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The phrase meiosis originates from the Greek language. It means to reduce; this refers back to the lessening of the variety of chromosomes inside the cell. Meiosis is the method of chromosomal discount in eukaryotic cells (crops, animals, and fungi), which results in the manufacturing of germ cells (gametes/intercourse cells) wanted for sexual copy. In meiosis, a double set of chromosomes (diploid) is diminished to a single set of chromosomes (haploid) to provide germ cells or spores. When these mix in sexual copy, the ensuing zygote is a diploid. On this manner, the chromosomal variety of the species is conserved by way of sexual copy.


It comes all the way down to chromosomal numbers. If a human, with n = 46 chromosomes, or two pairs of n = 23 chromosomes, had been to breed with out chromosomal discount, the egg cell and sperm cell would each have n = 46 chromosomes. When these fuse to grow to be a gamete, the zygote (embryo) would have n = 92 chromosomes, or double the quantity crucial! This may end in genetic abnormalities within the youngster. Additional, think about if this youngster had been to breed with one other youngster with n = 92 chromosomes: their youngster would have 184 chromosomes! This quantity can be ever-increasing. Thus, a chromosomal discount is critical for every species’ continued existence.

Studying: why is meiosis necessary for sexual copy

Earlier than meiosis begins, the chromosomes within the nucleus of the cell bear replication. It is because meiosis produces 4 daughter cells with half the chromosomes of the guardian cell; or 4 haploid cells from a single diploid cell. Keep in mind, haploid and diploid consult with the variety of chromosomes within the cell: haploid cells include one set of chromosomes (n) whereas diploid cells include two full units of chromosomes (2n). As you’ll be able to see, the maths doesn’t fairly work out: the guardian cell should first be transformed to a 4n (tetraploid) cell earlier than division begins. So a cell with n = 46 chromosomes might be transformed to a cell with n = 92 chromosomes, which, after meiosis, will produce 4 cells with n = 23 chromosomes.

Meiosis begins a lot the identical as mitosis does. After chromosomal replication, all chromosomes separate into sister chromatids (the equivalent two halves of a chromosome). Nevertheless, right here the similarities finish. In meiosis, an extra course of happens: that of recombination or crossing over. In recombination, the pairs of chromosomes line up and recombine, so that every chromosome has a bit of one other in it. On this manner, genetic range is ensured.

Thus, meiosis makes use of recombination to provide 4 haploid daughter cells that are not equivalent to their diploid guardian cell or one another.

The Phases of Meiosis

Meiosis is cut up into two elements, or divisions, every of which consists of a number of phases. These are prophase I, metaphase I, anaphase I and telophase I in meiosis I; and prophase II, metaphase II, anaphase II and telophase II in meiosis II. You’ll need some terminology to know these phases:

  • Bivalent – a pair of homologous chromosomes held collectively by a chiasma.
  • Chiasma – level of crossing over when chromosomes alternate genetic materials.
  • Centromere – the purpose of constriction of a chromosome.
  • Dyad – half of a tetrad; one half of a synapsed pair of homologous chromosomes.
  • Homologous chromosomes – the pair of chromosomes shaped by an unique chromosome and its duplication. These chromosomes are usually not equivalent.
  • Metaphase plate – the midline of the cell.
  • Monad – after separation, every chromosome of a tetrad types a monad. A dyad with out the synapsis to its homologous chromosome.
  • Nuclear envelope – the double membrane which encloses the nucleus.
  • Nucleolus – the middle of rRNA manufacturing inside the nucleus.
  • Sister chromatids – the 2 equivalent chromatids which kind a chromosome.
  • Spindle fibers – a bundle of microtubules working from one pole of the cell to a different, alongside which chromosomes transfer.
  • Synapse/synapsis – the method whereby two homologous chromosomes come into bodily contact with each other.
  • Tetrad – a pair of homologous chromosomes held collectively by a chiasma.

Meiosis I

Here’s a full image of meiosis I (determine 1), we’ll undergo every part intimately. It is best to have the ability to perceive the numerous of every part and the explanation for every step. When you perceive this, you can be ready for answering examination questions on meiosis I.


