Prophase one of the busiest phases of meiosis. Many things happen in this phase.
In meiosis there is the concept of homologous chromosomes. There are 23 chromosomes that came from mother and 23 chromosome that came from Father. And similar chromosomes from mother and father are like a couple. They come close to each other and they exchange their parts that happens in prophase 1. This chromosome pairs are called Homologous chromosomes.
Note that there is no demarcation of homologous chromosomes in mitosis.
In the first stage of prophase is like mitosis. Before prophase starts, genetic material, the DNA, is in the form of thread like structure called chromatin. As the cell division starts, it condenses to form chromosomes.
So, chromosomes are further supercoiled form of chromatin and their formation demarcates the first stage that is leptotene of prophase one.
How you can remember that this is the first phase. Well! it is amazingly simple. Just think of it that it is a beginning of love story of a beautiful couple, not just a single couple but 23 couples or 23 homologous chromosomes.
So, when a love story began to form in couple, they love to put their heads into lap of each other and that is the first phase of love Story. And because they are putting heads in lap that is called Leptotene. Actually, lepto means thin and we say it lepto because initially the chromatin is thin, and they condense to form Chromosome. In leptotene, condensation of chromatin occur to form chromosomes.
Then, as the love bond in between them becomes stronger, they become close to each other and they pair up with each other, and as they pair up with each other one of them lie on top of the other. When they do so a special bond is formed between them and that is not just a bond of love but that is the bond of Proteins.
A protein complex is formed between paternal chromosome and maternal chromosomes of each homologous Chromosomes pair. This complex that is formed it is called synaptonemal complex.
Nemal means thread. This thread-like proteinaceous complex is synaptonemal complex and this process of pairing it is called Synapses.
So, this process of pairing of paternal chromosome with maternal chromosome this is called synapses and it is demarcated by presence of synaptonemal complex and because they are pairing with each other we call it Zygotene phase or zygotene stage.
Zygo means to pair up with each other.
Now what happens when a couple lie on top of one another? Of course, they will do something (pun intended 😉) and while doing something some material is exchanged in between them. i.e. the genetic material.
So, there is some exchange of genetic material and this process of exchange of genetic material is called crossing over. So, in this phase in which crossing over occurs it is called Pachytene
The couples are packed up with each other to do something.
Of course, after a couple has done something, they will have to separate from each other. They cannot all the time remain attached with each other, lying on the top of each other.
So, they must detach from each other and when they detach from each other their complex is lost. Previously they had become a single entity. Now they are separated, and they are two different things, and we call it Diplotene. Di means two, so from a single entity, paired up in zygotene and packed up in Pachytene, now they are two different entities.
But remember they are still attached at point of exchange of genetic material, and they form a cross like structure. This cross like structure is called chiasma. Chiasma means cross. So, chiasma Formation (chiasmata formation) occurs in Diplotene.
There are usually more than one chiasma and that is called chiasmata formation. The homologous chromosomes are also called bivalents. In each bivalent, there are four chromatids, so if we talk with reference to number of chromatids, there are four chromatids, so we can also call it a tetrad.
So, homologous chromosomes are bivalent as well as tetrads. Bivalent as far as chromosomes are concerned and Tetrads as far as chromatids are Concerned.
After the Diplotene stage, they move apart from each other, and this is called diakinesis.
Diakinesis mean to move through. Kinesis, as you might know, means movement or kinetic energy.
Finally, in diakinesis what happens that nuclear membrane is also lost, so that in subsequent phases that is in metaphase and anaphase they could easily separated from, they could be easily separated from each other.