Test: Mitosis and its Significance (June 10) - 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.

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.

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.

A root tip is a rapidly growing part of a plant. So many cells will be in different stages of mitosis. At a time one can observe cells at these many stages. The commonly used roots are of onion, lentil and barely. Also, the chromosomes are large and very dark when stained. So root tip is used to study mitosis in laboratories.

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.

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.

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.

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.

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.