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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 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