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GROWTH AND DEVELOPMENT IN PLANTS AND ANIMALS
Biology Form 3
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Meiosis I

(a) Interphase I

The cell undergoes activities that prepare it for division. These activities include;

  • Replication of chromosomes.
  • Synthesis of organelles.
  • Buildup of energy.

 

(b) Prophase I

The nucleolus disappears.

Centrioles (if present) become arranged at opposite sides of the nucleus.

Chromosomes condense by shortening and thickening hence become more visible.

The homologous chromosomes lie side by side in the process of synapsis forming pairs called bivalents.

As prophase proceeds the chromosomes may become coiled around each other and later their chromatids may remain in contact at points called chiasmata (singular-chiasma). Homologous chromosomes associate and crossing over may occur at the chiasma. Important genetic exchanges usually take place at the chiasma.

 

(c) Metaphase I

The nuclear membrane disappears hence chromosomes are free in the cytoplasm.

Spindle fibres are fully formed.

Homologous chromosomes align at the equator of the spindle.

The homologous chromosomes still associated as bivalents move to the equator of the spindles. They attach to the spindles by their centromeres such that the two homologous chromosomes orientate towards opposite poles.

 

(d) Anaphase I

Homologous chromosomes separate and move towards opposite poles of the cell.

This is as a result of shortening of the spindle fibres hence pulling the chromosomes.

 

(e) Telophase I

The cell constricts across the middle.

The cell divides across the middle.

The set of chromosomes has arrived at each pole.

 

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