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Determine 1: The 4 levels of meiosis I with the prophase stage additional separated into 4 sub-phases

1. Prophase I

Prophase I is characterised by three fundamental occasions: the condensation of chromatin into seen chromosomes, the synapsis of chromosomes in every homologous pair, and crossing over of genetic materials between these synapsed chromosomes. Prophase I is additional subdivided into 5 discrete phases: leptonema, zygonema, pachynema, diplonema and diakinesis (determine 2).


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Determine 2: The totally different levels of the prophase of meiosis I


Often known as the leptotene stage, this part is characterised by the condensation of chromatin to kind seen chromosomes. The homology search begins.


This part is also called the zygotene stage. The homology search continues, with homologous chromosomes aligning into tough pairing, forming bivalents. The synaptonemal advanced begins to kind.


Often known as the pachytene stage, this part contains additional improvement of the synaptonemal advanced between homologous pairs of bivalents, resulting in synapsis. At this part, it’s clear that every bivalent comprises two pairs of sister chromatids. The sister chromatids of 1 pair are nonsister chromatids to the sister chromatids of the opposite pair. Collectively, the 4 chromatids are generally known as a tetrad. Crossing over or recombination of genetic materials between pairs of nonsister chromatids happens.


This part is also called the diplotene stage. The pairs of sister chromatids start to separate. Nonsister chromatids stay in touch at factors generally known as chiasmata (singular chiasma), the place the genetic alternate has occurred throughout crossing over.


Chromosomes separate additional however are nonetheless hooked up by way of chiasmata of the nonsister chromatids. Separation results in the chiasmata shifting in the direction of the ends of the chromatids, a course of generally known as terminalization. The nuclear envelope and nucleolus deteriorate, and the centromeres of every chromosome connect to spindle fibers, earlier than lining up on the metaphase plate. The chromosomes are nonetheless in pairs, which kind tetrads.

2. Metaphase I

This part is much like the metaphase of mitosis. The spindle fibers hooked up to the centromere of every tetrad align the chromosomes in order that one half of every tetrad is oriented in the direction of every pole.


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Determine 3: The alignment of tetrads throughout metaphase I of meiosis I

3. Anaphase I

At anaphase I, the chromosomes don’t cut up into their sister chromatids, however every tetrad is cut up into its chromosome pairs (dyads). These are pulled to reverse poles in a course of generally known as disjunction. Anaphase ends with the identical variety of dyads at every pole because the haploid variety of the guardian cell.


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Determine 4: The splitting of chromosome dyads throughout anaphase I

4. Telophase I

In some organisms, telophase I is entered and a nuclear membrane types across the dyads at every pole, earlier than a brief interphase interval is reached. In different organisms, telophase I is skipped, and meiosis II is entered.


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Determine 5: Formation of nuclear membrane isolating the 2 dyads

Meiosis II

As soon as once more, we’ll discover the second meiosis part very like the primary one. Here’s a full image of meiosis II following telophase I:

Product of Meiosis1

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Determine 6: The 4 levels of meiosis II, with 4 haploid cells on the finish of this part

1. Prophase II

Sister chromatids kind dyads related by a centromere. These are located on the middle of the cell. No condensation of chromatic materials or dissolving of nuclear membranes want happen.

prophase II

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Determine 7: The sister chromatids from the dyads are hooked up by a centromere throughout prophase II

2. Metaphase II

Spindle fibers hooked up to the centromere of every sister chromatid align the dyads on the metaphase plate, with one half of the dyad dealing with towards every pole.

metaphase II

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Determine 8: The metaphase plate types separating every half of the dyad

3. Anaphase II

The spindle fibers hooked up to every sister chromatid shorten, and every is pulled to an opposing pole of the cell.

anaphase II

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Determine 9: Like in anaphase I, the sister chromatids are pulled in the direction of reverse ends

4. Telophase II

The chromatids (monads) are located on the poles of the cell. Cytokinesis happens, whereby a nuclear membrane types round every set of chromosomes, and the cell divides into two cells with a haploid variety of chromosomes. Thus, 4 haploid gametes are shaped, which may now recombine throughout sexual copy to kind a zygote.

