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 10-12 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

---- C ---- 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

223 = 8.4 x 106 combinations

- X-Y pairing at pseudoautosomal region

2.6 Mb at tips of short arms (obligatory recombination320 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