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CONTENTS:
UNIT I DIVERSITY IN THE LIVING WORLD
Chapter 2 : Biological Classification
UNIT II STRUCTURAL ORGANISATION IN PLANTS AND ANIMALS
Chapter 5 : Morphology of Flowering Plants
Chapter 6 : Anatomy of Flowering Plants
Chapter 7 : Structural Organisation in Animals
UNIT III CELL : STRUCTURE AND FUNCTIONS
Chapter 8 : Cell : The Unit of Life
Chapter 10 : Cell Cycle and Cell Division
UNIT IV PLANT PHYSIOLOGY
Chapter 11 : Photosynthesis in Higher Plants
Chapter 12 : Respiration in Plants
Chapter 13 : Plant Growth and Development
UNIT V HUMAN PHYSIOLOGY
Chapter 14 : Breathing and Exchange of Gases
Chapter 15 : Body Fluids and Circulation
Chapter 16 : Excretory Products and their Elimination
Chapter 17 : Locomotion and Movement
The cell cycle is divided into two basic phases:
Interphase: The phase between subsequent cell divisions is called the interphase. The interphase lasts for more than 95% of the cell cycle.
M Phase (Mitosis phase): The actual cell division takes place in the M phase. The M phase lasts for less than 5% of the cell cycle. The M phase is composed of two major steps, viz. karyokinesis and cytokinesis. Division of nucleus happens during karyokinesis. Division of cytoplasm happens during cytokinesis.
The interphase is further divided into three phases, which are as follows:
G1 phase (Gap 1): During this phase, the cell is metabolically active and continuously grows.
S phase (Synthesis): During this phase, DNA synthesis or replication takes place. The amount of DNA becomes double during this phase, but the number of chromosomes remains the same.
G2 phase (Gap 2): During this phase, protein synthesis takes place.
Quiescent Stage (G0): Cells which do not divide further, exit G1 phase to enter an inactive stage. This stage is called quiescent stage (G0) of the cell cycle. The cells in this stage remain metabolically active but do not undergo division. But these cells can resume division as and when required.
M PHASE
Mitosis is divided into four stages, viz. Prophase, Metaphase, Anaphase and Telophase
Prophase
Condensation of chromosomal material takes place. A chromosome is seen to be composed of two chromatids. The chromatids are attached together at the centromere.
Spindle fibres are formed.
Various cell organelles; like golgi bodies and ER cannot be seen during this staged. Nucleolus and nuclear envelope also disappear.
Metaphase
All the chromosomes come to lie at the equator.
In each chromosome, one chromatid is connected to the spindle fibre from one pole and another chromatid is connected to the spindle fibre from another pole.
The plane of alignment of chromosomes during this phase is called metaphase plate.
Anaphase
Centromeres split which results in separation of chromatids.
After that, chromatids move to opposite poles.
TelophaseThe chromosomes form clusters at opposite poles. They become inconspicuous. Nuclear envelope is formed around the chromosome clusters. Nucleolus, golgi complex and ER are also formed.
CytokinesisDivision of cytoplasm is achieved by cytokinesis. In animal cell, a furrow appears in the plasma membrane. The furrow gradually deepens and finally joins in the centre. Thus, the cytoplasm is divided into two parts. In plant cells, cell wall formation begins in the centre. This grows outwards to meet the existing lateral walls and thus, the cytoplasm is divided into two parts.
Significance of MitosisMitosis results in the formation of new cells which are required for growth and repair. Mitosis results in the formation of two daughter cells; which have identical genetic makeup, like the mother cell.
MEIOSIS
Meiosis involves two sequential cycles of nuclear and cell division, but only a single cycle of DNA replication. Meiosis is divided into meiosis I and meiosis II.
Meiosis I begin after the S phase, and meiosis II follows meiosis I.
Pairing of homologous chromosomes happens during meiosis which results in recombination of genes.
Four haploid daughter cells are formed at the end of meiosis.
Meiosis I
Prophase I:
Prophase in meiosis I is typically longer and more complex than the prophase in meiosis II. Prophase I is subdivided into five phases, viz. Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.
Leptotene:
During this stage, the chromosomes become gradually visible under light microscope. Compaction of chromosomes continues throughout this phase.
Zygotene:
Chromosomes start pairing together. This process is called synapsis. The paired chromosomes are called homologous chromosomes.
