Aim: This paper discusses the structure, function, cell division and genetic variation of chromosomal behavior.
Structure of Human Chromosome:
A chromosome is a complex package of DNA found in the nucleus of a cell. A chromosome is a structure that carries DNA from a cell. Chromosomes are complex in structure, containing elements needed for processes such as multiplication and division. DNA binds proteins called histones to form units known as nucleosomes. These dense units become chromatin fiber, which thickens and forms the chromosome.
Figure:01: DNA sequencing into chromatin and chromosome (Britannica,2022).
Chromosomes have a unique set of in terms of number and order. For example, humans have 23 pairs of chromosomes - 22 pairs of chromosomes with numbers called autosomes, 1 to 22, and one pair of mating chromosomes, X and Y. Each parent contributes one chromosome for each couple to the offspring. The chromosome is a complex and well-organized DNA structure with the help of the proteins of histones H1, H2A, H2B, H3 and H4. This is the structure that can be seen during metaphase cell division. This concise package allows long DNA in eukaryotes to be inserted into the cell nucleus. There are two arms known as the p-arm and the q-arm. There is a constriction known as the centromere, in which a kinetochore is formed to connect the spinning fibers during cell division. The location of the centromere determines the type of chromosome such as metacentric, submetacentric, acrocentric and telocentric. There is a telomere at the end of the arm which is a very rotating region like the centromere. There is a high-density heterochromatin and slightly euchromatin regions.
Figure:02: Structure of Human chromosome (NH GRI,1988).
The Function of Human Chromosome:
The primary function of chromosomes is to carry DNA and to pass on genetic information from parents to offspring. Chromosomes play an important role in cell division. They protect DNA from damage. Chromosomes are essential for the process of cell division, replication, division, and the formation of daughter cells. Chromosomes are often referred to as 'packaging materials' because they firmly bind DNA and protein to eukaryotic cells.
Figure :03: Function of human chromosome (BYJU'S,2011).
~How and why the structure of the human chromosome relates to the function?
Centromere, a structure on the chromosome that binds together two chromatids. Chromatid is one of the components of a recurring chromosome. The autosome has any number of chromosomes, as opposed to mating chromosomes. The mating chromosome, is a chromosome that differs from a normal autosome in shape, size, and behavior. Human cells have 23 pairs of chromosomes (22 pairs of autosomes and one pair of mating chromosomes), giving a total of 46 pairs per cell. Homologous says the same thing about the nature, structure, and origin of flexibility, but not in function. Non-homologous is related to chromosomes that are not part of the same pair.
Preparation of slide to study cell division:
• Place a healthy onion bulb on a beaker with water at the base.
• Within three to four days the root will come out from the base (nearly 1-2cm long)
• Collect these roots for observation.
• Collect an onion root tip from the stock table and place it carefully on a clean glass slide.
• Locate the area of the tip where the milky white color changes to a dull white. At this point, cut off the tip and place it into a watch glass.
• With a clean dropping pipette, add IM HCl to cover the tip completely.
• Keep the tip in the solution for 6 to 8 minutes.
• Now, Cover the entire tip with distilled water. Allow the tip to soak in the water for 2 minutes.
• With a clean dropping pipette, add IM HCl to cover the tip completely.
• Allow the tip to soak in the water for 2 minutes. Carefully transfer the tip to a clean glass slide, after 2 minutes.
• Add one to two drops of aceto-carmine stain to the tip.
• With the razor blade, cut the tip into small pieces.
• Push the material stick on the razor blade back into the solution with the help of a toothpick.
• Add one more drop of aceto-carmine.
• Place it over the slide and cover it with glass cover slip.
• Roll it with a piece of blotting paper.
• Press it down firmly with the thumb, inside the blotting paper.
• The stain coming out from the cover glass will be absorbed by the blotting paper.
• Now carefully remove the blotter paper that has absorbed excess stain from the edges of the cover glass.
• The slide is placed on the stage of the microscope to observe the stages of Mitosis cell division.
• Locate the cells of the root tip under the microscope at low power.
• Slowly focus the objective looking for small darkly stained structures inside the cells which appear large and rounded.
• Rotate the nose piece to fix the high-power objective. Find the same cell(s).
• Constantly make fine adjustments of focusing to view the chromosomes clearly.
• Identify the stage of the cell under division.
Observation in Mitotic stages:
Mitosis has the following stages;
1. Interphase:
The Chromosomes are indistinguishable from one another.
2. Prophase:
Chromosomes are prepared for division by shortening and thickening of chromatids.
