Lecture 18 - Chromosome Structure
I. So what is a chromosome?
A. composed of chromatin - DNA + protein; forms
stable, ordered structure
1.
2.
II. Genomes – a brief overview
A. chromosome number can vary greatly
B. genome size
Genome
Virus
SV40
Herpes simplex
T4
Bacteria
E. coli
Eukaryotes
S. cerevisiae
C. elegans
Arabadopsis
D. melanogaster
human
maize
Amphiuma
size in kb
form
estimated # genes
5
152
165
circular
linear
linear
300
4,700
circular
4,000
13,000
100,000
100,000
165,000
3,000,000
4,500,000
76,500,000
16 chromo.
6 chromo.
5 chromo.
4 chromo.
23 chromo.
10 chromo.
14 chromo.
1.
2.
3.
III. How do you get that much DNA into a
cell?
A. Here’s the problem: DNA in
human chromosome I estimated 82
mm long, but nucleus only ~10 µm
(0.01 mm) in diameter
B. Solution: careful packageing, how?
1. linear DNA (human chromosomes = 2 meters/cell)
- human chromosome 1 = 82 mm
2. nucleosome – first level of compaction
- main components are histones – small, basic
proteins
(H1, H2A, H2B, H3 and H4)
1
5
6,000
19,000
25,000
35,000
3. solenoid (= 30 nm fiber)
- may be helix with 6
nucleosomes per turn
4. higher order
- appears to involve
formation of loops of 30
nm fiber around central
scaffold
IV. Features of chromosomes
A. chromomeres
B. centromeres
C. telomeres
D. types of chromatin
1. heterochromatin-densely staining, highly compacted, relatively
transcriptionally inactive
2. euchromatin-weakly staining, less compact, transcriptionally
active
E. banding patterns-treatment with various stains
reveals pattern of bands; can be used to ID
chromsomes or chromosome fragments
V. A specialized type of chromosome: polytene
chromosome
A. what is it?
B. how is it produced?
C. how is it useful? – characteristic banding pattern
of chromosome
- can be used to pinpoint location of gene
2
permits identification of regions