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what is a chromosome.docx

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McMaster University
Bhagwati Gupta

October 7 , 2013 Biology 2C03: Genetics What is a Chromosome? Chromosomal Theory of Inheritance - A collection of studies revealed that genes are on chromosomes. In fact, multiple genes are present on each chromosome What is a Chromosome? How many do we have? - Haploid: one copy of genetic material subdivided into chromosomes  Can still replicate and form sister chromatids  Single copy - Diploid: two copies of genetic material subdivided into chromosomes  Same genes but may have different alleles - Ploidy: the number of complete chromosome sets - Homologous chromosomes: a pair of matching chromosomes, one homolog from each parent - Most eukaryotes: diploid adults, haploid gametes (sperm/ovum) - Why diploid?  Diploid or polyploidy have genetic buffering - What about haploid and polyploidy? - Many plants are polyploidy - Some animals (e.g., certain fishes and amphibians) are also polyploid Chromosome Structure - Single chromosome with two sister chromatids - Two copies of chromosomes: undergo duplication in S phase so each chromosome have two sister chromosomes - Telomeres: structures at the top - Centromere: holding sister chromsomes together - Kinetochore: wehre spindle microtubules attach - Centromere is not defined as the middle of chromsomes as they may exist on all parts of the chromosome  Metacentric: located at the center  Submetacentric: slightly off centered (chromosomes 1 and 3)  Acrocentric: far on the edge, peaks at the top but not completely (13, 14, 15, 21, 22)  Telocentric: no telocentric chromosomes in humans; centromere located at the end - Eukaryotic chromosomes exist in four major types based on the position of the centromere Human Karyotype - Karyotype: G-banding  Dark regions are heterochromatic  Light regions are euchromatic, early replicating - Karyotype: painting probes - Human chromosome set: 23 chromosomes. Here we see two copies of the set (diploid) Autosomes and Sex-Chromosomes - Chromosomes can be distinguished as:  Autosomes: present in the same copy number in both males and females; number and morphology of autosomes is species-specific  Sex-chromosomes: present in different copies in males versus females; single pair of sex chromosomes or a single sex chromosome (XX female; XY male) Questions we need to Answer - How do we know that genes are on chromosomes? - What is the structure of a chromosome? - How does the structure of a chromosome affect gene expression? Chromosomal Basis of Heredity - Walter Flemming, 1882  First to detail the process of chromosomal movement during mitosis  Identified steps of mitosis from the condensation of chromosomes into coiled ‘threads’ (prophase) thorugh the separation of these threads into two ‘skeins of yarns’, when the scaffold of the nucleus reappears  Termed process Karyomitosis (‘threadlike metamorphosis of the nucleus’) The Chromosome Theory of Inheritance - The union of cytology and genetics - Cytology: the study of cell structure (chromosomes) - Genetics: the study of heritable factors (genes) - 1902: Walter Sutton and Theodor Boveri developed the chromosome theory of inheritance - Genes are located on chromosomes (chromosomes and genes show similar patterns of inheritance) - What is the accumulated proof for this theory? Evidence for the Chromosome Theory of Inheritance - Much evidence that suggested that heritable information is found on chromosomes: 1) Chromosome number is species specific 2) Blakeslee: Jimsonweed (Datura), plants with different chromosome sets display different phenotypes  Induced aneuploidy  Mutants were trisomy for different chromosomes  Associated phenotype with number of chromosome present 3) Eleanor Carothers: meiosis in grasshoppers, first cytological evidence for independent assortment chromosomes 4) Sex chromosomes: chromosomes represented differently in the two sexes 5) Patterns of chromosome movements parallel Mendel’s patterns of inheritance What is the Function of Mitosis? - Replication of identical cells – one cell produces tw
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