Chapter 2 Human Chromosomes DNA content: DNA content of cells (ploidy, cell cycle,life cycle) C (DNA content) = n (haploid number) C for humans = 3.5 x.

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Chapter 2 Human Chromosomes DNA content: DNA content of cells (ploidy, cell cycle,life cycle) C (DNA content) = n (haploid number) C for humans = 3.5 x g n = 23 all mammals are diploids In mitosis: 2C (before S phase) > 4C (after S phase) In meiosis: 2C (before S phase) > 4C (after S) ---- C 4 haploids ---- C

Development: - fertilization ---> cell division ---> differentiation -->morphogenesis ----> programmed cell death (apoptosis) - X-inactivation (Lyonization) occurs in human females. - is a means of dosage cpompensation at the last blastocyst stage - inactivate all X chromosomes except one - for X-linked genes, males are constitutionally hemizygous while females are functionally hemizygous. - Inactive X chromosome forms a Barr Body

Structure & functions of chromosomes: - human or any eukaryotic chromosomes only require 3 classes of DNA sequences: - centromeres - telomeres - origins of replication - packaging of DNA into chromosomes requires several levels of folding

- Centromeres are a primary constriction in the chromosome - Origins of replication (using yeast plasmid to trap promoters) - Telomeres - maintain the structural integrity of chromosomes - guarantees complete replication of chromosome ends - establish the cytoskeleton of the nucleus

Heterochromatin (genes not expressed) Euchromatin (genes may be expressed)

Mitosis & Meiosis: Mitosis

Meiosis: - independent assortement of paternal & maternal chromosomes 2 23 = 8.4 x 10 6 combinations - X-Y pairing at pseudoautosomal region 2.6 Mb at tips of short arms (obligatory recombination 320 kb at tip of long arm (no obligatory recombination genes in pseudoautosomal regions do not show X-linked pattern of inheritance and are not subject to X- inactivation

Human chromosomes: - best seen in lymphocytes - chromosomes identified by centromere position and banding pattern

Chromosome abnormalities: Most caused by: - miss-repair of broken chromosomes - improper recombination - malsegregation of chromosomes during mitosis or meiosis Types: - polyploidy (triploid) - aneuploidy (monosomic and trisomic - structural - chromosome breaks lead to deletion, insertion, inversion and translocation

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