Biology Unit 2 Notes From AQA TextBook AS Level. Chapter 7 and 8

Biology Unit 2 Notes From AQA TextBook AS Level.

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  • Created by: Jenessa
  • Created on: 27-05-10 10:27

7.1, Investigating Variation

Interspecific variation: If one species differs from another.

Intraspecific variation: If the members of the same species also differ from each other.


Sampling: involves taking measurements of individuals, selected from the population of organisms which is being investigated.

There could be sampling bias. Or it could be by chance.

To prevent sampling bias, you need to eliminate any human involvement in choosing samples. This can be achieved by random sampling.

1) Divide the study area into a grid of numbered lines.

2) Using random numbers, obtain a series of coordinates.

3) Take samples at the intersection of each pair of coordinates.

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7.1, Investigating Variation

We cannot completely remove chance from the sampling process, but we can minimise it by:

using a large sample size: the more individuals the smaller the probability that chance will influence the result. The greater the sample size, the more reliable the data will be.

analysis of the data collected: use statistical tests to determine the extent to which chance may have influenced the data.

Causes of variation:

Genetic differences and Environmental influences.

Genetic differences are due to the different genes that each individual organism possesses.

Genetic variation arises as a result of:

Mutations: sudden changes to genes and chromosomes that are passed on to the next generation.

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7.1, Investigating Variation

Meiosis: nuclear division forms the gametes. This mixes up the genetic material before it is passed into the gametes.

Fusion of gametes: offspring inherit some characteristics of each parent.

Variety in asexually reproducing organisms can only be increased by mutation.

Sexually reproducing organisms increase variation by all three methods.

Environmental influences:

Environmental influences include; climate conditions, soil conditions, pH and food availability.

Variation is due to the combined effects of genetic differences and environmental influences.

It is difficult to draw conclusions about the causes of variation, and any conclusions are treated with caution.

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7.2, Types of Variation

Variation is the result of either genetic differences or the influence of the environment.

Variation due to genetic factors:

- are usually controlled by a single gene.

- environmental factors have little influence on this type of variation.

Variation due to environmental influences:

- form a continuum.

- examples are height and mass.

- controlled by many genes (polygenes).

- shown on a bell-shaped curve (normal distribution curve).

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7.2, Types of Variation

Mean and Standard variation:

A normal distribution curve always has the same basic shape.

- two measurements; maximum height and width.

Mean: is the measurement at the maximum height of the curve, and provides and average value.

Standard Deviation: (s) is the measurement of the width of the curve. An indication of the range of values either side of the mean. A standard deviation is the distance from the mean to the point where the curve changes from being convex to concave.

Calculating standard variation:

(look in AQA AS biology textbook for formula) p127.

Calculate the mean value, Subtract the mean value, Square all the numbers, Ass the squared numbers, Divide this number by the original number of the measurements, and square root.

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8.1, Structure of DNA

The double helix structure of deoxyribonucleic acid (DNA) makes it immediately recogniseable.

DNA: is the chemical that determines inherited characteristics and it contains vast amounts of information in the form of the genetic code.

DNA is made up of just three basic components that combine to form a nucleotide.

Nucleotide Structure:

- made up of 3 components: deoxyribose sugar, phosphate group and organic base.

Organic bases: -single-base rings - cytosine (C) and thymine (T).

- double base rings - adenine (A) and guanine (G).

The organic bases are combined as a result of condensation reactions, to give a single nucleotide (mono-nucleotide). Two mono-nucleotides combine as a result of a condensation reaction between the deoxyribose sugar of one mono-nucleotide and the phosphate group of another to form a new structure a di-nucleotide. The linking of mono-nucleotides forms a long chain, known as a polynucleotide.

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8.1, Structure of DNA

DNA structure:

James Watson, Francis Crick and Rosalind Franklin.

DNA is made up of two strands of nucleotides (polynucleotides), joined together by hydrogen bonds formed between certain bases.

Pairing of bases:

Organic bases contain; nitrogen and are of two types. Double ring structures (adenine and guanine) have longer molecules than single ring structures (cytosine and thymine).

- adenine always pairs with thymine by 2 hydrogen bonds.

- guanine always pairs with cytosine by 3 hydrogen bonds.

Adenine is said to be complementary to thymine and guanine is said to be complementary to cytosine. Quantities of adenine and thymine in DNA are always the same, as are guanine and cytosine. The ratio of adenine and thymine to guanine and cytosine varies from species to species.

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8.1, Structure of DNA

The double helix:

You need to imagine that DNA is arranged like a ladder, with the two polynucleotide chains being twisted. The uprights of phosphate and deoxyribose wind around one another to form a double helix. They form the structural backbone of the DNA molecule.

Function of DNA:

DNA is the hereditary material responsible for passing genetic information form cell to cell and generation to generation. There are around 3.2 million base pairs in the DNA of a typical mammalian cell. DNA provides immense genetic diversity within living organisms.

- it is very stable and can pass from generation to generation without change.

- its two separate strands are joined by hydrogen bonds, which allow them to separate during DNA replication and protein-syntesis.

- it is an extremely large molecule and therefore carries immense amount of genetic information.

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this is great, thank you :)


aw thanks :)

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