General Biology/Cells/Sexual Reproduction

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Sexual

• 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

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

• 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

• 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

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

Microtubules and anaphase I

• 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

• 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

• 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

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

Reference

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