Meiosis and Sexual Reproduction

Meiosis: The Basis of Sexual Reproduction

Case Study: The Rainbow Connection

  • The Giddings family showcases diverse traits among siblings (hair color, eye color, skin tone).
  • Parents Tess (blue eyes, curly brown hair, olive skin) and Chris have children with remarkably different appearances.
  • The family's diversity even led to initial concerns about a potential baby switch at the hospital, necessitating a DNA test for confirmation.
  • The case study highlights the ability of sexual reproduction to mix inherited characteristics, resulting in a wide variety of offspring.

10.1 How Does Sexual Reproduction Produce Genetic Variability?

  • Asexual Reproduction: Produces genetically identical offspring through mitotic cell division.
    • Examples: Paramecium, Amoeba, Hydra, aspen trees.
  • Sexual Reproduction: Offspring are produced through the union of gametes (sperm and egg), resulting in genetic differences between offspring and parents.
    • The production of gametes requires a specialized form of cell division known as meiotic cell division.
  • Asexual reproduction was the original method of reproducing.
  • Genetic Variability Originates as Mutations in DNA
    • Hereditary information is stored in DNA molecules, which are packaged into chromosomes.
    • A gene is a sequence of nucleotides at a specific location (locus) on a chromosome.
    • Alleles are slightly different nucleotide sequences of a gene.
    • Alleles arise from mutations which are changes in a cell's DNA sequence.
    • Mutations can occur during DNA replication or due to environmental factors (e.g., UV light, chemicals).
    • Mutations in sperm or eggs can be passed down through generations.
  • Sexual Reproduction Generates Genetic Variability Between the Members of a Species
    • Different individuals within a species have different combinations of alleles, leading to variations in traits.
  • Eukaryotic Chromosomes Usually Occur in Pairs Containing Similar Genetic Information
    • Karyotype: The complete set of chromosomes in a single cell.
    • Humans have 23 pairs of chromosomes (46 total) per cell.
    • Homologous Chromosomes (Homologues): Pairs of chromosomes that contain genes controlling the same inherited characteristics.
    • Same genes, but may have different alleles.
    • Diploid: Cells with pairs of homologous chromosomes.
      • One homologue is inherited from the mother (maternal homologue), and the other from the father (paternal homologue).
    • Autosomes: Pairs of chromosomes with nearly identical DNA sequences, found in diploid cells of both sexes.
      • Humans have 22 pairs of autosomes.
    • Sex Chromosomes: Determine sex (e.g., XX in females, XY in males in humans and other mammals).
      • Small portions of X and Y chromosomes are homologous to each other.
    • Haploid: Cells containing only one member of each pair of homologues.
      • Sperm and eggs are haploid.
      • The bread mold Neurospora has haploid cells for most of its life cycle.
    • Haploid Number (n): The number of different types of chromosomes in a species.
      • For humans, n=23n = 23. Diploid cells contain 2n2n chromosomes.
    • Polyploid: Organisms with more than two copies of each homologous chromosome in each cell.
      • Examples: Many plants (tetraploid, hexaploid).
      • Tetraploid: 4n4n, Hexaploid: 6n6n

