Course 9.2Lesson 1 of 3
Sexual reproduction needs cells with half the usual number of chromosomes. Meiosis makes them. Its first division begins with a long and busy prophase I, in which the two chromosomes of each pair come together and swap pieces. This lesson follows the five stages of prophase I.
Try the code and the simulations beside the lesson.
A human body cell has 46 chromosomes. A sperm and an egg join in fertilisation. If each of them had 46, the zygote would have 92, and the next generation 184. The number would double in every generation.
This does not happen, because gametes are made by a special division that halves the number of chromosomes. It is called meiosis. It takes place in special diploid cells, the meiocytes, which are the gamete mother cells of the testis and ovary, and the spore mother cells of plants. Meiosis makes the haploid phase of a life cycle, and fertilisation brings back the diploid phase.
A body cell has its chromosomes in pairs. In each pair, one chromosome came from the mother and one from the father. They are homologous chromosomes, or homologues. They have the same length, the same centromere position and the same genes in the same order. They may carry different forms of a gene. Sister chromatids are not homologues. They are identical copies of one chromosome.
Prophase I is much longer and more complex than the prophase of mitosis. It is divided into five stages, named by what the chromosomes look like.
| Stage | What happens |
|---|---|
| Leptotene | The chromosomes become gradually visible as thin threads. Compaction goes on through the stage. |
| Zygotene | The homologues start to pair. This pairing is synapsis. A synaptonemal complex forms between them. |
| Pachytene | The pair is a bivalent, and its four chromatids are clear. Recombination nodules appear, and crossing over takes place. |
| Diplotene | The synaptonemal complex dissolves. The homologues start to separate, but stay joined at X-shaped chiasmata. |
| Diakinesis | The chiasmata move towards the ends (terminalisation). The chromosomes are fully condensed, the spindle is built, the nucleolus and the nuclear envelope disappear. |
A way to remember the order: Lazy Zebras Pace Down Dusty roads. It gives leptotene, zygotene, pachytene, diplotene, diakinesis.
The first two stages are short. Pachytene lasts longer. In the oocytes of some vertebrates, diplotene can last for months or years.
A pair of synapsed homologues is called a bivalent, because it is two chromosomes. It is also called a tetrad, because it holds four chromatids. NCERT uses both names for the same thing. Many A-Level and IB books use bivalent for the pair of chromosomes and tetrad for the four chromatids. Count the bivalents to count the pairs. Count the tetrads to count the sets of four chromatids. Both give the same number.
In pachytene, a chromatid of one homologue and a non-sister chromatid of the other break at the same place, swap the pieces and join again. This is crossing over. An enzyme, recombinase, helps in the process. Crossing over makes chromatids that carry genes from both parents, which is called recombination. The place where it happened is later seen as a chiasma.
NCERT and NEET name all five stages of prophase I. Many A-Level, IB and AP books describe prophase I as synapsis, crossing over and chiasmata, and name the stages less often. The events are the same.
You should see the two chromosomes come together at zygotene, pair fully at pachytene with the crossing over, and part again at diplotene except at the chiasma.
The program lists the five stages of prophase I with their main event.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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