Cell Cycle and Chromosomes

• The pairing of homologous chromosomes during the zygotene of prophase I and formation of tetrads are characteristic of meiosis; not mitosis. 
• Both mitosis and meiotic divisions exhibit the disappearance of the nuclear membrane and spindle formation.

Mitosis and meiosis are the two types of cell division which occur in a living cell. The meiosis is characterized by the pairing of homologous chromosomes, crossing over and separation of homologous chromosomes. Because the homologous chromosomes separate, the chromosome number is reduced to one-half during meiosis. In mitosis, the homologous chromosomes do not pair or exchange segments. There is no separation of homologous chromosomes consequently the chromosome number is not reduced. The homologous chromosomes remain separate or independent throughout mitosis.

Meiosis is a reduction division because the end result of this special type of cell division is four haploid cells. The first meiotic division occurs which results in two cells, each with 46 chromosomes. Then, the second round of meiotic division occurs and each of these two cells divides resulting in four haploid cells. 

Meiosis II is similar to Mitosis as sister chromatids of the haploid set of chromosomes split during Anaphase II of Meiosis II. 

 Mitosis occurs in all somatic cells. It is responsible for the growth of that organism and no exchange of genetic material takes place between the two chromosomes. In meiosis the number of chromosomes in the parent cell is reduced by half and produces four gamete cells, hence also called reductional division. In meiosis crossing over of chromosomes takes place during prophase 1 that results in an exchange of genetic material between the two chromosomes. This crossover is called synapsis and is responsible for genetic variation in the offspring. Thus the correct answer is option B.

The complete knowledge of cell division, that is, mitosis and meiosis make us identify that synapsis of homologous chromosomes takes place during prophase I of meiosis where the exchange of segments takes place between the homologous chromosome which is the main cause of variation. The spindle fibers disappear completely in the telophase of mitosis as the nuclear membrane is reformed to separate the two daughter cell. Nucleoli reappear at telophase I of meiosis.

• In mitosis, the number of chromosomes in the parent and progeny cells is the same. It is also known as equational division. 
• There is no crossing over taking place in mitotic cell division which leads to the formation of identical daughter cells.
  

• DNA duplication os required so that the genetic material in the cell is doubled and can be equally distributed between the 2 daughter cells. 
• On mitosis the DNA is duplicated during the S phase of the interphase after which the cell enters the cell division and the genetic material is distributed between the daughter cells equally. 
• In meiosis the cells undergoes meiosis 1 and meiosis 2 where the genetic material is already duplicated in the S phase of the interphase before entering the  cell division after the first meiosis the genetic material is halved of the mother cell. After this the cell enters the meiosis 2 where the stages are almost as same as the mitosis where the genetic material is distributed between the cells during the anaphase. 
• Therefore the answer option 'Meiosis I and mitosis' is correct. 

In a meiotic cell division, the different types of haploid cells produced will be 2$^n$, where n is the haploid chromosome number. In case, the diploid chromosome number is 12, the haploid chromosome number n will be 6. Thus different types of cells produced will be equal to 2$^6$ or 64 types.

Behaviour of homologous chromosomes is different in meiosis and mitosis. In meiosis the homologous chromosomes pair, exchange segments and then separate from each other. Exchange of segments between homologous chromosomes is called crossing over. In mitosis, homologous chromosomes do not pair and remain independent of each other.

if the number of bivalents is 8 in metaphase 1, then the number of chromosomes after meiosis -1 is 8 as the diploid stage of the cell persists.

In meiosis , synapsis (Pairing of homologous chromosome), Crossing over (exchange of chromosomal segment between nos sister chromatids) occurs which does not occur in mitosis.

The germ cells in gonads multiply by both mitotic and meiotic divisions. The germ cells proliferate mitotically before undergoing reduction division or meiosis. For example, in human males, the spermatogonia present on the inside wall of seminiferous tubules multiply by mitotic division and increase in numbers before undergoing meiosis.

• During mitosis, chromosome number remains the same and cells increase in number. Mitosis is the cell division in meristematic tissue. 
• Daughter cells produced after meiosis are haploid. Meiosis is the cell division that occurs in gametangia to produce gametes.

• Meiosis is the type of cell division in which homologous chromosomes separate after an exchange of fragments. As a result of meiosis, four haploid cells are produced which are all genetically different from each other. 
• This is due to the process of crossing over or exchange of segments between homologous chromosomes.