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BIOLOGY 2C03 (138)
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Lecture 5

Lecture 5 - February 5, 2013.docx
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Department
Biology
Course
BIOLOGY 2C03
Professor
Joe Kim
Semester
Winter

Description
Course Code Year Lecture 5 – Feb 5, 2013 Chromosomal Theory of Inheritance  A collection of studies revealed that genes are on chromosomes – in fact multiple genes are present on each chromosome  Homologous Chromosomes – a pair of matching chromosomes; one homolog from each parents  Ploidy – the number of complete chromosome sets  Most eukaryotes – diploid adults, haploid gametes Classification of eukaryotic chromosomes by structure:  Position of the centromere – constriction along length of chromosome; attaches to spindle fibers during mitosis and meiosis  Length of chromosome  Human chromosome set: 23 chromosomes  Figure – two copies of the set (diploid)  Chromosomes can be distinguished as: o Autosomes: present in the same copy number in both males and females; number and morphology of autosomes is species- specific o Sex-chromosomes – present in different copies in males vs. females; single pair of sex chromosomes or a single sex chromosome 1 Course Code Year  Questions we need to answer: o How do we know that genes are on chromosomes? o What is the structure of a chromosome? o How does the structure of a chromosome affect gene expression?  Chromosomal basis of hereditary o Review of mitosis and meiosis – connection between chromosome dynamics and Mendelian principles o Development of the chromosomal theory of inheritance – connecting Cytology and Genetics  The chromosome theory of inheritance – the union of cytology and genetics o Cytology – the study of cells (chromosomes that we can see) o Genetics – the study of heritable factors (genes) o 1902: Sutton and Boveri - genes are on chromosomes  What is the accumulated proof for this theory?  Much evidence that suggested that heritable information is found on chromosomes: 1) Chromosomes number is species specific o But, Sable Antelope: 46 o Karyotypes differ – structures differ not just number 2) Blakeslee: Jimsonweed (Datura) plants with different chromosome sets display different phenotypes o 12 chromosomes in Datura stramonium o 12 variant phenotypes, each associated with a single extra chromosome (each showing trisomy for a different chromosome) o Observation – extra chromosome; changes phenotype o Mendel – phenotypes determined by genes o Hypothesis – genes must be located on chromosomes 2 Course Code Year 3) Sex chromosomes – chromosomes represented differently in the two sexes o Different chromosomes set in males and females o How does this provide proof for chromosomal theory of inheritances? o See different phenotype in males vs. females (morphological behavioural; at the cellular level have a different chromosomal set o The difference between males and females must lie in the chromosome 4) Elinor Carothers – meiosis in grasshoppers, independent assortment of chromosomes o Homologous pair of autosomes that look slightly different o Equal likely that the longer autosome segregated with X or Y o Pair of homologous chromosomes were similar along most of their length, but one partner had a little extra at the end o Still partnered together and segregated in meiosis o Followed 2 sets of chromosomes that have different characteristics o Watched X and Y chromosomes o Watched autosomal chromosomes too 5) Patterns of chromosome movements in meiosis parallel Mendel’s patterns of inheritance (Walter Sutton) o Follow chromosome in meiosis o Show how they are consistent with pattern of behaviour o 2 divisions occur  Homologs segregate  Sister chromatids segregate o Segregation of pairs – consistent of segregation of alleles Cell Division Mitosis  Mitosis – replication of identical cells; one cell produces two cells that are genetically identical to one another and to the original mother cell  One diploid cell (2n)  two diploid cells (2 x 2n)  All cells of the body reproduce and multiply by mitosis 3 Course Code Year  Non-disjunction – chromosomes fail to separate  Homologues – two chromosomes carrying the same gene sequence o Paternal and maternal copies of the same chromosome are homologues  Centromere – point of attachment of sister chromatids and attachment of microtubules  Replicated chromosome – after DNA replication, a chromosome contains two sister chromatids  Chromosome – DNA and proteins Meiosis  Meiosis – germ line cells undergo meiosis in order to produce haploid gametes  Take diploid (2n) and produce collection of haploid cells (n)  Can be divided into two stages – two distinct cell divisions  Meiosis-specific events help to explain many observations in genetics  Meiosis I – first division; increase DNA molecules (2n4n) o Interphase I to Prophase I – replicated chromosomes o Prophase I – potentially tetraploid cell that can go on to produce 4 haploid cells o Metaphase I – homologous chromosome pair o Anaphase I – reduction division (2n  n)  Homologues separate and therefore alleles separate  Meiotic spindle attach to homologous chromosome o Telophase – divide; one homologous chromosome in each cell o Mendel’s First Law: Principle of Equal Segregation  Meiosis II – second division o Each cell ends up with one replicated sister chromatid o End of meiosis 2 we have 4 haploid cells o Anaphase II – equational division  Now replicated, paired sister chromatids separate  Metaphase I – assembly of spindle is completed; each chromosome pair (bivalent) aligns across the equatorial plane of the spindle o homologous chromosome pair o Paired centromeres of homologues assemble on spindle 4 Course Code Year  Metaphase-Ana
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