MEIOSIS
Prepared By Dr Bhabesh Nath
Assistant Professor
Department of Zoology
B N College (A) Dhubri
Learning Objectives
· Define meiosis and explain why it is essential in sexually reproducing organisms.
· Describe the events of premeiotic interphase, Meiosis I and Meiosis II.
· Explain the five substages of Prophase I in their correct sequence.
· Describe synapsis, crossing over, recombination and chiasmata.
· Explain reductional and equational divisions.
· Understand independent assortment and its contribution to genetic variation.
· Compare meiosis with mitosis and explain the consequences of meiotic errors.
· Relate meiosis to spermatogenesis, oogenesis and chromosome number maintenance.
1. Introduction
Meiosis is a specialized type of cell division that occurs in organisms undergoing sexual reproduction. Its principal function is to reduce the chromosome number by half and to generate genetically different reproductive cells. Unlike mitosis, meiosis involves two successive nuclear divisions after only one round of DNA replication. The process therefore converts a diploid cell into haploid cells while reshuffling genetic material.
The significance of meiosis extends beyond chromosome reduction. During Prophase I, homologous chromosomes pair and exchange corresponding segments of DNA. In addition, homologous chromosome pairs orient independently at Metaphase I. These processes create new combinations of alleles and are major sources of variation in populations.
2. Definition of Meiosis
Meiosis may be defined as a specialized form of cell division in which a diploid parent cell undergoes one round of DNA replication followed by two successive nuclear divisions to produce haploid cells. Meiosis I separates homologous chromosomes, whereas Meiosis II separates sister chromatids.
3. Important Features of Meiosis
· There is one round of DNA replication before the first meiotic division.
· There are two successive nuclear divisions, Meiosis I and Meiosis II.
· Meiosis I is called reductional division because homologous chromosomes separate.
· Meiosis II is called equational division because sister chromatids separate.
· Homologous chromosomes pair during Prophase I.
· Crossing over occurs between non sister chromatids of homologous chromosomes.
· The usual final products are four haploid cells.
· The products are genetically different from one another.
4. Premeiotic Interphase
Before entering meiosis, the cell passes through a preparatory period. During the S phase of this interphase, the DNA of every chromosome is replicated. Consequently, each chromosome consists of two sister chromatids joined at the centromere. There is no second round of DNA replication between Meiosis I and Meiosis II.
5. Meiosis I: Reductional Division
Meiosis I is the first meiotic division and is responsible for reducing the chromosome number. Homologous chromosomes, rather than sister chromatids, are separated. Meiosis I consists of Prophase I, Metaphase I, Anaphase I and Telophase I.
5.1 Prophase I
Prophase I is the longest and most elaborate phase of meiosis. It is characterized by chromosome condensation, pairing of homologous chromosomes, crossing over and formation of chiasmata. For detailed study, Prophase I is divided into five substages.
Leptotene
· Chromosomes begin to condense from diffuse chromatin into thin, visible structures.
· Each chromosome has already undergone DNA replication and therefore contains two sister chromatids.
· The chromosomes become progressively shorter and thicker as condensation continues.
Zygotene
· Homologous chromosomes begin to pair closely along their lengths.
· This pairing is called synapsis.
· The synaptonemal complex develops between homologous chromosomes and helps maintain their close association.
· A paired homologous chromosome unit is called a bivalent.
Pachytene
· Synapsis becomes complete and the homologous chromosomes remain closely paired.
· Each bivalent contains four chromatids and is therefore also described as a tetrad.
· Crossing over occurs between non sister chromatids of homologous chromosomes.
· Crossing over involves reciprocal exchange of corresponding DNA segments and produces recombinant chromatids.
Diplotene
· The synaptonemal complex gradually disappears.
· Homologous chromosomes begin to separate from each other.
· They remain connected at visible sites called chiasmata.
· Chiasmata represent the cytological manifestation of previous crossing over.
Diakinesis
· Chromosomes become highly condensed and more readily visible.
· Chiasmata undergo terminalization and appear closer to the ends of chromosome arms.
· The nucleolus disappears and the nuclear envelope breaks down.
· The spindle apparatus develops and the chromosomes become prepared for Metaphase I.
5.2 Metaphase I
· Bivalents become arranged along the equatorial plane of the cell.
· The orientation of each homologous pair is independent of the orientation of other pairs.
