Cell Division
Can you recall? Page No. 76
1. How do your wounds heal?
Answer:
a. A wound is an injury to living tissue.
b. Healing of wound take place by mitosis.
c. Repetitive mitotic divisions near the site of injury results in healing of wound.
2. What is the difference between growth of non-living material and living organism?
Answer:
| Aspect | Non-Living Material | Living Organism |
|---|---|---|
| Definition | Growth occurs by external addition of material (accretion) on the surface. | Growth involves an internal increase in cell number, size, or both. |
| Mechanism | Physical or chemical deposition (e.g., crystal growth, stalactite formation). | Biological processes like cell division (mitosis) and cell differentiation. |
| Regulation | Not regulated; depends on environmental conditions (e.g., availability of minerals). | Genetically and metabolically regulated by internal cellular mechanisms. |
| Energy Requirement | No metabolic energy required; driven by external physical/chemical forces. | Requires metabolic energy (e.g., ATP) from nutrients and biochemical reactions. |
| Cellular Involvement | No cells involved; purely inanimate processes. | Involves cells, organelles, and biomolecules (e.g., proteins, nucleic acids). |
| Growth Location | Occurs externally on the surface of the material. | Occurs internally within the organism (e.g., tissue growth). |
| Outcome | Increase in size or mass without functional complexity. | Increase in size, complexity, and functional specialization (e.g., organ formation). |
| Reversibility | Growth is irreversible; material does not self-repair or reorganize. | Growth can be reversible (e.g., tissue repair); organisms can regenerate or adapt. |
| Reproduction | No reproduction; growth does not lead to new entities. | Growth often linked to reproduction (e.g., formation of gametes or offspring). |
| Examples | Crystal growth (e.g., salt crystals), stalactite formation in caves. | Plant growth (e.g., stem elongation), animal growth (e.g., muscle development). |
Internet my friend Page No. 77
What is Karyogram or Karyotype?
Answer: A karyotype is the complete set of chromosomes in an organism, organized and displayed according to their size, shape, and banding pattern, typically observed during metaphase of cell division. A karyogram is the visual representation (photograph or diagram) of the karyotype, showing chromosomes arranged in pairs, numbered, and used to study chromosome number, structure, and abnormalities (e.g., Down syndrome). It helps in identifying genetic disorders and determining an organism’s chromosomal profile.
Can you tell? Page No. 79
1. What is cell a cycle?
Answer: The cell cycle is the series of stages a cell undergoes from its formation to its division into two daughter cells, consisting of interphase (growth and preparation) and the M phase (mitosis and cytokinesis). It ensures cell growth, DNA replication, and division for tissue maintenance, growth, and repair in multicellular organisms. The cycle is tightly regulated by checkpoints to prevent errors.
2. Which processes occur during interphase?
Answer: Interphase, the longest phase of the cell cycle, includes three sub-phases:
- G1 Phase (First Gap): Cell grows, synthesizes proteins, and prepares for DNA replication.
- S Phase (Synthesis): DNA replication occurs, duplicating the genome to ensure each daughter cell receives an identical set of chromosomes.
- G2 Phase (Second Gap): Cell continues to grow, checks for DNA errors, and prepares for mitosis by synthesizing proteins needed for cell division.
3. Which are the steps of mitosis?
Answer: Mitosis is the process of nuclear division, divided into four stages:
- Prophase: Chromosomes condense, the nuclear membrane breaks down, and spindle fibers form from centrioles.
- Metaphase: Chromosomes align at the cell’s equatorial plane, attached to spindle fibers at their centromeres.
- Anaphase: Sister chromatids are pulled apart to opposite poles as spindle fibers shorten.
- Telophase: Chromosomes decondense, nuclear membranes reform around each set, and spindle fibers disassemble.
Internet my friend Page No. 79
How the life span of a cell is decided?
Answer: The life span of a cell is determined by its type, function, and regulatory mechanisms within the organism. Some cells, like skin cells, have a short life span (days to weeks) due to constant replacement via mitosis, while others, like neurons, may last a lifetime with limited division. Factors such as genetic programming, cell cycle checkpoints, telomere shortening (limiting cell divisions), and external signals (e.g., hormones, growth factors) regulate the cell’s life span, balancing growth, repair, and apoptosis (programmed cell death).
Curiosity box: Page No. 81
1. What is exact structure of synaptonemal complex?
Answer: The synaptonemal complex is a proteinaceous, ladder-like structure formed between homologous chromosomes during prophase I of meiosis (specifically in the zygotene stage). It consists of two lateral elements (each associated with a homologous chromosome), a central element connected by transverse filaments, and recombination nodules (protein complexes facilitating crossing over). This tripartite structure stabilizes chromosome pairing (synapsis) and enables genetic recombination.
