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Meiosis: Process, Types and Significance, Practice Problems and FAQs

Meiosis: Process, Types and Significance, Practice Problems and FAQs

Do you know how sexual reproduction occurs in higher organisms such as plants and animals? It occurs by the fusion of male and female gametes obtained from the parent organisms which results in the formation of a zygote which grows into a new organism.

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Fig: Sexual reproduction

You must be aware that all the cells of an organism, belonging to a particular species, contain a fixed number of chromosomes. Now, if we consider the process of sexual reproduction, a new individual is formed by the fusion of two gametes (cells) coming from each parent. So do you think, each time an offspring is born, he has double the number of chromosomes compared to that of the parents? No, right? Then how is the chromosome number maintained in the offspring?

The answer to this question lies in the fact that the gametes carry half the number of chromosomes of the parents, i.e, they are haploid in nature. Hence, when they fuse the diploid chromosome number is restored. But how are these haploid gametes formed? The formation of haploid cells from diploid cells occurs by a special kind of division called meiosis. Come let us learn more about it.

Table of contents:

  • Introduction to meiosis
  • Why not mitosis for gamete formation?
  • Events in meiosis
  • Meiosis I
  • Interkinesis
  • Meiosis II
  • Types of meiosis
  • Significance of meiosis
  • Difference between meiosis I and meiosis II
  • Meiosis II vs Mitosis
  • Practice problems
  • FAQs

Introduction to meiosis

Definition

The type of cell division that leads to the formation of haploid cells from diploid cells is known as meiosis. It is also known as reductional division as the chromosome number is reduced to half in this type of cell division.

Fig: Meiosis

Key features

Meiotic division is mainly observed during gametogenesis in animals and plants which results in formation of haploid gametes required for sexual reproduction. Nuclear division and cell division occurs in two sequential phases known as meiosis I and meiosis II while DNA replication occurs in only one phase. Initiation of meiosis I occurs after replication of parental chromosomes occurs in the S phase of interphase of the cell cycle. Immediately after S phase, the DNA content of the cell becomes doubled but the chromosome number remains the same. Chromosomes are duplicated to form sister chromatids only after meiosis is initiated. Pairing of homologous chromosomes, formation of sister chromatids and recombination between non sister chromatids of homologous chromosomes occurs during meiotic division. Meiosis results in production of four haploid cells.

Why not mitosis for gamete formation?

A human cell has 46 chromosomes, hence, each human gamete should have 23 chromosomes.

If we assume that gamete formation occurs by mitotic division, the following process will occur:

Fig: Gamete formation by mitosis

After mitotic division, gamete mother cells will form gametes with the same number of chromosomes i.e. 46. When gametes with 46 chromosomes fuse, it results in the formation of a zygote with 92 chromosomes. When a gamete mother cell with 92 chromosomes forms a zygote, it results in the formation of a zygote with 184 chromosomes. Thus, reproduction by mitotic division will result in an increase of chromosome number with every new progeny.

Events in meiosis

Meiosis occurs in two phases:

  • Meiosis I
  • Meiosis II

Fig: Events in meiosis

Meiosis I

In meiosis I, the homologous chromosomes separate from each other and go to two different daughter cells. This reduces the number of chromosomes from diploid to haploid condition. Thus, meiosis I is known as heterotypic division or reduction division. It has two stages - karyokinesis I (nuclear division) and cytokinesis I (cytoplasmic division). Karyokinesis I is divided into - prophase I, metaphase I, anaphase I and telophase I.

Prophase I

Prophase in meiosis is much longer and more complex as compared to prophase of mitotic division. On the basis of chromosomal changes, prophase I is divided into 5 phases:

Fig: Events of Prophase I of Meiosis I

Leptotene

It is the first stage of prophase I. It is also called the ‘thin thread stage’. Chromosomes start appearing in this phase. Condensation of chromatin fibres into chromosomes continues through this phase. At this stage, the chromosomes often possess a linear series of darkly stained swollen areas known as chromomeres which are regions of extra condensation. The leptotene chromosomes are replicated but the chromatids are not distinguishable. In animal cells, the centrosome duplicates and each centrosome moves to the opposite pole and starts developing the astral rays.

