Biology Notes of Chapter 5 "Cell Cycle" Long and Comprehensive Answers

Mastering Cell Division: Comprehensive Biology Notes for Class 9 Board Exams 

    Preparing for Class 9 Board or Competitive Exams like ETEA, NEET, NMDCAT, CSS, PCS, PMS? Look no further! This article presents comprehensive Biology notes on the topic of Cell Cycle and Cell Division. Get ready to master the key concepts of Interphase, Mitosis and its Phases, and Meiosis (I and II) and its phases with these detailed answers. Designed for Grade IX students, these notes will help you score higher marks in your exams and ace any competitive exams with ease. So, gear up for success and dive into the world of Cell Cycle and Cell Division with these expertly crafted notes.

A detailed guide on Interphase, Mitosis, and Meiosis
From Notes Library of H.E.S (Health, Education, and Skills)

 

Describe the Interphase of the cell cycle.

Interphase

The period between the end of mitosis and the start of the next prophase is called Interphase.

Explanation

    In simple words, Interphase is a step of the cell cycle in which the cell grows, accumulates nutrients for division, and duplicates its DNA. In Interphase the most significant event is the replication (making copy) of DNA. Interphase is the longest part of the cell cycle which can be divided into the following three sub-phases.

1.    G1-phase (G for Gap).

The phase or period in which newly produced cells grow in size undergoes an internal chemical change and prepares the daughter cells for DNA replication is called G1-phase of Interphase.

       The following points are worth noting while describing the G1 phase.

• It is the longest sub-phase of Interphase.
• Depending on cell type it may extend from a few hours to several days.
• RNA ribosomes and several enzymes are synthesized in the G1 phase.

2. S-phase (Synthesis phase)

The phase or period in which the replication of DNA (production of an exact copy of genetic material) occurs is called the S-phase of Interphase.

    The following points are worth noting while describing S-phase.

• It is the most important event of the cell cycle.
• In addition to chromosome replication, a number of events take place in S-phase.
• Cell growth continues throughout the S-phase.
• Proteins and enzymes required in DNA synthesis also occur in S-phase.
• After DNA replication, two identical chromosomes become part of the cell and begin to be ready to enter the G2 phase.

3. G2-phase

The period or phase in which the condensation of chromosomes and construction of the microtubule framework (spindle fibers) take place is called the G2 phase.

    The following points are worth noting while describing the G2 phase.

• The period from the completion of DNA synthesis to the start of active mitosis is actually the G2 phase.
• The most important event of the G2 phase is the construction of a microtubules framework known as “Spindle fibers”.

    At the final stage of Interphase, the replication of centrioles and their movement towards both ends of the nucleus occur because of spindle fibers radiating from centrioles at each end and the cell gets ready for the division phase.

Define cell division. Describe the two types of cell division in detail.

  Cell Division

A process by which mature parent cells divide into two daughter cells and get the same number of chromosomes to that of parent cells is called cell division.

Explanation

    There are two distinct phases of cell division.

a. Karyokinesis

The division of nuclear chromosomes is called Karyokinesis.

b. Cytokinesis:

The division of cytoplasm is called Cytokinesis.

Types of cell division

    There are two types of cell division i.e. (I) Mitosis and (II) Meiosis.

I) Mitosis

A type of cell division in which a single mature cell divides into two daughter cells having the same DNA composition is called mitosis.

OR

A type of cell division in which a mature cell undergoes a series of events and produces exact copies in the form of daughter cells having the same DNA composition is called mitosis.

Explanation

    Mitosis is a continuous process and is more complicated than direct cell division. It occurs when new cells form and facilitate the growth of hair, the body, some internal organs, etc. Mitosis ensures the physical division of the cell into two daughter cells. For convenience, mitosis is divided into four phases.

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

II)    Meiosis or Reduction division

The type of cell division in which the diploid (2n) parental chromosomes reduce to haploid (n) chromosomes in daughter cells is called meiosis.

Explanation

    The term meiosis is also referred to as reduction division because the double number of chromosomes reduces to half of it and so a single diploid cell gives rise to four haploid cells. Meiosis occurs in the formation of male and female gametes through a process known as “Gametogenesis” in animals, while in plants the process is known as “Sporogenesis”. 

Define mitosis and also write down the phases of mitosis in detail.

Mitosis

A type of cell division in which a single mature cell divides into two daughter cells having the same DNA composition is called mitosis.

Phases of mitosis

    For convenience, mitosis is divided into the following four phases.

I) Prophase

The initial phase of mitosis in which chromatin material condenses and chromosomes appear is called the prophase of mitosis.

