A multicellular organism is formed from a single cell. Cell
is the basic unit of life. Either meiosis or mitosis occurs during cell
division. When a cell divides mitotically, it increases its number only. When a
cell divides meiotically, the chromosome number becomes half. The process of
mitosis leads to the formation of two daughter cells and the process of meiosis
leads to the formation of four daughter cells. Meiosis as well as mitosis
consists of four phases namely prophase, metaphase, anaphase, and telophase. A
nucleus is located in the middle of a eukaryotic cell. Chromatin is a
thread-like structure found in nuclei. As chromosomes develop, chromatin
condenses.
Chromosomes are thread-like structures that are found in the nucleus of
eukaryotic cells. They are made up of DNA and proteins and contain genes, which
are the basic units of inheritance that transmit traits from one generation to
the next. During cell division, chromosomes can be seen under a microscope
during the metaphase stage. Chromosomes are referred to as "colored bodies"
because they retain stain or color after staining.
Humans have 23 pairs of chromosomes. 22 pairs are autosomes and 1 pair is sex
chromosome. Sex chromosomes are responsible for the formation of a specific
gender i.e. male and female. Sex chromosomes include X and Y chromosome.
Gametes are reproductive cells that are formed through a process called
gametogenesis. In females, the ovum (also called an egg or secondary oocyte) is
the gamete produced through oogenesis, while in males, sperm is the gamete
produced through spermatogenesis. Both sperm and ova are haploid, meaning they
have half number of chromosomes (23). Gametes are produced by the meiotic
division in a diploid cells. Diploid cells have 23 pairs of chromosomes. In
females, ova are produced by oogonia, which is a diploid cell found in the
ovary, while in males, sperms are produced by spermatogonia, which are diploid
cells found in the testis.
When a sperm fertilizes an ovum, the resulting cell is called a zygote, which is
a single-celled, diploid structure with 46 chromosomes. The zygote will then
develop into an embryo and eventually into a complete oraganism.
An individual carrying XX sex chromosome will form a female and individual
having an XY sex chromosome will form a male individual. Sex of an organism,
i.e. male or female depends upon a male. The male have X and Y sex chromosome
and female have X and X sex chromosome. A female gamete will always contribute X
chromosome (22 autosomes and 1sex chromosome). Male gamete can contribute either
X or Y ( 22+X or 22+Y), but not both. If a male contributes an X chromosome and
female will contribute X, then the new individual develops into a female having
44 autosomes and 2 sex chromosomes i.e. XX (44+XX). If a male contributes a Y
chromosome and a female contributes an X chromosome, then the new individual
will develop into a male, having 44 autosomes and 2 sex chromosomes i.e.
(44+XY). Thus a particular gender is determined by 'Y' chromosome.
Meiosis is a type of cell division that occurs during sexual reproduction and is
responsible for producing gametes. It is also known as "reductional division"
because the number of chromosomes is halved during the process. Meiosis occurs
in two stages: meiosis I and meiosis II. During meiosis I, the prophase is
divided into five stages: leptotene, zygotene, pachytene, diplotene and
diakinesis. During the pachytene stage, a process called "crossing over" occurs,
in which the genes of one parent are combined with those of the other. This
process contributes to the variation between individuals by rearranging genes.
99.9% of DNA in all human beings is same and only 0.1% is different. This
difference is due to variation. Sexual reproduction and meiosis are thus
responsible for the variation in human beings. We are all similar, but not
identical.
About the Author: Amina Firdous has completed her Master's degree in Botany from
Baba Ghulam Shah Badshah University Rajouri. She belongs to Poonch district of
Jammu and Kashmir.