Meiosis
Meiosis is a type of cell division that produces sex cells, each with half the number of
chromosomes as the parent cell. In humans, meiosis occurs in the sex organs. The
sex cells that are made during meiosis are the sperm and the egg, which are also
known as gametes. Egg and sperm cells are called haploids with a chromosome
number of n. In fertilization, a haploid egg nucleus and haploid sperm nucleus fuse and
form a diploid (2n) cell called a zygote.
However, there is a significant difference between meiosis and other types of cell
division like mitosis or binary fission. In meiosis, the parent cell divides and produces
four gametes where genetic material has been exchanged between homologous
chromosomes (crossing over); whereas, in mitosis, the parent cell produces identical
daughter cells, which can undergo division on their own. Let's take a look at the different
stages of meiosis.
Different Stages in the Process of Meiosis
There are two stages of meiosis because there are two cell divisions, namely, meiosis I
and meiosis II. The parent cell or the dividing cell undergoes a preparatory phase,
known as interphase, before entering the two stages of meiosis. In the interphase, the
parent cell synthesizes more DNA (deoxyribonucleic acid) and proteins, increasing the
overall size and mass of the cell. As a part of the preparatory phase, the dividing cell
duplicates or doubles its chromosomes. With these major changes, the parent cell
enters the first stage of meiosis. Following is a brief discussion about the two stages
and their phases.
Meiosis I: Meiosis I is divided into four phases, which are as follows:
Step I
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
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The chromosomes condense
Each chromosome is made up of two identical chromatids, known as
sister chromatids.
The homologous chromosomes interchange equivalent sections of
chromatids which is a process known as crossing over.
The chromosomes undergo thickening and move away from the nuclear
envelope.
The nuclear membrane dissolves.


Homologous chromosomes pair up
The chromosome pairs separate and migrate


Cytokinesis (division of cytoplasm) takes place.
Two daughter cells with haploid chromosome number are formed.


Step II
Step III
Meiosis II: Meiosis II comprises the following four stages:
Step IV

The chromosomes are aligned at the center of the cell.

Chromosomes separate.

The process of cytokinesis is used to create four daughter cells or
gametes.
Step V
Step VI
The resulting four gametes are haploid, meaning that they contain half the number of
chromosomes. This is the reason as to why meiosis cell division is also referred to as
reduction division. During fertilization, two gametes, one from the mother and another
from the father, fuse, thus resulting in doubling of chromosome number. The fusion of
gametes leads to the production of a zygote that has the same chromosome number of
the parents. Variation occurs in the resulting zygote due to the process of meiosis and
fertilization of gametes. Zygote after attaining maturity, is capable of dividing into
daughter cells.
Questions: Circle the correct answer
1. What type of cells does meiosis produce?
Body cells
Sex cells
2. Where does meiosis occur in the human body?
Bloodstream
Sex Organs
Muscles
3. What are the sex cells called that are made during meiosis?
Somatic cells
Gametes
4. Egg and sperm cells are called haploid/diploid because they have a
chromosome number of n.
5. When a haploid egg fuses together with a haploid sperm during
fertilization, it results in this?
Embryo
Fetus
Zygote
6. How many gametes are made during meiosis?
Two
Four
Six
7. What is the process called where genetic material is exchanged
between homologous chromosomes?
Diffusion
Respiration
Crossing Over
8. In mitosis the daughter cells created are different/identical.
9. How many stages are there in meiosis?
Why?
One cell division
or
One/Two
Two cell divisions
10.In which stage during Meiosis I does crossing over occur?
Prophase I
Metaphase I
Prophase II
11.Meiosis leads to increased genetic variation/limited genetic variation.