Introduction
Chromosomes are compact, rod-shaped structures formed during cell division. Each chromosome is made up of a single, long DNA molecule wrapped around proteins.
In a non-dividing cell, DNA is already present in a coiled and compact form as chromatin. Chromatin is formed when negatively charged DNA is wrapped around positively charged histone proteins. Due to this special packing, it is possible for a very long DNA molecule to fit inside the nucleus. DNA carries genetic information in the form of specific sequences of nucleotides called genes. So, it is very important that at the time of cell division, the DNA remains intact and gets evenly distributed among the daughter cells. To ensure this, the chromatin is further condensed into long, compact, rod-shaped structures called chromosomes.
Structure of chromosome
Chromatid
Each chromosome consists of either a single chromatid or a pair of chromatids (depending on the phase of the cell cycle). When a pair of chromatids is present in a chromosome, they are called sister chromatids, as their DNA content is exactly the same.
Centromere
The centromere is the narrow, constricted part of the chromosome. It bears proteinaceous, disc-like structures called kinetochores, which serve as the site of attachment of spindle fibres during cell division. The centromere is also called the primary constriction.
Chromosomes are classified by the position of the centromere: metacentric (centromere in the middle, two equal arms), sub-metacentric (slightly away from the middle, one shorter arm), acrocentric (close to one end, one extremely short arm) and telocentric (at the very end).
Arms
The parts of the chromatid present on either side of the centromere are called the arms of the chromosome. A chromosome can have 1–4 arms.
Telomeres
Telomeres are repetitive, guanine-rich DNA sequences located at the ends of chromosomes. In many cells, a part of the telomere is lost every time the DNA is replicated.
A chromosome can have 1–4 telomeres.
Constriction
Secondary constrictions are constricted or narrow regions found at any point on the chromosome other than the centromere.
The secondary constriction marks the site of a Nucleolar Organizer Region (NOR), a region containing multiple copies of the 18S and 28S ribosomal RNA genes, which code for the ribosomal RNA required by ribosomes.
The part of the chromosome present beyond the secondary constriction is called the satellite.
Chromosomes that carry a satellite are called SAT chromosomes (satellite chromosomes). Their secondary constriction, the NOR, is where the nucleolus forms in the nucleus.
Don’t confuse the centromere with the kinetochore: the centromere is the constricted region of the chromosome, while kinetochores are the disc-shaped protein structures on either side of it where spindle fibres attach.