
Deoxy- Organic Base
The deoxyribose sugar, phosphate group and organic base are combined as a result of
condensation reactions, to give a single nucleotide.
Two mononucleotides may, in turn, be combined as a result of a condensation reaction
between the deoxyribose sugar of one of the nucleotides and the phosphate group of
The new structure is called a dinucleotide.
The continued linking of mononucleotides in this way forms a long chain known as a

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DNA Structure
DNA is made up of two strands of nucleotides.
Each of the two strands is extremely long, and they are joined together by hydrogen
bonds formed between certain bases.
DNA can be thought of as a ladder in which the phosphate and deoxyribose molecules
alternate for form the uprights and the organic bases pair together to form the rungs.…read more

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Pairing of Bases
The organic bases contain nitrogen and are of two types.
Single-ring bases: cytosine and thymine
Double-ring bases: adenine and guanine
Those with a double ring structure have longer molecules than those with a single ring
It follows that, if the rungs of the DNA ladder are to be the same length, the base pairs must
always be made up of one of each type.…read more

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The Double Helix
In order to appreciate the structure of DNA, you need to imagine the ladder-like
arrangement of the two polynucleotide chains being twisted.
In this way, the uprights of phosphate and deoxyribose wind around one
another to form a double helix.
They form the structural backbone of the DNA molecule.
For each complete turn of this helix, there are ten bases.…read more

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Function of DNA
DNA is the hereditary material responsible for passing genetic information from cell to
cell and generation to generation.
In total, there are around 3.2 billion base pairs of DNA of a typical mammalian cell.
This vast number means that there is almost an infinite variety that provides the immense
genetic diversity within living organisms.…read more

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Eukaryotic and Prokaryotic
DNA is stored differently in different organisms
Although the structure of DNA is the same in all organisms, eukaryotic and prokaryotic cells
store DNA in slightly different ways.
Eukaryotic DNA is linear and associated with proteins
Eukaryotic cells contain linear DNA molecules that exist as chromosomes.
The DNA molecule is really long so it has to be wound up so it can fit into the nucleus.
The DNA molecule is wound around proteins called histones.
Histone proteins also help support the DNA.…read more

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DNA versus Protein
As chromosomes only become visible during cell division , scientists focused their
attention on them as the sites of the hereditary material.
Chromosomes were shown to be made up of proteins and DNA.
To produce the extensive variety of cells and organisms that exists, the hereditary
material clearly needed to be diverse so that each organism could have its
own specific type.…read more

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Evidence that DNA is the Hereditary Material
Scientists work by using observations and current knowledge to form a hypothesis.
From this they make predictions about the outcome of a particular investigation.
By carrying out this investigation a number of times, they collect the experimental evidence
that allows them to accept or reject their hypothesis.
One investigation to test the hypothesis that DNA was the hereditary material involved
experiments using mice and a bacterium that can cause pneumonia.…read more

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There are three possible explanations for this:
1. Experimental error ­ the harmful forms in the mixture were not all killed
2. The living safe form had mutated into the harmful form
this is possible but extremely unlikely, especially given that the experiment was
repeated many times with the same result
3.…read more

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Other experiments provided further proof that DNA was the hereditary material and also
suggested a mechanism by which it could be transferred from one bacterial cell to another.…read more



This is amazing. Thank you XXX

arianator 4 life

WOW amazing resource :):):)



This set of notes is very well presented, with clear annotated diagrams in colour and covers the AQA AS specification unit 2 that covers DNA structure and function, genes, the triplet code, meiosis, mitosis and the cell cycle. These notes could be used by any biology student needing to study these topics at GCE level. 


It says 'the stages of meiosis' when it talks about prophase, metaphase, anaphase and telophase. I think that's suppose to be mitosis not meiosis. Other than that its definitely worth having a look through.

Sophie Atkinson

I used your resources (primarily the PDF documents) for my AS biology exam this summer and they really are so helpful, they condensed the information from the textbook in a way which I was unable to get my head around. I was hoping to use your A2 resources this year, however unfortunately you appear to have removed them from your account. Are they going back up or have you removed them permanently? :)


hi, according to a lot of people your notes are very helpful so I was wondering if your AQA AS Biology notes could go up?

Chen Boya

It is very useful


This resource is very helpful a lot of good information supported with images.

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