Ch 13 Meiosis and Sexual Life Cycles

Sexual Reproduction

• two parents each contribute half of their offspring’s genetic makeup

  • offspring receives half of their chromosomes from mom, other half from dad

  • via meiosis to make gametes

Genetic Variation

• offspring is never genetically identical to parent

• more generic variation, more likely a species will be able to evolve

Asexual Reproduction

• one parent produces offspring that is a clone

  • eggs are result of fusion between a haploid egg and haploid polar body

• offspring is genetically identical to parent

• no genetic variation

Haploid (n)

cell has only one copy of each chromosome

• gametes

Diploid (2n)

cell has two copies of each chromosomes

• somatic cells

Meiosis

• cellular division that reduces chromosome number from diploid to haploid

• creates gametes (eggs and sperm), each with 23 chromosomes

• diploid parent cell produces four haploid daughter cells

  • daughter cells are genetically unique from parent cells and from each other

• used to produce gametes

Meiosis I

• first stage of Meiosis

• separates homologous chromosomes

• produces 2 cells that are haploid

phases: interphase (G1, S, G2), prophase I, metaphase I, anaphase I, telophase I / cytokinesis I

Meiosis II

• second stage of Meiosis

• separates sister chromatids

• produces 4 cells that are also haploid (gametes)

phases:

• no interphase after telophase/cytokinesis i

  • DNA is not replicated again

• prophase II, metaphase II, anaphase II, telophase II/cytokinesis II

Interphase

• identical to mitosis

• gap 1 phase (“first gap”)

  • growth & normal cell functions

• S phase (“synthesis”)

  • DNA is replicated

• Gap 2 phase (“second gap”)

  • growth continues

  • cell prepares to divide

Homologous Chromosomes in Meiosis

• in prophase I of Meiosis I (after S phase), homologous chromosomes pair up to form tetrads

  • called synapsis

• tetrads bond with each other

  • non-sister chromatids from homologous chromosomes zipper together

Crossing Over (homologous chromosomes in meiosis)

non-sister chromatids exchange genetic material with each other

• physical location of crossing over is called chiasma (plural, chiasmata)

  • each pair of non-sister chromatids have at least on chiasma

• produces recombinant chromosomes

  • chromosomes that carry genes from both parents

Meiosis I: Prophase I

• nuclear membrane begins to disintegrate

• chromosomes condense

  • each chromosome is comprised of two identical sister chromatids

• homologous chromosomes synapse into tetrads

  • crossing over occurs between non-sister chromatids of the tetrads at chiasma

• centrosomes move to poles of the cell

• mitotic spindle begins to form from centrosomes

Meiosis I: Metaphase I

• homologous chromosomes align on the metaphase plate

  • one chromosome of each pair facing a pole

• sister chromatids of one homologous are attached to the microtubule from one pole

• sister chromatids of other homologous are attached to microtubule from the opposite pole

  • attached at kinetochores

Meiosis I: Anaphase I

• homologous chromosomes separate and move towards opposite poles of the cell

• sister chromatics remain intact and attached to each other

Meiosis I: Telophase I/Cytokinesis I

• at beginning of this phase, half of the cell has a haploid set of duplicated chromosomes

  • each chromosome is still composed of sister chromatids

• nuclear membrane forms around haploid sets of chromosomes

• cytokinesis occurs to separate cell into two daughter cells

  • daughter cells are haploid but still have chromosomes composed of sister chromatids

  • are not genetically identical to parent cell or to each other

Meiosis II: Prophase II

• there are now two haploid daughter cells

• there is no interphase between Meiosis I and II

  • DNA not replicated again

• centrosomes once again move to poles of the cell

  • mitotic spindle begins to form from centrosomes again

Meiosis II: Metaphase II

• centrosomes are at opposite poles

• microtubules are attached to kinetochores of sister chromatids

• sister chromatids align on the metaphase plate

Meiosis II: Anaphase II

• cleavage of cohesion proteins holding sister chromatids together

• sister chromatids separate from each other

  • are now separate “daughter” chromosomes

• microtubules shorten

  • pull daughter chromosomes to opposite poles of the cell

Meiosis II: Telophase II/Cytokinesis II

• nuclei form around all four sets of daughter chromosomes

• chromosomes are become less condense

• microtubules depolymerize

• cytokinesis separates cells into four haploid daughter cells

  • daughter cells are genetically different from each other and from original parent cell

