Biology Unit 2 revison AQA

  • Created by: jessica
  • Created on: 06-01-12 10:47



DNA stands for deoxyribonucleic acid and RNA for ribonucleic acid. They are polymers (long chain molecules) made from nucleotides.



·         a phosphate group, which is negatively charged.

  • a pentose sugar, which has 5 carbon atoms in it. In RNA the sugar is ribose. In DNA the sugar is deoxyribose.
  • a nitrogenous base. There are five different bases (you don't need to know their structures). The bases are usually known by there first letters only, you don't need to learn the full names. The base thymine is found in DNA only and the base uracil is found in RNA only.

Nucleotides polymerise by forming bonds between the carbon of the sugar and an oxygen atom of the phosphate. The bases do not take part in the polymerisation, so the chain is held together by a sugar-phosphate backbone with the bases extending off it. This means that the nucleotides can join together in any order along the chain. Many nucleotides form a polynucleotide.

A polynucleotide has a free phosphate group at one end and a free OH group at the other end.

Main features of the three-dimensional structure of DNA are;

  • DNA is double-stranded, so there are two polynucleotide stands alongside each other.
  • The two strands are wound round each other to form a double helix.
  • The two strands are joined together by hydrogen bonds between the bases. The bases therefore form base pairs, which are like rungs of a ladder.
  • The base pairs are specific. A only binds to T (and T with A), and C only binds to G (and G with C). These are called complementary base pairs. This means that whatever the sequence of bases along one strand, the sequence of bases on the other strand must be complementary to it. (Incidentally, complementary, which means matching, is different from complimentary, which means being nice.)

DNA therefore has two essential functions: replication and expression.


  • Replication means that the DNA, with all its genes, must be copied every time a cell divides.
  • Expression means that the genes on DNA must control characteristics. A gene is a section of DNA that codes for a particular protein. Characteristics are controlled by genes through the proteins they code for, like this:


Sum total of all the genes in an organism is called the genome.


RNA is a nucleic acid like DNA, but with 4 differences:

  • RNA has the sugar ribose instead of deoxyribose
  • RNA has the base uracil instead of thymine
  • RNA is usually single stranded
  • RNA is usually shorter than DNA



The sequence of bases on DNA codes for the sequence of amino acids in proteins. But there are 20 different amino acids and only 4 different bases, so the bases are read in groups of 3. This gives 64 combinations, more than enough to code for 20 amino acids. A group of three


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