Anaphase I

Meiosis, also known as reductional division, is the type of cell division in which a single cell divides two times to form the product of four haploid daughter cells. This process is divided into two halves, meiosis 1 and meiosis 2. Each of these contains four steps: first prophase, second metaphase, third anaphase, and fourth and last, telophase.

The third step of the process of meiosis 1 after prophase 1 and metaphase 1 is anaphase 1. This step is characterised by the transfer and movement of the chromosomes towards the poles of the cell through spindle fibres. It forms two separate groups of homologous chromosomes.

Process of anaphase 1 

During the first two stages of meiosis 1, i.e., prophase 1 and metaphase 1, the chromosomes start to condense, adapt a compact structure, and align at the center of the cell at the equatorial plates, in the form of bivalents. During this time, the microtubules from the two opposite poles reach out and join the centromeres of pairs of homologous chromosomes through a kinetochore microtubule. Anaphase 1 begins from this moment.

Anaphase 1 is known for the movement of chromosomes to the opposite poles of the cell. This happens from the breakdown of the microtubule network, which reduces the size of microtubule fibres leading to the chromosomes settling at the two opposite poles. 

There are two stages of anaphase: anaphase A and anaphase B. These stages occur simultaneously in the cell during meiosis in anaphase 1.

Anaphase A

Anaphase A is the first stage of anaphase 1, where the fibres connected to the microtubules reduce and shorten. This results in the movement of kinetochores in two directions, either up or down. This occurs in a motion where the kinetochores face the pole and the chromosomes, facing the other direction, follow the kinetochores. Also, to prevent directional instability, the kinetochores need to keep moving.

Anaphase B

Anaphase B is the second part of anaphase 1, where the spindle network gets elongated. This separates the chromosomes even further. Then, the sections which overlap the microtubules and the ends arising from the two opposite poles slowly move apart. Further, the cell firmly elongates in size leading to the poles moving even far from each other. This step is essential to prevent any incomplete cell division. This is the part that is very heavily regulated. Slowly, a complete set of homologous chromosomes reaches either side of the pole of the cell. This marks the end of anaphase 1 and the beginning of telophase 1.

Difference between anaphase 1 and anaphase 2

Anaphase 1 is the third step of meiosis 1, whereas anaphase 2 is the third step of meiosis 2. During anaphase 1, a single pair of the homologous chromosome, which forms a bivalent, is moved towards the opposite poles of the cell with the help of spindle fibres attached to the centromere through kinetochores. This leads to half the number of chromosomes on each side of the two poles. 

On the other hand, anaphase 2 is characterised by the separation or segregation of the sister chromatids of the chromosome. This occurs due to the division of the centromere. These sister chromatids then move towards extremely opposite poles in the same way as in mitosis. This results in one chromatid at each pole of the cell with an equal number of chromosomes.

Another difference is that in anaphase 1, the spindle fibres are joined to the centromere of the chromosomes, which belong to two different sets. In contrast, in anaphase 2, the spindle fibres are joined to the single chromosome containing two sister chromatids through one centromere.

Further, anaphase 1 can be described as a phase in which the separated homologous chromosomes are not modified in any particular way. On the other hand, anaphase 2 is the phase in which the separated genetic material is modified for the final stage and gets converted into chromosomes.

In anaphase 1, the centromere does not split. Instead, it remains as a whole. This is because the complete chromosome moves to the opposite poles, whereas in anaphase 2, the centromere splits and that too of both of the homologous chromosomes. This is because, this time, instead of a whole chromosome, the sister chromatids move toward the two different sides.

Conclusion

Anaphase 1 is the process occurring in diploid cells. It is the third step of meiosis 1 in which the chromosomes move in opposite directions with their kinetochores leading them with the help of microtubules or spindle fibres. Anaphase 1 is characterised by two phases, anaphase A and anaphase B, which occur simultaneously in the cell.