The Serious of events that take place in a eukaryotic cell leading to its division is called the cell cycle.
Periods of the cell cycle:
There are two broad periods of cell cycle
B. M-phase (division phase) I-Interphase:
The period between the end of one mitosis and the start of next mitosis is called interphase. Typically, interphase lasts for at least 90% of the total time required for the cell cycle therefore interphase is called the longest phase/ resting phase/ or growth phase of the cell cycle.
Cells that have temporarily or permanently stopped dividing are called G0 Phase.
Q2: Define Mitosis? Describe various stages of mitosis in detail?
A German Biologist Walther Fleming in the 1880s gave a detailed account of the stages of cell division. He observed that in a dividing cell the nucleus passes through a series of changes which he called mitosis.
Stage of mitosis:
The process of mitosis can be divided into two main stages.
It is the division of the nucleus. “Karyo” means nucleus and “kinesis” mean division.
It is the division of cytoplasm. “cyto” mean cell and “kinesis” mean division.
Various phases of Karyokinesis:I. Prophase:
i. Condensation of chromatin network occurs and thread like chromosomes appear.
ii. Each chromosome consists of two chromatids attached with each other at centromere. iii. Nuclear membrane disappears.
iv. Centrioles move to the opposite poles.
v. Three sets of spindles fibers arise from each centriole.
a. Astral microtubules:
Microtubules radiate outward and form star shaped structure called aster.
b. Kinetochore Microtubules:
Kinetochore microtubules are attached to the kinetochore of chromosome.
c. Polar microtubules:
Polar microtubules arise from one pole and come in contact with the microtubules of other poles.
ii. Metaphase:Main events:
i. During this phase chromosomes arranged itself at the center of cell to form line of chromosome called metaphase plate or equatorial plate.
ii. Two spindle fibers from both sides attach with one chromosome.
iii. Anaphase:Main events:
i. The centromere of each chromosome splits into two parts.
ii. The spindle fibers contract and they pull the chromatids (daughter chromosomes) towards their respective pole.
iii. Cytokinesis begins in anaphase.
vi . Telophase:Main events:
i. In telophase, spindle fibers breakdown.
ii. Chromosomes reach to the respective pole.
iii. Chromosomes uncoil to become thin chromatin networks.
iv. Two daughter nuclei are formed, each with the same number of chromosomes as were present in the parent nucleus.
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Understanding The Concepts
Q.1) What is cell cycle and what are its main phases ?
Answer: Cell Cycle: “Cell cycle is the series of events from the time a cell is produced until it completes mitosis and produces new cells”. Main phases of cell cycle: Cell cycle consists of two major phases i.e. 1) Interphase 2) Mitotic phase (M phase) 1) Interphase: lnterphase is the time when a cell’s metabolic activity is very high, as it performs its various functions. It is divided into three phases: i) G1 (first gap) ii) S (synthesis) iii) G2 (second gap) i) G1 phase: After its production, a cell starts its cell cycle in the G1 phase. During this phase, cell increases its supply of proteins, increases its supply of proteins, increases the number of organelles (such as mitochondria, ribosomes) and grows in size. This phase is also marked by the synthesis of various enzymes that are required in the next phase i.e. S- phase for the duplication of chromosomes.
Read more: Federal G9 Biology Notes Chapter 2 Pdf Download ii) S phase: In this phase, the cell duplicates its chromosomes. As a result, each chromosome consists of two sister chromatids. iii) G2 phase: In the G2 phase, cell prepares proteins that are essential for mitosis, mainly for the production of spindle fibers. After the G2 phase of interphase, the cell enters the division phase i.e. M phase. It is characterized by mitosis, in which cell divides into the two daughter cells. Go Phase: “Cells that have temporarily or permanently stopped dividing are said to have entered a state of quiescence, called Go phase”. In multicellular eukaryotes, cells enter Go phase from G1 and stop dividing. Some cells remain in Go for an indefinite period e.g. neurons. Some cells enter Go phase semi-permanently e.g. some cells of liver and kidney. Many cells do not enter Go and continue to divide throughout an organism’s life, e.g. epithelial cells. 2) Mitotic phase: Mitotic phase is a relatively short period of the cell cycle. It alternates with the much longer interphase, where cell prepares itself for the division.
Q.2) The S-phase of interphase is important and a cell can never divide without it. Justify.
Answer: Mitosis is the type of cell division in which a cell divides into two daughter cells, each with the same number of chromosomes as were present in the parent cell. Because each resultant daughter cell should be genetically identical to the parent cell, the parent cell must make a copy of each chromosome before mitosis. This occurs during the S phase of interphase. The S-phase is known as the replication phase of the cell cycle. During this phase, the cell’s genetic material or ‘DNA’ is copied. Since the DNA contains all of the information that a cell needs to produce essential materials, like proteins and enzymes, a cell can never divide without first going through this phase. DNA must be duplicated in the S-phase before cell division occurs. If not, the result will be 2 daughter cells with missing/un-equal amounts of genetic information. Consequently, the daughter cells will not be able to function properly and are likely to succumb to apoptosis (programmed cell death).