· Spindle fibres attach to the chromosomes so that homologues are connected toward opposite poles.
· Random orientation of bivalents contributes to independent assortment of chromosomes.
5.3 Anaphase I
· Homologous chromosomes separate from one another.
· One chromosome from each homologous pair moves toward each pole.
· Centromeres generally do not divide at this stage.
· Sister chromatids remain attached to one another.
· The chromosome number of each resulting group is reduced by half.
5.4 Telophase I
· Chromosomes reach the opposite poles.
· Chromosomes may partially decondense.
· Nuclear envelopes may reform depending on the organism.
· Cytokinesis may divide the cytoplasm, producing two haploid cells.
6. Interkinesis
Interkinesis is the brief interval between Meiosis I and Meiosis II in many organisms. It resembles an interphase in appearance, but it is fundamentally different because DNA replication does not occur. The chromosomes retain their two sister chromatids as the cell prepares for the second meiotic division.
7. Meiosis II: Equational Division
Meiosis II begins with the haploid products of Meiosis I. It resembles mitosis in the basic pattern of chromosome separation. The major difference is that Meiosis II begins with haploid cells and is not preceded by another round of DNA replication.
7.1 Prophase II
· Chromosomes condense again if they had undergone partial decondensation.
· The spindle apparatus develops.
· The nuclear envelope, if it has reformed, breaks down.
7.2 Metaphase II
· Chromosomes align individually at the equatorial plate.
· Spindle microtubules attach to the kinetochores of sister chromatids from opposite poles.
7.3 Anaphase II
· Centromeres divide.
· Sister chromatids separate from one another.
· Each separated chromatid is now considered an independent chromosome.
· The daughter chromosomes move toward opposite poles.
7.4 Telophase II and Cytokinesis
· Chromosomes reach the poles and become less condensed.
· Nuclear envelopes reform around the chromosome sets.
· The spindle apparatus disappears.
· Cytokinesis produces four haploid cells from the original meiotic cell.
8. Crossing Over and Genetic Recombination
Crossing over is the exchange of corresponding DNA segments between non sister chromatids of homologous chromosomes. It takes place during pachytene of Prophase I. The exchanged segments can contain different alleles, so crossing over creates chromatids carrying new combinations of genetic information.
The resulting recombinant chromosomes are important because they increase genetic diversity within a population. The process depends on controlled DNA breakage and repair mechanisms and is closely associated with homologous chromosome pairing.
9. Chiasmata
Chiasmata are visible points where homologous chromosomes remain associated after the synaptonemal complex has disappeared. They are particularly prominent during diplotene. Chiasmata help maintain the physical association of homologues until they separate during Anaphase I.
10. Independent Assortment
At Metaphase I, each pair of homologous chromosomes can orient in either of two directions toward the cell poles. The orientation of one pair is independent of other pairs. As a result, maternal and paternal chromosomes are distributed into daughter cells in many possible combinations. This is known as independent assortment and is another major source of genetic variation.
11. Meiosis in Spermatogenesis
During spermatogenesis, a primary spermatocyte is diploid and undergoes Meiosis I to form two haploid secondary spermatocytes. Each secondary spermatocyte undergoes Meiosis II to form two haploid spermatids. Thus, one primary spermatocyte gives rise to four haploid spermatids, which later differentiate into spermatozoa.
12. Meiosis in Oogenesis
During oogenesis, meiotic divisions involve unequal distribution of cytoplasm. A primary oocyte undergoes Meiosis I to produce a large secondary oocyte and a small first polar body. The secondary oocyte undergoes the second meiotic division, associated with formation of the mature ovum and additional polar body products. The unequal cytokinesis allows most of the cytoplasm and organelles to remain in the developing female gamete.
