Lecture 2.docx

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University of Auckland
Biological Sciences
Craig Miller

BIOSCI202 Lecture 2: Structure of the chromosome Chromosome number The chromosome set presented in organisms from the same species has a characteristic chromosome number and appearance  Haploid number (n) – most fungi and algae have only one set of chromosomes  Diploid number (2n) – most plants and animals have two sets of chromosomes Variation in chromosome number  There is no relationship between chromosome number and the complexity of a species, BUT the more chromosome a species has, there are potentially more variations it can produce via independent assortment Slender golden week can produces 2 , whereas the fern can produce 2 500different kinds of gametes  However, there is a balance for the number of chromosome a species can have e.g. Stalked adder’s tongue fern has a 2n of ~1000, but it reproduces asexually as its gametes are unviable very large number of chromosome does not segregate properly during meiosis  Number of chromosome is not related to genome size (total amount of DNA in the nucleus) Genome sizes are also very variable, e.g. in flowering plants, there is a >2500 fold range between smallest and largest Chromosome size The chromosomes of a single genome may differ considerably in size Humans have 46 chromosomes, 22 pairs of autosome and 1 pair of sex chromosome There is a range of chromosome sizes in humans with no clear discontinuity, the chromosomes grade from one to another In the human genome there is about a three to four fold range in chromosome size. Chromosome 1 is the largest chromosome while chromosome 21 is the smallest. 1 BIOSCI202 Chromosome topography Karyotype – is the morphology of all the chromosomes at mitotic metaphase Eukaryotic chromosome Each functional chromosome has a centromere (spindle fibres attach) and two telomeres (stabilise the chromosome) Chromosome morphology Most important feature is centromere position; centromere is the primary constriction in the chromosome Chromosomes are classified into three basic types:  Telocentric – centromere at one end I-shaped  Acrocentric – centromere off center  J-shaped  Metacentric – centromere in the middle  V-shaped Position of telomeres  Telomeres are specialised regions located at the ends of chromosomes  Generally there is no visible structure that distinguishes the telomeres from the rest of the chromosome, BUT, at the DNA level, the telomere is characterised by the presence of distinct DNA sequences e.g. in the ciliate Tetrahymena the telomere repeat is TTGGGG and in humans it is TTAGGG Position of nucleolar organiser  Nucleoli are spherical organelles within the nucleus that contain ribosomal RNA and components of ribosomes, the nucleoli resides next to a slight constriction in the chromosome Nucleolar organiser (sometimes called secondary constriction)  Nucleolar organiser contains a cluster of genes that code for the ribosomal RNA, these genes are tandem repeats  Every set of the chromosome will have at least one nucleolar organiser, BUT not on all chromosome
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