Unit 2, AS AQA Biology Notes For Chapter 7 and 8.

If one species differs from another, this is called interspecific variation.

lf members of the same species differ from each other, this is called intraspecific variation.

sampling: involves taking measurements of individuals, selected from the population of organisms which is being investigated. There are two types of sampling:

1) sampling bias

- random sampling

2) chance

- use a large sample size

- analyse the data collected

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.

meiosis. nuclear division forms the gametes and mixes up the genetic material before it is passed into the gametes.

fusion of gametes. offspring inherit some characteristics of each parent.

environmental influences include:

- climatic conditions

- soil conditions

- pH

- food availabilty

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

(look on p127 of AS AQA biology textbook for standard deviation formula)

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

nucleotide structure

deoxyribose sugar, phosphate group and organic base.

There are two organic bases..

single-ring bases: cytosine (C) + thymine (T)

double-ring bases: adenine (A) + guanine (G)

All three components are combined by condensation reactions to give a single nucleotide (mono-nucleotide).

Two mono-nucleotides form a di-nucleotide.

The linking of mono-nucleotides forms a long chain known as a polynucleotide.

Hydrogen bonds form between certain bases.

(James Watson, Francis Crick and Rosalind Franklin) - worked out the structure of DNA.

organic bases contain nitrogen.

- the double-ring structures (A) + (G) have longer molecules.

- the single-ring structures (C) + (T) have shorter molecules.

Adenine always pairs with Thymine by two hydrogen bonds.

Guanine always pairs with Cytosine by three hydrogen bonds.

adenine is said to be complementary to thymine.

guanine is said to be complementary to cytosine.

quantities of adenine and thymine in DNA are always the same, and so are the quantities of guanine and cytosine, however the ratio of the two paired bases varies from species to species.

there are around 3.2 million base pairs in the DNA of a typical mammalian cell.

functions of DNA

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

- it's two separate strands are joined only with hydrogen bonds, which allow them to separate during DNA replication and protein-synthesis.

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

- by having the base pairs within the helical cylinder of the deoxyribose-phosphate backbone, the genetic information is to some extent protected from being corrupted by outside chemical and physical forces.

genes: are sections of DNA that contain the coded information for making polypeptides.

coded information is in the form of a specific sequence of bases along the DNA molecule.

polypeptides combine to make proteins.

genes determine the proteins of an organism.

enzymes are proteins.

enzymes control chemical reactions.

genes determine the nature and development of all organisms.

a polypeptide is a sequence of amino acids.

there must be a minimum of three bases that code for each amino acid.

- only 20 amino acids regularly occur in proteins.

- each amino acid must have its own code of bases on the DNA.

- only four different bases (A, T, G, C) are present in DNA.

- three bases produce 64 different codes, more than enough to satisfy the requirements of 20 amino acids.

Three bases is called a Triplet code.

Some amino acids have more than one code.

In eukaryotes much of the nuclear DNA does not code for amino acids.

- a few amino acids only have a single triplet code.

- the remaining amino acids have between two and six triplet codes each.

- the triplet code is always read in one particular direction along the DNA strand.

- the start of the sequence is always the same triplet code. This codes for the amino acid methionine. If this first methionine molecule does not form part of the final polypeptide, it is later removed.

- three triplet codes do not code for any amino acid. These are called 'stop codes' and mark the end of a polypeptide chain.

- the code is non-overlapping, i.e. each base in the sequence is read only once. e.g. 123 and 456.

- the code is universal, i.e. with a few minor exceptions it is the same in all organisms.

Prokaryotic Cells: (bacteria)

DNA molecules are smaller

Form a circle (circular)

Not associated with proteins

No chromosomes

Eukaryotic Cells:

DNA molecules are larger

Form a line (linear)

Associated with proteins

Chromosomes present

chromosomes are only visible as distinct structures when a cell is dividing.

they are widely dispersed throughout the nucleus.

chromosomes appear as two threads, joined at a single point.

each thread is called a chromatid.

the DNA in chromosomes is held in position by proteins.

they are highly coiled and folded.

DNA is a double helix.

helix is wound around proteins to fix it into position.

DNA-protein complex is then coiled.

humans have 46 chromosomes.

chromosomes occur in pairs.

homologous pair: is always two chromosomes that determine the same genetic characteristics.

'but determining the same characteristics' is not the same as being identical.