Mitosis

• diploid parent cell produces two diploid daughter cells

  • daughter cells are genetically identical to the parent cell

• used for growth, repair, regeneration (and sometimes asexual reproduction)

Meiosis and Genetic Variation

• siblings don’t look alike

• species evolve

• meiosis and sexual reproduction provide 3 opportunities for genetic variation: crossing over, Independent Assortment of chromosomes, random fertilization

• mutation provides a fourth type of genetic variation

Crossing Over during Prophase I

• form of genetic variation

• produces recombinant chromosomes

  • genetic material from both parents on same chromosome

Law of Independent Assortment of Chromosomes

• random orientation of homologous chromosomes along the metaphase plate during metaphase I of meiosis

• there are 2ⁿ possible combinations of daughter cells after meiosis I where n = haploid number

  • humans have 2²³(8.4 mil) possible daughter cells

• during metaphase I, each tetrad’s orientation is independent of all other when they line up on the metaphase plate, meaning variations of genes(alleles) for a specific trait segregate independently

Random Fertilization

• form of genetic variation

• random sperm fertilizes random egg

• 2²³ × 2²³ = 70 trillion possible combinations

Evolutionary Significance of Genetic Variation

• populations evolve through differential reproductive success of individuals

• individuals best suited (genetically) to their environment leave more offspring than individuals less suited to their environment

• natural selection accumulates genetic variation most adapted to the environment

• as environment changes, the population will survive if at least some members have genes that allow success

• mutations are the original source of different alleles

  • these new alleles are mixed and matched during meiosis

  • this shuffling of alleles provides potential for some combinations that work better in the newly changed environment

• sexual reproduction is favored in an unstable environment

Abnormal Chromosome Numbers

• during anaphase I of meiosis I, microtubules from centrosomes should pull homologous chromosomes apart and towards the poles of the cell

• during anaphase ii of meiosis ii, microtubules from centrosomes should pull sister chromatids apart and towards opposite poles of the cell

• then, during telophase ii / cytokinesis ii, those separated sister chromatids (daughter chromosomes) will be completely separated into the resulting 4 daughter cells

• nondisjunction

Nondisjuction

an error in which homologous chromosomes and/or sister chromatids are not properly separated

• gametes contain an extra chromosome or are lacking a chromosome

• fertilization with these gametes results in an individual with extra or missing chromosomes

  • this is called aneuploidy

  • having only one chromosome of a homologous pair is called monoploid

  • having three (or more) chromosomes of a homologous pair is called polyploidy

Genetic Disorders caused by Nondisjunction

• autosome disorders

  • trisomy 13

  • trisomy 18

  • trisomy 21

• sex chromosome disorder

  • trisomies, monosomy

Trisomy 13

Patau Syndrome

• 3 copies of chromosome 13

• serious brain/organ defects

• 50% survival rate at 7-10 days

• 13% survival rate at age 10

Trisomy 18

Edwards Syndrome

• 3 copies of chromosome 18

• serious organ/joint defects

• 50% survival rate at 7-10 days

• 10% survival rate at age 10

Trisomy 21

Down Syndrome

• 3 copies of chromosome 21

• can also be caused by a translocation error between 14 & 21

• developmental delay

  • can be mild or severe

• heart defects

• life expectancy is approximately 60 years

  • most survivable of all trisomy conditions

Sex Chromosome Disorder

• involve X and/or Y chromosomes

• trisomies:

  • XXX

  • XXY

  • XYY

• monosomy:

  • X0

    • only known survivable monosomy

X0 Monosomy

Turner Syndrome

• individuals are female but only have one X chromosome

• conditions vary among females, most common is sterility

• only known survivable human monosomy

Alterations in Chromosome Structure

• errors in Meiosis

  • breakage of chromosomes

• one main type with three possible outcomes:

  • deletion error: part of the chromosome is broken off

    • duplication error: the deleted part of the chromosome attaches to its sister chromatid or non sister chromatid of homologous chromosome

    • inversion error: deleted part reattaches to same chromosome but inverted

    • translocation error: deleted part reattaches to non homologous chromosome