Type and functions of Chromosomes
What is chromosomes
The chromosomes are capable of self–reproduction and maintaining morphological and physiological properties through successive generations. They are capable of transmitting the contained hereditary material to the next generation. Hence these are known as ‘hereditary vehicles’.
Discovery of chromosomes
Hofmeister (1848) : First observed chromosomes in microsporocytes (microspore mother cells) of Tradescantia.
Flemming (1879) : Observed splitting of chromosomes during cell division and coined the term, 'chromatin'.
Roux (1883) : He believed the chromosomes take part in inheritance.
W.Waldeyer (1888) : He coined the term ‘chromosome’.
Benden and Boveri (1887) : They found a fixed number of chromosomes in each species.
Kinds of chromosomes
(i) Viral chromosomes : In viruses and bacteriophages a single molecule of DNA or RNA represents the viral chromosome.
(ii) Bacterial chromosomes : In bacteria and cyanobacteria, the hereditary matter is organized into a single large, circular molecule of double stranded DNA, which is loosely packed in the nuclear zone. It is known as bacterial chromosome or nucleoid.
(iii) Eukaryotic chromosomes : Chromosomes of eukaryotic cells are specific individualized bodies, formed of deoxyribonucleo proteins (DNA + Proteins).
Chromosomal theory of inheritance:
It was proposed independently by Sutton and Boveri in 1902. The chromosome theory of inheritance proposes that chromosomes are vehicles of hereditary information and expression as Mendelian factors or genes.
(i) Bridge between one generation to the next are sperm and ovum.
(ii) Both sperm and ovum contribute equally in heredity. Sperm provides only nucleus for fertilization. Therefore, heredity must be based in nuclear material.
(iii) Nucleus possesses chromosomes. Therefore, chromosomes must carry hereditary characters.
(iv) Chromosomes, like hereditary factors are particulate structures, which maintain their number, structure and individuality in organisms from generation to generation.
Chromosome number is n = 2 in Mucor hiemalis, 2n=4 in plant Haplopappus gracilis. Chromosome number is 14 (n=7) in Pea, 20 in Maize, 46 in human beings. Maximum number of chromosomes is known for Adder’s Tongue Fern (Ophioglossum reticulatum, 2n = 1262) and Aulocantha (2n=1600). Number of chromosomes is not related to complexity or size of organism e.g., Domestic Fowl and Dog both possess 78 chromosomes. Study of chromosome structure is performed at metaphase and study of chromosome shape at anaphase.
Chromosome cycle and cell cycle
Chromosomes exhibit cyclic change in shape and size during cell cycle. In the non-dividing interphase nucleus, the chromosomes form an interwoven network of fine twisted but uncoiled threads of chromatin, and are invisible. During cell division the chromatin threads condense into compact structures by helical coiling.
Different regions (structures) recognized in chromosomes are as under.
(i) Pellicle : It is the outer thin but doubtful covering or sheath of the chromosome.
(ii) Matrix : Matrix or ground substance of the chromosome is made up of proteins, small quantities of RNA and lipid. It has one or two chromonemata (singular - chromonema) depending upon the state of chromosome.
(iii) Chromonemata : They are coiled threads which form the bulk of chromosomes. A chromosome may have one (anaphase) or two (prophase and metaphase) chromonemata. There are three view points about the constitution of chromonema and chromosome.
(iv) A Primary Constriction and Centromere (kinetchore) : A part of the chromosome is marked by a constriction. It is comparatively narrow than the remaining chromosome. It is known as primary constriction. The primary constriction divides the chromosome into two arms. It shows a faintly positive Feulgen reaction, indicating presence of DNA of repetitive type. This DNA is called centromeric heterochromatin.
(v) Centromere : Centromere or kinetochore lies in the region of primary constriction. The microtubules of the chromosomal spindle fibres are attached to the centromere. Therefore, centromere is associated with the chromosomal movement during cell division. Kinetochore is the outermost covering of centromere.
(vi) Secondary constriction or nucleolar organizer : Sometimes one or both the arms of a
chromosome are marked by a constriction other than the primary constriction. During interphase this area is associated with the nucleolus and is found to participate in the formation of nucleolus. It is, therefore, known as nucleolar organizer region or the secondary constriction.
Nucleolar organizer region (NOR) : In certain chromosomes, the secondary constriction is (In human beings 13, 14, 15, 20 and 21 chromosome are nucleolar organizer) intimately associated with the nucleolus during interphase. It contains genes coding for 18S and 28S ribosomal RNA and is responsible for the formation of nucleolus. Therefore, it is known as nucleolar organizer region (NOR).
(vii) Telomeres : The tips of the chromosomes are rounded and sealed and are called telomeres which play role in Biological clock. The terminal part of a chromosome beyond secondary constriction is called satellite. The chromosome with satellite is known as sat chromosome, which have repeated base sequence.
(viii) Chromatids : At metaphase stage a chromosome consists of two chromatids joined at the common centromere. In the beginning of anaphase when centromere divides, the two chromatids acquire independent centromere and each one changes into a chromosome.
Types of chromosomes based on number of centromeres
Depending upon the number of centromeres, the chromosomes may be:
(i) Monocentric with one centromere.
(ii) Dicentric with two centromeres, one in each chromatid.
(iii) Polycentric with more than two centromeres.
(iv) Acentric without centromere. Such chromosomes represent freshly broken segments of chromosomes, which do not survive for long.
(v) Diffused or non-located with indistinct centromere diffused throughout the length of chromosome. The microtubules of spindle fibres are attached to chromosome arms at many points. The diffused centromeres are found in insects, some algae and some groups of plants (e.g. Luzula).
Types of chromosomes based on position of centromere
Based on the location of centromere the chromosomes are categorised as follows:
(i) Telocentric : These are rod-shaped chromosomes with centromere occupying a terminal position. One arm is very long and the other is absent.
(ii) Acrocentric : These are rod-shaped chromosomes having subterminal centromere. One arm is very long and the other is very small.
(iii) Submetacentric : These are J or L shaped chromosomes with centromere slightly away from the mid-point so that the two arms are unequal.
(iv) Metacentric : These are V-shaped chromosomes in which centromere lies in the middle of chromosomes so that the two arms are almost equal.
(9) Molecular organisation of chromosome : Broadly speaking there are two types of models stating the relative position of DNA and proteins in the chromosomes.For more details about the Biology you can refer zoology class 11 Notes prepared by academic team of Physics Wallah.