Cell : Meiosis and Gametogenesis

Sexual reproduction starts in the formation of gametes.  The process that leads to the formation of gametes is known as meiosis.  These gametes join in the process of fertilization and a new individual is formed.  What is the importance of meiosis?  Imagine! If the gametes had the diploid chromosome number, this would double during fertilization.  The fertilized egg (zygote) would then be tetraploid, 4n (2n + 2n = 4n).  The chromosome number will keep on doubling during the next generations and the cells would be nothing but chromosomes!  In meiosis, the diploid chromosome number is reduced so that each gamete contains the haploid number.  When a sperms and an egg combine, the diploid chromosome number is restored (n + n = 2n).  The zygote then undergoes mitosis and develops into an adult individual.

Stages of Meiosis

Meiosis requires two nuclear divisions: meiosis I and meiosis II.

Meiosis I reduces the chromosome number into haploid, thus, it is also known as reduction division. 

Meiosis I : Reduction of Chromosome Number 

1. Prophase I.  Just like in mitosis, double-stranded chromosomes and spindle fibers become visible.  The nuclear membrane and the nucleolus disintegrate.  In prophase I, something happens that does not occur in mitosis.  This is the pairing of homologous chromosomes, a process called synapsis.  Homologous chromosomes are chromosomes that look alike.  They have the same length and centromere position.  Synapsis produces a tetrad (four chromatids).  Crossing over, or exchange of genetic material between the adjacent chromatids, may occur.  After crossing over, the sister chromatids of each chromosomes are no longer identical.
2. Metaphase I.  The pairs of homologous chromosomes (tetrads), with their centromeres attached to spindle fibers, independently align around the center of the cell.  The independent alignment of the chromosomes makes possible the different combinations of the chromosomes in daughter cells.
3. Anaphase I.  The pair of homologous chromosomes in a tetrad separates from each other and moves toward the opposite poles.  Notice that each chromosome is still double-stranded.
4. Telophase I.  The nuclear membranes and the nucleoli reform.  Cytokinesis may or may not occur.  Telophase I is absent in some species and metaphase II follows.
5. Interkinesis I.  After meiosis I, a period of growth, known as interkinesis, occurs.  Interkinesis is similar to interphase, except that the chromosomes do not duplicate.  This is because the chromosomes are still in their duplicate form.

Meiosis II deals with the separation of the two chromatids in a chromosome.  At the end of meiosis, four haploid cells are formed.  The different phases of each division are similar to those of mitosis. 

1. Prophase II.  The double-stranded chromosomes and spindle fibers reappear in each new cell.
2. Metaphase II.  The double-stranded chromosomes, with their centromeres attached to spindle fibers, align at the center of the cell.
3. Anaphase II.  The centromeres divide, and the chromatids are pulled by spindle fibers toward the opposite poles.  The two chromatids give rise to two daughter-chromosomes.
4. Telophase II.  The spindle fibers disappear and the nuclear membranes and nucleoli reform.  Cytokinesis completes the division.  At the end of telophase II, there are four daughter cells, each with a haploid number of chromosomes.

Overview of meiosis

Gametogenesis

Meiosis is part of gametogenesis, the formation of gametes.  Following meiosis, haploid cells undergo changes in their structure so as to form specialized reproductive cells called gametes.  Spermatogenesis (sperm-formation) occurs in the testes of males while oogenesis (egg-formation) occurs in the ovaries of females.

1.      Spermatogenesis.  In the testes of males are the primary spermatocytes with diploid number of chromosomes.  Primary spermatocytes undergo the first meiotic division and produce two haploid secondary spermatocytes.  Secondary spermatocytes divide and form four haploid spermatids.  Each spermatid differentiates into a sperm.

2.      Oogenesis.  The ovary in females contains a number of diploid primary oocytes.  A primary oocyte divides meiotically, forming two haploid daughter cells.  The division is unequal because one of these cells, called the secondary oocyte, receives a greater mass of cytoplasm.  The other cell,which is much smaller, is called a  polar body.  The polar body may disintegrate or divide again.  The secondary oocyte undergoes meiosis II and produces two cells- one ootid and another polar body.  The ootid differentiates and forms the ovum (egg) while the three polar bodies disintegrate.  After oogenesis, only one egg cell is produced.

Tags: gametogenesis, meiosis, meiosis I, meiosis II, meiosis stages, mitosis, overview of meiosis, Reduction of Chromosome Number, stages of meiosis, types of cell reproduction, types of reproduction

Category: Biology