10- CELL CYCLE AND CELL DIVISION

CHAPTER NO.10 CELL CYCLE & DIVISON

A100

INTRODUCTION:

CELL CYCLE - The cell cycle is a process in which a cell grows and divides to

create a copy of it. Some organism reproduce through the cell cycle and in

complex multi cellular organism, the cell cycle is used to allow the organism to

grow, and to replace cells as they grow worn out. In animals, the whole cell cycle

takes around 24 hours from start to finish.

Cell cycle is ordered sequence of events that occurs in a cell in preparation of cell division .The cell cycle is four stage processes in which the cell increases in size,copies its DNA an: prepare to divide .The stages Gi, S, G2 make up

interphase which accounts for the span between successive cell divisions.

On the basis of stimulatory and inhibitory messages a cell receives, it decides

whether it should enter the cell cycle and divide.

Cell cycle consists of two parts;

1) Interphase

2) M-phase

1. INTERPHASE - It is the period between the end of one cell division to the

beginning of next cell division, so it is called resting phase but actually highly

metabolically active phase in which cell prepare itself for next cell division so

called preparatory phase.Interphase is the longest phase, in typically human

cell out of 90 hour interphase last for 89 hour. It is the resting phase of cell.

Resting refer to the rest from division. But the cell in the interphase, are

metabolically active. Metabolic activities are high in this phase. During this

phase, cells grow with MRNA and tRNA is synthesized, the chromosomes duplicate into two chromatids. The centrioles duplicate into two. Two centrioles are formed. The Centrosphere of centrioles microtubules arises.These microtubules form asters.

On the basis of synthesis activities, Interphase is divided into three sub phases;

(i) Go PHASE:-

a) It is the resting phase.

b) In these cells CYCLIN - D is in decreased concentration.

c) Growth factors stimulation takes the Gp cells to G; phase.

d) In this phase, cell prepares itself to the cell division.

e) The term post-mitotic is sometime used to refer both quiescent

and senescent cell.

(ii) Gi: PHASE: - It is the period between mitosis and initiation of DNA

replication. It is characterized by increase in cell size and nucleus. It

involves;

a) G stands for Gap.

b) It is also called Growth Phase.

c) Polling of amino acid for protein synthesis.

d) Polling of nucleotides for synthesis of RNA and DNA

e) Synthesis of enzymes and energy to unzip and synthesize DNA molecules.

f) This phase is the gap period between a mitotic phase and S phase of cell cycle.

g) It is the longer phase in which daughter cell grows and increase in size.

h) In this phase, 20 amino acids are formed from which millions of proteins and enzymes are formed.

i) During this phase, concentration of CYCLIN - D increases which results in phosphorylation and activation of necessary transcription protein.

(iii) S- Phase - It is phase between S & M phase.It is characterized by;

a) DNA replicates so each chromosome is formed of two sister chromatides joined at centromere.

b) Synthesis of histones (basic proteins).

c) Phosphorylation and activating proteins and enzymes that are involved in DNA Synthesis.

d) During this, DNA Synthesis occurs.

e) DNA molecules duplicates and all the chromosomes have been replicated.

f) Synthesis of histone proteins occur.

g) Specific events during cell divisions such as microtubules formation and chromatic remodeling occur in this phase.

(iv) Gz PHASE: - It involves;

a) Synthesis of spindle protein.

b) Synthesis of three types of RNA molecules.

c) Synthesis and storage of ATP molecules.

d) The G2 phase is the gap between S- phase and Mitotic phase of a cell cycle.

e) Itis the period of rapid cell growth and protein synthesis in which the cell ready it for mitosis.

f) The nucleus increases in volume.

g) This phase has double the number of chromosomes.

h) All the other cellular components are duplicated for two daughter cells.

i) Cyclin complexes are active which are necessary for cell to enter into

M-phase.

2. M-PHASE (MITOSIS) :

(i) Mitosis is the distribution of the two sets of chromosomes into two separate and equal nuclei.

(ii) This is the division stage and during this phase cell divides.

(iii) This phase has a short duration.

(iv) This phase has two sub-phases called Karyokinesis and Cytokinesis. Karyokinesis refer to the division of nucleus into two daughter nuclei which has four sub-stages;

a) Prophase

b) Metaphase

c) Anaphase

d) Telophase

(v) Cytokinesis refers to division of cytoplasm resulting in two

daughter cells.

“LET US KNOW WHAT WE HAVE LEARNT!”

PART: A —- VERY SHORT ANSWER TYPE QUESTIONS:

MULTIPLE CHOICE QUESTIONS:

Q1. During G: phase of cell division:

(a) RNA and proteins are synthesized.

(b) DNA and proteins are synthesized.

(d) Cell undergoes duplication.

Q2. In cell cycle DNA replication occurs during:

(a)G1 Phase

(b)G2 Phase

(d)S- Phase

Q3. Which of following sequence is a correct one for a meiotic cell cycle?

(a)G1-S-G2-M-G1

(b)G1-G2-S-M-G2

(c)G2-S-G1-M-G2

(d)S-G1-G2-M-S

TRUE / FALSE:

Q1. Interphase is also called as resting phase.

Q2. Increase in cell size and nuclueus occur during G1 phase.

Q3. In animals cell cycle takes place 72 hours from start to finish.

FILL IN BLANKS:

Q1. G1, S and G2 are sub phases of .

