SEE Science (Cell Division) Notes

                    

             

          Cell Division

1. What is metaphase? Mention the main changes that occur in this stage. Also, draw a labeled diagram showing this stage of mitosis.

Ans: Metaphase is the second stage or middle stage of mitosis which lasts for 15 minutes.

The main changes which take place in the metaphase of mitosis are as follows:

i) The nucleolus and nuclear membrane disappear completely and the formation of spindle fibres completed.

ii) The centromeres of chromosomes are arranged at the equatorial plane.

iii) Chromosomes become short, thick and very clear.

 

                        

    Fig: Metaphase of mitosis

2. Write down the importance of mitosis cell division.

Ans: Mitotic cell division is important for unicellular and multicellular organisms because of the following reasons:

i) It is a method of asexual reproduction in unicellular organisms.

ii) It is responsible for growth and development of multicellular organisms.

iii) it helps to replace old and dead cells by new ones.

iv) It helps to maintain genetic stability.

3. Which type of cell division is indicated by the given diagram?

    Where does this type of cell division take place?

Ans: Mitosis cell division is indicated by the given diagram.

       This cell division takes place in somatic cells or vegetative cells.

4. What is meiosis? In which types of cell does this cell division occur?        

Ans: A type of cell division in which a diploid (2n) parent cell divides into four haploid (n) daughter cells is called meiosis.

  It occurs only in the reproductive cells or germ cells at the time of gamete (i.e. sperm and ovum) formation.

5. Name the type and phase of the cell division given in the figure.

Ans: Telophase-II of meiosis-II is shown in the given diagram.

Changes

a)     Spindle fibres begin to disappear.

b)     The nucleolus and nuclear membrane reappears around the chromosomes.

c)     Daughter chromosomes reach the four poles of the cell.

6. Write down the significance of meiotic cell division.

Ans: The significance of meiotic cell division is as follows:

i) It forms gametes for sexual reproduction.

ii) It helps in the formation of spores for asexual reproduction.

iii) It brings out genetic variations in daughter cells or offspring.

7. Write down any four differences between mitosis and meiosis.

Mitosis cell division	Meiosis cell division
1. Mitosis takes place in vegetative or somatic cells.	1. Meiosis takes place in germ or reproductive cell.
2. Number of chromosomes in daughter cells remains same as present in the parent cell.	2. The number of chromosomes is reduced to half in the daughter cells.
3. One diploid cell is divides into two diploid daughter cells.	3. One diploid cell is divides into four haploid daughter cells.
4. A single cell division takes place at a time.	4. Two cell divisions take place at a time
5. It helps in the growth and development of an organism.	5. It helps in the formation of gametes and spores and evolution.

8. What phase and type of cell division is given in the figure?

Ans: Metaphase of mitosis is shown in the given diagram.

 

9. Which types and stage of cell division is represented by the given diagram?

   Which reproduction is facilitated by this division?

Ans: Metaphase stage of mitosis cell division is represented by the given diagram.

Asexual reproduction is facilitated by this type of cell division.

 

10. Which type of cell division and phase is shown in the figure? Write one change that takes place in that phase.

Ans: Anaphase of mitosis is shown in the given diagram.

       

Changes

a)     The centromere of each chromosome divides into two parts so that two

       daughter chromosomes are formed.

b)     Daughter chromosome migrated toward the opposite pole by the contraction

       of spindle fibers.

c)     Chromosomes become shorter and thicker and take the shape of 'U' or 'V'.

  

11. The number of chromosomes in a mother cell is equal to the number of chromosomes in each daughter cell in mitosis. How is it possible?

Ans: In mitotic cell division, the number of chromosomes in daughter cells becomes equal to the number of chromosomes in the mother cell in mitosis. It is possible due to duplication of chromosome in interphase. During interphase of mitosis, duplication of chromosomes takes i.e. synthesis of extra duplication of each chromosome. In karyokinesis, a half number of chromosomes will go to each daughter cell. Thus, each daughter cell will get same number of chromosomes as present in the mother cell.  

