Chapter 4

  • How can an extra chromosome cause a wide range of phenotypic effects?
    • Chromosomes transmit genetic info
    • The type and amount of genetic material is important for normal development
  • Diploid vs haploid
    • most body cells are diploid (2n)
  • Fertilization is the union of haploid gametes to produce diploid zygotes
    • Meiosis generates haploid gametes
    • gametes fuse during fertilization
    • zygotes are diploid
  • Metaphase chromosomes can be classified by centromere position
    • metacentric: centromere is in the middle
    • acrocentric: centromere is near one end
    • sister chromatids: identical copies of replicated chromosome
    • homologs: same set of genes, but different alleles
    • nonhomologs: unrelated sets of genes
  • Karyotype: micrograph of strained chromosomes arranged in homologous pairs
    • homogolous chromosomes: same size, shape, and banding
    • autosomes: all chromosomes except x and y
    • sex chromosomes: unpaired x and y
  • Sex determination
    • heterogametic sex: gender with 2 different gametes (XY or ZW)
    • Homogametic sex: gender with 2 of the same gametes (XX or ZZ)
    • some species depend on environment (ex. temp)
    • SRY: primary determinant of maleness
  • Cell cycle: repeating pattern of cell growth and division
    • 3 parts of interphase: G1, S, G2
    • centrosome: microtubule organizing venter near the nuclear envelope
    • centrioles: core of centrosome, not found in plant cells
    • G1: cell growth, gap before chromosome duplication
    • S: chromosome replication, creation of sister chromatids
    • G2: Growth, synthesis of proteins for mitosis
  • Prophase: chromosomes condense and become visible, centrosomes move apart toward opposite poles, nucleoli disappears
  • Prometaphase: spindle forms and sister chromatids attach to microtubules from opposite centrosomes, nuclear envelope breaks down, microtubules invade nucleus and connect to kinetochores in centromere of each chromatid
  • Metaphase: chromosome align at cell equator with sister chromatids facing opposite poles, forces on chromosomes from each pole are at equilibrium
  • Anaphase: centromeres divide simultaneously, kinetochore microtubules shorten and pull separated sister chromatids to opposite poles
  • Telophase: chromosomes de-condense and are enclosed in two nuclei, nuclear envelope forms around each group of chromatids, nucleoli re-form, spindle fibers disperse
  • Types of microtubules: astral, kinetochore, and polar
  • Cytokinesis: parent cells split into 2 daughter cells, cytoplasm divides
  • Cytokinesis in animals: contractile ring helps form cleavage furrow
  • Cytokinesis in plants: cell plate forms near equator
  • Mitosis checkpoint 1: is cell sufficient size? proper signals received? then duplicate chromosomes and centrosomes
  • Mitosis checkpoint 2: have chromosomes been completely duplicated? then enter mitosis
  • Mitosis checkpoint 3: have all chromosomes arrived and aligned at metaphase plate? then initiate anaphase
  • Somatic cells: most cells in organism, in G0 or actively going through mitosis
  • Germ cells: precursors to gametes, set aside during embryogenesis, become incorporated into reproductive organs, undergo meiosis
  • Meiosis I: homologs pair, cross over, and then segregate
    • Prophase I: homologs pair and are help together by synaptonemal complex, crossing over occurs
    • Leptonene: first stage of prophase I, chromosomes thicken and become visible, centrosomes move toward opposite poles
    • Zygotene: second stage of prophase I, homologous chromosomes enter synapsis
    • Pachytene:thirdstageofprophaseI,synapsiscomplete,crossingoveroccursPachytene: third stage of prophase I, synapsis complete, crossing over occurs
    • Diplotene: fourth stage of prophase I, synaptonemal complex dissolves, tetrad of 4 chromatids visible, crossover points appear as chiasmata holding non-sister chromatids together
    • Diakinesis: fifth stage of prophase I, chromatids thicken and shorten, nuclear membrane breaks down, spindle forms
    • Metaphase I: tetrads line up along metaphase plate, chromosomes attach to fibers from opposite poles, sister chromatids attach to fibers from the same pole
    • Anaphase I: centromere does not divide, chiasmata dissolve, homologous chromosomes move to opposite poles
    • Telophase I: nuclear envelope re-forms, resultant cells have half the number of chromosomes
    • Interkinesis: similar to interphase, but without chromosomal duplication
  • Meiosis II: sister chromatids separate to opposite poles
    • Prophase II: chromosomes condense, centrioles move toward poles, nuclear envelope breaks down
    • Metaphase II: chromosomes align at metaphase plate, sister chromatids attach to spindle fibers from opposite poles
    • Anaphase II: centromeres divide, sister chromatids move to opposite poles
    • Telophase II: chromosomes begin to uncoil, nuclear envelopes and nucleoli re-form
    • Cytokinesis
    • cytoplasm divides, 4 new haploid cells formed
  • Crossing over during meiosis I forms a tetrad
  • Nondisjunction: mistakes in chromosome segregation during meiosis I or II
    • can result in inviable gametes or embryos, or abnormal chromosome numbers in viable individuals
  • Independent assortment and crossing over create genetic diversity
  • Gametogenesis: formation of gametes
    • involves meiosis and events before and after meiosis
    • different animals have variations
    • humans
    • oogenesis produces 1 ovum from each primary oocyte
    • spermatogenesis: 4 sperm from each primary spermatocyte
  • germ line: specialized diploid cells set aside during embryogenesis
  • Oogenesis
    • Oogonia: diploid germ cells in ovaries of female embryos
    • divide by mitosis and enter meiosis I to become primary oocytes
    • primary oocytes arrest in diplotene stage until after birth
    • At puberty, 1 primary oocyte per month resumes meiosis at ovulation
    • produces secondary oocyte and 1st polar body
    • secondary oocyte arrests in metaphase of meiosis II
    • At fertilization, meiosis II is completes and produces mature ovum and second polar body
    • long meiotic arrest may contribute to chromosome segregation errors
  • Spermatogenesis
    • spermatogonia: diploid germ cells found only in testis, divide by mitosis to form primary spermatocytes
    • after puberty, maturation begins to form sperm
    • primary spermatocytes undergo symmetrical division at meiosis I to produce 2 secondary spermatocytes
    • secondary spermatocytes under go symmetrical division at meiosis II to produce 2 spermatids
    • spermatids mature to become sperm
    • equal numbers of X and Y sperm are produced
    • four haploid sperm produced by symmetrical meiosis of each spermatocyte
    • mitosis and meiosis occur throughout adult life
  • Drosophila genetics
    • gene symbol identified by abnormal phenotype
    • wild-tupe allele superscript +
    • Recessive mutant lowercase
    • w+ or w
    • Dominant mutant uppercase
    • Bar+ or Bar
    • white gene located on x chromosome
    • crisscross inheritance occurs with x-linked recessive traits
    • daughters inherit father’s phenotype and sons inherit mother’s phenotype
  • x-linked recessive
    • mutation never passes from father to son
    • daughters of affected males are carriers, 1/2 sons of carriers will inherit the trait
  • x-linked dominant
    • trait seen in every generation
    • affected male will produce affected daughters and sons
  • y-linked: only in males
  • Dosage compensation
    • females have 2 chromosomes, one inactivated and condensed into Barr body
    • inactivated x chromosome random
  • sex-influenced traits: appear in both sexes, but hormonal differences may cause difference
  • sex-limited traits: affect a structure or process found in only 1 sex