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Transcript
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 Supercoiling of DNA • The tension induced in a circular DNA molecule causes it to become supercoiled • Supercoiling is the usual state for bacterial chromosomes, which consist of a number of independently supercoiled loops • The process is controlled by topoisomerase enzymes that can cut and re-join one strand of the DNA • Topoisomerases can also untangle DNA • Refer to figures 6.1 and 6.4 in Hartl Eukaryotic chromosomes • In metaphase of mitosis, chromosomes can be seen under microscope they have a compact rod-like structure • The ends of chromosome are called telomeres, function is to protect the ends of the DNA • Near the middle is the centromere, function is to attach to spindles during cell division and ensure correct segregation • Telomeres and centromeres contain special DNA sequences and associated proteins • Telomeres are replicated differently from the rest of the genome - see figure 6.27 in Hartl • Different regions of the chromosome can be stained with dyes (e.g. Giemsa) giving a characteristic banding pattern • See figure 6.20 in Hartl 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 Chromatin and histones • 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 • 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) • See figure 6.8 in Hartl Higher level DNA packing • To achieve packing ratio of 7000, chromatin is organised into several levels of complex folded and coiled structures • See figure 6.10 in Hartl Unique and repeated DNA • If eukaryotic DNA is melted and allowed to re-anneal, it does so in 3 distinct phases • See figure 6.18 in Hartl • The explanation is that there is highly repetitive DNA (which re-anneals quickly), moderately repetitive DNA (intermediate) and unique or single copy DNA (re-anneals slowly) Classes of eukaryotic DNA • Highly repetitive: – Bits of old virus genomes – Simple sequence repeats e.g. CACACA…. – Special sequences such as centromeres • Moderately repetitive: – Other old virus genomes – Multi-gene families, e.g. ribosomal RNA • Single-copy: – Most “normal” genes Stability of genomes • Most DNA is very stable - inherited without changes from parents (except 1/106 mutation rate) and does not change in the lifetime of the cell • Some DNA is unstable, i.e. can move about in the genome - called transposable elements or transposons • First observed by Barbara McLintock in the 1940s, as different coloured segments in corn cobs (see section 6.8 in Hartl) • Explanation was transposon in the maize genome that affects expression of genes controlling pigment - jumps to different locations in DNA of different segments of the corn-cob, switching colour in those segments • Many other examples, in many organisms (including humans). Often they are repetitive in the genome