13. Comparison Between Meiosis I and Meiosis II
Feature | Meiosis I | Meiosis II |
Type of division | Reductional | Equational |
Chromosomes that separate | Homologous chromosomes | Sister chromatids |
Centromere division | Generally absent | Occurs during Anaphase II |
Synapsis | Occurs during Prophase I | Absent |
Crossing over | Occurs during Prophase I | Absent |
Starting cells | Diploid meiotic cell | Haploid cells produced by Meiosis I |
14. Comparison Between Mitosis and Meiosis
Feature | Mitosis | Meiosis |
Number of nuclear divisions | One | Two |
DNA replication | One round before division | One round before Meiosis I |
Synapsis | Absent | Present during Prophase I |
Crossing over | Normally absent | Present during Prophase I |
Chromosome number | Usually maintained | Reduced by half |
Usual products | Two daughter cells | Four haploid cells |
Genetic relationship | Usually similar to parent cell | Genetically varied |
Major biological role | Growth, repair and cell replacement | Formation of haploid reproductive cells and generation of variation |
15. Errors in Meiosis
Accurate chromosome segregation is essential for the formation of normal gametes. Failure of chromosomes to separate properly is called nondisjunction. It can occur during Meiosis I or Meiosis II and may result in gametes containing an abnormal number of chromosomes.
· Nondisjunction in Meiosis I involves failure of homologous chromosomes to separate.
· Nondisjunction in Meiosis II involves failure of sister chromatids to separate.
· Fertilization involving an abnormal gamete can produce aneuploid embryos.
· Trisomy 21 is a well known example of a chromosomal condition associated with meiotic nondisjunction.
16. Biological Significance of Meiosis
· Meiosis produces haploid reproductive cells from diploid precursor cells.
· It prevents chromosome number from doubling in every generation after fertilization.
· Crossing over creates recombinant chromosomes.
· Independent assortment produces different combinations of maternal and paternal chromosomes.
· Genetic variation generated by meiosis provides raw material for natural selection and evolution.
· Meiosis is therefore essential for the continuity and genetic diversity of sexually reproducing populations.
17. Frequently Asked Questions
What is meiosis?
Meiosis is a specialized cell division involving one round of DNA replication followed by two successive nuclear divisions, normally producing four haploid cells.
Why is Meiosis I called reductional division?
It separates homologous chromosomes, reducing the chromosome number from the diploid condition to the haploid condition.
Why is Meiosis II called equational division?
Sister chromatids separate during Meiosis II, so the chromosome number of the daughter cells is maintained.
How many times does DNA replication occur during meiosis?
DNA replication occurs once, before Meiosis I. It does not occur between Meiosis I and Meiosis II.
What is synapsis?
Synapsis is the close pairing of homologous chromosomes during zygotene of Prophase I.
What is a bivalent?
A bivalent is a paired unit consisting of two homologous chromosomes during Prophase I.
What is a tetrad?
A tetrad is the four chromatid structure formed when two replicated homologous chromosomes pair.
When does crossing over occur?
Crossing over occurs during pachytene of Prophase I.
What are chiasmata?
Chiasmata are visible connections between homologous chromosomes that reflect sites of crossing over.
What happens during Anaphase I?
Homologous chromosomes separate and move toward opposite poles, while sister chromatids remain joined.
What happens during Anaphase II?
Centromeres divide and sister chromatids separate to form independent chromosomes.
What is interkinesis?
Interkinesis is the interval between Meiosis I and Meiosis II during which DNA replication does not occur.
Why is meiosis important for evolution?
It produces genetic variation through crossing over and independent assortment, providing variation on which evolutionary processes can act.