Formation of synapsis is accompanied by the formation of synaptonemal complex.
The synaptonemal complex by a pair of homologous chromosomes is called a bivalent or a tetrad.
Pachytene:
Bivalent chromosomes clearly appear as tetrads, at this stage.
Recombination nodules appear. These nodules are the sites at which crossing over takes place between non-sister chromatids of the homologous chromosomes.
Exchange of genetic materials between two homologous chromosomes takes place during crossing over. This leads to recombination of genetic materials on the two chromosomes.
Diplotene:
Synaptonemal complex is dissolved at this stage.
The recombined homologous chromosomes of the bivalent separate from each other; except at the site of crossing over.
The X-shaped structures; thus formed; are called chiasmata.
Diakinesis:
Chiasmata is terminated at this stage.
Meiotic spindles are formed to prepare the homologous chromosomes for separation.
Nucleolus disappears and nuclear envelope breaks down by the end of diakinesis.
Metaphase I:
The bivalent chromosomes are aligned on the equatorial plate.
Spindle fibres from opposite poles attach to the pair of homologous chromosomes.
Anaphase I:
Homologous chromosomes separate, but sister chromatids remain attached at their centromeres.
Telophase I:
Nuclear membrane and nucleolus reappear. This is followed by cytokinesis and this stage is called the diad of cells. The stage between the two meiosis divisions is called interkinesis. Interkinesis is usually short lived.
Meiosis II
Prophase II:
Meiosis II resembles the mitotic cell division. It begins immediately after cytokinesis. Nuclear membrane disappears. Chromosomes again become compact.
Metaphase II:
The chromosomes align at the equator. Spindle fibres from the opposite poles get attached to the kinetochores of sister chromatids.
Anaphase II:
Centromeres split and sister chromatids move towards the opposite poles.
Telophase II:
The two groups of chromosomes get enclosed by nuclear envelope. This is followed by cytokinesis; resulting in the formation of four daughter cells.
Significance of Meiosis:
Conservation of specific chromosome number of each species is achieved across successive generations in sexually reproducing organisms through meiosis. Meiosis helps in increasing the genetic variations in the population of organisms from one generation to the next.
NCERT Solutions
Cells are the basic building blocks of life. comprehending them is essential for comprehending many other biological concepts.
The learner is exposed to the critical principles of cell cycle and cell division in this chapter. They learn the distinction between meiosis and meitosis, as well as the relevance of each. They learn about the cell cycle phases, such as interphase, prophase, metaphase, anaphase, telophase, and cytokinesis.
1. What is the average cell cycle span for a mammalian cell?
Solution:
Average cell cycle span for a mammalian cell is 24 hours.
2. Distinguish cytokinesis from karyokinesis.
Solution:
Cytokinesis- It is the cell division of cytoplasm occurs during the M phase of the cell cycle.
Karyokinesis- It is the separation of daughter chromosomes corresponding to M phase of cell cycle
3. Describe the events taking place during interphase.
Solution:
Events taking place during interphase are as follows:
G1 phase (Gap 1) – During this stage, the cell is metabolically active. It grows and prepares the DNA to replicate.
S phase (Synthesis) – During this stage, the synthesis of DNA takes place. The DNA quantity doubles whereas the number of chromosomes remains unchanged
G2 phase (Gap 2) – During this phase, the cell advances to grow and prepare itself for division. It is during this stage that the RNA and proteins that are required for mitosis are generated.
4. What is Go (quiescent phase) of cell cycle?
Solution:
In adult animals some cell will not exhibit the cell division, and many other cells occasionally divide when there is need to replace cells that have lost because of injury or cell death. These cells exit the G1 phase to enter inactive stage of the cell cycle called G0 phase. Cells in G0 phase does not proliferate unless called on to do so. Hence, the cells in this phase tend to become inactive, stop dividing, and become specialized through the differentiation process.
5. Why is mitosis called equational division?
Solution:
Mitosis is called equational division because the number of chromosomes in the parent and progeny cells is the same.
6. Name the stage of cell cycle at which one of the following events occur:
(i) Chromosomes are moved to spindle equator.
(ii) Centromere splits and chromatids separate.
(iii) Pairing between homologous chromosomes takes place.
(iv) Crossing over between homologous chromosomes takes place.