Q2. Replication occurs during .

ANSEWER KEY: PART-A

MULTIPLE CHOICE QUESTIONS:

ANS:1. (a)

ANS:2. (d)

ANS:3. (a)

TRUE OR FALSE:

ANS:1. True

ANS: 2. True

ANS:3. False

FILL IN THE BLANKS:

ANS: 1. Interphase.

ANS: 2. S — Phase

PART:B SHORT ANSWER TYPE QUESTIONS:

Q1. Define cell cycle?

Q2. Write a short note on Interphase.

Q3. Show cell cycle with the help of diagram.

PART:C LONG ANSWER TYPE QUESTIONS:

Q1. Explain different phases of cell cycle.

A101

INTRODUCTION:MITOSIS

1. It is the cell division occurring in somatic cells.

2. It is also called as equational division as the number of chromosomes in the parent and progeny cells is the same.

3. Mitosis is generally seen in diploid cells. It also occurs in haploid cells of some lower plants and some social

insects.

4. It involves major reorganization of all cell components.

5. The KARYOKINESIS (division of nucleus) in mitosis has four stages: Prophase, Metaphase, Anaphase and Telophase.

6. The CYTOKINESIS; (division of nucleus) of mitosis is different in plant cell and animal cell.

MITOSIS; THE SOMATIC CELL DIVISION

KARYOKINESIS: THE DIVISION OF NUCLEUS

1. PROPHASE

It is the longest phase in mitosis. It follows the S and G2 phases of

interface in the S and G2 phases, DNA molecules are intertwined.Chromosomal materials are untangled and condensed to form mitotic chromosomes. They are seen to be composed of two chromatids attached together at the centromere.Centrosomes begin to move towards opposite poles of the cell.Each centrosome radiates out microtubules called asters. The two

asters together with spindle fibres form mitotic apparatus.Cells at the end of the prophase do not show Golgi complexes,

endoplasmic reticulum, nucleolus and nuclear envelope.

2. METAPHASE The nuclear envelope completely disintegrates so the chromosomes

spread through the cytoplasm of the cell.

Chromosomes condensation is completed. They can be observed and studied easily under the microscope. They will have two sister

chromatids.Chromosomes come to lie at the equator. The plane of alignment of

the chromosomes at the metaphase is called metaphase plate.The spindle fibres from both poles are connected to chromatids by

their kinetochores in the centromere.

3. ANAPHASE It is the shortest phase in the mitosis.Centromere of each chromosome divides longitudinally resulting in

the formation of two daughter chromatids (chromosomes of the future daughter nuclei).As the spindle fibres contract the chromatids move from the equator to the opposite poles.

4. TELOPHASE Chromosomes cluster at opposite poles and uncoil into chromatin

fibres.Nuclear envelope develops around the chromosome clusters at each pole. So, two daughter nuclei are formed.Nucleolus, Golgi complex and ER reappear.The spindle fibres disappear.

CYTOKINESIS It is the division of cytoplasm to form two daughter cells.It starts when TELOPHASE is in progress.

CYTOKINESIS IN ANIMAL CELL:Here, a cleavage furrow is appeared in the plasma membrane.

It gradually deepens and joins in the centre dividing the cytoplasm

into two.CYTOKINESIS IN PLANT CELL:It is different from cytokinesis in animal cells due to the presence of cell wall.In plant cells, the vesicles formed from Golgi bodies accumulate at the equator. it grows outwards and meets the lateral walls. They

fuse together to form cell-plate. It separates the two daughter cells.

Later, the cell plate becomes the middle lamella.During cytokinesis, organelles like mitochondria and plastids get

distributed between the daughter cells.

In some organisms, Karyokinesis is not followed by Cytokinesis. As a result, multinucleate condition (syncytium) arises. eg. liquid endosperm in coconut.

SIGNIFICANCE OF MITOSIS It produces diploid daughter cells with identical genome.

It helps to retain the same chromosome number in all somatic cells.It helps in the body growth of multicellular organisms. Mitosis in the meristematic tissues helps in a continuous growth of plants

throughout the life.It restores the nucleo-cytoplasmic ratio that disturbed due to cell growth.It helps in cell repair & replacement. e.g. cells of the upper layer of the epidermis, lining of the gut and blood cells.

Dear students,

LET US KNOW WHAT WE HAVE LEARNT!

A) MCQs

1. Mitosis is characterised by:

a. Reduction division

b. both (a) and (b)

c. Equal division

d. Pairing of homologous chromosomes

2. Cleavage differs from mitosis in:

a. synthetic phase

b. both (a) and (b)

c. growth phase

d. none of these

3. In mitosis centromere divides at:

a. Metaphase

b. Anaphase

c. Telophase

d. Prophase

4. During cell division, nuclear envelope reappears in:

a. Interphase

b. Telophase

c. Prophase

d. G2 phase

5. DNA duplication takes place in:

a. S- phase of cell cycle

b. G2 phase of cell cycle

c.G; phase of cell cycle

d. None of these

B) TRUE /FALSE

1. The cell cycle involves amitosis, mitosis and meiosis.

2. Mitosis occurs only in multicellular organisms.

3. Anastral spindle occurs in animal cells.

C) FILL UPS

1. Somatic cells multiply by ...............

2. ............. Chromosomes are exactly alike.

