Cell Cycle and Cell Division Test - 3

The correct answer is OPTION 1: (i) S-Phase (ii) Interphase (iii) Prophase (iv) Mitosis

• During the S phase of interphase, centrioles replicate and begin to migrate to the opposite pole of the cell during the prophase stage of mitosis.
• Interphase is the time between the end of one cell division and the beginning of the next.
• It is also known as the non-dividing stage and the resting phase.
• It is a very active metabolic phase that is followed by the dividing M phase.
• Mitosis is an indirect cell division, also known as reductional cell division, in which the somatic cell divides in such a way that the chromosome number in the daughter cell remains constant.
• The longest phase of mitosis or karyokinesis is prophase.

Metaphase is the best phase to count total number of chromosomes in any species and detailed study of morphology of chromosome. Idiogram (arrangement of chromosomes in a series of decreasing length) can be drawn in the stage.

In anaphase, the centromeres of chromosomes start to divide into two, forming daughter chromosomes with centromere in each. Daughter chromosomes are repulsive, so they migrate towards opposite poles. Spindle fibres attached to the centromeres shorten and pull the chromosomes to the poles.

In anaphasic movement of chromosomes, the centromeres lead the path while the arms trail behind. As a result the anaphasic chromosomes appear V., L, J- and I-shaped. The shapes are formed respectively in metacentric, submetacentric, acrocentric and telocentric chromosomes.

Colchicine is an alkaloid widely used in plant breeding for doubling the chromosome number. The alkaloid does not allow the formation of spindle because it prevetns assembly of microtubules. It is, therefore, called 'mitotic poison'. Colchicine holds holds the cells in metaphase. Colchicine does not inhibit chromosome replication.

During telophase, the individual chromosomes are no longer seen and chromatin material tends to collect in a mass at the two poles. Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements. Nuclear envelope assembles around the chromosome clusters. Nucleolus, Golgi complex and ER reform.

In higher plants cytokinesis is takes place by cell plate method. In this method small vesicles of Golgi complex are assembled at the equator. Here spindle persists for some time called phragmoplast. All vesicles fuse to form two sheets which enclose a matrix or film. This film becomes solidified to form cell plate or middle lamella. It grows centrifugally and finally comes in contact with lateral walls of parent cell. Now phragmoplast disappear. Cellulose, hemicellulose and pecting are deposited on either side of cells plate. It forms primary cell wall.

The sites of mitotic cell division in a plant are meristematic regions. Mitosis can be easily studied in smears of root and stem tips. Which contain rapidly dividing apical meristems.

Spindle fibres are structures formed from mictrobubules in the cytoplasm during cell division. Microtubules are made up of tubulin protein. Spindle fiberes move chromatids or chromosomes diametrically apart and gather them in two d=clusters at opposite ends (poles) of the cell.

Microtubule is a microscopic tubular structure, with an external diameter of 24 nm and of variable length, found in a wide range of eukaryotic cells. Microtubules are composed of numerous subunits of the globular protein tubulin and occur singly or in pairs, triplets, or bundles. Microtubules help cells to maintain their shape. They also occur in cilia and eukaryotic flagella and the centrioles and form the spindle during nuclear division. A further role is in the intracellular transport of materials and movement of organelles.

The key feature of metaphase is the attachment of spindle fibers to kinetochores of chromosomes. Kinetochores are disc-shaped structure at the surface of the centromeres. These structures serve as the site of attachment of spindle fibres to the chromosomes that are moved toward poles.

Each cycle produces 2 daughter cells. It is calculated by the formula (2^{number of cycles}= number of daughter cells). Ten cycles are required to produce 1024 cells as follows:

2¹⁰ = 1024.

During telophase of mitosis viscosity of cytoplasm decreases. A new nuclear membrane is formed (either from older nuclear envelope or ER) around each set of chromosomes. Chromosomes overlap farming chromatin. The nucleolar organiser region of satellite chromosomes produce nucleolus for each daughter nucleus. Nucleoplasm surrounds in the area of chromatin. The gel state of spindle is converted into sol state and disappears. In this way two daughter nuclei are formed at the poles of spindle. Hence this phase is just reverse of prophase. 

During G₁ phase, DNA content is 2C.S phase is marked by replication of DNA and the amount of DNA per cell is doubled. During G₂ phase, synthesis of RNA and proteins takes place. During mitosis, DNA content gets equally distributed among two daughter cells.

In 20 mins. Culture tube 1/8th filled with cells. Each cell divides every minute producing two daughter cells.
Hence, 21 min = 1/8 × 2 = 1/4
In 22 min 1/4 × 2 = 1/2
In 23 min 1/2 × 2 = 1
That means, in 23 minutes the culture tube will be completely filled.

Figure (i) shows metaphase II and figure (iii) shows telophase II, Hence, the missing figure is that of anphase II.

Kinetochore is a plate-like structure by which microtubules of the spindle attach to the centromere of a chromosome during nuclear division. The centromere of each chromosome divides into two, so that each chormatied comes to have its own centromere. The two chromatids now start repelling each otherand separate completely to become daughter chromosmes. the daughter or new chromosmes. The daughter or new chromosomes move towards the poles of spindle along the path of their chromosome fibres.

The availability of mitogen and energy rich compounds decide wheter G₁ phase will be arrested (G0 phase) or undergo S-phase. This point is called check point or G₁ cyclin or G₁ point. Once the check point of G₁-phase is crossed and cell has entered S-phase, cell cycle will go on further division till completion.

Amitosis is a method of direct cell division in which the nucleus divides into two daughter nuclei without showing differentiation of chromosomes and development of spindle. It is commonly seen in prokaryotes.

All the daughter cells of multicellular organism have the same number and type of chromosomes as parent cells due to equitable distribution of all chromosomes in mitosis. This helps in ensuring genetic continuity.