Mitosis and Meiosis
These are basically types of cell division. Cell division occurs by mitosis and meiosis. Mitosis is a general cell division found in every living organism. Meiosis is a type of cell division occurring only in the gametes (sperm and ovary). These actually refer to the division of nucleus.
Mitosis is a general cell division. How it occurs is that there are four phases of cell division. The first phase being the inter-phase. In this the chromatin forms the chromosomes. The structure of chromosomes is like this, there are two chromatids attached to the centromere and on the opposite side is another pair of chromatids.
The second phase is the metaphase in which the nuclear envelop collapses and forms the spindle. In the anaphase these spindles then pull on each pair of chromatids which are separated from the other pair. In the telophase, the spindle again forms the nuclear envelop around the chromosomes and forms two new nucleus just before the cell division. The cytoplasm of the cell divides and the plasma membrane comes between them forming two new daughter cells. In a plant cell the cell wall would grow in between the nuclei forming two new daughter cells.
Meiosis is the cell division of the gametes. In the case of meiosis in humans we should know that the cells of a human body has 46 chromosomes while the gamete cells of the human body has 23 chromosomes. In a baby human he or she would have 46 chromosomes in each cell, 23 from father and 23 from mother. This number is called the haploid number.
During cell division there are two processes first being meiosis I while the second being the meiosis II. In the first process, chromosomes are present each with two chromatid pairs. The chromosomes join together and two chromosomes form a pair called the homologous pair. The same thing happens here as in the case of mitosis. The spindle forms and pulls the pairs but in this case instead of pulling the chromatid, it pulls the chromosomes breaking the pair and forming two new nucleus. These then divide into two daughter cells which completes meiosis I and it again tries to divide. This time the spindle pulls on the chromatids and break it off. Then 4 new daughter cells are formed with 23 chromosomes in each new gamete cell. This completes the meiosis II and the whole process.
Haploid number and Diploid number
The total number of chromosomes found in an organism’s nucleus in every cell of its body (except for gamete cells) is called its diploid number [2(n)]. In humans the diploid number is 46. The haploid number (n) is half the diploid number of chromosomes found in the nucleus of the gametes (sperm cells and ovum/egg cells). The haploid number of a human gamete is 23 being the half of 46. The reason for this difference in number is because when the ovum fertilizes by the sperm cell and forms the zygote, it combines the nucleus of the egg and the sperm. The egg and the sperm have 23 chromosomes in each of its nucleus. When they combine these chromosomes add and create a diploid number of chromosomes. This results in a normal baby. However, if the chromosome number in any of the gamete were even a single chromosome more or less the baby born would have been abnormal.