Genetics Topic3

Chromosomes, genes, alleles, and mutations

homologous chromosome chromatin

Eukaryotic chromosomes are made of DNA and proteins ·The DNA double helix is wound around special proteins called histones

·Each chromosomes is divided into a number of different sections of DNA called genes - Gene = heritable factor that controls a specific characteristic or trait > The different forms of a gene are called alleles (e.g. allele for blue eyes vs. allele for green eyes) > Each allele of a gene differs from other alleles by one or a few bases only and occupies the same locus (position on the chromosome) as other alleles of the same gene

·The whole of genetic information of an organism (all of its chromosomes) is referred to as its genome

Gene mutations ·The term mutation refers to a change in the base sequence of a gene ·Usually only one or two bases are changed: - Insertion = extra base is added - Deletion = base is removed - Inversion = bases are put in the wrong order - Substitution = The wrong base is put in (e.g. T gets put in where C should be)

Inversion: Substitution:

Mutation Consequences: ·May lead to incorrect amino acid being inserted during translation ·May result in a non-functional or malfunctional protein ·If the protein does fail to function properly the result can be either beneficial to the organism (by helping it survive better in it’s environment), benign, or harmful Causes of Mutation: ·Most occur as an accident during transcription, but some are a result of mutagens such as chemicals and radiation

Sickle Cell Anemia: example of a harmful gene mutation ·Mutation is a substitution of one base pair in the gene that codes for the production of haemoglobin - change from GAG (glutamate) to GTG (valine) - the different properties of valine cause the haemoglobin molecule to fold up differently and changes the shape and function

·The hemoglobin protein that is produced is said to look like a sickle as a result of this mutation ·shape causes it to be unable to carry oxygen efficiently ·impairs blood flow - blood cells are less flexible - stick in capillaries and block or slow blood ·Individuals with two copies of the recessive allele are often anemic and suffer from extreme weakness and fatigue due to the lack of oxygen

sickle cell anemia and malaria http://www.pbs.org/wgbh/evolution/library/01/2/l_012_02.html