GIF: Leptotene

Zygotene

It is also called the ‘paired thread stage’. Homologous chromosomes, which are morphologically similar to each other and have been received from each parent, pair up in this stage. The ends of homologous chromosomes which are in contact with the nuclear envelope, approach each other and the chromosomes are brought close together. A nucleoprotein complex named synaptonemal complex is formed which help in the adherence and attachment of the homologous chromosomes throughout their length. This process is known as synapsis. It results in the centromeres (point of attachment of sister chromatids), chromomeres and alleles of the homologous chromosomes lying exactly opposite to each other. Synapsis results in the formation of bivalents. Their number is one half of the total number of chromosomes.

Fig: Homologous chromosomes

Fig: Bivalent formation

The asters continue to move away from each other in this phase of prophase I which is quite short-lived.

GIF: Zygotene

Pachytene

It is also known as the ‘ thick thread stage’ of prophase I. It is the longest stage of prophase I. It is a substage of chromosome thickening and crossing over. Each bivalent formed at the end of leptotene is actually formed of two homologous chromosomes, each of which have a sister chromatid attached to it. However, the chromatids or the individual chromosomes are not clearly visible at the bivalent stage due to the presence of the synaptonemal complex. Further condensing of the chromosomes during pachytene makes the sister chromatids more visible and they appear as tetrads.

Fig: Tetrads visible during pachytene stage

Recombination nodules are formed on the non-sister chromatids of homologous chromosomes.

Fig: Formation of recombination nodule

Recombination nodules are the sites of crossing over where the genetic information is exchanged between the non-sister chromatids of homologous chromosomes. Recombinase enzyme aids the process of crossing over.

Fig: Crossing over

By the end of pachytene, recombination is completed, leaving the non-sister chromatids linked at the site of recombination.

GIF: Pachytene stage

Diplotene

It is also known as the ‘twin thread stage’ of prophase I. Dissolution of the synaptonemal complex occurs at this stage. Except for the site of crossing over, recombined homologous chromosomes separate from each other. The points of attachment between the homologous chromosomes after their separation appear as X shaped chiasmata.

Fig: X-shaped chiasma

This phase can last for months or years in the oocytes of some vertebrates.

Fig: Human Oocyte

Diakinesis

It is the last stage of prophase I. Terminalisation of chiasmata occurs during which the chiasmata start moving towards the outer end of the chromosomes. Fully condensed chromosomes can be observed at this stage. Spindle apparatus assembles in order to execute separation of homologous chromosomes further. Breakdown of the nuclear membrane occurs. Nucleolus of the cell disappears.

GIF: Diakinesis

Metaphase I

Spindle fibres align at the opposite poles and microtubules attach at the kinetochore of the chromosomes. The two chromosomes of a bivalent get attached to different spindle fibres belonging to different poles in the region of their kinetochores. These spindle fibres contract and bring the chromosomes to the equatorial plate.

GIF: Metaphase I

Anaphase I

Spindle fibres pull each member of the homologous chromosomes to the opposite poles which results in their separation and the chiasmata disintegrates. This process is known as disjunction. After disjunction, the separated homologous chromosomes are known as univalents or dyads and are made up of two sister chromatids. Sister chromatids remain associated with the help of centromere.

As paternal and maternal chromosomes are randomly arranged at the equatorial plate, the assortment of the two members of a homologous pair at the respective poles is independent of the assortment of other pairs. This forms the basis of Mendel’s law of independent assortment.

The random assortment of maternal and paternal chromosomes in the daughter cells is one of the major reasons for the development of genetic variability in the chromosomes carried by the gametes. During sexual reproduction, these variations are passed on to the offspring.

GIF: Anaphase I

Telophase I

In this stage, two polar groups of dyad chromosomes organise themselves into two daughter nuclei. For the formation of the nuclei, the chromosomes elongate without dispersion. Nuclear membrane and nucleolus reappears during this stage.

GIF: Telophase I

Cytokinesis I

After the chromosomes have separated, cytoplasm divides with the help of the cytokinesis stage of meiosis I. Cytokinesis generally occurs through cleavage and rarely through cell plate formation. In plant cells, cell wall material is deposited in the furrow. At the end of cytokinesis I, two daughter cells are formed which have a haploid number of elongated dyad chromosomes in their nuclei.

