General Biology/Cells/Sexual Reproduction

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General Biology | Getting Started | Cells | Genetics | Classification | Evolution | Tissues & Systems | Additional Material


Sexual[edit]

  • Exclusively eukaryotes
  • Fusion of two haploid genomes
    • Fertilization (= syngamy)
    • Forms new individuals in multicellular organisms as result of fusion of egg and sperm
  • Plants
  • Animals
  • Meiosis yields haploid genomes at some point in life cycle

Sexual life cycle[edit]

Typical animal life cycle

  • Meiosis occurs in germ line cells in gonads producing haploid gametes
  • All other cells are somatic cells
  • Alternation of generations
  • Sexual intercourse

Meiosis[edit]

  • Gives rise to genetic variation
  • Reduction division: 2n to n
  • Preceded by one round of DNA (chromosome) replication
  • Two rounds of nuclear (& usually cell) division
    • Meiosis I
  • Synapsis of homologs
  • Segregation of homologs
  • Reduction division, 2n to n
    • Meiosis II
  • No chromosome replication
  • Segregation of sister chromatids
  • Formation of 4 haploid (n) cells
  • Two nuclear divisions, usually 2 cell divisions, only one round of replication
  • Meiosis I
    • Prophase: synapsis and crossing over
    • Metaphase
    • Anaphase: chromosome segregation
    • Telophase
  • Meiosis II (mitosis-like)
    • Prophase
    • Metaphase
    • Anaphase: sister chromosome segregation
    • Telophase

Prophase I: synapsis[edit]

  • Complete alignment of replicated homologs
  • Synapsis occurs throughout the entire length of a pair of homologs
  • Key to chromosome segregation
  • Synapsis, crossing over
  • Subdivided into 5 continuous stages

Crossing over[edit]

  • Reciprocal, physical exchange between nonsister chromatids
  • Type of recombination; mixes maternal and paternal genes
  • Visual evidence: chiasmata

Microtubules and anaphase I[edit]

  • During prophase microtubules attach to kinetochores on one side of centromere
  • The metaphase checkpoint insures proper attachment
  • A phosphorylation event initiates motor activity and anaphase

Meiosis II[edit]

  • Cytologically similar to mitosis
    • No preceding DNA replication
    • Chromatids segregate and move to opposite poles as chromosomes
    • 4 haploid cells produced
  • In animals, these cells differentiate into gametes
  • In plants and many other organisms, these cells divide by mitosis, followed some time later by gamete formation

Evolution of sex[edit]

  • Asexual reproduction: all offspring genetically identical to parent
  • Sex: recombination destroys advantageous combinations
  • So why sex?
    • Many hypotheses
    • Effect repair of genetic damage?
  • Much pachytene repair as well as gene conversion
  • Some protists form diploid cells in response to stress
    • Recombination breaks up combinations of genes favoring parasites, thus reducing parasitism?

Consequences of sex[edit]

  • Recombination: generates genetic diversity
    • Crossing over
    • Independent assortment
  • Random fertilization
    • Qualities of gamete usually do NOT reflect qualities of genes enclosed in gamete

Reference[edit]

This text is based on notes very generously donated by Paul Doerder, Ph.D., of the Cleveland State University.