Explanation

    The prophase of mitosis occurs step by step as given below

• First of all, each centriole divides into two which move apart but remain close to the nucleus.
• The microtubule framework called “Spindle fibers” appears between each centriole. Spindle fibers are composed of a protein known as “Tubulin”.
• Following three sets of spindle fibers appear from each centriole.
• Astral microtubules: This set of spindle fiber radiates outward and forms a star-like structure “Aster”.
• Kinetochores microtubules: This set of spindle fiber attaches to “Kinetochore” (a protein on centromeres) and move the chromosomes towards poles.
• Polar microtubules: This set of spindle fiber extends from one pole and come in contact with polar microtubule from the opposite pole.
• The nuclear membrane disappears and chromosomes get free. Each chromosome has two subunits called “Chromatids” which are attached at a region called “Centromere”.

II) Metaphase

The phase of mitosis in which chromosomes arrange themselves at the center of the cell and reach its maximum thickness is called metaphase.

 Explanation

    The metaphase of mitosis occurs step by step as given below

• Chromosomes arrange themselves in the region known as the metaphase plates or equatorial plates.
• Kinetochore microtubules attach to the chromosomes but Polar microtubules remain to detach.

III) Anaphase

The phase of mitosis in which daughter chromosomes move to the opposite poles is called anaphase.

Explanation

    The anaphase of mitosis occurs step by step as given below

•  The Centromere (region of chromatid attachment) of each chromosome divides due to the pull of the spindle fiber’s contraction.
• Cytokinesis (a division of the cytoplasm) begins in anaphase.

IV) Telophase

The phase of mitosis in which many of the events of prophase reverses is called Telophase.

    The Telophase of mitosis occurs step by step as given below

• Telophase begins when chromosomes reach the poles of the daughter cell.
• The chromosomes uncoil.
• A nuclear membrane around the daughter cell appears.
• The spindle apparatus breaks down.
• The nucleolus reappears.

    At the end of Telophase Cytokinesis is completed.

Define meiosis, and also write down the phases of mitosis in detail.

Meiosis or Reduction division

The type of cell division in which the diploid (2n) parental chromosomes reduce to haploid (n) chromosomes in daughter cells is called meiosis.

    Meiosis starts after the end of the Interphase and can be divided into the following two stages i.e.
A) Meiosis I and B) Meiosis II.

A. Meiosis I

    Meiosis I comprises Prophase I, Metaphase I, Anaphase I, and Telophase I their detail is given as

1. Prophase I

    The following events take place in Prophase I, which is the longest phase (takes more than 90% time in meiosis).

• The chromosomes become visible.
• Two homologous chromosomes pair up or synapse to form tetrads (a pair of chromosomes having four chromatids)or bivalents (because of having two chromosomes).
• At this point, the chromosomes cross over allowing the chromosomes to exchange genetic material.
• Then like the prophase of mitosis nuclear membrane disappears, spindle fibers move to the center and the tetrads attach to the spindle fiber by Kinetochore.
  

2. Metaphase I

    Following events take place in Metaphase I.

• Homologous chromosomes arrange themselves on the metaphase plate.
• Spindle fiber from one pole of the cell attaches to one chromosome of each pair while that from the opposite pole attaches to the other homolog.

3. Anaphase I

    The following events take place in Anaphase I.

• Spindle fibers pull the separated homolog towards opposite poles of the cell.
• The sister chromatids (which form the duplicated homolog) remain attached and move as one unit.

    This phase is the key event in creating new genetic combinations in the sexually reproducing species through a process called “Genetic recombination”.

4. Telophase I

    The following events take place in Telophase I.

• After chromosomes reach their poles, the nuclear membrane reappears along with nucleoli.
• Each pole receives a haploid number of chromosomes.
• At the end of meiosis I the cytoplasm divides into two cells.

B. Meiosis II

    Meiosis II resembles mitosis where simply the separation of chromosomes takes place, however, it differs from mitosis in that a haploid number of chromosomes are present in each cell formed as a result of Meiosis I.

1. Prophase II

The chromosomes become more prominent. Each chromosome comprises a centromere and two chromatids. The spindle fibers also appear.

2. Metaphase II

    Chromosome arranges themselves on the equator and forms an equatorial plate.

3. Anaphase II

    The centromere of each chromosome divides, thus each daughter chromosome moves to opposite poles.

4. Telophase II

    The daughter chromosome or chromatids reach the opposite poles, the nucleoli appear and the nuclear membrane reappears. The cytoplasm then divides to form four haploid daughter cells and two from each cell entering meiosis II.