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Lab 13. Mitosis and Meiosis Purpose The purpose of this lab is to understand the process of mitosis and melosis in animal and plant cells. Learning Objectives After completing this lab, you will be able to: 1. Describe the stages of mitosis. 2. Identify the stages of mitosis in an onion root or whitefish blastula using a light microscope. 3. Describe how cytokinesis differs in plant vs. animal cells. 4. Name and describe the phases of meiosis I and II with attention to the movement of chromosomes. 5. Explain how meiosis reduces the number of chromosomes. 6. Compare the outcomes of mitosis and meiosis. Overview and Background Mitosis Cellular division is a complex process involving the replication and division of genetic material, cytoplasm, and cellular organelles. Mitosis is a form of cellular division that results in two new identical cells called daughter cells, each of which has the same number of chromosomes as the original parent cell. The daughter cells are clones; they are genetically identical to one another. Mitosis allows the body to grow and replace wom out and damaged cells. In this lab, the study of mitosis will entail an examination of dividing plant and animal cells. The onion root tip is a common material for the study of mitosis because it is easy to obtain, prepare, and the chromosomes are relatively easy to see. The root tip is divided into several different regions, not all of which will contain cells undergoing mitosis. The end of the root is covered by a protective root cap. Located immediately behind the root cap is the meristematic tissue, which contains many rapidly dividing cells. When mitosis has been concluded, plant cells will form a cell plate in order to complete cytokinesis. The whitefish blastula offers an excellent opportunity to observe mitosis in animal cells. After the whitefish egg is fertilized by the sperm it will begin a series of mitotic divisions which eventually produces a spherical mass of cells called a blastula. In an animal cell, when mitosis has been completed, cytokinesis is accomplished by a cleavage furrow. Meiosis Meiosis is a form of cellular division in which the chromosome number is reduced by half during the production of gametes (sperm or eggs). In the case of meiosis, the nucleus of the parent cell (i.e. the germ-line cell) about to undergo meiosis has the diploid (2n) number of chromosomes. After meiosis is complete, the four daughter nuclei have the haploid (n) number. In this lab, the study of meiosis will include the building of chromosome models

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Stages of Mitosis

Right before prophase, the cell spends most of its life in the interphase, where preparations are made before the beginning of mitosis (the DNA is copied). However, since the actual process involves the division of the nucleus, prophase is technically the first stage of this process.

The different stages of mitosis occurring during cell division are given as follows-

Interphase

Before entering mitosis, a cell spends a period of its growth under interphase. It undergoes the following phases when in interphase:

• G1 Phase: This is the period before the synthesis of DNA.
• S Phase: This is the phase during which DNA synthesis takes place.
• G2 Phase: This is the phase between the end of DNA synthesis and the beginning of prophase.

Prophase

Prophase immediately follows S and G2 phase of the cycle and is marked by condensation of the genetic material to form compact mitotic chromosomes composed of two chromatids attached at the centromere.

The completion of prophase is characterised by the initiation of the assembly of the mitotic spindle, the microtubules, and the proteinaceous components of cytoplasm that help in the process.

The nuclear envelope starts disintegrating.

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Prophase

Prometaphase

In the prometaphase, the nuclear envelop disintegrates. Now the microtubules are allowed to extend from the centromere to the chromosome. The microtubules attach to the kinetochores which allow the cell to move the chromosome around.

Metaphase

At this stage, the microtubules start pulling the chromosomes with equal force, and the chromosome ends up in the middle of the cell. This region is known as the metaphase plate. Thus, each cell gets an entire functioning genome.

Metaphase

Anaphase

The splitting of the sister chromatids marks the onset of anaphase. These sister chromatids become the chromosome of the daughter nuclei. The chromosomes are then pulled towards the pole by the fibres attached to the kinetochores of each chromosome. The centromere of each chromosome leads at the edge while the arms trail behind it.

Anaphase

Telophase

The chromosomes that cluster at the two poles start coalescing into an undifferentiated mass, as the nuclear envelope starts forming around it. The nucleolus, Golgi bodies and ER complex, which had disappeared after prophase start to reappear.

Telophase

Telophase is followed by cytokinesis, which denotes the division of the cytoplasm to form two daughter cells. Thus, it marks the completion of cell division.