Sexual reproduction is a fundamental biological process in
both animals and plants. It ensures the continuation of life through the fusion
of special cells known as gametes—sperm and egg in animals, or spores in
plants. When these gametes unite, they form a zygote, which develops
into a new organism. A remarkable aspect of this process is that the chromosome
number remains constant across generations. But how is this stability
achieved?
Over a century ago, August Weismann proposed a
groundbreaking hypothesis: there must be a special type of cell division that reduces
the chromosome number by half before gametes are formed. This idea laid the
foundation for what we now know as meiosis.
Somatic vs. Germline Cells
In multicellular organisms, cells are broadly categorized
into:
- Somatic
cells: These make up the body's tissues and organs.
- Germline
cells: These give rise to gametes.
Both cell types typically carry a diploid (2n) set of
chromosomes—meaning two sets, one from each parent. However, gametes must be haploid
(n), containing only one set, so that upon fertilization, the resulting zygote
is diploid again.
This reduction in chromosome number is achieved through meiosis,
often referred to as reduction division.
What is Meiosis?
Meiosis is a specialized type of cell division
that reduces the chromosome number by half, creating four haploid cells from
one diploid cell. This ensures genetic consistency across generations while
introducing variation.
The Two Phases of Meiosis
Meiosis occurs in two sequential stages:
- Meiosis
I: The reduction division, where homologous chromosomes
separate.
- Meiosis
II: Similar to mitosis, this stage separates the sister
chromatids.
Meiosis I: Reducing the Chromosome
Number
Interphase I
Before meiosis begins, the cell undergoes DNA replication,
creating duplicate copies of each chromosome. Unlike in mitosis, there is no
G₂ phase in this interphase.
Prophase I: A Detailed and Dynamic
Process
This is the longest and most complex stage, further divided
into five substages:
1. Leptotene – The Thread Stage
Chromosomes begin to condense and appear as thin threads.
The nucleus enlarges, and homologous chromosomes—pairs with the same
shape and gene sequence—begin moving closer together.
2. Zygotene – The Pairing Stage
Homologous chromosomes align precisely in a process called synapsis,
forming paired structures.
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| Prophase: 1 |
3. Pachytene – The Thickening Stage
Synapsis completes, and chromosomes become shorter and
thicker. Each pair now consists of four chromatids (two from each
homologue), forming a structure called a tetrad or bivalent.
Crossing Over
At this stage, genetic material is exchanged between
chromatids of homologous chromosomes. This process, called crossing over,
occurs at chiasmata—visible X-shaped structures—and contributes to
genetic variation.
4. Diplotene – The Separation Begins
Homologous chromosomes start to repel each other and begin
to separate, but they remain connected at the chiasmata. This stage can last
for an extended period in some species.
5. Diakinesis – Moving Apart
Chromosomes continue to condense and prepare for alignment.
The nucleolus disappears, and the nuclear envelope remains intact. Each
chromosome is now composed of two chromatids attached at a single centromere.
Metaphase I
The nuclear envelope breaks down, and spindle fibers
form. Homologous chromosome pairs align along the equator of the cell.
Each homologue is attached to spindle fibers from opposite poles.
Anaphase I
Spindle fibers shorten and pull homologous chromosomes
apart toward opposite poles. Each pole receives one chromosome from each
pair, reducing the chromosome number by half.
 |
| Crossing Over |
Telophase I and Cytokinesis
Chromosomes arrive at opposite poles. The cell divides into two
haploid cells, each containing a single set of chromosomes. Though still
duplicated, each chromosome consists of two sister chromatids.
Meiosis II: Separating the Sister
Chromatids
Meiosis II resembles a typical mitotic division and
occurs in each of the two haploid cells formed during meiosis I.
Interphase II
This stage is brief and does not involve DNA replication.
Prophase II
Nuclear structures prepare for the second division, though
the complex events of Prophase I do not occur here.
Metaphase II
Chromosomes align at the equator in a single row. Spindle
fibers attach to the centromeres of each chromatid.
Anaphase II
Centromeres divide, and the sister chromatids are
pulled apart to opposite poles. Each separated chromatid is now considered
a full chromosome.
Telophase II and Cytokinesis
Nuclear membranes re-form around each set of chromosomes.
Cytokinesis follows, resulting in four genetically unique haploid cells,
each with a single set of chromosomes.
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Meiosis
in Animal Cell
|
What Happens to the Haploid Cells?
These haploid cells serve different roles across
organisms:
- In
animals, they mature into gametes (sperm or egg).
- In
plants, fungi, and some protists, they may continue to divide
through mitosis to form spores or gametophytes.
Meiosis is essential not just for
reproduction but also for maintaining the genetic stability of species.
By halving the chromosome number, it ensures that each generation starts with
the correct genetic blueprint. At the same time, crossing over and independent
assortment during meiosis introduce variation—fueling the diversity of
life.
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