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 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 interphasemetaphaseinterphase chromosomes Mitotic spindle chromosomes nucleus 1 23

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 Mitotic spindle

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 = 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