ANSWER KEY: PART -A

A) MCQs

Ans. 1:

In this division the chromosomes number remains same in the daughter

cells.

Ans. 2: ¢

Cleavage differs from mitosis in growth phase. There is no growth phase

in cleavage.

Ans. 3: b

In Anaphase stage centromere divides.

Ans. 4: b

Nucleolus and nuclear membrane reappear during Telophase.

Ans. 5: a

DNA duplication takes place during S- phase or synthetic phase of cell division.

B) TRUE & FALSE

1. False: The cell cycle involves INTER PHASE and M-PHASE.

2. False: Mitosis occurs in all the somatic cells of all the organisms.In unicellular organisms it is the source of reproduction also.

3. False: Anastral spindle occurs in higher plants.

C) FILL UPS

1. Mitosis

2. Homologous

1. Why is mitosis known as equational division?

2. Why is so called resting stage, the interphase considered the most

active stage of cell cycle?

3. Write a short note on spindle apparatus.

1. What is the significance of mitosis?

A102

INTRODUCTION:MEIOSIS

It is the division of diploid germ cells that reduces the chromosome number

by half forming haploid daughter cells (gametes).It occurs during gametogenesis.

It leads to the haploid phase in the life cycle of sexually reproducing

organisms.Fertilisation restores diploid phase.

KEY FEATURES OF MEIOSIS:It involves two cycles (meiosis | and meiosis Il) but only a single cycle of DNA replication.

It involves pairing of homologous chromosomes and recombination between their non-sister chromatids.Meiosis | begin after replication of parental chromosomes to form

identical sister chromatids at the S -phase.In DIPLOID cell after Interphase the number of chromosomes remains the same( 2n) BUT each chromosome change its shape

from single stranded (MONAD) to double stranded(DYAD).In a DIPLOID cell the amount of DNA content changes from 2c to

Ac.Four haploid cells are formed at the end of the Meiosis Il.

It is typically longer and more complex.

It includes 5- phases based on chromosomal behaviour:Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.

a. Leptotene (Leptonema):Chromatin fibres become long slender chromosomes.Nucleus enlarges.

b. Zygotene (Zygonema):Chromosomes become more condensed.Similar chromosomes start pairing together (synapsis) with the help of a complex structure called synaptonemal complex.The paired chromosomes are called homologous chromosomes.Each pair of homologous chromosomes is called a bivalent.

c. Pachytene (Pachynema):Comparatively longer phase.Bivalent chromosome split into similar chromatids. This stage is

called tetrads. During this, recombination nodules appear at which crossing over occurs. It leads to genetic recombination on homologous chromosomes.Crossing over: The exchange of genetic material between non sister chromatids of two homologous chromosomes in the presence of an

enzyme, recombinase.Recombination is completed by the end of the pachytene.

d. Diplotene (Diplonema):Dissolution of the synaptonemal complex occurs.The recombined homologous chromosomes of the bivalents

separates from each other except at the sites of cross overs.These X shaped structures are called Chiasmata.

e. Diakinesis:Terminalisation of chiasmata.

Chromosomes are fully condensed.The meiotic spindle fibres originate from the poles to prepare the homologous chromosomes for separation.Nucleolus and Nuclear envelope disappear.

METAPHASE Spindle formation in completed.

The chromosomes align on the equatorial plate.The microtubules from the spindle attach to the pair of : homologous chromosomes.

ANAPHASE The homologous chromosomes separate while sister chromatids

remain associated at their centromeres.

TELOPHASE The nuclear membrane and nucleolus reappear.Two polar groups of DYAD chromosomes organise themselves

into two daughter haploid nuclei with 2n amount of DNA.Spindle fibres and astral rays disappear.After this, cytokinesis may or may not occur.After a short interphase, it is followed by Meiosis Il.This short stage between the two meiotic divisions is called interkinesis. DNA replication does not occur in this phase.

SIGNIFICANCE OF MEIOSIS It conserves the chromosome number of each species. It causes genetic variation (due to crossing over) in the population of organism.

It is important for evolution.

Dear students,

LET US KNOW WHAT WE HAVE LEARNT!

A) MCQs

1. Meiosis results in:

a. Production of gametes

b. Introduction of variation

c. Reduction in chromosomes

d. All of the above

2. During anaphase of meiosis:

a. Homologous chromosomes separate

b. Non-homologous autosomes separate

c. Sister chromatids separate

d. Non-sister chromatids separate

3. Terminalisation is related to:

a. Meiosis |

b. Meiosis II

c. Meiosis

d. Cytokinesis

4. Bivalent are arranged at the equator in:

a. Prophase-ll

b. Metaphase-ll

c. Metaphase-l

d. Anaphase-|

5. A bivalent of meiosis-| consists of:

a. Two chromatids and one centromere

b. Two chromatids and two centromere

c. Four chromatids and two centromere

d. Four chromatids and four centromere

B) TRUE & FALSE:

1. Two cycles of DNA replication occurs in meiosis.

2. Chiasmata are observed in metaphase I.

3. Meiosis occurs in root tip.

C) FILL UPS

1. In meiosis haploid condition is reached by ........... stage.

2. Chiasmata are the expressions of ..............

PART A: VERY SHORT ANSWER TYPE QUESTIONS

A) MCQs

Ans. 1: d

It involves production of gametes, reduction in chromosomes and introduction

of variation.