12.   Write down a reason for the number of chromosomes in each daughter cell is just half than that of number of chromosomes of mother cell in meiosis cell division. Give any one advantage of this change in chromosome number.

Ans: The number of chromosomes in each daughter cell is just half than that of number of chromosomes of mother cell in meiosis cell division because in meiosis cell division, two successive cell divisions takes place in which one diploid cell divides twice to form four haploid cells. As a result, number of chromosomes in each daughter cell will get half number of chromosomes as present in the mother cell.

13.   Which stage and type of cell division are shown in the diagram?

What is the number of chromosomes in the cell formed by this division?

Ans: Anaphase stage of mitotic cell division is shown in the given figure.

The number of chromosomes is diploid (2n) in the cell formed by this division.

14. Name the objects X and Y in the give figure. Which kind and stage of cell division is seen in the figure.

Ans: In the given diagram,

X-Nuclear membrane

Y- Chromosome

Telophase stage of mitosis cell division is shown in the given diagram.

                    

Changes

a)       Spindle fibers begin to disappear.

b)       The nucleolus and nuclear membrane reappears around the chromosomes.    

c)       Daughter chromosomes reach to their respective poles of the cell.

14.   Identify the stage of cell division in the give figure and write any two important changes that take place in this stage. How is stability of living beings maintained by meiosis?

Ans: Anaphase-I of meiosis-I is shown in the given figure.

Any two important changes of this stage:

    i)        The centromeres of the chromosomes does not divide but they just

         separate from the pair.

ii)      The overlapped chromosomes move towards the opposite poles of

         the cell by the contraction of spindle fibres.

iii)    The number of chromosomes in the daughter nuclei is reduced to half.

Meiosis results in the formation of haploid sex cells (gametes i.e. sperms or ova), which after fertilization restores the original diploid number in the zygote. Thus, it is responsible for maintaining genetic stability by keeping the fixed number of chromosomes in a species.

  

15.   From the given figure, name the type and phase of the cell division. Write one change that is seen in this phase.

Ans: Metaphase-I of meiosis-I is shown in the given figure.

One change that is seen in Metaphase-I                                                    

i)      The centromere of bivalent gets attached with spindle fibres.

ii)     The nucleolus and nuclear membrane disappear completely

   and the formation of spindle fibres completed.

iii)    The bivalent arrange themselves in the equatorial plane.

16.   Mitosis cell division is also called equational division, why?

Ans: After the completion of mitosis cell division, parent cell divides into two daughter cells, each having the same number of chromosomes as present in the parent cell. Therefore, mitosis cell division is also called equational division.

17.   What is the importance of mitosis in unicellular and multicellular organisms?

Ans: In unicellular organism, by the mitosis, one cell is divided into two daughter cells. Therefore, due to the mitosis two new organisms are formed. However, in multicellular organism, by the mitosis one cell is divided into two daughter cells. Thus, it helps in growth and development of the body.

18.   What is synapsis? Name the sub-stage, stage and type of cell division in which this process occurs.

 Ans: The phenomenon of pairing of homologous chromosomes is called synapsis.

                     It takes place in the Zygotene sub-stage of Prophase I of meiosis cell division. 

19.   What is crossing over? Name the sub-stage, stage and type of cell division in which this process occurs.

Ans: The process of exchanging genetic material between two non-sister chromatids is called crossing over.

Importance:

a) It helps in organic evolution.

b) It helps in mutation.

It takes place in the Pachytene sub-stage of Prophase I of meiosis cell division. 

20.   Define:

a)    Chromosome: The thread like structure found in the nucleus of all the cells and carry hereditary information is called chromosome.

