10.1 Meiosis


10.1.1  Describe the behaviour of the chromosomes in the phases of meiosis

Interphase:  Cell growth and DNA replication (duplication of DNA creates sister chromatid chromosomes)

Meiosis I

  • Prophase I:  DNA supercoils and chromosomes condense, nuclear membrane dissolves, homologous pairs form bivalents, crossing over occurs
  • Metaphase I:  Spindle fibres from centrioles (at poles) attach to centromeres of bivalent, bivalents line up along the equator of the cell
  • Anaphase I:  Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
  • Telophase I:  Chromosomes decondense, nuclear membranes may reform, cell divides (cytokinesis) forming two haploid daughter cells

Interkinesis:  An optional rest period between meiosis I and meiosis II, no DNA replication occurs in this stage

Meiosis II

  • Prophase II:  Chromosomes condense, nuclear membrane dissolves (if reformed), centrioles move to opposite poles (perpendicular to previous poles)
  • Metaphase II: Spindle fibres from centrioles attach to centromeres of chromosomes, chromosomes line up along the equator of the cell
  • Anaphase II:  Spindle fibres contract and split the chromosome into sister chromatids, chromatids (now called chromosomes) move to opposite poles
  • Telophase II:  Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) resulting in four haploid daughter cells


  • Meiosis is the division of a cell to form four haploid gametes, all of which may be genetically distinct if recombination occurs in prophase I

Overview of Meiosis

10.1.2  Outline the formation of chiasmata in the process of crossing over

  • Crossing over involves the exchange of segments of DNA between homologous chromosomes during Prophase I of meiosis
  • The process of crossing over occurs as follows:
    • Homologous chromosomes become connected in a process called synapsis, forming a bivalent (or tetrad)
    • Non-sister chromatids break and recombine with their homologous partner, effectively exchanging genetic material (crossing over)
    • The non-sister chromatids remain connected in an X-shaped structure and the positions of attachment are called chiasmata
  • Chiasma hold homologous chromosomes together as a bivalent until anaphase I
  • As a result of crossing over, chromatids may consist of a combination of DNA derived from both homologues - these are called recombinants

Crossing Over in Prophase I

10.1.3  Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I

  • During anaphase I, homologous chromosomes separate, such that each resultant daughter cell (and subsequent gametes) contains a chromosome of either maternal or paternal origin
  • The orientation of these homologues in metaphase I is random, such that there is an equal probability of the daughter cell having either the maternal or paternal chromosome
  • As humans have a haploid number of 23 chromosomes, this means that there is 223 potential gamete combinations (over 8 million combinations)
  • Crossing over in prophase I results in entirely new chromosome combinations, as recombination through gene exchange produces wholly original chromosomes containing both maternal and paternal DNA, resulting in near infinite genetic variability
  • Other sources of genetic variation include random fertilisations, DNA mutations, chromosome mutations and non-disjunction

10.1.4  State Mendel's law of independent assortment

Gregor Mendel was a 19th century Moravian monk who demonstrated that the inheritence of traits (i.e. genes) followed particular laws:

  • Law of Segregation:  Each hereditary characteristic is controlled by two alleles, which segregate and pass into different reproductive cells (gametes)
  • Law of Independent Assortment:  The separation of alleles for one gene will occur independently of the separation of alleles for another gene 
    • According to the law of independent assortment, different allele combinations should always be equally possible
    • However this law only holds for genes that are on different chromosomes - the law of independent assortment does not apply to linked genes

10.1.5  Explain the relationship between Mendel's law of independent assortment and meiosis

  • The law of independent assortment relates to the random orientation of homologous chromosomes in metaphase I of meiosis
  • Because the orientation of a homologous pair is random, and does not affect the orientation of any other homologous pair, any one of a pair of alleles on a chromosome has an equal chance of being paired with, or separated from, any one of a pair of alleles on another chromosome
  • This means the inheritance of two different traits will occur independently of each other (provided the genes aren't linked)

Linked versus Unlinked Genes