Chromosomes are thread-like structures located in the nucleus of each cell that bundle the DNA molecule.DNA is tightly wrapped around proteins called histones that support the shape of each chromosome. Strasburger (1815) was the first to define chromosomes, whereas Waldeyer (1888) coined the name “chromosome.” When cells are stained with an appropriate basic dye and observed under a light microscope during the metaphase phase of mitosis, they appear as rod-shaped dark stained entities.
Eukaryotes have numerous large, linear chromosomes that are found in the cell nucleus. Each chromosome usually has one centromere and one or two arms that protrude from it. Each chromosome is divided into three components structurally:
2. The Matrix
It is the chromosome’s outer membrane that protects the chromosome’s substance. It is made up of achromatic chemicals and is exceedingly thin.
The chromonemata are present in the ground substance of the chromosome.
The chromonemata are two identical spirally coiled threads embedded in the matrix of each chromosome. The two chromonemata are similarly coiled to the point that they appear to be a single thread with a thickness of about 800A. Each chromonemata consists of around 8 microfibrils, each of which contains two DNA double helixes.
The following structural features (excluding the chromomere) can be detected under a light microscope in mitotic metaphase chromosomes: Chromatid, Chromonema, Chromomeres, Centromere, Secondary constriction or Nucleolar organizer, Telomere, and Satellite.
The centromere is a tiny structure in the chromonema that is distinguished by constriction and is considered a permanent chromosome component. The two chromonemata are fused at this moment. Centromere, kinetochore, or main constriction are all terms used to describe this phenomenon. The chromosome is divided into two portions, or “arms,” by it. The short chromosomal arm is referred to as the “P arm.” The long arm of the chromosome is referred to as the “Q arm.”
Its location is fixed for each chromosomal type and serves as a distinguishing feature. The kinetochore has a trilaminar structure in thin electron microscopic sections, with a dense outside proteinaceous layer of 10 nm thickness, a low-density middle layer, and a dense inner layer closely attached to the centromere. During cell division, the chromosomes are connected to spindle fibers in this region.
Nucleolar Organiser or Secondary Constriction
Aside from the primary constriction at the centromere, the chromosome has a secondary constriction at any point along its length. These constrictions are important for identifying specific chromosomes in a set because they are consistent in their position and extent.
Satellite chromosomes, often known as sat-chromosomes, are chromosomes with secondary constrictions. The nucleolus is always associated with secondary constriction of sat-chromosomes. As a result, secondary constrictions are also known as nucleolus organizer regions (NOR) and sat-chromosomes as nucleolus organizer chromosomes.
These are specialized chromosomal ends that show physiological differentiation and polarity. Telomeres are the ends of the chromosomes. Due to a lack of telomere, if a chromosome breaks, the split ends can merge.
Types of Chromosomes
A. on the basis of the number of centromeres
1. One centromere monocentric.
2. Two centromeres, dicentric.
3. Centromeres with more than two centromeres are polycentric.
4. Without a centromere, acentric.
5. Diffuse or non-located, with indistinct centromeres strewn across the chromosome’s length.
B. Depending on Centromere Position
1. Telocentric chromosomes are rod-shaped chromosomes with the centromere at the end, leaving the chromosome with only one arm.
2. Acrocentric chromosomes are rod-shaped chromosomes with a sub-terminal centromere. One arm is lengthy, while the other is rather short.
3. Sub-metacentric chromosomes have a centromere that is slightly off-center, causing the two arms to be unequal.
4. Metacentric chromosomes have a V-shaped centromere in the middle and nearly equal arms.
The Role and Importance of Chromosomes
Storage of Genetic Code: The genetic material essential for the organism’s development and growth is stored in the chromosome. A series of components called genes make up DNA molecules. Genes are regions of DNA that code for certain proteins that the cell needs to function properly.
Sex Determination: Humans have 23 pairs of chromosomes, one of which is designated as the sex chromosome. The gender of a child is determined by the chromosome passed down by the father. The child will be female if the X chromosome is passed out of the XY chromosome, and a male child will develop if the Y chromosome is passed out of the XY chromosome.
Control of Cell Division: During the process of mitosis, chromosomes check for successful cell division. The parent cells’ chromosomes ensure that the necessary information is passed on to the daughter cells that the cell needs to grow and develop properly.
Protein Synthesis and Storage: Chromosomes direct the sequences of proteins synthesized in our bodies and also keep DNA in order. The proteins are also kept in the chromosomes’ coiled shape. These proteins that are linked to DNA aid in the correct packaging of DNA.