Function: It transmits hereditary characteristics of parents to their offspring with the               help of genes.

b)    Cell: Cell is defined as the structural and the functional unit of life.

c)    Cell division; The process in which two or more daughter cells of same kind are formed from pre-existing cells is called cell division.

d)    Chromosome: The thread like structure in the nucleus of a cells and carry hereditary information.

e)    Genes: A tiny unit of heredity located in chromosome is called genes.

f)    Spindle fibres: A Thread like structure formed at the metaphase stage in which chromosomes are attached with centromere is called spindle fibres.

g)    Centromere: A round like structure found in the chromosomes, which contain gene is called centromere.

h)    Interphase: The change that occur in newly formed cells till they are ready for the further division.

i)     Karyokinesis: The process of division of nucleus during cell division.

j)    Cytokinesis: The process of division of cytoplasm during cell division.

k)    Sex chromosome: Those chromosomes which occur singly or in a pair and carry genes in determining the sex of the individual in unisexual organisms.

l)     Autosomes: Those chromosomes which regulates the growth and development of the body of organism.

m)   Chromatin fiber: The chromosome materials in the form of long and very loosely coiled thread are called chromatin fiber.

n)    Mitosis Cell Division: The type of cell division in which a single diploid mother cell divides to form two diploid daughter cells is called mitosis cell division.

o)    Haploid cell: The cell which contains single chromosome for each character is called haploid cell.

p)    Diploid cell: The cell which contains two chromosomes for each character is called diploid cell.

q)    Amitosis or Direct cell division: The type of cell division in which a cell divides into two daughter cells without the formation of spindle fibres and condensation of chromatin fibres is called amitosis or direct cell division.

r)    Homologous chromosomes: The identical male and female parent chromosomes occur in pair is called homologous chromosomes.

s)    Chiasmata: The point on which crossing over takes place is called chiasmata.

21.   How is zygote formed?

Ans: When male gamete fuses with female gamete, the zygote is formed.

22.   What is somatic cell?

Ans: Any cell of a plant or an animal other than a germ cell or reproductive cell is called somatic cell.

23.   What is the number of chromosome present in each cell of different living beings?

Living organism	Number of chromosome
Human cell	46 (23 pairs)
Frog	26 (13 pairs)
Mice	40 (20 pairs)
Onion	16 (8 pairs)
Rice	24 (12 pairs)
Potato	48 (24 pairs)
Sugarcane	80 (40 pairs)
Dog	78 (39 pairs)

24.   Meiosis cell division is also called reductional cell division, why?

Ans: After completing the meiosis cell division, one parent cell divides into four daughter cells, each having the half number of chromosome as present in the parent cell. Therefore, meiosis cell division is also called reductional cell division.

25.   "Interphase is the resting stage."

Ans: Interphase is the resting stage only with respect to division, but it is metabolically very active. During this phase, cell grows to its maximum size and synthesis RNA, protein and ATP. DNA replication also takes place. Centrioles also replicate to form two pairs of centrioles.

26.   What is the condition of chromosome in Interphase of mitosis cell division?

Ans: The chromosome material is in the form of long and very loosely coiled thread in the Interphase of mitosis cell division.

27.   Why mitosis cell division is also called somatic cell division?

Ans: Mitosis cell division is also called somatic cell division because this cell division takes place only in somatic or vegetative parts of an organism.

28.   Differences between metaphase and anaphase of mitosis cell division.

S.N	Metaphase of mitosis cell division	S.N	Anaphase of mitosis cell division
a)	Chromosomes are arranged at the equatorial plane.	a)	Chromosomes migrate towards opposite poles of the cell.
b)	In metaphase of mitosis centromeres does not divide.	b)	In anaphase of mitosis centromeres  divides.

29.   Changes seen in prophase of mitosis cell division.

Ans: Changes seen in prophase of mitosis cell division are:

a)    The chromosomes become short and thick due to condensation.

b)    The nuclear membrane and nucleolus began to disappear and spindle fibers start to appear at the end of the phase.

c)    Centrosome divides into two centrioles and moves towards opposite poles of the cell.

d)    Each chromosome is double due to duplicating of the DNA.  

30.   How is stability of living being maintained by meiosis?

Ans: Meiosis result in formation of haploid sex cells (gametes i.e. sperm or ova), which after fertilization restores the original diploid number in the zygote. Thus, is responsible for maintaining genetically stability by keeping the fixed number of chromosome in a species.

31. How many chromosomes are there in human sperm and zygote, why?

Ans: Sperm contains haploid (n) and zygote contains diploid (2n) number of chromosomes because ovum is formed by meiosis cell division and zygote is formed by fusion of two haploid (n) gamete.