Chromosomal Structures
Eukaryotic Chromosomes are composed of DNA,
Histone and Nonhistone Proteins
Histones,H1,H2A,H2B,H3,H4,The
later 4 form nucleosomes (100 A) and
thus called core histones,H1 plays a
role in forming 300 A superhelix
Histones are rich in basic amino acids
Nonhistone proteins play a variety of
functions,
Scaffold,the structural
backbone,can be
revealed after mild
treatment of the
chromosome,
Nucleosomes,beads on a string
EM picture supports
the radial loop-
scaffold model.
SARs,scaffold-associated
regions
Specialized chromosomal elements
Replication
Telomere
Centromere
Telomere’s functions
1,keep chromosomes from shortening
2,prevent chromosome fusion
3,ageing?
4,cancer?
DNA Compaction (Chromosome Structure)
and the Regulation of Gene Activity
High condensation silences gene expression
Heterochromatin and euchromatin
Heterochromatin,Certain DNA dyes stain some
regions much darker than the others,These darker
regions appear much more condensed,often located
near centromeres and have very few active genes,
Constitutive heterochromatin,condensed at most
times in all cells,Facultative heterochromatin,not all
cells
Position-effect
variegation
Barr bodies,heterochromatin and loss of gene
activity
Polytene
chromsomes
Chromosomal Alteration and Genetics
Chromosomal
alterations
happen during
evolution
Deletions and
Phenotypes
Mapping genes with deletion
Duplication can also arise from unequal
crossovers
Extra copy of a gene can have consequences
Inversion heterozygotes have few recombinants
Transposable elements
Retroposon,tranposes via RNA intermediate
Experiment shows how retroposon works
Aneuploidy
in human
How to generate diploid gamete