Presentation on theme: "DNA, genes and chromosomes"— Presentation transcript:
1 DNA, genes and chromosomes Revise your BI1505/7 notesTextbook - Purves chapters on “Chromosomes…” and “DNA…”, Lehninger chapter 24These 3 lectures will cover the organisation of chromosomes and genomes in different organismsDetails 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 matTranscriptionTHE CAT SAT ON THE MATRNATranslationProteinLe chat s’est assis sur le petit tapis
3 Why DNA is the genetic material Can be replicated accuratelyHas capacity to carry information to direct organisation and metabolism in cellsCan undergo mutation to enable change and evolutionStudies in the 1920s-1940s proved DNA = genes ……..
7 Chromatin and histones In eukaryotes, the first level of DNA packing is the chromatin fibreChromatin 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 nucleosomesSee figures 24-23, 24-24, table 24-3 in LehningerChromatin 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)
9 Chromosome structure - packing ratio Packing ratio is the length of the DNA divided by the length into which it’s packagedSmallest human chromosome (21) has 4x107 bp of DNA, 10 times size of E. coli genomeEquivalent to 14mm of extended DNAIn most condensed state the chromosome is about 2mm longPacking ratio = 14000/2 = 7000So, 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 structuresSee relevant figures in Purves or in Lehninger
12 Position-effect variegation (PEV) State of chromatin (euchromatin, heterochromatin) can affect gene expressionA gene could be moved to a heterochromatic region by an inversionHeterochromatin’s structure tends to switch off gene expression
13 An example of PEVA 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 patchesThis is because of PEV switching off w gene in some cell lines in the eyeThe boundary between heterochromatin and euchromatin is not exactly the same in all cell lines, hence eyes are mosaic
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