Skill: Drawing diagrams to show chiasmata formed by crossing over

Drawing tips

• Use two coloured pens/pencils to show chromosomes/chromatids of maternal or paternal origin
  • One chromosome of each colour makes a homologous pair
  • Blue and red are conventionally used for this purpose, but any colour choices that show good contrast are acceptable
  • Draw each homologous chromatid as a long line

Stage 1: Synapsis

• All 4 chromatids of a pair of homologous chromosomes align closely together
• Draw this as 4 lines in close proximity, 2 red and 2 blue
  • Remember to include the centromeres

Stage 1 : Synapsis. A bivalent (tetrad) forms from two homologous chromosomes. There are 4 chromatids aligned against each other

Stage 2: Cuts occur in the DNA of non-sister chromatids

• During?coiling and shortening of DNA?in prophase 1, the DNA is stressed/placed under tension
• This causes a?cut in the DNA?of one of the chromatids, catalysed by?endonuclease?enzymes
  • In fact,?many cuts occur simultaneously?within the same bivalent
• One such cut is shown below
• The adjacent non-sister chromatid also breaks?at the same point?as it has the same base sequence at the point of breakage
  • And is cut by the same endonuclease enzyme

Stage 2: Cuts occur in the DNA of non-sister chromatids

Stage 2: Cuts occur in the DNA of non-sister chromatids

Stage 3: Formation of Chiasmata

• There are?loose, cut ends of DNA?within the bivalent with short sections of exposed, unpaired bases
• These bases?re-form hydrogen bonds?to complementary bases quickly, however,
• They can base-pair to cut ends from a?different chromatid
• This can occur with a non-sister chromatid because the non-sister chromatid will have a very similar,?almost identical sequence of bases
• This is how crossing-over leads to?swapping of alleles?between non-sister chromatids
• When the chromosomes condense and shorten again, the chiasmata continue to hold non-sister chromatids together
• This causes the overall chromosome shape to feature?X-shapes?at the chiasmata, viewable under an?electron microscope

Stage 3: Appearance of the recombinant bivalent, with chiasma showing as an X-shape

Electron micrograph of a bivalent in prophase 1, showing multiple chiasmata