Chromosomes

CHROMOSOMES - Dr. P. Saranraj, Assistant Professor, Department of Microbiology, Sacred Heart College (Autonomous), Tirupattur, Vellore District, Tamil Nadu, India.

1. CHROMOSOMES Dr. P. Saranraj M.Sc., M.Phil. , Ph.D., NET Assistant Professor Department of Microbiology Sacred Heart College (Autonomous) Tirupattur, Vellore District. Mobile: [phone] E.mail: [email protected]

2. CHROMOSOMES In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called Chromosomes.  Chromosomes were first described by Straubberg in 1875. The trem Chromosomes was coined by Waldeyer in 1888. Each chromosome is made up of DNA tightly coiled proteins called Histones that support its structure. Chromosomes are not visible in the cell’s nucleus not even under a microscope when the cell is not dividing. However, the DNA that makes up chromosomes becomes more tightly packed during cell division and is then visible under a microscope. Human cells have 46 Chromosomes (23 Identical pairs) (44 Autosomes and 2 Sex Chromosomes) ( Autosomes are the chromosomes which are not involved in sex determination)

3. Organisms Number of chromosomes Human 46 Chimpanzee 48 Dog 78 Horse 64 Chicken 78 Goldfish 94 Fruit fly 8 Mosquito 6 Nematode 11(m), 12(f) Round worm 2

4. Organisms Number of Chromosomes Onion 16 Fungal Mold 16 Carrot 20 Tomato 24 Tobacco 48 Rice 24 Maize 20

5. STRUCTURE OF CHROMOSOMES Centromere – It is a part of chromosomes that links sister chromatids . Centromere gives the characteristic shape to the chromosomes and helps describe the location of specific genes . “P” arm – It is a short arm which is present in upward . “Q” arm - It is a long arm which is present in downward .

6. Telomere – It is present in the tips of chromosome . Chromatids – Two identical copies formed by the replication of two chromosomes . Chromatins – Chromatin is a complex of DNA and Protein found in the eukaryotic nucleus which packages chromosomes. The structure of the chromatin varies significantly between different stages of cell cycle according to the requirement of DNA

7. TYPES OF CHROMOSOMES Chromosomes are of four types. They are: 1) Telocentric 2) Acrocentric 3) Sub – metacentric 4) Metacentric

8. 1.TELOCENTRIC In Telocentric, the Centromere is present at the end of the Chromosome . CENTROMERE LONG ARM

9. 2. ACROCENTRIC The Centromere is almost terminal . It has one large and another very small arm . LONG ARM CENTROMERE SHORT ARM

10. 3. SUB - METACENTRIC The Centromere is not at the middle position of the chromosomes. The arms are unequal and it is ‘L-Shaped’ in appearance . CENTROMERE SHORT ARM LONG ARM

11. 4. METACENTRIC The Centromere is at the middle position . The arms are equal and it is ‘V-Shaped’ in appearance. TWO EQUAL ARMS CENTROMERE