Biology Unit 2 Revision notes

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  • Created on: 20-04-11 16:36

The Genetic Code

1. DNA (deoxyribonucleic acid) consists of a sequence of nucleotide, each of which has:

a. A phosphate group.

b. A sugar group (deoxyribose sugar).

c. A base – adenine (A), thymine (T), cytosine (C) or guanine (G).

2. The sugar and phosphate groups form a sugar-phosphate backbone to DNA. The bases in the two 

complementary DNA strands bond together by hydrogen bonding – A bonds to T and C bonds to 

G. These strands then twist around to form a double helix structure.

3. DNA is arranged into chromosomes:

a. There are 46 (23 pairs) in humans.

b. The chromosomes are arranged in homologous pairs  – i.e. two with the same types of 

gene along them.

c. Alleles for a gene will occupy the same locus on each chromosome.

4. There is only 0.1% difference in DNA between humans. SNPs (single nucleotide polymorphisms) 

are responsible for individual characteristics.

5. DNA is a successful molecule:

a. It is a huge, coiled, molecule, containing masses of information in a small space.

b. Characteristics are accounted for by small variations in the structure.

c. It is very stable, and can take physical stress.

d. It can reproduce very accurately.

6. DNA replication occurs during the interphase, and is semi-conservative replication:

a. Helicase enzymes break hydrogen bonds between bases.

b. DNA polymerase enzymes join free bases in the cytoplasm to ach strand.

c. This produces two identical strands, with ½ old and ½ new DNA (i.e. semi-conservative).

7. Meselson and Stahl’s experiment gave evidence for semi-conservative replication:

a. Bacterial DNA was radioactively labelled with heavy nitrogen.

b. The bacteria were allowed to replicate, and a sample was taken from each generation.

c. In generation 0, the entire DNA was 



d. In generation 1, the entire DNA was a mixture of half 


N and half 




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