Chromosomal theory of Inheritance pdf

Aug 09, 2022, 16:45 IST

Chromosome Theory of Inheritance

• It was proposed independently by Sutton and Boveri.
• The theory was expanded by Morgan, Sturtevant and Bridges. This theory proposes that chromosomes are vehicles of hereditary information and expression as genes are present over them.Resolve all Your Biology Doubts from Physics Wallah.
• The salient features of chromosome theory of inheritance are as follows –
• Bridge between one generation and the next is sperm and ovum.
• Both Sperm and ovum contribute equally in heredity. Sperm provides only nucleus for fertilization. Therefore heredity must be based in nuclear material.
• Nucleus possesses chromosomes. Therefore, chromosomes must carry hereditary characters.
• Chromosomes have a definite role in heredity as loss or deficiency results in structural or functional deficiency in organisms.
• Chromosomes, like hereditary factors are particulate structures which maintain their number, structure and individuality in organisms from generation to generation.
• Somatic diploid cells possess two chromosomes as well as two mendelian factors or genes of each type. A gamete has only one chromosome as well as one mendelian factors out of homologous pair.
• Synapsis and random independent separation of chromosomes constitute the quantitative basis of independent assortment of mendelian factors/genes.
• Paired condition of chromosomes or mendelian factors is restored during fertilization.
• Sex of certain organisms is determined by specific chromosomes called sex chromosomes.

Kinds of Chromosomes

Viral Chromosome

• In viruses and bacteriophages DNA or RNA represents the viral chromosome. The DNA containing viral chromosomes occur mostly as linear complex and is found in animal viruses and bacteriophages while RNA containing viral chromosomes are single stranded and occur in plant viruses.

Prokaryotic Chromosome

• It is also called nucleoid, genophore, prochromosome or chromoneme. It consists of single double stranded circular molecule of DNA.
• In Escherichia coli about long DNA molecule is accommodated in a space of 1 – 2 . DNA duplex is covalently closed circular structure having several loops each of which is super coiled. Coils are stabilized by RNA and polyamines.

As histones are absent, prokaryotic DNA is therefore called naked.

Eukaryotic Chromosome

• DNA of a chromosome is associated with positively charged histones and some less positively charged nonhistone proteins. Association between DNA and histones produces linear chains of nucleosomes. A nucleosome itself is made of octamer of 4 histone proteins (2 molecules of each type) – H₂A, H₂B, H₃ and H₄.
• DNA is wrapped over the octamer forming 1.75 coils.
• DNA present between two nucleosomes is called inter breed or linker DNA. It bears H₁ histone molecule.
• Nucleosome chain provides chromatin a thickness of about 10nm. It is often coiled in a solenoid fashion with each coil having about 6 nucleosomes and a diameter of 30nm or 300A⁰.

Chromatin mostly occurs in 30nm thickness state. It condenses in the form of radial loops over a scaffold of a nonhistone nuclear protein during the formation of a chromosome.

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