Drawing Chiasmata
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