Ans. 2: a

Homologous chromosomes separates from centromere.

Ans. 3: a

Terminalisation is completed during diakinesis.

Ans. 4. ¢

Bivalent chromosomes align on the equatorial plate.

Ans. 5: ¢

Synapsis between homologous chromosomes leads to formation of bivalent.

B) TRUE & FALSE

1. False: One cycle of DNA replication occurs before meiosis 1 only.

2. True: A few Chiasmata are observed in metaphase |.

3. False: Mitosis occurs in root tip. Meiosis occurs only in germ cells to

form gametes.

C) FILL UPS

1. Anaphase - |

2. Crossing over

PART: B SHORT ANSWER TYPE QUESTIONS:

1. Describe the following: i. Synapsis ii. Bivalent iii. Chiasmata

2. What is crossing over? Give its significance.

3. What are homologous chromosomes? What happens to homologous

chromosomes during meiosis?

PART: C LONG ANSWER TYPE QUESTIONS

1. What is meiosis? Explain various stages of prophase - | only.

A103

INTRODUCTION:MEIOSIS

It is the division of diploid germ cells that reduces the chromosome

number by half forming haploid daughter cells (gametes).It occurs during gametogenesis.It leads to the haploid phase in the life cycle of sexually reproducing

organisms.Fertilisation restores diploid phase.

KEY FEATURES OF MEIOSIS:It involves two cycles (meiosis | and meiosis Il) but only a single cycle of DNA replication.It involves pairing of homologous chromosomes and recombination between their non-sister chromatids.Meiosis | begins after replication of parental chromosomes to form

identical sister chromatids at the S phase.

Four haploid cells are formed at the end of the Meiosis II.

MEIOSIS Il It resembles the mitosis. It has the following phases.

PROPHASE II:It is initiated immediately after cytokinesis of meosis-1 The chromosomes again become compact.

Nucleolus and nuclear membrane disappear in both nuclei.

METAPHASE II:The chromosomes align at the equator and the microtubules from

opposite poles of the spindle get attached to the kinetochores of sister chromatids. Metaphasic plate is formed.

ANAPHASE II:It begins with the simultaneous splitting of the centromere of each

chromosome which was holding sister chromatids together.So the separated chromatids move towards opposite poles of the cell by shortening of microtubules attached to kinetochores.

TELOPHASE II:The two groups of chromosomes get enclosed by a nuclear envelope.

CYTOKINESIS follows resulting in the formation of tetrads of cells,i.e., 4- haploid daughter cells.

SIGNIFICANCE OF MEIOSIS II:

Meiosis - Il is the second division of meiosis in which the replicated

chromosomes (in Meosis-1) split and single stranded chromosomes (chromatids) pass on to daughter cells. So it maintains haploid number of chromosomes obtained after Meiosis-l, changing 2n DNA content to 1n DNA content of the cells. It is called HOMOTYPIC or EQUATIONAL division as it

maintains the same number of chromosomes.

Dear students,

LET US KNOW WHAT WE HAVE LEARNT!

PART A: VERY SHORT ANSWER TYPE QUESTIONS:

A) MCQs:

1. At which stage of meiosis does the genetic constitution of gametes is

finally decided:

a. Metaphase — |

b. Anaphase - II

c. Metaphase - II

d. Anaphase - |

2. During which stage of spermatogenesis the chromosomes are associated with tetrads?

a. Pachytene

b. Diplotene

c. Zygotene

d. Leptotene

3. Diploid chromosome number is 8. What shall be the number of chromatids in each daughter cell after meiosis- 1?

a. 8

b. 4

c. 2

d. 16

4. In eggs, active period of growth and metabolism is:

a. Leptotene

b. Zygotene

c. Pachytene

d. Diplotene

5. Sister chromatids separate and move to the opposite poles in:

a. Anaphase - II

b. Anaphase - |

c. Telophase -— ||

d. Telophase — |

B) TRUE / FALSE:

1. Meiosis involves two successive divisions, resulting in 4 daughter cells.

2. Chiasmata are observed in metaphase - |.

3. Division of cytoplasm is called cytokinesis.

C) FILL UPS:

1. Gi, S and G2 are sub phases of ............

2. Meiosis is best studied in smears of young .............

ANSWER KEY: PART -A

A) MCQs:

Ans. 1: b

In this, splitting of centromere takes place.

Ans. 2: a

In this stage, bivalent chromosomes appear as tetrads.

Ans. 3: a

Number remains same after meiosis.

Ans. 4: d

Diplotene can last for months or years. So, it is a long stage.

Ans. 5: a

Splitting of centromere of each chromosome which hold the sister chromatids.

B) TRUE & FALSE:

1. True

2. True

3. True

C) FILL UPS:

1. Interphase

2. Anthers

PART: B SHORT ANSWER TYPE QUESTIONS:

1. Meiosis-Il is not mitosis. Why?

2. What is the significance of meiosis — II?

3. What is the need of meiosis — II?

PART: C LONG ANSWER TYPE QUESTIONS:

1. Explain in detail meiosis — II.

A104

Dear students,

LET US KNOW WHAT WE HAVE LEARNT!

