Meiosis and Sexual Life cycles

• Meiosis


  A type of cell division that reduces the number of chromosomes in the daughter cells by half, resulting in genetically distinct daughter cells
• Sexual life cycle


  The generation-to-generation sequence of stages in the reproductive history of an organism that involves the alternation of meiosis and fertilization
• Living organisms are distinguished by their ability to reproduce their own kind
• Genetics


  The scientific study of heredity and variation
• Heredity


  The transmission of traits from one generation to the next
• Variation


  The differences in appearance that offspring show from parents and siblings
• In a literal sense, children do not inherit particular physical traits from their parents. It is genes that are actually inherited.
• Genes


  The units of heredity, made up of segments of DNA
• Gametes


  Reproductive cells (sperm and eggs)
• Locus


  The specific location of a gene on a chromosome
• Asexual reproduction


  A single individual passes genes to its offspring without the fusion of gametes
• Clone


  A group of genetically identical individuals from the same parent
• Sexual reproduction


  Two parents give rise to offspring that have unique combinations of genes inherited from the two parents
• Somatic cells


  Any cell other than a gamete
• Karyotype


  An ordered display of the pairs of chromosomes from a cell
• Homologous chromosomes


  Chromosomes in a pair that are the same length and shape and carry genes controlling the same inherited characters
• Sex chromosomes


  Chromosomes that determine the sex of the individual (X and Y)
• Autosomes


  The remaining 22 pairs of chromosomes in humans
• Diploid cell


  A cell with two sets of chromosomes
• Sister chromatids


  The two identical copies of a replicated chromosome
• Haploid


  A cell with a single set of chromosomes
• Fertilization


  The union of gametes (the sperm and the egg)
• Zygote


  The fertilized egg with one set of chromosomes from each parent
• Meiosis


  The cell division process that reduces the number of chromosome sets from diploid to haploid
• Meiosis I


  1. The reductional division that separates homologous chromosomes

  2. Meiosis II

  3. The equational division that separates sister chromatids
• Meiosis


  • It is preceded by chromosome replication
  • It results in four haploid daughter cells, rather than two diploid daughter cells as in mitosis
• Stages of meiosis I

  1. Prophase I

  2. Metaphase I

  3. Anaphase I

  4. Telophase I and cytokinesis
• Prophase I


  • Chromosomes condense
  • Homologous chromosomes pair up (synapsis)
  • Crossing over occurs between non-sister chromatids
• Metaphase I


  • Tetrads line up at the metaphase plate, with one chromosome facing each pole
• Anaphase I


  • Pairs of homologous chromosomes separate, with one chromosome moving toward each pole
• Meiosis I is preceded by interphase, when the chromosomes are duplicated to form sister chromatids
• Meiosis II involves the separation of sister chromatids, resulting in four haploid daughter cells
• The alternation of meiosis and fertilization is common to all organisms that reproduce sexually
• Types of sexual life cycles

  • Gametes are the only haploid cells in animals
  • Plants and some algae exhibit an alternation of generations
  • In most fungi and some protists, the only diploid stage is the single-celled zygote
• Depending on the type of life cycle, either haploid or diploid cells can divide by mitosis, but only diploid cells can undergo meiosis
• The halving and doubling of chromosomes in sexual life cycles contributes to genetic variation in offspring
• Prophase II


  1. Spindle apparatus forms

  2. Chromosomes (each still composed of two chromatids) move toward the metaphase plate
• Metaphase II


  1. Sister chromatids are arranged at the metaphase plate

  2. Kinetochores of sister chromatids attach to microtubules extending from opposite poles
• Anaphase II


  1. Sister chromatids separate

  2. Sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles
• Telophase II and Cytokinesis

  1. Chromosomes arrive at opposite poles

  2. Nuclei form, and the chromosomes begin decondensing

  3. Cytokinesis separates the cytoplasm