Root tip behaviour heory A process by which a parent cell divides into two or more daughter cells is called cell division. Cell division is a small part of the cell cycle. In normal eukaryotic cells the type of cell division is known as mitosis. Another type of cell division is present in reproductive cells of eukaryotes known as meiosis. Cell cycle is mainly classified into two segments: M-phase and Interphase. Interphase is the longer period of cell division. During this phase the cell prepares for its next stage.This is a period of diverse activities and these activities prerequisite for the next mitotic phase. Interphase is mainly divided into three phases: G1 phase, S phase and G2 phase. S phase is the period of replication. G1 and G2 are the two gap phases during which the cell grows, producing proteins and preparing the cells. These phases also have certain check points and the whole cell cycle is strictly regulated. M phase of the cell cycle stands for Mitosis or nuclear division. In eukaryotes, DNA replication is followed by a process called mitosis which separates the chromosomes in its cell nucleus into two identical sets, in two individual nucleus. Mitosis is followed by cytokinesis. The process of Mitosis is mainly divides into four stages: Prophase, Metaphase, Anaphase and Telophase. Prophase: During this stage, the chromosomes super coiled, condensed and become visible for first time during the cell cycle. The spindle fibers start forming. The nuclear membrane starts disintegrating. Metaphase: During this stage, the spindle fibers reach and attach to centromere of each sister chromatids. The chromosomes align along the center plane of the cell. The nuclear membrane disintegrates completely. Anaphase: During this stage, the centromeres start splitting and the sister chromatids begin to migrating towards the opposite poles of the cell. Telophase: During this stage, the chromosomes are clustered on the either ends of the cell. The nuclear membrane starts reforming. The cell plate (new cell wall) starts to form between the two daughter nuclei. This will be followed by cytokinesis. Mitosis in Onion Root Tip The genetic information of all organisms resides in the individual DNA molecules or chromosomes. An onion cell possesses eight chromosomes like forty six chromosomes of human cells. In 1842 C. Nägeli first seen chromosomes and in 1888 W. Waldeyer named it. Walther Flemming studied and named the process of cell division as mitosis. Cell division is rapid in growing root tips of sprouting seeds or bulbs. The most commonly used root tips in labs to study mitosis are onion, wheat, lentil, barley and alfalfa. An onion root tip is the rapidly growing part of the onion and thus many cells will be in different stages of mitosis. The onion root tips can be prepared and squashed in a way that allows them to be flattened on a microscopic slide, so that the chromosomes of individual cells can be observed easily. The super coiled chromosomes during different stages of mitosis present in the onion root tip cells can be visualized by treating with DNA specific stains like Feulgen stain and Acetocarmine stain.

Mitosis in Onion root tip Mitotic Index The percentage of cells undergoing mitosis or it is defined as the ratio of no. of cells in the dividing phase to the total number of cells observed. This will help to identify the region of most mitotic activities. Mitotic index help us to quantify the cell division. Mitotic index decreases with increasing distance from root tip. That means gradual decrease in cell division as move from the zone of cell division to the zone of cell elongation. The meristematic region in the root tip is the actively growing region and thus the mitotic index is high. Mitotic index = n/N × 100 Mitotic index is