A) MCQs

1. Mitosis is characterised by:

a. Reduction division

b. both (a) and (b)

c. Equal division

d. Pairing of homologous chromosomes

2. In mitosis centromere divides at

a. Metaphase

b. Anaphase

c. Telophase

d. Prophase

3. Main feature of Anaphase of mitosis is:

a. Arrangement of chromosomes at equator

b. Spindle formation

c. Separation of daughter chromosomes

d. Cytokinesis

4. Chiasma formation occurs in

a. Mitosis

b. Meiosis

c. Prophase

d. G2 phase

5. Meiosis occurs in organisms during

a. Sexual reproduction

b. Asexual reproduction

c. Both a and b

d. None of these

B) TRUE & FALSE

1. Mitosis produces four daughter cells.

2. Mitosis occurs only in multicellular organisms.

3. Synapsis is present in meiosis.

C) EILL UPS

1. Somatic cells multiply by .

2. Mitosis produces no .

PART A: VERY SHORT ANSWER TYPE QUESTIONS

A) MCQs

Ans. 1:¢

EXPLANATION: in this division the chromosome remains same in the daughter

cells.

Ans. 2:b

EXPLANATION: centromere divides and chromatids separate.

Ans. 3:¢

EXPLANATION: in this phase daughter chromosomes separate

Ans. 4:b

EXPLANATION: in prophase-1 of meiosis chiasma formation takes place.

Ans. 5: a

EXPLANATION: sexually reproducing organisms divides by meiosis.

B) TRUE & FALSE

1. False

2. False

3. True

C) FILL UPS

1. Mitosis

2. Variations

1. Why is mitosis known as equational division?

2. What is the significance of meiosis?

1. Write down the difference between mitosis and meiosis.

A105

INTRODUCTION:MITOSIS

1. It is the cell division occurring in somatic cells.

2. It is also called as equational division as the number of chrosomes in the parent and progeny cells is same.

3. The karyokinesis of mitosis has four stages: Prophase,Metaphase, Anaphase and Telophase.

1. PROPHASE It is the longest phase in mitosis. It follows the S and G2 phases of

Chromosomes come to lie at the equator. The plane of alignment of the chromosomes at the metaphase is called metaphase plate.

The spindle fibers from both poles are connected to chromatids by their

kinetochores in the centromere.

Centromeres split and chromatids separate.Chromatids move to opposite poles.

Nuclear envelope develops around the chromosome clusters at each pole. So, two daughter nuclei are formed.The spindle fibers disappear.

It is the division of cytoplasm to form two daughter cells.It starts when telophase is in progress.

In plant cells, cytokinesis takes place by cell plate formation.In animal cells, cytokinesis takes place by cell furrow.

Dear students,

LET US KNOW WHAT WE HAVE LEARNT!

A) MCQs

1. Mitosis is characterised by:

a. Reduction division

b. both (a) and (b)

c. Equal division

d. pairing of homologous chromosomes

2. In which stage chromosomes arrange in the equatorial plate

a. prophase

b. metaphase

c. anaphase

d. none of these

3. In mitosis centromere divides at

a. Metaphase

b. Anaphase

c. Telophase

d. Prophase

4. Which statement is correct for anaphase

a. Chromatids move towards periphery.

b. Nuclear membrane appears.

c. Chromosomes become distinct.

d. None of these

5. Chromosome number remains same in

a. S phase of cell cycle

b. Mitosis

c. G2 phase of cell cycle

d. Meiosis

B) TRUE & FALSE:

1. Cytokinesis is the division of cytoplasm.

2. Four daughter cells are produced in mitosis.

3. Chromatids move to opposite poles in anaphase.

C) FILL UPS:

1. Mitosis is division.

2.Karyokinesis is the division of .

A) MCQs

Ans. 1:¢

EXPLANATION: In this division the chromosomes number remains same

in the daughter cells.

Ans. 2: b

EXPLANATION: chromosomes lie at equator plate in metaphase.

Ans. 3: b

EXPLANATION: In Anaphase stage centromere divides.

Ans. 4. a

EXPLANATION: Chromatids move towards periphery in anaphase.

Ans. 5:6

EXLANATION: Because mitosis is equational division.

B) TRUE /FALSE:

1. True

2. False

3. True

C) FILL UPS:

1. equational

2. nucleus

1. Draw a labeled diagram of metaphase stage of mitosis.

2. Identify following figures and name the stage.

1. Draw well labeled diagrams of mitosis.

A106

INTRODUCTION:MEIOSIS -|

It is the division of diploid germ cells that reduces the chromosome number by half, forming haploid daughter cells (gametes).

It is divided into two parts that are KARYOKINESIS and CYTOKINESIS.

KARY OKINESIS has four phases:

PROPHASE -I

It is typically longer and more complex.

It includes 5 phases based on chromosomal behaviour:Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.

a. LEPTOTENE (LEPTONEMA):Chromatin fibres become long slender chromosomes.Nucleus enlarges.

b. ZYGOTENE (ZYGONEMA):Similar chromosomes start pairing together (synapsis) with the help of a complex structure called synaptonemal complex.Each pair of homologous chromosomes is called a BIVALENT.

c. PACHYTENE (PACHYNEMA):Bivalent chromosome split into similar chromatids.

Crossing over: The exchange of genetic material between non sister chromatids of two homologous chromosomes in the

presence of an enzyme, recombinase.

d. DIPLOTENE (DIPLONEMA):Chiasma formation takes place.

e. DIAKINESIS:Terminalisation of chiasmata.

