Download Human karyotype

Chromosomes and gene regulation
• DNA is just information - a “blueprint”
• No use unless you know the rules for using
the information
• DNA is organised (in chromosomes)
• The expression of genes is regulated
• These 2 lectures cover these 2 issues
Giant chromosome
from Drosophila
(From course textbook)
Human chromosomes
• Human body cells have 46 chromosomes in
pairs:
– 22 pairs of autosomes (numbered 1 to 22)
– 2 X chromosomes (female) or X and Y (male)
• Germ cells (sperm and egg) have 23
chromosomes
– 22 autosomes and a Y or an X
• Body cells are diploid and germ cells are
haploid
Human karyotype
(picture of
chromosomes)
drawn from a
photograph of
chromosomes
taken through a highpower microscope.
Routine lab method.
Used in hospitals.
DNA is packaged into chromosomes
• Each human cell contains 2 metres of DNA
(3,000,000,000 bases in a haploid cell)
• Nucleus is 5 microns (0.005 mm) diameter
• DNA must be properly packaged, not just
tangled up and stuffed into nucleus
• Packaging involves coiling and folding the
DNA in specific ways
• Special proteins are associated with DNA together called chromatin
Cell cycle
nucleus
1
chromosomes
interphase
Mitotic spindle
2
3
chromosomes
metaphase
interphase
• Cell cycle is the means by which cells divide
• Stages:
1. Chromosome replication
2. Mitosis
3. Cell division
• Chromosomes are extended during interphase, but
condensed and visible under microscope during metaphase
Features of the chromosome
• Centromere is
Mitotic
telomere
required to attach to
spindle
spindle at mitosis,
so chromosomes
segregate into new
centromere
cells
• Telomeres protect
the ends of
mitosis
telomere
chromosomes
replication
2 new cells • Replication origins
are where DNA
replication starts
Nucleosomes & chromatin
• Extended DNA under electron microscope
looks like “beads on a string”
• The beads are DNA coiled around a core of
proteins (histones). DNA and proteins
together are chromatin
• Each of these structures is a nucleosome
• Nucleosomes are the basic type of
organisation of DNA in a chromosome
Structure of nucleosomes
Beads-on-string chromatin
Digestion with nuclease enzyme
Single nucleosomes
Histone proteins (8 subunits)
146 bp DNA fragments
Overall packaging of DNA
2 nm
11 nm
30 nm
DNA double helix
Nucleosomes
Fibre of packed nucleosomes
300 nm
Extended form of chromosome
700 nm
Condensed form of chromosome
1400 nm
(1 nm = 0.000001 mm)
Entire chromosome
Chromatin and gene activity
• Stained chromosomes show bands
• Bands are due to different forms of
chromatin
• Euchromatin is where the active genes are
• Heterochromatin has no or few active
genes, found near centromeres and
telomeres
Gene activity affected by position
White eye gene
inversion
euchromatin
heterochromatin
Chromosomes in nucleus
• During interphase
chromosomes are organised
within nucleus
• Individual chromosomes
probably attached at points
to wall of nucleus
• Part of chromosomes are in
a region called nucleolus