Prepare a temporary mount of onion root tip to Study Mitosis

Experiment 2: Our aim is to prepare a temporary mount of onion root tip to study mitosis

Theory

All organisms are made of cells. For an organism to grow, mature and maintain tissue, new cells must be made.  All cells are produced by division of pre-existing cells. Continuity of life depends on cell division.  There are two main methods of cell division: mitosis and meiosis. In this tutorial we will learn about mitosis.

What is Mitosis?

Mitosis is very important to life because it provides new cells for growth and replaces dead cells. Mitosis is the process in which a eukaryotic cell nucleus splits in two, followed by division of the parent cell into two daughter cells. Each cell division consists of two events: cytokinesis and karyokinesis.  Karyokinesis is the process of division of the nucleus and cytokinesis is the process of division of cytoplasm.

Events during Mitosis

1. Prophase:

   1. Mitosis begins at prophase with the thickening and coiling of the chromosomes.
   2. The nuclear membrane and nucleolus shrinks and disappears.
   3. The end of prophase is marked by the beginning of the organization of a group of fibres to form a spindle.

2. Metaphase

   1. The chromosome become thick and two chromatids of each chromosome become clear.
   2. Each chromosome attaches to spindle fibres at its centromere.
   3. The chromosomes are arranged at the midline of the cell.
3. Anaphase
   1. In anaphase each chromatid pair separates from the centromere and move towards the opposite ends of the cell by the spindle fibres.
   2. The cell membrane begins to pinch at the centre.
4. Telophase
   1. Chromatids arrive at opposite poles of cell.
   2. The spindle disappears and the daughter chromosome uncoils to form chromatin fibres.
   3. The nuclear membranes and nucleolus re-form and two daughter nuclei appear at opposite poles.
   4. Cytokinesis or the partitioning of the cell may also begin during this stage.

The stage, or phase, after the completion of mitosis is called interphase.  It is the non dividing phase of the cell cycle between two successive cell divisions.  Mitosis is only one part of the cell cycle. Most of the life of a cell is spent in interphase.  Interphase consist of three stages call G1, S and G2.

Mitosis in Onion Root Tip

The meristamatic cells located in the root tips provide the most suitable material for the study of mitosis. The chromosome of monocotyledonous plants is large and more visible, therefore, onion root tips are used to study mitosis. Based on the kind of cells and species of organism, the time taken for mitosis may vary. Mitosis is influenced by factors like temperature and time

Viva-Voce [Study of Mount  Onion Tip Mitosis ]

As Per Pattern of CBSE (2018-19)  

1. 1. Why mitosis is also known as equational division?

Ans.   The mitosis is also known as equational division because the number of chromosomes in the daughter cell is equal to the chromosome number in parent cell.

2. 2. Why do we use onion root tips or grasshopper larva for studying mitosis?

Ans.   Onion root tips or grasshopper larva are often used for studying mitosis because these tissues have   actively dividing cells and we can easily show up all the stages of mitotic cell divisions in them.

3. 3. Why is early morning considered as an ideal time to harvest the onion root tips for study of mitosis?

Ans.   This is because root tips have active cell divisions in morning. When we cut such material we get all stages of cell division in one material.

4. 4. Suggest the name of few tissues which are suitable for the study of mitosis

Ans.   We can select the dividing cells of

• Tail of tadpole larvae.
• Epithelial cells of gills of fishes.

(Hi) Bone marrow tissue of any vertebrate.

• Root tip cells of any herbaceous plant.
• Shoot apices of plants.
• Grasshopper larvae.

5. 5. Why do we prefer monocotyledonous material for cytological studies?

Ans.   Monocotyledonous plants are preferable because they have large sized chromosomes which are better visible in light microscope.

6. 6. Which stain is used for studying chromosomes?

Ans.    Acetocarmine is generally used for studying chromosomes as it stains chromosome deep red while cytoplasm remains unstained.

7. 7. From where do spindle fibres originate?

Ans.   Spindle fibres originate from centrioles in case of animal cells, but in plant cells, these originate from the cytoplasm. These are proteinaceous in nature.

8. 8. In which stage of cell division chromosomes are seen best?

Ans.    Chromosomes are best seen in metaphase stage, because at this stage chromosomes are thickest and shortest and exist in condensed form.

9. 9. Suppose a cell has 44 chromosomes. How many chromosomes will it have after mitosis?

Ans.    In mitosis, each of the daughter cells produced have the same number of chromosomes (44  chromosomes) as that of parent cell.

Q.10.      What do you mean by the term 'karyokinesis'?

Ans.    Karyokinesis means the division of nucleus

Q.11.   What shape would a metacentric and sub-metacentric chromosome exhibit during

the anaphase stage?

Ans.

Types of chromosome		Shape	Centromere position
Metacentric		V	Median
Sub-metacentric		L	Sub-median
Acrocentric		J	Sub-terminal
Telocentric		1	Terminal

 

 

 

 

Q.12.   In which stage do the chromatids start moving towards opposite poles?

Ans.        Anaphase.

Q.13.   Where do spindle fibres attach on the centromere during cell division?