The meiotic spindle fibres originate from the poles.Nucleolus and Nuclear envelope disappear.

METAPHASE -|Spindle formation.The Bivalent chromosomes align on the equatorial plate.

ANAPHASE - |The homologous chromosomes separate while sister chromatids remain associated at their centromeres.

TELOPHASE -|The nuclear membrane and nucleolus reappear and two haploid daughter nuclei are formed.After this, cytokinesis may or may not occur.

Dear students,

LET US KNOW WHAT WE HAVE LEARNT!

A) MCQs

1. Meiosis results in:

a. Production of gametes

b. Introduction of variation

c. Reduction in chromosomes

d. All of the above

2. During anaphase — | of meiosis:

a. Homologous chromosomes separate

b. Non-homologous autosomes separate

c. Sister chromatids separate

d. Non-sister chromatids separate

3. Terminalisation is related to:

a. Meiosis |

b. Meiosis II

c. Meiosis

d. Cytokinesis

4. Bivalent are arranged at the equator in:

a. Prophase-ll

b. Metaphase-ll

c. Metaphase-l

d. Anaphase-|

5. A bivalent of meiosis-| consists of:

a. Two chromatids and one centromere

b. Two chromatids and two centromere

c. Four chromatids and two centromere

d. Four chromatids and four centromere

B) TRUE & FALSE

1. Bivalent is formed during zygotene stage.

2. Chiasmata are observed in metaphase I.

3. Meiosis occurs in germ cells.

C) FILL UPS

1. In meiosis haploid condition is reached by ..............stage.

2. Chiasmata are the expressions of ..............

A) MCQs

Ans. 1: d

EXPLANATION: It involves production of gametes, reduction in chromosomes and introduction of variation.

Ans. 2: a

EXPLANATION: Homologous chromosomes separate from centromere.

Ans. 3:a

EXPLANATION: Terminalisation is completed during diakinesis of Prophase-1

of Meiosis -1.

Ans. 4: ¢

EXPLANATION: Bivalent chromosomes align on the equatorial plate.

Ans. 5:¢

EXLANATION: Synapsis between homologous chromosomes leads to formation of bivalent.

B) TRUE & FALSE

1. True

2. False: Chiasmata are observed in Diplotene of prophase - I.

3. True

C) FILL UPS

1. Anaphase - |

2. Crossing over

PART: B SHORT ANSWER TYPE QUESTIONS:

1. Draw diagram of prophase-1.

2. Draw diagram where Chiasmata is formed.

PART: C LONG ANSWER TYPE QUESTIONS

1. What is meiosis? Draw diagrams of meiosis-2 only.

A107

INTRODUCTION:CELL DIVISION is a process in which parent cell after replication of its component s,divides to form new (daughter) cells. A cell divides by two methods: MITOSIS and MEIOSIS.In this cell division daughter cells have equal number of chromosomes and amount of DNA as that of the parent. Itis known as [A or ED |t occurs in somatic cells.

1. Daughter cells are identical to the parents as itis an equational cell division having equal number of chromosomes and same amount of DNA as the parent.

2. Restores the proper size of the cell.

3. Related to the growth of an individual as occurs by increasing the number of cells.

4. Helps in vegetative propagation and asexual reproduction of organisms.

5. In unicellular organisms lead to asexual reproduction.

6. Responsible for healing wounds, repairing cells and regeneration of body cells.

7. Occurs in somatic cells and leads to somatic variations.

8. Maintains haploid and diploid genetic constitution.

It is reductional cell division as chromosomes are reduced from diploid to haploid ,occurs in germ cells of gonads (testis/ovary) during the formation of gametes.

1. Essential in the life cycle of all the sexually reproducing organisms.

2. Helps in maintaining the same number of chromosomes in a species.

3. Responsible for variations and evolution as it develops new varieties and brings

about gene mutations.

4. Similar in all the sexually reproducing organisms which shows their basic similar

evolutionary relationship.

LET US KNOW WHAT WE HAVE LEARNT!

PART: A VERY SHORT ANSWER TYPE QUESTIONS:

A) Multiple choice questions:

1. Mitosis is characterized by :

(a) Reduction division

(b) Equational division

(d) None

2. Meiosis occurs in organisms during:

(a) Sexual reproduction

(b) Vegetative reproduction

(d) None

3. Meiosis results in:

(a) Production of gametes

(b) Introduction of variations

(d) all of the above

4. A somatic cell divides by:

(a) Mitosis

(b) Meiosis

(d) Vegetative propagation

5. Identify the wrong statement about meiosis:

(a)Occurs in sexually reproducing organisms

(b)Reduction in the chromosome numbers to half

(c)Responsible for gene mutations

(d)Occurs in somatic cell

B) Fill in the blanks:

1. Somatic cells multiply by ;

2. Mitosis results in the formation of nucleus having number of chromosomes.

3. Mitosis helps in vegetative reproduction and reproduction.

C) True/False:

1. Meiosis is responsible for the development of new varieties.

2. Meiosis is an equational division.

ANSWER KEY: PART-A

A)Multiple choice questions:

1. (b) In mitosis daughter cells have equal number of chromosomes and amount

of DNA as that of parent cell.

2. (a) Meiosis occurs in sexually reproducing organisms.

3. (d)Meiosis leads to variations, number of chromosomes are reduced to half,

occurs in gonads.

