Cell Division 

 Biology 

CELL DIVISION:

Biology for 9 & 10

Chapter - 3

Q. 1. What is cell division?

Ans:

Every living body is made up of of a cell or cells. The life of every organism starts with a single cell. After the fertilization of an egg, the structure of a human is achieved with crores of cells dividing through the process of cell division.

2. What is equational cell division?

Ans: In mitosis cell division, chromosome number of mother cell and daughter cell remains equal. Qualitative features of the chromosome of mother cell is transferred to the daughter chromosome without any change. Again the size and shape of the daughter cell is just like the mother cell. For such reason, mitosis cell division is called equational cell division.

3. Describe the different stages of mitosis with labeled diagrams.

Ans: 
Mitosis is a continuous process. It can be divided into five stages. They are-

a) Prophase:

This is the first stage of mitosis. At the onset of the stage, the nucleus becomes a little larger and chromatin fibers short, thick and tightly coiled structures called chromosomes. At this stage, chromosomes can be seen with a compound microscope. Though every chromosome then divides into two sister chromatids, at the centromere they remain attached. 

b) Prometaphase:

Early in this stage, spindle apparatus with two poles is developed in plant cells from fiber protein. The middle plane of the spindle apparatus is called the equator. Some fibers of the spindle  apparatus, made of microtibules of cytoskeleton, are stretched from one pole to the other. These are the called Spindle fibers. At this stage, kinetochores in the centromeres of chromosomes attach to some fibers of the spindle apparatus. The nuclear membrane and nucleolus start to disintegrate and disappear.

c) Metaphase:

At the onset of the stage, all the chromosomes assemble at the equator, from the two poles. The centromere of each chromosome remains on the equator but the two arms take positions towards the poles. The Nuclear membrane and nucleolus disappear completely.

d) Anaphase:

The sister chromatids become the two sister chromosome and they are pulled apart. The cleaved centromere moves to the pole, while the chromatids trail behind. At the end of the anaphase stage, the daughter chromosomes are completely pulled apart to the two poles and start their elongation.

e) Telophase:

This is the last stage of mitosis and a reverse of the prophase. Chromosomes become decondensed and surrounded by new nuclei. The Nucleolus also reappears. The Nuclear membrane reappears around the nuclear reticulum. So, ultimately, two new nuclei are formed in the two poles. The spindle apparatus disintegrate and so the spindle fibers gradually disappear.

4. Discuss the significance of mitosis.

Ans:
The significance of mitosis in the living body of organisms is immense. It plays an important role in the vegetative reproduction of organisms and increasing the number of reproductive cells. Mitosis is essential to form new cells for growth and healing of injuries. 

Multiple Choice Question:

1. In which stage does a nucleus become larger?

a) prophase
b) anaphase
c) metaphase
d) telophase

2. Through meiosis-

i) the number of chromosomes is changed 
ii) haploid gametes are developed
iii) consistency of features in organisms is maintained
Which one of the following is correct?
a) i  & ii
b)  ii & iii
c) i & iii
d) i , ii & iii
Answer the following questions 3 and 4 based on the Figure given below.

3. In the cell division of Figure A -

i) the mother cell and the daughter cells bear same features
ii) the number of chromosome becomes half in the newly developed cell
iii) chromosomes divide once
Which one of the following is correct?
a)  i  & ii
b) ii  & iii
c) i  & iii
d)  i , ii   & iii

4. The division in the Figure  B is different from that of Figure A and so - 

a) the number of chromosomes remains the same in the daughter cells
b) the number of chromosomes increases
c) abnormal cells are produced
d) the normal growth of the body occurs

Creative Questions:

a) Where does mitosis occur?

Ans:   Mitosis cell division is found in bacteria, blue-green algae, yeast etc.

b) Write in detail why meiosis is called 'reductional division'.

Ans: Meiosis is a type of cell division by which a Eukaryotic cell division into four cells by a special method. In this process the nucleus divides twice but the chromosome divides once. The number of chromosome in the newly formed daughter cell reduces to half of the number of mother cell. As the number of chromosome reduces to half, this process is also termed as educational division.

c) In stage B what changes occur? Explain.

Ans: 

Step B in the above stem is the telophase stage of mitosis cell division. It is the last state of mitosis. The changes of this stage are:

i) The daughter chromosomes take position in the two opposite poles.
ii) The chromosomes re-absorb water and gradually become thin and long by loosing shrinkage.
iii) Nucleolus re- appears.
iv) The spindle apparatus is disintegrated and so, spindle fibers gradually disappear.

d) Explain what problems may emerge if the process expressed in the figures above is not accomplished properly.

At the last phase of telephase a cell wall gradually develops along the equatorial zone and as a result two daughter cells are formed by the division of the mother cell. The significance of mitosis in the living organisms is immense. The balance in between the nucleus and cytoplasm of a cell in terms of volume and amount is maintained by the process of cell division mitosis. Different problems may emerge if mitosis process does not accomplish properly. 

These problems are discussed below:
i) The number of cells within an organism increased 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 a multicellular body. So, if the mitosis process does not work properly, body formation and development will be hindered.

ii) By this division the number and properties of a chromosome in each cell of a multicellular body remains constant. This balance of chromosome will be hampered if any kind of disturbance occur in above mentioned process.

iii) By mitosis process sex organs are formed. As a result, continuity of reproductive sequence is maintained. So, if mitosis cannot function properly, sex organs will not be formed properly and it will hamper reproduction process.

iv) By producing new cells this process repairs the various types of damages of multicellular organisms. Therefore, healing of injuries will be hindered if above shown process cannot accomplish properly.

v) By mitosis division the definite shape and size of the cell remains constant. This balance will be damaged if mitosis cannot function properly.

Q.1. How can the microtubules be shortened?

Ans: The centromere is needed for the separation of chromatids during mitosis. It is visible as a constriction and is the site of attachment of spindle microtubules. Each metaphase chromosome has two kinetochores at its centromere, one on each chromatid. These are made of protein molecules which bind specifically to the DNA in the centromere and also bind to microtubules. Bundles of microtubules called spindle fibres extend from the kinetochores to the poles of the spindle during mitosis. Construction of kinetochores begins before nuclear division starts and they are lost again afterwards.
The microtubules attached to a given kinetochore pull the kinetochore, with the rest of its chromatid dragging behind, towards the pole. This is achieved by shortening of the microtubules, both from the pole end and from the kinetochore end.

The poles of the spindle are where the centrosomes are located, one at each pole. The centrosome is an organelle found in animals cells that acts as the microtubule organising centre (MTOC) for construction of the spindle.