Solution:
i) Chromosomes are moved to the spindle equator in the Metaphase.
ii) Centrosomes split and chromatids separate in the Anaphase
iii) Pairing between homologous chromosomes take place in the Zygotene stage of prophase 1 in meiosis
iv) Crossing over between homologous chromosomes takes place during the Pachytene stage of prophase 1in meiosis
7. Describe the following:
(a) synapsis (b) bivalent (c) chiasmata
Draw a diagram to illustrate your answer.
Solution:
a) Synapsis – Homologous chromosomes pair together during Zygotene of prophase-I of meiosis. This pairing is called synapsis.
b) Bivalent or tetrad is the pair of complex formed by a pair of synapsed homologous chromosome during the zygotene of prophase I of meiosis.
c) Chiasmata
During diplotene, the paired chromosomes form an X-shaped structure known as chiasmata. At chiasmata, the crossing over between two non-sister chromatids takes place.
8. How does cytokinesis in plant cells differ from that in animal cells?
Solution:
Plant cytokinesis
Animals cytokinesis
Occurs by cell plate formation
Takes place by cleavage
Cell plate moves to the centre and extends towards the exterior
Cleavage begins at the periphery and advances inwards
The fusion of vesicles originates in cell plate formation
Cleavge starts with contraction of a peripheral ring of microfilaments
Midbody is not formed
Midbody is formed with dense material in the middle of the cell.
9. Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.
Solution:
During formation of male gametes in human beings (sperms), four daughter cell formed during meiosis are equal in size. The formation of female gamete (ovum) during meiosis results in formation of four daughter cells, unequal in size. The unequal daughter cells are – one big mature ovum and 3 small polar bodies.
10. Distinguish anaphase of mitosis from anaphase I of meiosis.
Solution:
The differences are as follows:
Anaphase of mitosis
Anaphase I of meiosis
Centromere splits and chromatids separate
centromere does not split and sister chromatids remain associated at their centromere
11. List the main differences between mitosis and meiosis.
Solution:
The differences are as follows:
Mitosis
Meiosis
Occurs in somatic cells
Occurs in germ cells
Number of chromosomes stays the same as the parent cell
The number of chromosomes gets halved in comparison to parent cells.
Two daughter cells are formed
Four daughter cells are formed
Chromosomes replicate before each mitotic division
Chromosomes does not replicate before each meiotic division
12. What is the significance of meiosis?
Solution:
Significances of Meiosis:
It conserves specific chromosome number of each species achieved across generation.
Enhances the genetic variability in the population of organisms from generation to generations. These variation are significant for the evolution process.
It produces gametes for sexual reproduction
Promotes crossing over. It introduces a new combination of variations or traits
Chromosomal mutations can occur due to abnormalities during meiosis. Few of these can be beneficial to organisms.
13. Discuss with your teacher about
(i) haploid insects and lower plants where cell-division occurs, and
(ii) some haploid cells in higher plants where cell-division does not occur.
Solution:
i) Haploid insects where cell division occurs is drones of honey bee and lower plants are Spirogyra, Chlamydomonous, Pteridophytes. These haploid gametes are produced by them through mitosis and not meiosis.
ii) Spermatozoa and ova of higher animals and microspores of higher plants will not undergo cell division.
14. Can there be mitosis without DNA replication in ‘S’ phase?
Solution:
During S phase, DNA synthesis or replication of DNA takes place. DNA replication is essential for cell division.
Without DNA replication, cell division will not take place.
15. Can there be DNA replication without cell division?
Solution:
Yes, DNA replication can take place without cell division. In order to prepare for cell division, DNA replication is necessary. Cell division is the succeeding logical step that occurs pots cell division.
16. Analyse the events during every stage of cell cycle and notice how the following two parameters change
(i) number of chromosomes (N) per cell
(ii) amount of DNA content (C) per cell
Solution:
i) Yes, DNA replication can take place in G1 phase of cell cycle. The number of chromosomes remain the same and each chromosome is formed from one chromatid. In the S phase, chromosome is formed by two sister chromatids joined at the centromere. Similar conditions continue in the G2 phase, while in M phase, sister chromatids separate and move to different cells. The number of chromosomes stays the same in mitosis.
ii) Amount of DNA content in the cell remains the same in G1 phase but in S phase it doubles as the DNA replication takes place. It remains double in G2 phase but halved in the M phase of the cell cycle.
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