Brent Cornell

    
Synapsis

During prophase I of meiosis, homologous chromosomes become connected in a process known as synapsis

• The connected homologues are known as a bivalent (bi = two chromosomes) or a tetrad (tetra = four chromatids)
• The chromosomes are connected by a protein-RNA complex called the synaptonemal complex
  

While autosomes always undergo synapsis during meiosis, sex chromosomes often remain unpaired

Pairing of Homologous Chromosomes via Synapsis

Chiasmata

While in synapsis, non-sister chromatids may break and recombine with their homologous partner (crossing over)

These non-sister chromatids remain physically connected at these points of exchange – regions called chiasmata

Chiasmata (singular = chiasma) hold the homologous chromosomes together as a bivalent until anaphase I

• Chiasmata formation between non-sister chromatids can result in the exchange of alleles

Electron Micrograph of Chiasmata

    
Chiasmata are X-shaped points of attachment between two non-sister chromatids of a homologous pair

Chiasmata form as a result of crossing over and hence non-sister chromatids should show an exchange of genetic material

• The exchange of genetic material is most easily shown if homologous chromosomes are differentially colour-coded

Chiasmata