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Unformatted text preview: FRIDAY APRIL 11, 2008 The Chromosome Theory of Inheritance Outline of Chromosome Theory of Inheritance Observations and experiments that placed the hereditary material in the nucleus on the chromosomes Mitosis ensures that every cell in an organism carries same set of chromosomes. Meiosis distributes one member of each chromosome pair to gamete cells. Gametogenesis, the process by which germ cells differentiate into gametes Meiosis generates diversity in genotypes MONDAY: Validation of the chromosome theory of inheritance Evidence that Genes Reside in the Nucleus 1667 Anton van Leeuwenhoek
Microscopist Semen contains spermatozoa (sperm animals). Hypothesized that sperm enter egg to achieve fertilization 1854-1874 confirmation of fertilization through union of eggs and sperm Recorded frog and sea urchin fertilization using microscopy and time-lapse drawings and micrographs Evidence that Genes Reside in Chromosomes 1880s innovations in microscopy and staining techniques identified thread-like structures Provided a means to follow movement of chromosomes during cell division Mitosis two daughter cells contained same number of chromosomes as parent cell (somatic cells) Meiosis daughter cells contained half the number of chromosomes as the parents (sperm and eggs) Sutton Bovari theory Walter Sutton Studied great lubber grasshopper Parent cells contained 22 chromosomes plus an X and a Y chromosome. Daughter cells contained 11 chromosomes and X or Y in equal numbers. Bovari studied cancer cells. At Fertilization, Haploid Gametes Produce Diploid Zygotes. Gamete contains one-half the number of chromosomes as the zygote.
Haploid cells that carry only a single chromosome set Diploid cells that carry two matching chromosome sets n the number of chromosomes in a haploid cell 2n the number of chromosomes in a diploid cell diploid vs haploid cell in Drosophila melanogaster
Emphasis! Each chromosome consists of 1 DNA double helix, running from one chromosome end to the other end. Human chromosome 1 likely represents the world's longest single covalent molecule (300 million bases) in a single strand of the helix.
Fig. 4.2 The number and shape of chromosomes vary from species to species.
Drosophila melanogaster Drosophila obscura Drosophila virilus Peas Macaroni wheat Giant sequoia trees Goldfish Dogs Humans n
4 5 6 7 14 11 47 39 23 2n
8 10 12 14 28 22 94 78 46 Anatomy of a chromosome Metaphase chromosomes are classified by the position of the centromere
Fig. 4.3 Homologous chromosomes match in size, shape, and banding patterns. Homologous chromosomes (homologs) contain the same set of genes. Genes may carry different alleles. Nonhomologous chromosomes carry completely unrelated sets of genes. Karyotypes can be produced by cutting micrograph images of stained chromosomes and arranging them in matched pairs Human male karyotype
Fig 4.4 Mitosis ensures that every cell in an organism carries the same chromosomes. Cell cycle repeating pattern of cell growth and division Alternates between interphase and mitosis Interphase period of cell cycle between divisions/cells grow and replicate chromosomes G1 gap phase birth of cell to onset of chromosome replication/cell growth S synthesis phase duplication of DNA by SEMICONSERVATIVE REPLICATION G2 gap phase end of chromosome replication to onset of mitosis The cell cycle Fig. 4.7a Chromosome replication during S phase of cell cycle
Synthesis of chromosomes Note the formation of sister chromatids Fig. 4.7 b Mitosis see Fig 4.8 Metaphase middle stage Chromosomes move towards imaginary equator called metaphase plate Fig. 4.8 c Mitosis - continued Anaphase Separation of sister chromatids allows each chromatid to be pulled towards spindle pole connected to by kinetochore microtubule.
Fig. 4.8 d Meiosis produces haploid germ cells. Somatic cells divide mitotically and make up vast majority of organism's tissues Germ cells specialized role in the production of gametes
Arise during embryonic development in animals and floral development in plants Undergo meiosis to produce haploid gametes Gametes unite with gamete from opposite sex to produce diploid offspring. Meiosis
Chromosomes replicate once. Nuclei divide twice. Fig. 4.12 Meiosis I Metaphase and Anaphase Meiosis Prophase II and Metaphase II Meiosis Anaphase II and Telophase II Meiosis - Cytokenesis Fig. 4.13 Review tables 4-3 and 4-4 DANCE The chromosome theory correlates Mendel's laws with chromosome behavior during meiosis. Chromosome Behavior Each cell contains two copies of each chromosome Chromosome complements appear unchanged during transmission from parent to offspring. Homologous chromosomes pair and then separate to different gametes. Maternal and paternal copies of chromosome pairs separate without regard to the assortment of other homologous chromosome pairs. At fertilization an egg's set of chromosomes unite with randomly encountered sperm's chromosomes. In all cells derived from a fertilized egg, one half of chromosomes are of maternal origin, and half are paternal. Behavior of genes Each cell contains two copies of each gene. Genes appear unchanged during transmission from parent to offspring. Alternative alleles segregate to different gametes. Alternative alleles of unrelated genes assort independently. Alleles obtained from one parent unite at random with those from another parent. In all cells derived from a fertilized gamete, one half of genes are of maternal origin, and half are paternal. APPLICATIONS TO HUMANS Gametogenesis involved mitosis and meiosis. Oogenesis egg formation in humans
Diploid germ cells called oogonia multiply by mitosis to produce primary oocytes. Primary oocytes undergo meiosis I to produce one secondary oocyte and one small polar body (which arrests development). Secondary oocyte undergoes meiosis II to produce one ovum and one small polar body. Polar bodies disintegrate leaving one large functional gamete Oogenesis in humans Fig 4.18 OOGENESIS ARRESTS AT MI IN EMBRYOS MEIOSIS ARREST MEIOSIS I RESUMES, COMPLETED FOR CELL ONLY AT OVULATION, ARRESTS AT M II M II RESUMES, COMPLETED UPON FERTILZATION Gametogenesis Spermatogenesis in humans
Symmetrical meiotic divisions produce four functional sperm. Begins in male testis in germ cells called spermatogonia Mitosis produces diploid primary spermatocytes. Meiosis I produces two secondary spermatocytes per cell. Meiosis II produces four equivalent spermatids. Spematids mature into functional sperm. Spermatogenesis in humans Fig. 4.19 MAALE GERM LINE DNA CONTINOUSLY REPLICATED AFTER PUBERTY MOST MUTATIONS OCCUR DURING REPLICATION GERM LINE IN OLDER MALES HAS GREATER NUMBER OF REPLICATIONS, TRANSMITS MORE MUTATIONS Meiosis contributes to genetic diversity in two ways. Independent assortment of nonhomologous chromosomes creates different combinations of alleles among chromosomes. Crossing-over between homologous chromosomes creates different combinations of alleles within each chromosome. Fig. 4.17 Meiosis Prophase I Feature Figure 4.13 Meiosis Prophase I continued Meiosis Prophase I continued Crossing over during prophase produces recombined chromosomes. Fig. 4.14 a-c Fig. 4.14 d, e How crossing over produces recombined gametes Fig. 4.15 ...
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- Winter '08