Cytogenetics: Chromosome Mutations, Aberrations & Evolution

Chromosomes Prokaryote Eukaryote

Eukaryotic chromosomes

Human karyotype

Why do we care? Many diseases and birth defects are a direct result of missing, broken, or extra chromosomes. Down Syndrome Cri du chat Syndrome Patau Syndrome

Mutations at the level of the homologous pair EUPLOIDY: "true" ploidy, meaning two members of each homologous pair. ANEUPLOIDY: "not true" ploidy, meaning more or fewer members than two of each homologous pair. MONOSOMY - one homolog; partner is missing TRISOMY - three homologs NULLISOMY- one entire homologous pair is missing.

Monosomy and Trisomy

Down Syndrome

Figure: 07-04 Caption: Incidence of Down syndrome births contrasted with maternal age.

How does it happen? Nondisjunction Figure: 07-01 Caption: Nondisjunction during the first and second meiotic divisions. In both cases, some of the gametes formed either contain two members of a specific chromosome or lack that chromosome. Following fertilization by a gamete with a normal haploid content, monosomic, disomic (normal), or trisomic zygotes are produced. Each chrom. has two chromatids

Trisomy: Patau Syndrome 1/20,000 births severe mental retardation heart and organ defects polydactyly death by the age of one year

Structural Changes Deletions (deficiencies) Duplications Inversions Translocations

Deletions (deficiencies) w

How can chromosomes break? Ionizing radiation (production of free radicals, which act like little atomic "cannon balls", blasting through strands of DNA or c'somes. Chemical insult. Why do they rejoin? Break points of chromosomes are highly reactive ("sticky"), whereas normal ends of c'somes are capped by telomeres, which do not readily bond to other molecules.

Breaks that occur ______ __________________ __________________ will affect both newly formed chromatids, & all daughter cells arising from them. Breaks that occur ______ ____________________ ____________________ may affect only one chromatid. (Thereafter, only the progeny carrying the broken chromatid will be affected.)

Cri-du-chat Syndrome _________________ Mental retardation Slow motor skill development Low birth weight and slow growth Small head (microcephaly) Partial webbing of fingers or toes Wide-set eyes (hypertelorism) High-pitched cry

Structural Changes Deletions (deficiencies) Duplications Inversions Translocations

Duplication

Duplications

Bar eye: caused by duplication

Duplications: source of evolutionary novelty? Duplication is a source of new genes over evolutionary time: e.g., gene families like globins and MHC genes ______________

Structural Changes Deletions (deficiencies) Duplications Inversions Translocations

Inversions