Ans.        Spindle fibres attach themselves on the kinetophores during cell division.

Q.14.   What does kinetophore consist of?

Ans.        It consists of protein and specific sections of chromosomal DNA at the centromere.

Q.15.   How does cytokinesis differ in animal and plant cells?

Differences between animal and plant cytokinesis	are:
Animal cytokinesis	Plant cytokinesis
It occurs by cleavage.	It occurs commonly by cell plate formation.
Spindle starts disappearing in early telophase stage.	Spindle persists till half cytokinesis.
A mid-body of dense material is formed at the middle of the cell.	Mid-body is not formed.

Ans.             

 

 

 

 

Q.16.   Name the chemical which can inhibit cell division during metaphase stage.

Ans.        Colchicine inhibits the formation of spindle fibres during metaphase stage.

Q.17.   Name the source of colchicine.

Ans.     It is obtained from a plant Colchicum autumnale of the family-Liliaceae.

Q.18.   What is a phragmoplast?

Ans.     It is a cell plate formed in plant cell during cytokinesis. It is formed in the centre which grows outwardly until it divides the cell into two identical daughter cells completely.

Q.19.     How chromatin changes into chromosomes ?

Ans.    By losing water and supercoiling. the chromatin becomes compact.

Q.20.   Why mitosis is called equational division ?

Ans.    Equal chromosome number is maintained in daughter cells.

Q.21.   Where do mitotic divisions take place ?

Ans.    Mitotic divisions take place in vegetative cells.

Q.22.   Differentiate mitosis in plants and animals.

Ans..

S.No.	In plants	S.No.	In animals
1.	Cell wall is present.	1.	Cell wall is absent.
2.	Centriole is absent.	2.	Centriole is present.
3.	No astral rays are formed.	3.	Astral rays are formed.

           

 

 

 

Q.23.   Why root tips should be cut and fixed in early morning hours ?

 

Ans.     Root tip cells have mitotic activity during morning hours when temperature is about 28°C.

 

Q.24.   Why root tips are treated with 1 N HCI ?

 

Ans.     For hydrolysis so that middle lamellae between the cells dissolve.

 

Q.25.   Why we select root tips for above experiment ?

Ans.     Root tips have meristematic activity.

Q.26.   Why monocots are used to show mitosis for cytological studies 7

Ans.      Chromosomes of monocotyledonous plants are large sized so monocots are used for cytological studies.

Q.27.   Name three types of meiosis ?

Ans.     Zygotic, gametic and sporic meiosis.

Q.28.   What is synapsis ?

Ans.     Pairing of homologous chromosomes is called synapsis.

Q.29.    What is the significance of crossing over ?

Ans.     It helps in exchange of characters and is significant in evolution of new recombinations.

Q.30.   Differentiate Mitosis and Meiosis ?

Mitosis	Meiosis
(C) Only 1 cell division. (D)   It occurs in somatic cells. (E)     Prophase is short, simple. (F)      No synapsis and crossing over. (G)   Only one plane of metaphasic plate. (H)     Centromeres divide during anaphase. (I)         Equational division.	(D)               Two cell divisions. (E) It occurs in reproductive cells. (F)   Prophase is long composed of 6 stages. (G)   Crossing over takes place to exchange the segments. (H) 2 parallel planes of metaphasic plates. (I)     Centromeres divide only in anaphase II. (J)      Reductional division.

Ans.

           

 

 

 

 

Q.31.   When does crossing over take place ?

Ans.     In pachytene stage.

Q.32.   What is metaphasic plate ?

Ans.        During metaphase centromere lie in a single line thus making a metaphasic plate.

Q.33.   Who described the process of mitosis and meiosis for the first time and in which year?

Ans.     Mitosis was first described by W. Flemming in 1882; while meiosis was first described by Winiwarter (1900) and Farmer and Moore coined the term meiosis in 1905.

Q.34.    Can a haploid cell divide meiotically?

Ans.     No, a haploid cell cannot divide meiotically.

Q.35.    What is the significance of crossing over in meiosis?

Ans.     During crossing over in meiosis exchange of genetic material between non-sister chromatids occurs; as a result, the genotype of the cells is altered. So it results in the genetic recombination and finally helps in the formation of new species.

Q:36.    What do you mean by the term karyokinesis?

Ans.     Karyokinesis means the division of the nucleus.

Q.37.    What is a meiocyte cell?

Ans.     A cell that is to divide by meiosis is known as a meiocyte cell.

Q.38.    How do you differentiate the chromosomes of mitosis and meiosis?

Ans.     Meiotic chromosomes are comparatively longer and thinner. They show a characteristic beaded appearance by the presence of chromomeres.

Q.39.    From one parent cell how many daughter cells will be produced after meiosis and after mitosis respectively?

Ans.     From one parent cell two cells will be produced after mitosis; and four cells after meiosis.

Q.40.    At what stage of cell division does nuclear membrane reappear?

Ans.     Telophase.

Q.41.   In which stage of cell division chromosomes are seen best?

Ans.     At metaphase stage.

Q.42.   Name the chemical that can arrest the process of cell division at metaphase stage?