Q.3) How would you state the events of prophase of mitosis?
Answer: Prophase: Prophase is the first stage in mitosis and starts after the completion of interphase. Normally, the genetic material in the nucleus is in a loose thread-like form called chromatin. Condensation of chromatin: At the onset of prophase, chromatin condenses into highly ordered structures called chromosomes. Structure of the chromosome: Since the genetic material has already been duplicated earlier in S phase, each chromosome is made of two sister chromatids, bound together at the same centromere. Each chromosome also has a kinetochore at the centromere. The kinetochore is a complex protein structure that is the point where spindle fibers attach. Duplication of centrioles and spindle formation: There are two centrioles (collectively called a centrosome) close to the nucleus. Each centriole duplicates and thus two daughter centrosomes are formed. Both centrosomes migrate to the opposite poles of the cell. Here, they give rise to microtubules by joining tubulin proteins present in the cytoplasm. The microtubules thus formed are called spindle fibers. Complete set of spindle fibers is known as mitotic spindle. Degradation of nuclear envelop: By this time, nucleolus and nuclear envelope have degraded, and spindle fibers have invaded the central space. Prophase in plants: In highly vacuolated plant cells, the nucleus has to migrate to the center of the cell before prophase. The cells of plants lack centrioles. So, spindle fibers are formed by the aggregation of tubulin proteins on the surface of the nuclear envelope during prophase. Prophase in prokaryotes: Prokaryotes do not have a proper nucleus and do not form spindles during division. That is why their division is not called mitosis.
Q.4) Make a list of the events of mitosis.
Answer: Events of mitosis: The process of mitosis is complex and highly regulated. There are two major phases: 1. Karyokinesis-the division of nucleus 2. Cytokinesis-the division of cytoplasm Karyokinesis is further divided into four phases: i. Prophase ii. Metaphase iii. Anaphase iv. Telophase
Q.5) How is mitosis significant?
Answer: Significance of mitosis: Importance of mitosis is the maintenance of chromosomal set i.e. each daughter cell receives chromosomes that are alike in composition and equal in number to the chromosomes of the parent cell. Following are the occasions in the lives of organisms where mitosis happens: 1) Development and growth: The number of cells within an organism increase by mitosis. This is the basis of the development of a multicellular body from a single cell i.e. zygote and also the basis of the growth of the multicellular body. 2) Cell replacement: In some parts of the body, e.g. skin and digestive tract, cells are constantly sloughed off and replaced by new ones. New cells are formed by mitosis and so are exact copies of the cells being replaced. Similarly, red blood cells have a short lifespan (about 4 months) and new red blood cells are formed by mitosis. 3) Regeneration: Some organisms can regenerate parts of their bodies. The production of new cells is achieved by mitosis. For example, sea star regenerates its lost arm through mitosis. 4) Asexual reproduction: Some organisms produce genetically similar offspring through asexual reproduction. Mitosis is a means of asexual reproduction. For example, hydra reproduces asexually by budding. The cells at the surface of hydra undergo mitosis and form a mass called bud. Mitosis continues in the cells of bud and it grows into a new individual. The same division happens during asexual reproduction (vegetative propagation) in plants.
Q.6) Describe the events that occur during the phases of meiosis-l.
Answer: Phases of Meiosis I: In meiosis I, the homologous chromosomes in a diploid cell separate and so two haploid daughter cells are produced. It is the step in meiosis that generates genetic variations. Meiosis I occurs in two main steps: 1) Karyokinesis 2) Cytokinesis. 1) Karyokinesis of Meiosis I: The karyokinesis of Meiosis I is subdivided into: i) Prophase I ii) Metaphase I iii) Anaphase I iv) Telophase I i) Prophase I: Prophase I is the longest phase in meiosis. During this stage, chromatin condenses into chromosomes. Synapsis: The homologous chromosomes line up with each other and form pairs by a process called synapsis. Bivalent: Each pair of homologous chromosomes is called bivalent. Tetrad: Each bivalent has four chromatids, so it may also be called a tetrad. Chiasmata: The two non-sister chromatids of homologous chromosomes join each other at certain points along their length. These points of attachment are called chiasmata. Crossing over: In the next stage, the non-sister chromatids of homologous chromosomes exchange their segments and the phenomenon is known as crossing over. Genetic recombination: The exchange of segments results in the recombination of genetic information. After crossing over, each pair of homologous chromosomes remains as bivalent. Chromosomes condense further, the nucleoli disappear, and the nuclear envelope disintegrates. Centrioles, which were duplicated during interphase, migrate to the two poles and form spindle fibers. The kinetochore spindle fibers attach with the kinetochores of chromosomes. While the non-kinetochore spindle fibers from both sides interact with each other. Two kinetochore spindle fibers (from the opposite poles) attach with a pair of chromosomes.
ii) Metaphase I: The pairs of homologous chromosomes align along an equatorial plane, forming the metaphase plate. iii) Anaphase I: Kinetochore spindle fibers shorten. It results in pulling apart the chromosomes of each pair. Since one chromosome is pulled toward one pole, two haploid sets are formed. Each chromosome still contains a pair of sister chromatids. iv) Telophase I: Chromosomes arrive at the poles. Each pole now has half the number of chromosomes but each chromosome still consists of two chromatids. Spindle network disappears, and the nuclear envelope is formed around each haploid set. Chromosomes uncoil back into chromatin. 2) Cytokinesis: Cytokinesis (the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells) occurs and the creation of two haploid daughter cells is completed.