GIF: Cytokinesis I

Interkinesis

Daughter cells formed after meiosis enter into a short-lived stage known as interkinesis. There is no DNA replication during this phase. Proteins and other biochemicals are synthesised during this phase and in animal cells, the centrosomes may replicate.

Fig: Daughter cells in interkinesis

Meiosis II

It is the second phase of meiosis in which the sister chromatids of the chromosomes will split and single stranded chromosomes will be passed on to the daughter cells. Meiosis II maintains the haploid number of chromosomes obtained after meiosis I. Thus, it is known as homotypic division or equational division. Meiosis II also has a karyokinesis II (nuclear division) and a cytokinesis II (cytoplasmic division) stage. Karyokinesis II is divided into - prophase II, metaphase II, anaphase II and telophase II.

Prophase II

This phase is less complex than prophase I and is initiated after cytokinesis of prophase I. Nuclear membrane begins to disappear in early prophase II. The elongated dyad chromosomes condense into chromosomes. As the cells proceed into late prophase II, the nuclear membrane disappears and chromosomes become more compact. Centrioles move towards the opposite poles of the cell.

GIF: Prophase II

Metaphase II

Each chromosome gets attached to two spindle fibres, one from each pole, at their kinetochores. During this phase, the spindle fibres contract and cause the chromosomes to align at the equatorial plane of the cell.

GIF: Metaphase II

Anaphase II

During this phase, the centromere splits as the spindle fibres continue to contract. The sister chromatids separate and move towards the opposite poles.

GIF: Anaphase II

Telophase II

It is the concluding stage of meiosis II. Nuclear membrane and nucleolus reappear during this phase. Chromosomes decondense to chromatin fibres. The astral rays and spindle fibres disappear.

GIF: Telophase II

Cytokinesis II

Division of cytoplasm of daughter cells occurs in this stage. Tetrad of haploid cells are formed by the end of cytokinesis II.

GIF: Cytokinesis II

Types of meiosis

Zygotic meiosis

Zygotic meiosis occurs in some algae, fungi and protists in which the zygote is the only diploid phase of the life cycle and undergoes meiosis immediately to form haploid spores that develop into haploid organisms. Such organisms are said to have a haplontic life cycle.

Gametic meiosis

This occurs in organisms in which the gamete is the only diploid stage of the life cycle and the adult organisms are diploid. The adult diploid organism bears sex organs which have diploid gamete mother cells that would undergo meiotic division to form haploid gametes. This is seen in all animals, some protists and some algae. Such organism are said to have a diplontic life cycle.

Sporic meiosis

This occurs in organisms which exhibit alternation between a diploid sporophyte and haploid gametophyte in their life cycle. The sporophyte bears diploid spore mother cells which undergo meiosis to form haploid spores that would develop into haploid gametophytes. The gametophytes bear sex organs and are reponsible for the formation of gametes. This is more commonly seen in bryophytes and pteridophytes which have a haplodiplontic life cycle.

Significance of meiosis

It helps in maintaining the chromosome number of future generations of sexually reproducing organisms.

The independent assortment of maternal and paternal chromosomes in the daughter cells is one of the major reasons for the development of genetic variability in the chromosomes carried by the gametes. During sexual reproduction, these variations are passed on to the offspring.

Crossing over of genetic material between non-sister chromatids of homologous chromosomes also helps to bring in variations in the genetic composition of the chromosomes inherited by the offspring from the parents. The accumulation of these genetic variations over many generations can lead to evolution of a new species.

The chances of gene mutations increase during meiosis due to the action of multienzyme complex of the recombination nodules. This agains helps in introducing variations in the genetic material that is to be passed on to the offspring via the gametes.

Difference between meiosis I and meiosis II

Meiosis I

Meiosis II

DNA replication occurs in the interphase.

No DNA replication occurs.

The centromere remains intact.

Centromere splits into two halves.

Result of meiosis I is dyad of cells.

Result of meiosis II is tetrad of cells.

Meiosis II vs Mitosis

Similarities

Both meiosis II and mitosis are equational divisions. Separation of sister chromatids occurs during the anaphase to become the chromosomes of the daughter cells. Microtubules assemble at the opposite poles of the cell and attach to the centromere of each sister chromatid. Condensation of chromosomes occurs during the telophase stage.