10.2 How Does Meiotic Cell Division Produce Genetically Variable, Haploid Cells?

  • Sexual reproduction involves two key steps:
    • Meiotic Cell Division: A diploid cell gives rise to haploid daughter cells, each containing a single member of each pair of homologues.
    • Fertilization: Fusion of a sperm and egg restores the diploid number of chromosomes in the offspring.
  • Meiotic cell division consists of:
    • Meiosis: A specialized type of nuclear division where a diploid nucleus divides twice, producing four haploid nuclei.
    • Cytokinesis: Packages the four nuclei into separate cells.
  • In mitotic cell division, there is one round of DNA replication followed by one nuclear division.
  • In meiotic cell division, there are two nuclear divisions, with DNA replicated only before the first division.
  • Meiosis I: Separates the pairs of homologous chromosomes, sending one homologue from each pair into two daughter nuclei (haploid).
  • Meiosis II: Separates the chromatids into independent chromosomes, parcelling one chromosome into each of two daughter nuclei.
  • Meiosis I Separates Homologous Chromosomes into Two Haploid Daughter Nuclei
    • The phases of meiosis are similar to those of mitosis but are distinguished by I or II to indicate the nuclear division in which they occur.
    • Before meiosis I, chromosomes are duplicated during interphase, and sister chromatids are attached at the centromere.
  • During Prophase I, Homologous Chromosomes Pair Up and Exchange DNA
    • Homologous chromosomes line up side by side, and their chromatids exchange segments of DNA.
    • Proteins bind maternal and paternal homologues together for precise alignment.
    • Enzymes cut and graft DNA, exchanging parts of chromatids.
    • Chiasmata: Crosses where chromatids of maternal and paternal chromosomes have exchanged parts.
    • Crossing Over: The mutual exchange of DNA between maternal and paternal chromosomes at chiasmata.
    • Spindle microtubules assemble outside the nucleus during prophase I.
    • The nuclear envelope breaks down, and spindle microtubules capture chromosomes by attaching to kinetochores.
  • During Metaphase I, Paired Homologous Chromosomes Line Up at the Equator of the Cell
    • Interactions between kinetochores and spindle microtubules move paired homologues to the cell's equator.
    • Homologous pairs of duplicated chromosomes line up along the equator.
    • The orientation of each pair is random (independent assortment), contributing to genetic diversity.
  • During Anaphase I, Homologous Chromosomes Separate
    • Sister chromatids of each duplicated homologue remain attached to each other and move to the same pole.
    • Chiasmata untangle, allowing homologues to separate and move to opposite poles.
    • Each pole receives a cluster of chromosomes containing one member of each homologous pair (haploid number).
  • During Telophase I, Two Haploid Clusters of Duplicated Chromosomes Form
    • Spindle microtubules disappear.
    • Cytokinesis occurs commonly.
    • Nuclear envelopes may re-form.
    • Meiosis II follows immediately, with no DNA replication in between.
  • Meiosis II Separates Sister Chromatids into Four Daughter Nuclei
    • Similar to mitosis in a haploid cell.
    • During prophase II, spindle microtubules re-form.
    • During metaphase II, duplicated chromosomes line up at the cell's equator.
    • During anaphase II, sister chromatids separate and move to opposite poles.
    • Telophase II and cytokinesis conclude meiosis II.
    • Nuclear envelopes re-form, chromosomes decondense, and the cytoplasm divides.
    • Each of the two daughter cells produced in meiosis I undergoes meiosis II, resulting in four haploid cells from the original diploid cell.

10.3 How Do Meiosis and Union of Gametes Produce Genetically Variable Offspring?

  • Mutations provide the original source of genetic variability.
  • The shuffling of homologues during meiosis creates novel combinations of chromosomes.
  • Crossing over creates chromosomes with novel combinations of genes.
  • Fusion of gametes adds further genetic variability to the offspring.
  • Shuffling the Homologues Creates Novel Combinations of Chromosomes
    • Random distribution of maternal and paternal homologues to daughter nuclei during meiosis I is a major source of genetic diversity.
    • During metaphase I, each pair of homologues lines up at the cell's equator, with the maternal and paternal chromosomes facing opposite poles.
    • Each homologue faces each pole randomly and is not affected by the orientation of homologues of other chromosome pairs.
  • Crossing Over Creates Chromosomes with Novel Combinations of Genes
    • Homologous chromosomes have different alleles of some genes.
    • Crossing over leads to genetic recombination, which is the formation of chromosomes with combinations of alleles that differ from those of either parent.
  • Fusion of Gametes Adds Further Genetic Variability to the Offspring
    • At fertilization, two gametes fuse to form a diploid offspring.