2. What is structure of chiasma?
Answer: A chiasma is the X-shaped point of physical contact between homologous chromosomes where crossing over occurs during prophase I of meiosis (visible in diplotene and diakinesis). It represents the site where non-sister chromatids exchange genetic material, held together by cohesin proteins until separation. Chiasmata ensure proper chromosome segregation and genetic diversity.
3. Which type of proteins are involved in formation of spindle fibres?
Answer: Spindle fibers are primarily composed of tubulin proteins, specifically α-tubulin and β-tubulin, which polymerize to form microtubules. These microtubules are organized by motor proteins (e.g., dynein and kinesin) and associated proteins (e.g., γ-tubulin in microtubule-organizing centers). These proteins work together to form the mitotic spindle, facilitating chromosome movement.
4. Why and how some spindle fibres elongate and some contract?
Answer: Spindle fibers elongate or contract to orchestrate chromosome movement during mitosis. Kinetochore microtubules contract by depolymerizing tubulin subunits at their ends, pulling chromosomes to opposite poles during anaphase, driven by motor proteins like kinesin and dynein. Polar microtubules elongate by adding tubulin subunits, pushing spindle poles apart, aiding cell elongation, regulated by motor proteins and microtubule dynamics.
5. What is the role of centrioles in formation of spindle apparatus?
Answer: Centrioles, located within centrosomes, serve as microtubule-organizing centers (MTOCs) during cell division. They duplicate and migrate to opposite poles, nucleating microtubules to form the spindle apparatus, with astral microtubules anchoring the spindle and polar microtubules aiding pole separation. In animal cells, centrioles ensure proper spindle orientation, though some cells (e.g., plant cells) form spindles without centrioles.
6. What would have happened in absence of meiosis?
Answer: In the absence of meiosis, organisms would lack a mechanism to produce haploid gametes, preventing sexual reproduction and genetic recombination. This would result in offspring with doubled chromosome numbers each generation, leading to genetic instability and non-viable organisms. Genetic diversity, essential for evolution and adaptation, would be severely limited, restricting species survival.
Can you tell? Page No. 82
1. What is difference between mitosis and meiosis?
Answer:
| Aspect | Mitosis | Meiosis |
|---|---|---|
| Definition | A single division process producing two identical daughter cells. | Two sequential divisions producing four non-identical haploid gametes. |
| Occurrence | Occurs in somatic (body) cells for growth, repair, and asexual reproduction. | Occurs in reproductive cells for gamete formation in sexual reproduction. |
| Number of Divisions | One division (prophase, metaphase, anaphase, telophase). | Two divisions (meiosis I and meiosis II). |
| Chromosome Number | Daughter cells are diploid (2n), same as parent cell. | Daughter cells are haploid (n), half the parent cell’s chromosome number. |
| Genetic Variation | No genetic variation; daughter cells are genetically identical. | Promotes genetic variation via crossing over and random assortment. |
| Stages | Prophase, metaphase, anaphase, telophase, followed by cytokinesis. | Meiosis I (reductional) and meiosis II (equational), each with four stages. |
| Purpose | Growth, tissue repair, and maintenance of multicellular organisms. | Production of gametes (sperm, egg) for sexual reproduction and diversity. |
2. What is difference between meiosis-I and meiosis-II?
Answer:
| Aspect | Meiosis-I | Meiosis-II |
|---|---|---|
| Nature of Division | Reductional division: Reduces chromosome number from diploid (2n) to haploid (n). | Equational division: Separates sister chromatids, maintaining haploid number. |
| Chromosome Behavior | Homologous chromosomes pair (synapsis) and separate, forming bivalents. | Sister chromatids of each chromosome separate, similar to mitosis. |
| Key Events | Crossing over (in prophase I) and random assortment (in metaphase I). | No crossing over; chromatids align and separate (like mitotic anaphase). |
| Stages | Prophase I (with substages: leptotene, zygotene, pachytene, diplotene, diakinesis), metaphase I, anaphase I, telophase I. | Prophase II, metaphase II, anaphase II, telophase II. |
| Outcome | Two haploid cells, each with one chromosome set (duplicated). | Four haploid cells, each with single chromatid chromosomes. |
| Genetic Variation | Introduces variation via recombination and independent assortment. | No further variation; separates chromatids for gamete formation. |
| Purpose | Reduces chromosome number to prepare for gamete formation. | Completes gamete formation by producing four distinct haploid cells. |
3. Elaborate the process of recombination.
Answer: Recombination, also known as crossing over, is a process during prophase I of meiosis (specifically in the pachytene stage) where homologous chromosomes exchange genetic material, increasing genetic diversity. It begins with the formation of the synaptonemal complex in zygotene, aligning homologous chromosomes precisely. In pachytene, recombination nodules containing enzymes (e.g., recombinases) facilitate the breakage and rejoining of non-sister chromatids at specific points, forming chiasmata (visible in diplotene). This exchange of DNA segments between maternal and paternal chromosomes creates new combinations of alleles, contributing to genetic variation in gametes, which is crucial for evolution and adaptation.
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