Chapters
In this article, we will explain the meanings of haploid (n) and diploid (2n). Besides this, we will also discuss what is meant by homologous pairs of chromosomes and discuss the need for a reduction division during meiosis in the production of gametes. In the end, we will discuss the behaviour of chromosomes in plant and animal cells during meiosis and the related behaviour of the nuclear envelope, the cell surface membrane, and the spindle. So, let us get started.
Diploid and Haploid Cells
The terms diploidy and haploidy are applicable to cells across various species. They tell the number of sets of chromosomes, rather than the total number of chromosomes.
Diploid Cells
A diploid cell refers to a cell that has two complete sets of chromosomes (2n). These chromosomes have the DNA required for the synthesis of protein and cell function.
Approximately all cells in the human body are diploid with 46 chromosomes (23 pairs) in their nucleus.
Haploid Cells
One complete set of chromosomes (n) is present in the haploid cells. It means that the number of chromosomes in haploid cells is half as compared to the diploid cells. Haploid cells in humans have 23 chromosomes in their nuclei. These haploid cells are referred to as gametes and they play their role in sexual reproduction. Humans include a male sperm and a female egg.
In the next section of the article, we will discuss the requirement for the reduction of division during meiosis.
The Requirement for Reduction Division During Meiosis
- The nuclei of gametes fuse together to create the core of the zygote during fertilization
- Both gametes should have the right number of chromosomes for the survival of the zygote. If several or very few chromosomes are present in the zygote, then it may not survive
- The gametes should be haploid, for a diploid zygote, i.e. n + n = 2n
- Haploid gametes are produced by meiosis during sexual reproduction
- Reduction division is the first cell division of meiosis
- Reduction division refers to a nuclear division that reduces the number of chromosomes of the cell
- In humans, this chromosome number is decreased from 46, i.e. diploid to 23, i.e. haploid
- The decrease in the number of chromosomes during meiosis guarantees that the gametes created are haploid
Homologous Chromosomes
- There are two complete sets of chromosomes in the nucleus in diploid cells
- Chromosomes have a fixed length, and their centromere is positioned in a specific location
- The above characteristic features enable us to identify each chromosome in a photomicrograph
- When observed in photomicrographs, chromosomes are usually seen grouped into their homologous pairs
- Homologous chromosomes contain the same genes in the same positions and are of the same shape
- A diploid zygote forms during the fertilization
- In a zygote, a single chromosome of each homologous pair comes from the female gamete and the other one comes from the male gamete
- The same genes in the same order facilitate homologous chromosomes to line up alongside each other during meiosis
In the next section of the article, we will discuss the behaviour of chromosomes in plant and animal cells during meiosis and the related behaviour of the nuclear envelope, the cell surface membrane, and the spindle.
Meiosis in Animal and Plant Cells
A kind of nuclear division that causes the production of haploid cells from diploid cells is known as meiosis.
As a result of meiosis, gametes are produced in animals and plants that are used in sexual reproduction. Meiosis is quite similar to mitosis, however, it has two divisions, i.e. meiosis I and meiosis II. The following stages occur within each division:
- Prophase
- Metaphase
- Anaphase
- Telophase
Prophase I
- DNA becomes visible as chromosomes after being condensed
- Since replication of DNA already took place, therefore each chromosome has two sister chromatids that are joined together by a centromere
- The chromosomes are arranged side by side in the form of homologous pairs and a pair of a homologous chromosome is referred to as bivalent
- Because the homologous chromosomes are extremely close to each other, the crossing over of non-sister chromatids may occur. The point at which the crossing over takes place is known as chiasma
- In prophase, I, the migration of centrioles to opposite poles and formation of spindle occurs
- Finally, the nuclear envelope breaks down and the nucleolus disintegrates
Metaphase I
- Along the equator of the spindle, the bivalent lines up. The spindle fibres are attached to the centromeres
Anaphase I
- The separation of homologous pairs of chromosomes occurs as microtubules pull entire chromosomes to the opposite ends of the spindle
- The division of centromeres does not occur at this stage
Telophase I
- The arrival of chromosomes to the opposite poles
- Spindle fibres begin to break down
- Nuclear envelopes are created around the two groups of chromosomes and reformation of nucleoli occurs
- Few plant cells go straight into meiosis II without reforming the nucleus in telophase I
Cytokinesis
- This is when the cytoplasm divides
- Cell organelles are distributed between two developing cells
- The cell surface membrane in animal cells pinches inside (inwards) to form a cleavage furrow in the middle of the cells. The cleavage furrow contracts and divides the cytoplasm in half
- Vesicles from the Golgi apparatus accumulate along the equator of the spindle in the plant cells
- The merging of vesicles with each other creates a new cell surface membrane. It also results in the secretion of the calcium pectate layer which becomes middle lamella. Cellulose layers are laid upon the middle lamella to create the primary and secondary walls of the cell
- In meiosis, I, the end product of cytokinesis is two haploid cells that contain half the number of centromeres
Second Division of Meiosis : Meiosis II
- Between meiosis I and meiosis II, there is no interphase, hence replication of DNA does not take place
- The second division of meiosis is approximately similar to the stages of mitosis
Prophase II
The breakdown of the nuclear envelope occurs along with the condensation of chromosomes. A spindle is created at the right angle to the old one.
Metaphase II
Along the equator of the spindle, the chromosomes line up in a single file.
Anaphase II
The division of chromosomes occurs, and the individual chromatids are pulled to opposite poles. This forms four groups of chromosomes that have half the number of chromosomes as the original parent cell
Telophase II
The formation of a nuclear membrane occurs around each group of chromosomes
Cytokinesis
Cytoplasm divides when the new cell surface membranes are created, resulting in the formation of four haploid cells. The cells still have the same number of centromeres as they have at the beginning of meiosis I, however, now they contain half the number of chromosomes.
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