4. (a) Asomatic cell divides by mitosis.

5. (d)Meiosis occurs in germ cells inside gonads..

B) Fill in the blanks:

1. Mitosis It occurs in somatic cells

2. Same Daughter cells formed during mitosis have same no. of chromosomes

and amount of DNA as that of parent cell.

3. Asexual.

C) True/False:

1. True

2. False: Meiosis is a reductional division.

PART: B SHORT ANSWER TYPE QUESTIONS:

1. Why is mitosis known as equational division?

2. Define meiosis.

3. What is cell division?

PART: C_ LONG ANSWER TYPE QUESTIONS:

1. What is the significance of Mitosis and Meiosis?

A108

RECAPITULATION:CELL DIVISION is a process in which parent cell after replication of its components,divides to form new (daughter) cells. A cell divides by two methods: MITOSIS and MEIOSIS.In this cell division daughter cells have equal number of chromosomes and amount of DNA as that of the parent. It is known as i or . |t occurs in somatic cells.

It is reductional cell division as chromosomes are reduced from diploid to haploid,occurs in germ cells of gonads (testis/ovary) during the formation of gametes.

NOW LET US DO NCERT QUESTIONS ANSWERS:

Question 1:What is the average cell cycle span for a mammalian cell?

Answer 1:The average cell cycle span for a mammalian cell is approximately 24 hours.

Question 3:Describe the events taking place during interphase.

Answer 3:Interphase involves a series of changes that prepare a cell for division. It is a period during which the cell experience growth and DNA replication in an orderly manner.Interphase is divided into three phases.G: phase S phase G2 phase

G, phase - It is the stage during which the cell grows and prepares its DNA for

replication. In this phase, the cell is metabolically active.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.

Question4:What is Go (quiescent phase) of cell cycle?

Answer 4:In adult animals some cells 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 G; phase to enter inactive stage of the cell cycle called Gp phase. Cells in Go phase do 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.

Question5:Why mitosis is called equational division?

Answer 5:Mitosis is called equational division because the number of chromosomes in the parent and progeny cells is the same.

Question 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.

Answer :6 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 takes place in the Zygotene stage of

prophase-1 in meiosis.

iv) Crossing over between homologous chromosomes takes place during the

Pachytene stage of prophase -1 in meiosis.

Question 7:Describe the following:

(a) synapsis

(b) bivalent

Draw a diagram to illustrate your answer.

Answer :7 a) SYNAPSIS — Homologous chromosomes pair together during Zygotene of prophase-| 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 | 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.

Question :9 Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.

Answer :9During 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.

Question :12 What is the significance of meiosis?

Answer :12 Significances of Meiosis:

It conserves specific chromosome number of each species achieved across

generations.Enhances the genetic variability in the population of organisms from generation to generation. These variations are significant for the evolution process.It produces gametes for sexual reproduction Promotes crossing over. It introduces a new combination of variations or traitsChromosomal mutations can occur due to abnormalities during meiosis. a few of these can be beneficial to organisms.

Question :13 Discuss with your teacher about

(i) haploid insects and lower plants where cell-division occurs.

(ii) some haploid cells in higher plants where cell-division does not occur.

Answer :13

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.

Question :14 Can there be mitosis without DNA replication in ‘S’ phase?

Answer :10 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.

Question :15 Can there be DNA replication without cell division?

Answer :15 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 post cell division.

Question :16Analyse 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

Answer :16

i) DNA replication can take place in G; phase of cell cycle. The number of

chromosomes remains 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 G: 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.

A109

INTRODUCTION:CELL DIVISION is a process in which parent cell after replication of its components,divides to form new (daughter) cells. A cell divides by two methods: MITOSIS and MEIOSIS.Dear students we have completed the last chapterno.10, in unit no.3 We have done all the possible different types of questions including NCERT exercise questions in the previous assignments.In this assignment we will revise all the diagrams included in this chapter.

A110

RECAPITULATION:Cell cycle: the sequence of events by which a cell duplicates its genome,synthesizes other constituents of the cell and divides into two daughter cells.Phases of cell cycle:SeG: phase: cell metabolically becomes active and grows continuously.S phase: synthesis of DNA but number of chromosomes remains same.

G» phase: synthesis of proteins while cell continues to grow.Rn © starts with nuclear division, separation of daughter

chromosomes (karyokinesis) ends with cytokinesis (division of cytoplasm)

EE nactive stage, cells do not differentiate but remain active Cell division is a part of cell cycle and occurs in dividing cells. A cell can divide

by two methods: mitosis and meiosis.

Mitosis: Equational (somatic) cell division, daughter cells have equal number

of chromosomes and equal amount of DNA as that of parent cell. Occurs in somatic cell. Flemming coined the term mitosis

Mechanism of mitosis: Two steps:

Karyokinesis -1 : Division of nucleus into nuclei .

(a) Prophase : It is of longest duration , chromatin threads appear ,chromosomes duplicate , chromatids are formed ,nucleus disappears nuclear membrane starts disintegrating nuclear membrane disappears

, spindle fibres are formed.

(b) Metaphase: Chromosomes arrange themselves around the equator of the spindle (congression).

(c) Anaphase =~: Centromere divides into two , chromatids of each chromosome move apart towards opposite poles of the spindle.

