1. 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

2. 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

3. 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 ……..

5. Genome size increases (roughly) with evolutionary complexity of organism
Organism Genome (kb) Form
Virus MS RNA
Virus l Linear DNA
Other viruses Circular DNA
Bacteria Circular DNA
Yeast 13,000 Linear DNA
Arabidopsis (plant) 100,000 arranged
Fruit fly 165,000 as
Mouse 3,000,000 several
Human 3,000,000 chromosomes

6. Human Chromosomes

7. 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)

8. Nucleosomes

9. 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

10. 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

11. Chromatin packaging

12. 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

13. 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