Test: Mitosis (NCERT) - NEET MCQ

In animal cells, during the S phase of interphase, DNA replication begins in the nucleus and the centriole duplicates in the cytoplasm. These duplicated centrioles move towards opposite poles of the cell in prophase of mitosis.

The best stage to count the number and study the morphology of chromosomes is during metaphase. During metaphase, the chromosomes are condensed and highly visible, making it easier to count and study their structure. The chromosomes are also lined up at the equatorial plane of the cell, which makes it easier to observe them under a microscope.

Metaphase is a stage in the process of cell division known as mitosis, during which the cell's nucleus divides into two identical daughter nuclei. At this stage, the chromosomes, which contain the cell's genetic material, are visible under a microscope as thin, elongated structures. They are arranged in pairs, with one chromosome from each pair being inherited by each daughter nucleus.

By counting the number of chromosomes and studying their morphology (shape, size, and structure), scientists can learn more about the genetic makeup of an organism and how it may be affected by various factors, such as environmental conditions or genetic mutations.

Line in NCERT: "This then, is the stage at which morphology of chromosomes is most easily studied."

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.

Line in NCERT: "At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously and the two daughter chromatids, now referred to as daughter chromosomes of the future daughter nuclei, begin their migration towards the two opposite 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.

Line in NCERT: "the centromere of each chromosome remains directed towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind."

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.

Line in NCERT: "At the beginning of the final stage of karyokinesis, i.e., telophase, the chromosomes that have reached their respective poles decondense and lose their individuality. The individual chromosomes can no longer be seen and each set of chromatin material tends to collect at each of the two poles. This is the stage which shows the following key events: Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements. Nuclear envelope develops around the chromosome clusters at each pole forming two daughter nuclei. Nucleolus, golgi complex and ER reform."

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.

Line in NCERT: "The two asters together with spindle fibres forms mitotic apparatus."

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.

Line in NCERT: "Small disc-shaped structures at the surface of the centromeres are called kinetochores. These structures serve as the sites of attachment of spindle fibres (formed by the spindle fibres) to the chromosomes that are moved into position at the centre of the cell."

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. 

Line in NCERT: "At the beginning of the final stage of karyokinesis, i.e., telophase, the chromosomes that have reached their respective poles decondense and lose their individuality."

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.

Line in NCERT: "The interphase is divided into three further phases: G1 phase (Gap 1), S phase (Synthesis), G2 phase (Gap 2)."

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.

Line in NCERT: "The stages through which a cell passes from one division to the next is called the cell cycle."

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.

Line in NCERT: "Centromeres split and chromatids separate."

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.

Line in NCERT: "S phase marks the phase of DNA replication and chromosome duplication."

Doubling of the number of chromosomes can be achieved by disrupting mitotic cell division soon after: Anaphase.

Line in NCERT: "Centromeres split and chromatids separate."

All the following key events take place during the telophase of mitosis except:

Answer: B: Chromosomes become discrete

During telophase:

Chromosomes cluster at opposite spindle poles

Nuclear envelope reforms around each set of chromosomes

Nucleolus, ER, and Golgi complex reform

However, chromosomes becoming discrete is a characteristic of early mitotic stages (prophase).

Line in NCERT: "Nuclear envelope develops around the chromosome clusters at each pole forming two daughter nuclei."

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.

Line in NCERT: "Mitosis usually results in the production of diploid daughter cells with identical genetic complement."

In eukaryotes, DNA replication takes place during the S phase (Synthesis phase) of the cell cycle. During this phase, the entire genome is replicated, resulting in two copies of each chromosome to ensure that both daughter cells receive a complete set of genetic information during cell division.

Line in NCERT: "S phase marks the phase of DNA replication and chromosome duplication."

- Centromeres split and chromatids separate: Anaphase
- Chromatids move to opposite poles: Anaphase

In the Anaphase of mitosis, centromeres split, allowing sister chromatids to separate and move towards opposite poles of the cell. This stage is characterized by the movement of chromatids due to the shortening of microtubules attached to the kinetochores at the centromeres. This ensures that each daughter cell receives an identical set of chromosomes.

Line in NCERT: "Centromeres split and chromatids separate. Chromatids move to opposite poles."