Q.7) Describe the significance of meiosis.
Answer: Significance of Meiosis: The significance of meiosis for reproduction and inheritance was described in 1890 by German biologist August Weismann. He pointed out that meiosis was necessary not only to maintain the number of chromosomes in the next generation but also to produce variations in next generation. 1) Maintenance of the chromosome number in next generation: Meiosis is essential for sexual reproduction. In humans, diploid gamete-mother cells or germline cells undergo meiosis to produce haploid gametes. Male and female gametes unite to form a diploid zygote, which undergoes repeated mitosis and develops into a new diploid human. Many haploid fungi and protozoans produce haploid gametes through mitosis. Plants’ life cycle shows alternation of generations.
The cells of diploid sporophyte generation undergo meiosis to produce haploid spores, which grow into haploid gametophyte generations. Gametophyte generation produces haploid gametes through mitosis. The gametes combine to produce diploid zygote. Zygote undergoes repeated mitosis to become diploid sporophyte. 2) Production of variations in next generations: The chromosome pairs of each parent undergo crossing over during meiosis. So daughter cells i.e. gametes have genetic variations. When gametes fuse and form zygote, its genetic make up is different from both parents. Thus meiosis allows a species to bring variations in the next generations. Beneficial variations help organisms to adapt to the changes in the environment.
Q.8) Contrast mitosis and meiosis, emphasizing the events that lead to different outcomes.
In this phase, homologous chromosomes do not form pairs. There is no crossing over.
Homologous chromosomes pair up. Crossing over takes place between these homologous chromosomes.
Single chromosomes align to form metaphase plate.
Homologous pairs align to form metaphase plate.
Chromosomes break and individual chromatids are pulled towards poles.
Individual chromosomes are pulled towards poles.
Daughter nuclei contain diploid number of chromosomes. Each chromosome has single chromatid
Daughter nuclei contain haploid number of chromosomes. Each chromosome has two chromatids
Chromosome number remains the same (2n) and yields two diploid cells
Chromosome number reduced to half (n), and yield four haploid cells
Occurs in somatic cells
Occurs in germ cells i.e. egg and sperms
Q.9) Describe necrosis and apoptosis.
Answer: Apoptosis: “Apoptosis is one of the main types of programmed cell death”. Process of apoptosis: During apoptosis, cell shrinks and becomes rounded due to the breakdown of the cytoskeleton by enzymes. Its chromatin undergoes condensation and nuclear envelope breaks. In this way, the nucleus spreads in the form of several discrete chromatin bodies. Cell membrane makes irregular buds known as blebs. Blebs break off from the cell and are now called apoptotic bodies, which are then phagocytosed by the other cells. Occurrence of apoptosis:
Apoptosis can occur when a cell is damaged or undergoes stress conditions.
Apoptosis removes the damaged cell, preventing it from getting further nutrients, or to prevent the spread of infections.
Apoptosis also gives advantages during development. For example, during the formation of fingers, the cells between them undergo apoptosis and the digits separate.
Necrosis: “Necrosis the accidental death of cells and living tissues”. Causes of necrosis: Necrosis is less sequential than apoptosis. There are many causes of necrosis including injury, infection, cancer etc. Necrosis may occur when a cell is given hypoxic (with less oxygen) environments. Process of necrosis: During necrosis, there is the release of special enzymes from lysosomes. Lysosomal enzymes break cellular components and may also be released outside the cell to break surrounding cells. Cells that die by necrosis may also release harmful chemicals that damage other cells.
Short Questions Fbise Notes class 9 biology
Q.1) A nerve cell does not divide after its formation. In which phase of cell cycle it is?
Answer: A nerve cell does not divide after its formation. The cell is called to be in “Go phase”, a phase of quiescence.
Q.2) How is cytokinesis different in plant cells as compared to animal cell?
Cytokinesis in plants
Cytokinesis in animals
The division of cytoplasm takes place by cell plate formation.
The division of cytoplasm takes place through cleavage.
Cell plate formation starts at the center of the cell and grows outwards, toward the lateral walls.
Cleavage starts at the periphery and then moves inwards, dividing the cells into two parts.
Q.3) What type of cell division occurs when our wounds are healed ?
Answer: Mitosis occurs when our wounds are healed. The number of cells within an organism increases by mitosis and this is the basis of development and growth.
Q.4) Plants do not make their gametes by meiosis. How is that ?
Answer: Plants make their gametes by mitosis. This is because in meiosis the daughter cells only contain half the number of gametes. Plants reproduce by asexual reproduction, so the cells will be identical to the plant. This is done by mitosis.
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