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telophase II

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Determine 10: 4 haploid daughter cells kind throughout telophase II

Overview of Meiosis

The small print of meiosis might be overwhelming; beneath we’ll spotlight some key factors from each phases of meiosis. Throughout recombination chromosomes from every guardian alternate the ideas of their homologous chromosomes. This fashion every homologous chromosome pair has a bit little bit of the opposite one. The purpose at which chromosomes alternate materials is named a chiasma.


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Determine 11: Genetic materials alternate throughout meiosis and the ensuing gametes

Why is Meiosis Necessary in Finding out Biology?

Meiosis is necessary for 3 fundamental causes: it permits sexual copy of diploid organisms, it allows genetic range, and it aids the restore of genetic defects.

1. Permits sexual copy of diploid organisms

As talked about beforehand, meiosis permits the discount of a diploid cell to a haploid gamete, which may then recombine with one other haploid gamete to create a diploid zygote.

2. Allows genetic range

The crossing over or recombination of genes which happens in meiosis rearranges the alleles current in every chromosome of a homologous pair, permitting the blending of paternal and maternal genes, both of which might be expressed within the resultant offspring. This enables genetic range in a inhabitants, which is a buffer to genetic defects, susceptibility of the inhabitants to illness and modifications within the setting. With out this recombination, the gene pool of populations would stagnate, and a single occasion may wipe out a whole inhabitants. Genetic range signifies that there might be sure people inside any given inhabitants that might be higher capable of survive a lack of habitat, a change in meals availability, a change in climate patterns, illnesses or different catastrophic occasions, making certain species continuity.

3. Aids the restore of genetic defects

The recombination which happens in meiosis can additional assist in the restore of genetic defects within the subsequent era. If a genetic defect is current on a sure allele of 1 guardian, recombination can substitute this allele with the wholesome allele of the opposite guardian, permitting wholesome offspring.

How is Meiosis Completely different from Mitosis?

Mitosis is the manufacturing of two genetically equivalent diploid daughter cells from one diploid guardian cell. Meiosis produces 4 genetically distinct haploid daughter cells from a single diploid guardian cell. These germ cells can then mix in sexual copy to kind a diploid zygote.

Meiosis solely happens in eukaryotic organisms which reproduce sexually, whereas mitosis happens in all eukaryotic organisms, together with these which reproduce asexually.

The desk beneath summarizes the similarities and variations between meiosis and mitosis.




Can solely happen in eukaryotes DNA replication happens first Manufacturing of daughter cells based mostly on guardian cell’s genetic materials Technique of cell replication in crops, animals, and fungi


Begins as diploid; ends as haploid Begins as diploid; ends as diploid Chromosome quantity is diminished Chromosome quantity is conserved Chromosome pairs bear synapsis No synapsis happens Used for sexual copy Used for progress/therapeutic/asexual copy 2 nuclear divisions 1 nuclear division 8 phases 5 phases Daughter cell not equivalent to guardian cell Daughter cell equivalent to guardian cell Ends in 4 daughter cells Ends in 2 daughter cells Produces germ cells Produces somatic cells Happens solely in sexual organisms Happens in asexual and sexual organisms

Wrapping Up Meiosis and Biology

We now know that meiosis is the method of chromosomal discount which permits the manufacturing of haploid germ cells crucial for sexual copy. Meiosis is moreover necessary for its function in enabling genetic range and facilitating the restore of genetic defects by way of recombination.

The advantages that meiotic copy offers over mitotic copy are that mitotic copy produces equivalent cells, conserving the chromosomal set and the genes inside, whereas meiosis permits for the expression of latest traits due to the method of crossing over. With out meiosis sustaining genetic range inside populations, organisms wouldn’t have the ability to adapt to swimsuit their setting, nor evolve, nor survive catastrophic occasions. A inhabitants’s genetic range is its most dependable software within the combat for the species’ survival.

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