Ans.     Colchicine.

Q.43.    What do you mean by cytokinesis?

Ans.     It is the division of the cytoplasm.

Q.44.    Suppose a cell has 44 chromosomes. How many chromosomes will it have after mitosis?

Ans.     44. In mitosis the daughter cells produced have the same number of chromosomes as the parent cell.

Q.45.    Suppose we take a cell from a leaf instead of root tip, will mitotic events be seen?

Ans.     No, because the cells of leaf do not divide.

Q.46.    Why is mitosis termed as ec]national division and meiosis as reduction division?

Ans.     Mitosis is equational division because the daughter cells formed are identical to the parent cell in all respects. Meiosis is reduction division because the daughter cells formed have half the number of chromosomes as the parent cell.

Q.48.    Can you name a process of nuclear division which is neither mitotic nor meiotic?

Ans.     Amitosis. In this nucleus simply splits into two halves without any event. It occurs in Amoeba and only macronucleus of Paramoecium divides amitotically.

Q.49.   Why are root tips used as a study material for mitosis?

Ans.     Root tips have the meristematic tissue that keeps dividing to form new cells. Hence these are a good material to study various stages of mitosis.

Q.50.   Why do we use acetocarmine as the stain?

Ans.     Acetocarmine stains the chromosomes dark red and leaves the cytoplasm unstained.

Q.51.   Why are the root tips hydrolysed?

Ans.     Hydrolysis with hydrochloric acid (HC1) soften the root tips by breaking down the middle lamellae between the cells.

Q.52.   Why is mitosis called an equational division?

Ans.     The two daughter cells formed at the end of mitosis have equal or same number of chromosomes. The chromosomes are similar qualitatively and quantitatively.

Q.53.   What is mitosis?

Ans.     Mitosis is a cell division by which a cell divides to produce two daughter cells containing same number of chromosomes as in the parent cell, thus maintaining the chromosome number constant.

Q.54.   What is karyokinesis?

Ans.     If refers to the division of nucleus.

Q.55.   What is cytokinesis?

Ans.     It refers to the division of cytoplasm.

Q.56.   Are centrioles present in the onion root tip cells?

Ans.     No.

Q.57.   What is the diploid number (2n) of chromosomes in an onion root tip cell?

Ans.     Sixteen (2n = 16).

Q.59.   At which stage of cell division, the chromosomes are best studied?

Ans.     Chromosomes are best studied at metaphase.

Q.60.   Why is it so?

Ans.     (i)        At metaphase, they are condensed and distinct. Each chromosome appears in the form of two

.                       distinct chromatids attached by a centromere.

(ii)        The chromosomes can be readily counted and distinguished from each other.

(iii)       The chromosomes reveal a distinct pattern and each has a specific size and shape.

Q.61.   Why do chromosomes appear to be of different shapes?

Ans.     Due to the position of centromere, chromosomes acquire different shapes.

Q.62.   Why do chromosomes at anaphase have J, V, L or rod shapes?

Ans.     These shapes are due to the position of centromere in the chromosomes.

-           J-shaped chromosomes have centromere slightly displaced from the centre and so both arms         are unequal.

-         V-shaped chromosomes have centromeres right in the centre and so both the arms are equal.

-         L or rod-shaped chromosomes have their centromere either at or near the extreme end and so their one arm is extremely short or negligible and the other very long.

Q.63.   Name the chemical that can arrest the process of cell division at metaphase.

Ans.     Colchicine.

Q.64.   If a cell lias 46 chromosomes (2n = 46), how many chromosomes would each daughter cell have after mitosis.

Ans.     46.

Q.65.   What is the importance of mitosis?

Ans.     (i)        It results in an increase in number of cells required for body growth.

(ii)        It helps in the repair and replacement of lost cells.

(iii)       Since the cells formed contain the same number and identical sets of chromosomes, it does not lead to variation in genetic information.

Q.66.   How do you identify the metaphase stage?

Ans.     The chromosomes get aligned at the centre of the cell forming an equatorial plate.

Q.67.   Name the stage at which splitting of the centromere takes place.

Ans.     Anaphase.

Q.68.   At which place are chromosome attached to spindle fibres?

Ans.     Centromere region.

Q.69.   Why should the onion root tips be cut in the early morning hours?

Ans.     It is a time of high mitotic activity in the onion root tips.

 

 

 

 

 

 

 

 

Significance & Overview Of Biology Class XII Practicals 

There are altogether twenty-five exercises in the present manual which are based on Biology curriculum for Class XII.

For each practical work, principle, requirements, procedure, precautions, observations, discussion and the questions are given in the book. The methodology of preparation of any reagent, if required, has been given along with the requirements, for the convenience of students and teachers. The questions are aimed to develop learner’s understanding of the related problems. Precautions must be well understood by the learners before proceeding with the experiments and projects. In addition to the core experiments enlisted in the syllabus for Class XII emphasis has also been given for pursuing Investigation Project Work. Appropriate appendices related to the observation and study of organisms are given along with the experiment. International symbols for units, hazards and hazard warnings are given at appropriate places in the book.