Differences

Ploidy of daughter cells after mitosis is 2n i.e., diploid and after meiosis II is n i.e., haploid.

Practice problems

Q1. Find the correct match.

A. Anaphase I- Chromosomes move to opposite poles
B. Metaphase II- Nuclear membrane and nucleolus reappears
C. Telophase I- Splitting of centromere
D. Prophase II- Chromosomes align at the equatorial plane

Solution: During anaphase I, spindle fibres pull the chromosomes to the opposite poles which results in separation of homologous chromosomes. Sister chromatids remain associated with the help of centromere.

Nuclear membrane and nucleolus reappears during telophase I and II.

Centromere splits during anaphase II.

Chromosomes align at the equatorial plate during metaphase I and II.

Hence, the correct option is a.

Q2. Which stage of meiosis I is shown in the given image?

A. Prophase I
B. Metaphase I
C. Telophase I
D. Anaphase I

Solution: Given image shows that the homologous chromosomes of a bivalent have separated and reached the opposite poles and are existing as univalents or dyads. The nuclear membrane also forms again. These events are observed during telophase I. During this phase, the nuclear membrane and nucleolus reappear.

Hence, the correct option is c.

Q3. This phase of meiosis I is a short lived stage. Daughter cells enter into this stage after meiosis I. Which stage is being referred to in the given statement?

A. Cytokinesis
B. Interkinesis
C. Leptotene
D. Zygotene

Solution: Daughter cells formed after meiosis I enter into a short-lived stage known as interkinesis. There is no DNA replication during this phase.

Hence, the correct answer is b.

Q4. ‘X’ and ‘Y’ cells undergo meiosis and mitosis respectively. Both the cells have 92 chromosomes each before division. What should be the number of chromosomes in the daughter cells produced by X and Y respectively?

A. 46, 46
B. 46, 92
C. 92, 46
D. 92, 92

Solution: Meiosis is a reductional division as the chromosome number is reduced to half in this type of cell division while mitosis is an equational division as the chromosome number stays the same after mitotic division. The daughter cells of X will have 46 chromosomes and daughter cells of Y will have 92 chromosomes.

Hence, the correct option is b.

FAQs

Q1. What is the difference between a centromere and a kinetochore?
Answer:
A centromere is the point of attachment of the sister chromatids of a chromosome and the kinetochore is the protein complex on the chromosome to which the spindle fibres bind during cell division.

Q2. How is meiosis in males different from that in females?
Answer:
Meiosis in males occurs in the seminiferous tubules at puberty while in females it occurs in the oogonia since birth. The sperms are released from the testes after they complete meiosis. In females meiosis I starts at the time of birth and results in the formation of primary oocytes which are arrested at the diplotene stage of meiosis I till puberty. Onset of puberty stimulates the completion of meiosis I and initiation of meiosis II in the follicles and the secondary oocyte is formed. However, meiosis II is not completed when the secondary oocyte is released from the ovaries during ovulation. It is completed only after the oocyte is fertilised by a sperm.

Q3. At which stage of meiosis is the secondary oocyte released from the ovaries?
Answer:
The secondary oocyte is arrested at the metaphase II stage of meiosis II when it is released from the ovaries.

Q4. What are meiocytes?
Answer:
Cells that undergo meiosis to form haploid daughter cells are known as meiocytes.

Q5. What are meiospores?
Answer:
Haploid spores that are formed by meiotic division of diploid spore mother cells are known as meiospores. These are commonly found in plants.

Q6. Are gametes always produced by meiosis?
Answer:
In plants such as bryophytes and pteridophytes, the life cycle is divided between a diploid sporophyte phase and a haploid gametophyte phase. The haploid gametophytes bear the sex organs antheridia (male) and archegonia (female) which are also haploid in nature. The gamete mother cells in these sex organs are hence haploid in nature and undergo mitotic division to form gametes. Thus, gametes are not always produced by meiotic division.

Q7. When does meiosis occur in Chlamydomonas?
Answer:
The zygote formed by the fusion of gametes in Chlamydomonas undergoes meiosis to form haploid spores that develop into haploid gametophytes.

 

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