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Q7(a):
Describe the following:
(a) synapsis
Draw a diagram to illustrate your answer.
Solution :
1. Theoretical Definition of Synapsis
Synapsis (also known as syndesis) is the fundamental genetic event occurring during the zygotene stage of Prophase I in Meiosis I. It is defined as the precise pairing of homologous chromosomes. During this process, the paternal and maternal chromosomes of a homologous pair align side-by-side along their entire length [Per the Zygonema pairing mechanism].
2. Molecular Mechanism: The Synaptonemal Complex
The physical association is facilitated by the formation of a proteinaceous, ladder-like structure termed the synaptonemal complex. This complex acts as a scaffold to hold the homologous chromosomes in close proximity (approx. 100 nm distance), which is essential for subsequent genetic recombination (crossing over) during the pachytene stage [Ref: Genetic Recombination Theory].
3. Characteristics of Synapsis
- Homology Requirement: Pairing occurs specifically between homologous chromosomes—chromosomes that share the same gene loci and centromere position.
- Bivalent/Tetrad Formation: The resulting structure, consisting of two homologous chromosomes, is referred to as a bivalent. Because each chromosome consists of two sister chromatids, the complex is also called a tetrad [Cytological convention].
- Directionality: Synapsis often initiates at the telomeres (zipper-like initiation) and progresses toward the centromeres, or it may occur simultaneously at multiple sites along the chromosome length.
4. Illustrative Representation of Synapsis
5. Biological Significance
The biological necessity of synapsis is twofold:
- Structural Integrity: It aligns homologous loci perfectly, ensuring that when the cell proceeds to anaphase, each daughter cell receives exactly one representative of each homologous pair.
- Genetic Diversity: By bringing homologous chromatids into intimate proximity, it sets the stage for the physical exchange of DNA segments (crossing over), which is the primary driver of variation in sexually reproducing organisms [Per Mendel’s Law of Independent Assortment and Meiotic variation principles].
Final Solution: Synapsis is the phenomenon of pairing of homologous chromosomes during the zygotene phase of prophase I of meiosis, stabilized by the synaptonemal complex, resulting in the formation of a bivalent (tetrad) structure essential for genetic recombination.
More Questions from Class 11 Biology Cell Cycle and Division EXERCISES
- Q1: What is the average cell cycle span for a mammalian cell?
- Q2: Distinguish cytokinesis from karyokinesis.
- Q3: Describe the events taking place during interphase.
- Q4: What is $G_0$ (quiescent phase) of cell cycle?
- Q5: Why is mitosis called equational division?
- Q6(i): Name the stage of cell cycle at which one of the following events occur: (i) Chromosomes are moved to spindle equator.
- Q6(ii): Name the stage of cell cycle at which one of the following events occur: (ii) Centromere splits and chromatids separate.
- Q6(iii): Name the stage of cell cycle at which one of the following events occur: (iii) Pairing between homologous chromosomes takes place.
- Q6(iv): Name the stage of cell cycle at which one of the following events occur: (iv) Crossing over between homologous chromosomes takes place.
- Q7(b): Describe the following: (b) bivalent Draw a diagram to illustrate your answer.
- Q7(c): Describe the following: (c) chiasmata Draw a diagram to illustrate your answer.
- Q8: How does cytokinesis in plant cells differ from that in animal cells?
- Q9: Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.
- Q10: Distinguish anaphase of mitosis from anaphase I of meiosis.
- Q11: List the main differences between mitosis and meiosis.
- Q12: What is the significance of meiosis?
- Q13(i): Discuss with your teacher about (i) haploid insects and lower plants where cell-division occurs, and
- Q13(ii): Discuss with your teacher about (ii) some haploid cells in higher plants where cell-division does not occur.
- Q14: Can there be mitosis without DNA replication in ‘S’ phase?
- Q15: Can there be DNA replication without cell division?
- Q16(i): Analyse the events during every stage of cell cycle and notice how the following two parameters change (i) number of chromosomes ($N$) per cell
- Q16(ii): Analyse the events during every stage of cell cycle and notice how the following two parameters change (ii) amount of DNA content ($C$) per cell
CBSE Solutions for Class 11 Biology Cell Cycle and Division
Chapters in CBSE - Class 11 Biology
- The Living World
- Biological Classification
- Plant Kingdom
- Animal Kingdom
- Morphology of Flowering Plants
- Anatomy of Flowering Plants
- Structural Organisation in Animals
- Cell – The Unit of Life
- Biomolecules
- Transport in Plants
- Mineral Nutrition
- Photosynthesis
- Respiration in Plants
- Plant Growth and Development
- Digestion and Absorption
- Breathing and Exchange of Gases
- Body Fluids and Circulation
- Excretory Products and Elimination
- Locomotion and Movement
- Neural Control and Coordination
- Chemical Coordination and Integration
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