Biology Topics related to disease and health provide critical insights into human physiology and medicine.
The Cell Nucleus – Molecular Expressions Cell Biology and Introduction to Chromosomes
Nature and Occurrence:
The nucleus is a large, centrally located spherical cellular component. It is bounded by two nuclear membranes, both forming a nuclear envelope. The nuclear envelope encloses a space between two nuclear membranes and is connected to a system of membranes called the ER (Endoplasmic Reticulum).
The nuclear envelope separates the nucleus from the cytoplasm. The nuclear envelope contains many pores (the nuclear pores) and encloses the liquid ground substance, the nucleoplasm. Nucleopores allow the transfer of materials between the nucleoplasm and the cytoplasm. Within nucleoplasm two types of nuclear structures are embedded – the nucleolus and chromatin material.
The nucleolus may be one or more in number and is not bounded by any membrane. It is rich in protein and RNA (ribonucleic acid) molecules and acts as the site for ribosome formation. Nucleolus is known as the factory of ribosomes. Ribosomes are helpful in protein synthesis in the cytoplasm.
Observation of Nucleus in the Animal
Take a glass slide and put a drop of water on it. This is done to put the material under microscopic observation. Using a toothpick or an ice-cream spoon you can scrape the inside surface of your cheek. With the help of a needle, you can transfer this material and spread it evenly on the cleaned glass slide. To colour this material, you can put a drop of methylene blue solution/stain on it.
Finally put a coverslip over this stained material and observe this temporary mount/slide under the high power of a microscope. You will observe a spherical or oval dot-like structure, called a nucleus near the centre of each cheek cell.
In the chapter on Tissues, you will learn that these cells are of squamous epithelium. You can draw diagrams of these cells on your observation sheet and label them. A similar structure (i.e., nucleus) has been observed in the onion peel cells.
The chromatin material is a thin, thread-like intertwined mass of chromosome material and composed of the genetic substance DNA (deoxyribonucleic acid) and proteins (i.e., histones). Basically, chromatin is formed of repeating subunits, the nucleosomes, each of which has a DNA molecule coiled around a disc of histones. DNA stores all the information necessary for the cell to function (i.e., metabolism), to grow, and to reproduce further cells of the next generation. Distinct segments of DNA are called genes. The chromatin is condensed into two or more thick ribbon-like chromosomes during the division of the cell.
Differences between Nucleus and Nucleolus
|1. It represents the whole eukaryotic complex that contains genetic information.||1. It is a component of the nucleus.|
|2. It is covered by a two-membrane envelope.||2. It does not have a covering membrane.|
|3. It controls the structure and working of cells.||3. It synthesizes ribosomal subunits.|
Structure of Chromosomes
Chromosomes are thread-like structures usually present in the nucleus that become visible only during cell divisions (mitosis and meiosis). Chromosomes contain hereditary information of the cell in the form of genes (hereditary units).
Each chromosome is made up of two components :
- DNA (deoxyribonucleic acid), and
- Proteins (e.g., histones and acidic proteins).
DNA is the most essential component of chromosomes. It is the material of genes (i.e., genetic material).
Most chromosomes consist of two arms that extend out from a specialized region of DNA, called the centromere (meaning” middle body”). Centromere or primary constriction gives a particular shape to chromosomes due to their position. The chromosome extremities or terminal regions on either side are called telomeres.
What is a Chromatid?
Before a cell divides, it duplicates its chromosomes (i.e., each chromosome DNA molecule of each chromosome makes a copy of itself and becomes associated with proteins). The two copies of chromosomes remain attached at their centromeres. As long as two copies of a chromosome are connected to the common centromere, they are called sister chromatids.
Both chromatids of a chromosome are identical, with identical genes. During cell division, the two sister chromatids separate and each chromatid becomes an independent daughter chromosome.
Diploid and Haploid Number of Chromosomes
Every eukaryotic species has a fixed number of chromosomes in its cells. The number of chromosomes varies from a minimum of two (in roundworms, Ascaris megalocephaly) to a few hundred in different species. In human beings, there are 46 chromosomes (44 autosomes + 2 sex chromosomes) in each body (or somatic) cell. There is always a pair of chromosomes of each kind.
The paired condition of chromosomes is known as diploid; a cell with the full number of chromosomes (i.e., two of each kind) is called a diploid cell. Body cells in human beings are diploid.
A set comprising unpaired chromosomes of each kind is said to be haploid and a cell that has half the number of chromosomes (i.e., one of each kind) is called a haploid cell. The gametes in human beings are haploid.
Significance of Diploidy
The diploid state of organisms originated during the process of fertilization of sexual reproduction. During fertilization, two haploid cells or gametes of different types: sperm of man and ovum of woman are fused together to produce a diploid egg (zygote). This egg divides by mitotic cell divisions to form numerous diploid body cells, making the body of the diploid organism.
Thus, 46 chromosomes (i.e., two of each kind or 23 pairs) in each body cell in human beings represent the diploid number, and 23 chromosomes (i.e., one of each kind) in the sex cells or gametes (sperm and ovum) in human beings represent a haploid number.
Cells that lack the Nucleus
- Red blood cells of humans and other mammals lose their nuclei and this enables them to carry more hemoglobin and hence pick up more oxygen.
- Phloem sieve tubes provide the transport system for sucrose in plants. They lose most of the cell organelles including their nuclei. This makes it easier for materials to flow through it.
Functions of Nucleus
- The nucleus controls all metabolic activities of the cell.
- If the nucleus is removed from a cell, the protoplasm ultimately dries up and dies.
- It regulates the cell cycle.
- It is concerned with the transmission of hereditary traits from the parent to offspring.