Chromosomes and gene regulation

1
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

2
Giant chromosome from Drosophila (From course
textbook)
3
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

4
Human karyotype (picture of chromosomes) drawn
from a photograph of chromosomes taken through a
high- power microscope. Routine lab
method. Used in hospitals.
5
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

6
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

7
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

8
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

9
Structure of nucleosomes
Beads-on-string chromatin
Digestion with nuclease enzyme
Single nucleosomes
Histone proteins (8 subunits) 146 bp DNA fragments
10
Overall packaging of DNA
DNA double helix
Nucleosomes
Fibre of packed nucleosomes
Extended form of chromosome
Condensed form of chromosome
Entire chromosome
(1 nm 0.000001 mm)
11
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

12
Gene activity affected by position
White eye gene
inversion
heterochromatin
euchromatin
13
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