chromosomes and gene regulation dna is just information - a blueprint no use unless you know the...
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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 chromosomestaken through a high-power 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
• 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
interphase metaphase interphase
chromosomes
Mitotic spindle
chromosomes
nucleus
1 2 3
Features of the chromosome
• Centromere is required to attach to spindle at mitosis, so chromosomes segregate into new cells
• Telomeres protect the ends of chromosomes
• Replication origins are where DNA replication starts
telomere
centromere
telomere
replication
mitosis
2 new cells
Mitoticspindle
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
300 nm
700 nm
1400 nm
(1 nm = 0.000001 mm)
DNA double helix
Nucleosomes
Fibre of packed nucleosomes
Extended form of chromosome
Condensed form of chromosome
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
heterochromatineuchromatin
White eye gene
inversion
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