10.4 When Do Mitotic and Meiotic Cell Division Occur in the Life Cycles of Eukaryotes?

  • The life cycles of almost all eukaryotic organisms involve:
    • Fertilization: Fusion of two haploid cells, bringing together genes from different parental organisms.
    • Meiotic Cell Division: Re-creating haploid cells.
    • Mitotic Cell Division: Growth of multicellular bodies or asexual reproduction.
  • Eukaryotic life cycles are named according to the relative dominance of diploid and haploid stages.
  • In Diploid Life Cycles, the majority of the cycle is spent as diploid cells.
    • Most animals spend virtually their entire life cycle in the diploid state.
    • Diploid adults produce short-lived haploid gametes by meiotic cell division.
    • Sperm and egg fuse to form a diploid zygote, which develops into the adult organism through mitotic cell division and differentiation.
  • In Haploid Life Cycles, the majority of the cycle is spent as haploid cells.
    • Many fungi and single-celled algae spend most of their life cycles as haploid.
    • Asexual reproduction by mitotic cell division produces a population of identical, haploid cells.
    • Under certain conditions, two haploid reproductive cells from genetically different strains fuse to form a diploid zygote, which immediately undergoes meiotic cell division to produce haploid cells again.
    • Mitotic cell division never occurs in diploid cells in organisms with haploid life cycles.
  • In Alternation of Generations Life Cycles, there are both diploid and haploid multicellular stages.
    • The life cycle of plants alternates between multicellular diploid and multicellular haploid stages.
    • Specialized cells of a diploid adult undergo meiotic cell division, producing haploid spores.
    • Spores undergo mitotic cell division and differentiation to produce a multicellular haploid adult.
    • Certain haploid cells differentiate into haploid gametes, which fuse to form a diploid zygote.
    • The zygote grows by mitotic cell division into another multicellular diploid adult stage.
    • In some plants, both haploid and diploid stages are free-living, independent plants, whereas flowering plants have reduced haploid stages represented only by the pollen grain and a small cluster of cells in the ovary of the flower.

10.5 How Do Errors in Meiosis Cause Human Genetic Disorders?

  • Errors in meiosis, called nondisjunction, can affect the number of sex chromosomes or autosomes in a gamete.
  • Most embryos arising from the fusion of gametes with abnormal chromosome numbers spontaneously abort.
  • Some embryos with abnormal chromosome numbers survive to birth or beyond.
  • Some Disorders Are Caused by Abnormal Numbers of Sex Chromosomes
    • Sperm contains either an X or a Y chromosome, and all eggs contain an X chromosome.
    • Nondisjunction of sex chromosomes in males produces sperm with either no sex chromosome (O sperm) or two sex chromosomes (XX, YY, or XY).
    • Nondisjunction of the sex chromosomes in females produces O or XX eggs.
    • The most common abnormalities are XO, XXX, XXY, and XYY.
  • Turner Syndrome (XO)
    • Occurs in about 1 in every 2,500 female babies.
    • Ovaries usually degenerate before birth, and girls do not undergo puberty.
    • Treatment with estrogen can promote secondary sexual characteristics; however, women with Turner syndrome are infertile.
    • Other characteristics: short stature, folds of skin around the neck, and increased risk of cardiovascular disease, kidney defects, and hearing loss.
  • Trisomy X (XXX)
    • Occurs in about 1 in every 1,000 women.
    • Most women have no detectable differences from XX women, except for a tendency to be taller and to have a higher incidence of learning disabilities.
    • Most trisomy X women are fertile and almost always bear XX and XY children.
  • Klinefelter Syndrome (XXY)
    • Occurs in about 1 in every 500 to 1,000 males.
    • Small testes that do not produce as much testosterone as the testes of XY men typically do.
    • At puberty, some have mixed secondary sexual characteristics, such as partial breast development, broadening of the hips, and thin beards.
    • Men may be infertile because of low sperm count but are not impotent.
  • Jacob Syndrome (XYY)
    • Occurs in about 1 male in every 1,000.
    • X chromosomes contain few active genes.
    • Most common effect that XYY males tend to be taller than average.
    • There may also be a slightly increased likelihood of learning disabilities.
  • Some Disorders Are Caused by Abnormal Numbers of Autosomes
    • Nondisjunction of the autosomes produces eggs or sperm that are missing an autosome or that have two copies of an autosome.
    • Embryos with only one copy of any of the autosomes almost always abort early in development.
    • Embryos with three copies of an autosome (trisomy) also usually spontaneously abort.
    • A small fraction of embryos with three copies of chromosomes 13, 18, or 21 survive to birth.
  • Trisomy 21 (Down Syndrome)
    • An extra copy of chromosome 21 occurs in about 1 of every 700 births.
    • Characteristics include weak muscle tone, a small mouth partially open, and distinctively shaped eyes.
    • More serious problems include varying degrees of mental impairment, low resistance to infectious diseases, and heart defects.
    • The frequency of nondisjunction increases with the age of the parents, especially the mother.
    • Can be diagnosed before birth by examining the chromosomes of fetal cells and, with less certainty, by biochemical tests and ultrasound examination of the fetus.