18. Multiple Choice Questions
1. The main purpose of meiosis is to:
· A. Produce identical diploid cells
· B. Reduce chromosome number and generate variation
· C. Increase chromosome number
· D. Prevent DNA replication
Answer: B
2. DNA replication before meiosis occurs:
· A. Twice
· B. Once before Meiosis I
· C. Once before each division
· D. During Meiosis II
Answer: B
3. Synapsis occurs during:
· A. Leptotene
· B. Zygotene
· C. Pachytene
· D. Diakinesis
Answer: B
4. Crossing over occurs mainly during:
· A. Leptotene
· B. Zygotene
· C. Pachytene
· D. Diplotene
Answer: C
5. Chiasmata are especially visible during:
· A. Zygotene
· B. Pachytene
· C. Diplotene
· D. Metaphase II
Answer: C
6. Homologous chromosomes separate during:
· A. Anaphase I
· B. Anaphase II
· C. Metaphase I
· D. Telophase II
Answer: A
7. Centromeres divide during normal meiosis at:
· A. Anaphase I
· B. Anaphase II
· C. Prophase I
· D. Metaphase I
Answer: B
8. A bivalent contains:
· A. One chromosome
· B. Two homologous chromosomes
· C. Two unrelated chromosomes
· D. Four homologous chromosomes
Answer: B
9. The four chromatid structure of a paired homologous chromosome unit is called:
· A. Dyad
· B. Tetrad
· C. Monad
· D. Centrosome
Answer: B
10. Independent assortment is associated mainly with:
· A. Random orientation of homologous pairs at Metaphase I
· B. DNA replication
· C. Telophase II
· D. Cytokinesis
Answer: A
11. Failure of normal chromosome separation is called:
· A. Synapsis
· B. Nondisjunction
· C. Terminalization
· D. Replication
Answer: B
12. One primary spermatocyte normally gives rise to:
· A. One sperm cell
· B. Two spermatids
· C. Four spermatids
· D. Eight diploid cells
Answer: C
13. Cytokinesis in oogenesis is generally:
· A. Equal
· B. Unequal
· C. Absent
· D. Random
Answer: B
14. Meiosis contributes to genetic variation through:
· A. Crossing over and independent assortment
· B. DNA replication only
· C. Cytokinesis only
· D. Chromosome condensation only
Answer: A
15. Meiosis II most closely resembles:
· A. DNA replication
· B. Mitosis
· C. G₁ phase
· D. Fertilization
Answer: B
19. Question Bank
A. Very Short Answer Questions
1. Define meiosis.
2. What is reductional division?
3. What is equational division?
4. What is synapsis?
5. Define bivalent.
6. Define tetrad.
7. What is crossing over?
8. What are chiasmata?
9. What is interkinesis?
10. Define nondisjunction.
11. When does centromere division occur during meiosis?
12. Name the five substages of Prophase I.
B. Short Answer Questions
13. Explain the important features of meiosis.
14. Describe the events of leptotene.
15. Explain synapsis during zygotene.
16. Describe pachytene and explain crossing over.
17. Explain the formation and significance of chiasmata.
18. Describe the events of Metaphase I and Anaphase I.
19. Write a note on interkinesis.
20. Describe the stages of Meiosis II.
21. Explain independent assortment.
22. Compare Meiosis I and Meiosis II.
23. Describe meiosis during spermatogenesis.
24. Explain unequal cytokinesis during oogenesis.
C. Long Answer Questions
25. Describe meiosis in detail with suitable headings and explain the events of Meiosis I and Meiosis II.
26. Describe Prophase I in detail, including leptotene, zygotene, pachytene, diplotene and diakinesis.
27. Explain crossing over, chiasmata and genetic recombination and discuss their significance.
28. Compare mitosis and meiosis with respect to chromosome behaviour, DNA replication, number of divisions and products.
29. Explain how meiosis maintains chromosome number across generations and contributes to genetic variation.
30. Discuss meiotic nondisjunction and its biological consequences.
20. Examination Tips
· Remember the sequence of Prophase I as leptotene, zygotene, pachytene, diplotene and diakinesis.
· Associate zygotene with synapsis.
· Associate pachytene with crossing over.
· Associate diplotene with visible chiasmata.
· Remember that homologous chromosomes separate during Anaphase I.
· Remember that sister chromatids separate during Anaphase II.
· State clearly that DNA replication occurs only once before Meiosis I.
· For a long answer, include a comparison between Meiosis I and Meiosis II and mention genetic variation.
21. References
· Alberts, B. et al. Molecular Biology of the Cell. Garland Science.
· Cooper, G. M. and Hausman, R. E. The Cell: A Molecular Approach. Oxford University Press.
· Lodish, H. et al. Molecular Cell Biology. W. H. Freeman.
· Campbell, N. A. et al. Campbell Biology. Pearson.
· Raven, P. H. et al. Biology. McGraw Hill.
· NCBI Bookshelf. Molecular and cellular biology resources. National Center for Biotechnology Information.
22. Quick Revision
Meiosis consists of one round of DNA replication followed by two divisions. Meiosis I is reductional and separates homologous chromosomes. Meiosis II is equational and separates sister chromatids. Prophase I has five substages: leptotene, zygotene, pachytene, diplotene and diakinesis. Synapsis occurs during zygotene, crossing over occurs during pachytene and chiasmata are prominent during diplotene. Independent assortment occurs through the random orientation of homologous pairs at Metaphase I. The usual result is four haploid cells with genetically varied chromosome combinations.
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