Meiosis


Meiosis is the process by which sex cells (gametes) are made in the reproductive organs

It involves the division of a germline cell (diploid) into four genetically distinct haploid daughter cells

Meiosis consists of two cell divisions:

  • The first division is a reduction division of the diploid nucleus to form haploid nuclei
  • The second division separates sister chromatids (this division is necessary because meiosis is preceded by interphase, wherein DNA is replicated)


Homologous Chromosomes

Homologous chromosomes are chromosomes that share: 

  • The same structural features (e.g. same size, same banding pattern, same centromere position)
  • The same genes at the same loci positions (while genes are the same, alleles may be different)


Overview of Meiosis

The process of meiosis involves two divisions, both of which follow the same basic stages as mitosis (prophase, metaphase, anaphase and telophase)

Meiosis is preceded by interphase, which includes the replication of DNA (S phase) to create chromosomes with genetically identical sister chromatids


Meiosis I

Homologous chromosomes must first pair up in order to be sorted into separate haploid daughter cells

In prophase I, homologous chromosomes undergo a process called synapsis, whereby homologous chromosomes pair up to form a bivalent (or tetrad)

  • The homologous chromosomes are held together at points called chiasma (singular: chiasmata)
  • Crossing over of genetic material between non-sister chromatids can occur at these points, resulting in new gene combinations (recombination)


The remainder of meiosis I involves separating the homologous chromosomes into separate daughter cells

  • In metaphase I, the homologous pairs line up along the equator of the cell
  • In anaphase I, the homologous chromosomes split apart and move to opposite poles
  • In telophase I, the cell splits into two haploid daughter cells as cytokinesis happens concurrently


Meiosis II

In meiosis II, the sister chromatids are divided into separate cells 

  • In prophase II, spindle fibres reform and reconnect to the chromosomes
  • In metaphase II, the chromosomes line up along the equator of the cell
  • In anaphase II, the sister chromatids split apart and move to opposite poles
  • In telophase II, the cell splits in two as cytokinesis happens concurrently


Because sister chromatids may no longer be genetically identical as a result of potential recombination, the process of meiosis results in the formation of four genetically distinct haploid daughter cells


Meiosis in Detail


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


Independent Assortment

  • During metaphase I, when homologous chromosomes line up at the equator, the paired chromosomes can randomly arrange themselves in one of two orientations (paternal left / maternal right  OR  maternal left / paternal right)
  • When the chromosomes separate in anaphase I, the final gametes will differ depending on whether they got the maternal or paternal chromosome
  • Independent assortment of chromosomes creates 2n different gamete combinations (n = haploid number of chromosomes)