3. Metaphase:
Chromosomes are arranged in a random manner on the equatorial plate of the cell.
4. Anaphase:
The centromere splits lengthwise in the chromosomes and the chromatids began to move towards the pole.
5. Telophase and cytokinesis:
The chromosomes have completed their movement towards the pole and begin to disperse inside the nuclear membrane.
Observation of meiosis stages:
Meiosis has two major stages;
1. Meiosis I :
It includes following stages;
• Interphase I:
Chromosomes are indistinguishable to each other. Replication of DNA. Thick chromosomes are X-shaped structures that can be easily seen under a microscope, and the chromosomes are identical in pairs. Regeneration occurs as homologous chromosomes interact with DNA. At the end of this phase, the nuclear membrane dissolves.
• Prophase I :
Synapsis and crossing over occur. Chromosomes are prepared for division by shortening and thickening chromatids. Coupled chromosomes form a line between cells.
• Metaphase I :
Tetrads lines up on the metaphase plate.
• Anaphase I :
Homologous pairs separate out. The chromosome pairs are different and go to opposing poles. Each pair can go on any plank.
• Telophase I :
Chromosomes are enclosed in nuclei. The chromosomes have completed their movement towards the pole and begin to disperse inside the nuclear membrane. Transformation of the nuclear membrane. Cells divide into 2 female cells, each with 23 chromosomes.
• Interkinesis I :
It is a period of rest that species of some cells enters during meiosis.
• Cytokinesis I:
The Cytoplasm of the original cell divides into two daughter cells.
2. Meiosis II:
It includes the following stages;
• Prophase II:
There are now 2 cells. DNA does not repeat itself.
• Metaphase II:
Chromosomes are arranged in a random manner on the equatorial plate of the cell. Individual chromosomes form a line between cells.
• Anaphase II:
The centromeres split lengthwise in the chromosome and the chromatids begin to move towards the pole. Copies of chromosomes (chromatids) disintegrate and proceed to oppose poles.
• Telophase II:
Four reproductive cells are arranged in a formation. Transformation of the nuclear membrane. There are 4 new haploid daughter cells. In males, 4 sperm cells are produced. For women, 1 egg and 3 polar bodies are produced. Polar bodies do not function as mating cells.
• Cytokinesis :
Four daughter cells are produced.
Difference between plants and animal cells in terms of cell division :
1. Formation of spindle fibers:
In plants:
Most plants do not have centrioles, but instead , have microtubule clusters that function to control the distribution of chromosomes. They also play a role in cell division during cytokinesis. During proposal, the plant cell begins to produce spindles from the planning centers that grow into a nuclear region and attach themselves to chromosomes. From there, they organize the organization and division of chromosomes between daughter cells during mitosis.
In Animals:
Animal cells consist of two sets of microtubules and centrioles, collectively called centrosomes, found in the stem cells. During proposal, microtubules within the centrosome extend in length to the chromosomes in the nucleus. Microtubules are called spindles at this point. Spindles arrange for careful planning and separation of chromosomes between female cells during mitosis. Other microtubules from the centrosome also contribute to cytokinesis after the last stage of mitosis.
2. In Cytokinesis:
In animals, the cell is separated from the outside by a contractile ring, which creates a tear duct. The layer of actin and myosin fibers under the plasma membrane at the center of the cell begins to deteriorate until the cell is squeezed in. In plants, a new cell wall builds up inside the growing cell until two new cells form. The fusion of new cell walls is made of vesicles filled with cellulose and lignin, which eventually merge to form a new cell wall, and the parent cell divides in two.
The Behavior of chromosomes leads to variation :
The common behavior of chromosomes in meiosis is that pairs are homologous synapses, reunite, and then divide into anaphase I. In anaphase II, the sister chromatids separate. However, studies of small corn chromosomes obtained from different sources often have chromatid junction failure in anaphase I. Reproduction produces endless opportunities for genetic diversity. In other words, reproduction produces a genetic variant. They are different from both parents and different from each other. This happens for many reasons. If homologous chromosomes form pairs during prophase I meiosis I, overdose is possible. Skip genetic interactions between homologous chromosomes. It results in a new set of genes in each chromosome. When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and separate chromosomes differentiate independently. This is called a private assortment. It results in gametes with a unique combination of chromosomes. During meiosis, equal chromosomes (1 per parent) pair in length. Chromosomes fall into areas called chiasma. In each chiasma, the chromosomes break down and reassemble, one and the same trajectory of their genes. This reunification results in genetic variation.
Figure no:04: Meiosis, inheritance , and variation (Science Learning Hub,2019).
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