Upload
isabella-bell
View
227
Download
6
Tags:
Embed Size (px)
Citation preview
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
DNA
RNA
Protein
Transcription
Translation
Language
The cat sat on the mat
THE CAT SAT ON THE MAT
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 Genome (kb) FormVirus MS2 4 RNAVirus 50 Linear DNAOther viruses 5-300 Circular DNABacteria 700-5000 Circular DNA
Yeast 13,000 Linear DNAArabidopsis (plant) 100,000 arrangedFruit fly 165,000 asMouse 3,000,000 severalHuman 3,000,000 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
2m 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