When does meiosis occur and how many times does it happen?

Question

I am a bit confused on the whole meiosis progress.What i want to know is- when does meiosis actually take place i.e is it happening all the time in our bodies constantly or is there a specific time when it happens and how many times is this progress repeated?? If it does happen all the time then what happens to the thousands of gametes that are produced from a result of this??Please help.

Answer 1

If all you are interested in is timing then this is quite easy, but it is different in the two sexes.

There are two phases to meiosis, known as meiosis 1, and 2. During meiosis 1 chromosomes pair and recombination occurs supplying the new chromosomes for the next generation. In meiosis 2, germ cells are produced, each containing a single set of chromosomes (haploid) which are ready to produce the next generation.


In females meiosis 1 occurs during embryonic development. A fixed number of germ cells are produced. At the onset of sexual maturity, meiosis 2 occurs and a mature egg cell is produced - once a month in humans - with the other half of the chromosomes going into another cell called a polar body, is discarded and not used in reproduction. Any egg cells which are not used are lost during menstruation. Once all the reproductive cells have matured and have either been fertilized or not used then she can no longer conceive.

In males, the generation of spermatozoa only occurs after the onset of sexual maturity. They are produced continously, although fertility declines with age. These cells will live in the reproductive tract of males for a limited amount of time, after which they will be broken down by the body's own system destroying old cells. Cells are destroyed after a certain amount of time because of the risk of damage with ageing.

This explanation applies to mammals but throughout the animal kingdom there are many different modes of reproduction some of which are markedly different to that described above.

Answer 2

In biology, meiosis is the process that allows one diploid cell to divide in a special way to generate haploid cells in eukaryotes. The word "meiosis" comes from the Greek meioun, meaning "to make smaller," since it results in a reduction in chromosome number.

Meiosis is essential for sexual reproduction. It therefore occurs in most eukaryotes, including single-celled organisms. A few eukaryotes, notably the Bdelloid rotifers, have lost the ability to carry out meiosis and acquired the ability to reproduce by parthenogenesis. Meiosis does not occur in archaea or prokaryotes, which reproduce by a sexual cell division processes.

During meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA called chromosomes, undergoes DNA replication followed by two rounds of division, resulting in haploid cells called gametes. Each gamete contains one complete set of chromosomes, or half of the genetic content of the original cell. These resultant haploid cells can fuse with other haploid cells of the opposite gender or mating type during fertilization to create a new diploid cell, or zygote. Thus, the division mechanism of meiosis is a reciprocal process to the joining of two genomes that occurs at fertilization. Because the chromosomes of each parent undergo genetic recombination during meiosis, each gamete, and thus each zygote, will have a unique genetic blueprint encoded in its DNA. In other words, meiosis is the process that produces genetic variation.

Biochemically, meiosis uses some of the same mechanisms employed during mitosis to accomplish the redistribution of chromosomes. There are several features unique to meiosis, most importantly the pairing and recombination between homologous chromosomes, which enable them to separate from each other

Answer 3

In females, meiosis occurs in precursor cells known as oogonia that divide twice into oocytes. These stem cells stop at the diplotene stage of meiosis I and lay dormant within a protective shell of somatic cells called the follicle. Follicles begin growth at a steady pace in a process known as folliculogenesis, and a small number enter the menstrual cycle. Menstruated oocytes continue meiosis I and arrest at meiosis II until fertilization. The process of meiosis in females is called oogenesis.

In males, meiosis occurs in precursor cells known as spermatogonia that divide twice to become sperm. These cells continuously divide without arrest in the seminiferous tubules of the testicles. Sperm is produced at a steady pace. The process of meiosis in males is called spermatogenesis.