So Mutations!  Any change to the quantity or structure of DNA of an organism is known as a mutation.  Mutations can occur in either somatic cells (body cell) and germ cells (those that produce the gametes (these can be passed on!)).  Changes in the structure or number of a whole chromosome is know as a chromosome mutation  Changes which affect a single locus and therefore produce a different allele are know as gene mutations.  They are the basis of discontinuous variation

Gene Mutations  There are three main types of gene mutation which you have to know about: 1.Base substitution 2.Base addition 3.Base deletion Base additions and deletions tend to have the most significant effect on the polypeptide produced as they cause frame shifts, in the code. More of a problem at the start of the sequence compared to the end. Base substitutions often = silent mutations. However sometimes they can have a very large effect e.g. ATG tyrosine to ATT = stop

Changes to a single base could result in  A nonsense mutation – when a change causes on of the three stops to be coded.  A mis-sense mutation – where a change in a base results in the coding of a different amino acid – the significance of this depends of the role of the original amino acid e.g. If it had a role in forming a bond.  A silent mutation – where the change still results in the same amino acid being coded for – has no effect

Sickle Cell Anaemia  First human disease understood on the molecular level (1949 Linus Pauling).  All related to the DNA that codes for the β – amino acid chain within haemoglobin.  Caused by a single substitution the base thymine to adenine (CTT to CAT) causing glutamic acid rather than valine.  This produces haemoglobin with a sticky patch. The molecules tend to stick to each other forming long fibres inside the red blood cell. The fibres distort the red blood cell making them inflexible and sickle shaped. The red blood cells are then unable to carry oxygen and block small capillaries. Sufferers become tired, listless, and have a shortened life span.

Phenylketonuria or PKU  Caused by an abnormal base sequence in part of the DNA molecule.  Codes for an enzyme  Phenylalanine is found in many foods  Tyrosine is converted to melanin.  So people with PKU have lighter skin and hair than normal.  Without the enzyme phenylalanine accumulates in the blood and tissues and can cause severe brain damage in young children even mental retardation.  However it can be treated by diet and the HUMAN GENOME PROJECT IS HELPING.

Mutagens or mutagenic agents  Agents that increase the rate of mutation (usually 1 – 2 mutations per genes) the main two are: 1. Chemicals 2.High energy radiation

Mutations – costs and benefits  Benefit  Can increase variation and genetic diversity  Cost  Tend to produce organisms which are less well suited to the environment.  Mutations that occur in body cells rather than gametes can disrupt normal cellular activities such as cell division

Genetic control of Cell division  Controlled by two genes:  Proto-oncogenes which stimulate cell division  Tumour suppressor genes – which slow cell division.  Use page 232 to write a job advertisement for each of the genes