Mitosis & Meiosis- Chapter 3 The somatic cell cycle- division & growth Interphase 3 phases: G1 (first gap phase) genes become active RNA & proteins synthesized cell expands length of G1 varies with cell type S (synthesis) can occupy as much as 35% of cell cycle DNA synthesis occurs chromosomes replicate into sister chromatids remain attached at centromere G2 (second gap phase) metabolic processes are finished DNA repair takes place here cells can also go into G0 a resting phase Mitosis produces cells that are identical to parent cell only about 5-10% of cell cycle 4 phases (based on observable chromosome changes): prophase chromosomes condense, become visible by end of prophase sister chromatids visible centrioles separate cytoskeleton organizes includes mitotic spindle kinetochore attaches to spindle nuclear membrane disappears metaphase condensed chromosomes pulled to metaphase plate anaphase centromeres divide sister chromatids become full chromosomes migrate toward separate poles telophase migrating chromosomes approach asters new nuclear envelope forms spindle disappears Cytokinesis cytoplasm divides cell physically separates into 2 daughter cells begins with formation of cell furrow (animal cells) or cell plate (plant cells) Eukaryotic Chromosomes can be studied in Mitosis Cytogenetics chromosome morphology location of centromere (see Figure 3.7) metacentric submetacentric acrocentric telocentric chromosome arms p-arm (short) q-arm (long) telomere end of chromosomes size presence of satellites number of chromosomes haploid (N) diploid (2N) karyotype characteristic number, size & shape in each species abnormalities detected by karyotype analyses amniocentesis leukemia chemical stains reveal patterns in chromosomes heterochromatin condense early in prophase heavy stain can vary with cell type constitutive heterochromatin always heterochromatin facultative heterochromatin euchromatic in some cells euchromatin condense later in prophase light stain actively expressed regions of chromosome Meiosis physical basis of Mendel's Rules of Inheritance only diploid cells go through meiosis diploid cells contain 2 copies of each chomosome known as homologues 2N to N via two cell division Meiosis I (Reductional Division) Prophase I (stages are determined by chromosome behavior) leptotene chromosomes become visible chromatids not distinguishable zygotene homologues make contact & pair begins at telomere anchored on nuclear envelope synapsis begins zipper-like connection forms synaptonemal complex pachytene synapsis is complete chromatids become visible 2 homologues called bivalent 4 sister chromatids called tetrad synaptonemal complex disintegrates homologues held together by chiasmata points of recombination betw. homologues diplotene begins when synaptonemal complex no longer visible chromosomes continue to condense all 4 chromatids clearly seen diakinesis chromatids at most condensed state chiasma appear to migrate nuclear envelope disappears spindle fibers reach kinetochores Metaphase I bivalents migrate to metaphase plate Anaphase I chiasmata dissociate centromeres do NOT divide chromosomes segregate- not chromatids Telophase I nuclear envelopes reform producing two haploid nuclei Cytokinesis sometimes occurs Interphase just G1 Meiosis II (Equational Division)- more closely resembles mitosis Prophase II chromosomes condense & shorten sister chromatids visible Metaphase II chromosomes migrate to metaphase plate Anaphase II centromeres divide sister chromatids separate Telophase II chromosomes approach spindle pole chromosomes elongate nuclear envelope reforms Chromosome Theory of Inheritance first evidence came from Morgan's reciprocal crosses proposed gene for white eye color was on X chromosome proposed that Y chromosome had few or no genes because only males have Y genes on Y have holandric inheritance males are hemizygous for genes on X sex-linked patterns of inheritance seen in other species opposite in birds, butterflies & moths males are homogametic sex sex-linked genes revealed in human pedigree analysis criss cross inheritance (Figure 3.24) ex: hemophilia, color blindness final proof of Chromsome Theory of Inheritance came from studies of nondisjunction by Calvin Bridges, a student of Morgan seen in Drosophila (eye color phenotype) found unusual complement of sex chromosomes due to segregation failure in anaphase I also revealed 2 methods of sex determination in animals Two methods of sex determination ex: Drosophila: ratio of X:A determines sex XX:AA = female X:AA = male presence of Y confers fertility on males indicates genes on X and A are important for sex determination in Drosophila ex: mammals sex determined by presence of Y in humans, XO is female (Turner's syndrome) XY is male XX is female XXY is male (Klinefelter's syndrome) so Y chromosome must contain genes for sex (extra sex chromosomes are caused by nondisjunction) Dosage compensation in mammals, extra sex chromes tolerated but not extra autosomes except for chromosome 21 (Down's syndrome) the smallest human chromosome has very few genes all individs have two copies of other chromes males have only 1 X so there's a method to balance sex chrome genes between males & females only one X chromosome is activated all other inactivated X inactivation proposed by Mary Lyon (called Lyonization) early stage of development inactivation is random effects can be seen in female tortoiseshell cats proved by Murray Barr (discovered Barr bodies) Barr bodies are totally heterochromatic