(d) Telophase : Chromatids are organized into chromatin material nuclear membrane is formed , nucleolus appears, 2- daughter nuclei are formed having same number and shape of chromosomes as parent nucleus .

Cytokinesis : division of cytoplasm after karyokinesis

Significance of mitosis : maintenance of chromosome number , growth or addition of cells ,regeneration , reproduction in unicellular organisms, repair and wound healing.

Meiosis :; Reduction of chromosomes from diploid to haploid conditions,occurs in germ cells of gonads (testis/ovary) during the formation of gametes

Mechanism of meiosis : Two meiotic divisions:

Meiosis 1: It is reductional division

Karyokinesis1 (a) Prophase 1 : long duration, has 5 substages:Leptotene =: nucleus increases in size , centrospheres becomes indistinct , centrioles separate , astral rays appear

Zygotene : pairing(synapsis) of homologous chromosomes (bivalent) ,nucleolus is distinct

Pachytene : chromosomes shorten and thicken, become indistinct ,tetrads appear , crossing over occurs, nucleolus still distinct , two asters move apart .

Diplotene: synaptonemal complex dissolves.

Diakinesis : bivalents separate , move towards the periphery of the nucleus , nucleolus disappears

(b) Metaphase 1 : bivalents get arranged at equator for equal distribution of

chromosomes , chromosomes replicate

(c) Anaphase 1: homologous chromosomes separate , dyads are formed.

(d) Telophase 1 : dyad chromosomes organize into two daughter haploid ,formation of nuclei , nucleoli , nucleoplasm , nuclear envelope

Cytokinesis : division of cytoplasm which occurs through cleavage, produces

2 daughter cells with nuclei having haploid number of dyad chromosomes .

Interkinesis : brief phase between meiosis 1 and meiosis 2

Meiosis 2

1. Karyokinesis 2:

(a) Prophase 2 : chromosomes shorten , divergent chromatids , nucleolus

disappears

(b) Metaphase 2_: spindle apparatus appears , chromosomes connect to spindle fibres which bring chromosomes in the equator of the spindle. At the end each poles has haploid no. of chromosomes .

(c)Anaphase 2: centromere of each chromosome splits allowing them to

move towards opposite poles of the cell . chromatids separate

(d) Telophase 2 : Each polar group forms nucleus. Astral rays and spindle

fibres disappears

2. Cytokinesis : occurs by cleavage

Significance of meiosis Formation of gametes in sexually reproducing organisms

Genetic variability Maintenance of chromosomal number

PART: A- VERY SHORT ANSWER TYPE QUESTIONS:

A) Multiple choice questions

1. Meiosis in diploid organisms results in:

(a) Production of gametes

(b) reduction in the number of chromosomes

(d) all of the above

2. At which stage of meiosis does the constitution of gametes is finally

decided:

(a) Metaphase - 1

(b) Anaphase - 2

(d) Anaphase - 1

3. Meiosis occurs in organisms during:

(a) Sexual reproduction

(b) vegetative reproduction

4. During anaphase 1 of meiosis:

(a) homologous chromosomes separate

(b) non homologous autosomes separate

(d) non-sister chromatids separate

5. Mitosis is not characterized by:

(a) Reduction division

(b) Equal Division

(d) none of the above

6. Identify the wrong statement about meiosis:

(a) Pairing of homologous chromosomes

(b) Four haploid cells are formed

the centromere.

(d) Two cycles of DNA replication occur

B) Fillin the blanks

7. During sds stage chromosomes are arranged in equatorial plate.

8. During meiosis chromatids of individual chromosomes separate in stage.

9. dissolves during diplotene stage of prophase -1 of meiosis.

C) True /False

10. Gois the inactive stage of cell cycle

11. Crossing over occurs between non - sister chromatids of a bivalent.

ANSWER KEY: (PART- A)

A) Mulitiple choice questions:

1. (d). in meiosis chromosomes are reduced to half , occurs in sexually reproducing organisms.

2.(d). during anaphase 1 two groups of dyad chromosomes are formed having half the number of chromosomes present in the

mother cell

3. (a). Meiosis occurs in sexually reproducing organisms

4. (a). In anaphase 1 homologous chromosomes separate

5. (d). Mitosis is an equational division in which the number of chromosomes is equal in both the daughter cells.

6. (d). In meiosis pairing of homologous chromosomes occur, four haploid cells are formed

B)Fill in the blanks:

1. Metaphase. During this ,chromosomes arrange themselves at the

equator

2. Anaphase. Chromatids separate and start moving apart from each other

3. Synaptonemal complex dissolves except at chiasmata during diplotene.

C) True /False:

1. True. Go is the inactive stage of cell division in which cell does not

differentiate

2. True. Crossing over occurs between noon sister- chromatids

PART: B SHORT ANSWER TYPE QUESTIONS:

1. Distinguish between anaphase of mitosis and anaphase -1 of meiosis.

2. Write the significance of meiosis.

3. Why is mitosis called equational division?

PART: C LONG ANSWER TYPE QUESTIONS:

1. Describe the events occurring during interphase.

2. Describe the following. Draw diagrams to illustrate the answer.

a) Synapsis

b)bivalent

c) Chiasmata.

Chapter 10 Cell Cycle and Cell Division

Plus One

Friday 5 February 2021

Chapter 10 Cell Cycle and Cell Division

Chapter 10 Cell Cycle and Cell Division

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