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DNA, genes and chromosomes
• Revise your BI1505/7 notes
• Textbook - Purves chapters on “Chromosomes…”
and “DNA…”, Lehninger chapter 24
• These 3 lectures will cover the organisation of
chromosomes and genomes in different organisms
• Details of DNA replication and gene expression
will be covered by other lecturers
The Central Dogma
Information flow in cells
Language
DNA
The cat sat on the mat
Transcription
THE CAT SAT ON THE MAT
RNA
Translation
Protein
Le chat s’est assis sur le petit tapis
Why DNA is the genetic material
• Can be replicated accurately
• Has capacity to carry information to direct
organisation and metabolism in cells
• Can undergo mutation to enable change and
evolution
• Studies in the 1920s-1940s proved DNA =
genes ……..
Genome size increases (roughly) with
evolutionary complexity of organism
Organism
Virus MS2
Virus l
50
Other viruses
Bacteria
Genome (kb)
4
5-300
700-5000
Form
RNA
Linear DNA
Circular DNA
Circular DNA
Yeast
Arabidopsis (plant)
Fruit fly
Mouse
Human
13,000
100,000
165,000
3,000,000
3,000,000
Linear DNA
arranged
as
several
chromosomes
Human Chromosomes
Chromatin and histones
• In eukaryotes, the first level of DNA packing is the
chromatin fibre
• Chromatin is formed by wrapping the DNA around
complexes of the 4 histone proteins (2 molecules each of
histones H2A, H2B, H3, H4) to form “beads on string”
arrangement - the beads are nucleosomes
• See figures 24-23, 24-24, table 24-3 in Lehninger
• Chromatin is of 2 different types - euchromatin (where most
of the active genes are) and heterochromatin (no active
genes). Some regions of genome can switch between these 2
states (facultative heterochromatin)
Nucleosomes
Chromosome structure - packing ratio
• Packing ratio is the length of the DNA divided by
the length into which it’s packaged
• Smallest human chromosome (21) has 4x107 bp of
DNA, 10 times size of E. coli genome
• Equivalent to 14mm of extended DNA
• In most condensed state the chromosome is about
2mm long
• Packing ratio = 14000/2 = 7000
• So, there must be an efficient packaging
mechanism
Higher level DNA packing
• To achieve packing ratio of 7000, chromatin
is organised into several levels of complex
folded and coiled structures
• See relevant figures in Purves or in
Lehninger
Chromatin
packaging
Position-effect variegation (PEV)
• State of chromatin (euchromatin, heterochromatin)
can affect gene expression
• A gene could be moved to a heterochromatic
region by an inversion
• Heterochromatin’s structure tends to switch off
gene expression
An example of PEV
• A mutant allele of the w gene in Drosophila causes
eyes to be white (wild-type is red)
• An inversion of part of the X chromosome causes
eyes to have red and white patches
• This is because of PEV switching off w gene in
some cell lines in the eye
• The boundary between heterochromatin and
euchromatin is not exactly the same in all cell
lines, hence eyes are mosaic