Biology OCR Unit 5

Nucleotide = the basic unit which makes a DNA or RNA strand. It contains - a five carbon (ribose) sugar, a phosphate group and a nitrogenous base.

There are four nitrogenous bases;

• Adenine, A and Guanine,G these are purines.
• Cytosine, C, Thymine, T and Uracil, U which replaces T in RNA these are pyrimidines.

Hydrogen bonds form between the bases to hold the strands together. This is called complementary base pairing. This is what allows DNA to be copied and be used as a genetic code.

The bases G and C pair together - forming three hydrogen bonds.

The bases T and A pair together - forming two hydrogen bonds.

DNA is contained within chromosomes - these are found in the nucleolus. Sections of the DNA strand are called genes.

The genetic code is used to make proteins. Proteins are made from amino acids.

Proteins are coded for by each group of three consecutive bases on the DNA strand. This is called a base triplet or a codon. Each base triplet codes for a different amino acid.

Some amino acids are coded for by more than one codon. For example; CGA, CGG, CGT and CGC all code for the amino acid arginine.

Only one strand of a a DNA helix will code for proteins, this is called the reference strand.

REMEMBER: DNA is contained within the nucleus - it cannot leave!!

Protein synthesis takes place at ribosomes - which are mainly floating free in the cytoplasm. In order for protein to be made the genetic code needs to reach the ribosomes. This is done using mRNA (messenger RNA).

Messenger RNA is a single strand of nucleotides which carries the genetic code from the DNA. mRNA is made in the process called transcription.