Chromosomes: structure and functions
July 14, 2018
Introduction
- Karl Wilhelm von Nägeli in 1842 discovered a thread like structure which was called chromosomes.
- They are the filamentous bodies typically present in the nucleus and become visible during cell division.
- They are the carriers of hereditary materials called genes.
- Most of the chromosomes on a cell are called heterosome and only one to two sex chromosomes are present.
Structure of chromosome
- Each chromosome consists of two chromatids held together at a point along their length.
- The point at which the two chromatids of a chromosome are held together appears as a constriction in the chromosomes called centromere.
- The chromosomes consist of various structures like centromere, secondary constriction, nuclear organisers, telomeres, satellites and matrix.
Image source: toppr
1. Chromonema
- Metaphasic chromosomes consist of two sub-units called chromatids.
- The chromatids consist of filamentous structure called the chromonema.
- The chromonemata form the gene bearing portion of the chromosomes.
- They are embedded in achromatic substance known as matrix.
- Matrix is enclosed in a sheet.
- The sheet is called pellicle. Pellicle is not uniformly formed.
- The chromonema may contain threads. The threads remain coiled with each other.
- The coil may be of two types: paranemic coils and plectonemic coils.
- When the thread separate easily that is called paranemic coils and when the thread cannot separate easily that is called plectonomic coils.
2. Centromere
- The constricted region of chromosome is called centromere.
- It consists of small granules for that centromere is also called kinetochore or primary constriction.
- Position of centromere is constant for particular chromosome.
- Depending upon the position of centromere, chromosomes are categorized as:
- Metacentric (V-shaped): This centromere is located at or near the midpoint so that the arm ratio is 1:1.
- Sub-metacentric: When the centromere is located slightly away from the midpoint so that the two arms are unequal, then the chromosome is called sub-metacentric.
- Acrocentric: Rod shaped chromosome where centromere is towards one end sub-terminal. So the chromosome has a short and a long arm.
- Telocentric: Rod shaped chromosome with centromere at one end during anaphase.
3. Secondary constriction:
- In addition to centromere as primary constriction, there is another constriction in the specific region of the chromosome; called secondary constriction.
- The location of secondary constriction is specific in its site for particular chromosome.
- Thus, it helps in identification of chromosomes.
5. Nucleolar organizer
- Normally in each diploid sets of chromosomes, two homologous chromosomes have additional constriction, i.e.called nucleolar organizer.
- They are called so because they are necessary for nucleolus.
6. Telomere
- The tips of the chromosomes are called telomeres.
- The telomeres differ from rest part of chromosome on structure and composition.
- They have unique non-sticking property with other parts of chromosomes and other telomere.
- Telomeres are region for the attachment to the nuclear envelope.
7. Satellite
- They are round, elongated or knob-like appendages with the chromosomes.
- Chromosomes bearing satellites at their terminal points are called satellite chromosome.
Functions of chromosomes
- These are the vital component of cell.
- These control almost all cellular activities at physiological levels.
- These maintain the identity of species.
- These help in determination of sex of species.
- These act as vehicle of hereditary characters from one generation to another.
- With the help of chemical constituents DNA and RNA synthesize protein and enzymes.
- Some species of chromosomes synthesize yolk in oocytes of many vertebrates.
References:
i) https://www.thoughtco.com/chromosome-373462
ii) https://study.com/learn/lesson/chromosome-function-types.html
Chromosomes: structure and functions
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Shailesh Koirala
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