question_answer

Living beings exhibit a large degree of similarity in chemical constitution, biochemical reactions and body functions. They provide a number of evidences of common ancestry and evolution of different groups of organisms. Discuss in detail.

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Study the schematic representation of the genes involved in the lac operon given below and answer the questions that follow.

(i) Identify and name the regulatory gene in this operon. Explain its role in switching off the operon.

(ii) Why lac operon is called an inducible operon system?

(iii) Name the inducer molecule and the products of the genes 'z' and 'y' of the operon. Write the function of these gene products.

Answer:

Evidences from biochemistry and comparative physiology which show common ancestry   and evolution of different groups of organism are as follows

· Nucleic acids and chromosomes The hereditary material is present in the form of DNA. DNA is usually organised into chromatin fibres in the nucleus and chromosomes in a dividing cell. It has the same chemical composition in all the organisms.

· Protoplasm All living beings are made of protoplasm, commonly called living matter. Its biochemical constitution is similar in all the organisms. About 90% of the protoplasm is formed of four elements, i.e., carbon, hydrogen, oxygen and nitrogen. Alongwith phosphorus and sulphur, they constitute most of the organic compounds of living matter, i.e. carbohydrates, proteins, lipids (fats) and nucleic acids.

· Hormones They are biochemicals produced by ductless or endocrine glands which in traces help in triggering reactions or functions in other parts of the body. The hormones of vertebrates are both chemically and functionally similar. In case of deficiency in human beings, the hormones obtained from other vertebrates are taken as injections, e.g. insulin, thyroxin.

· Enzymes An organism has a number of systems. A system possesses a similar set of enzymes in different organisms, so much so that Krebs' cycle has similar enzymes in both plants and animals.

· Photosynthetic pigments All eukaryotic autotrophic plants possess chlorophyll-a.

· Chlorophyll-b to occurs in green algae and embryophytes. The latter, therefore, must have orginated from green algae.

· Metabolism Different metabolic reactions like respiration, digestion, assimilation, muscle contraction, nerve conduction [in animals) and photosynthesis (in plants) show a physiological harmony in various living beings.

· Blood and lymph These are fluid connective tissues which have the same composition and function in most of the animals indicating a close relationship.

· Blood groups Human beings have four main blood groups, i.e. A, B, AB and O. AB grouping is also found in apes but not in monkeys, showing that man is more closely related to apes than to monkeys.

· Excretion Nitrogenous waste shows a progressive detoxification in vertebrates. It is ammonia in fishes, urea in amphibians, uric acid in reptiles and birds and a combination of urea, uric acid and other chemicals in mammals.

· Oxyhaemoglobin crystals Crystals formed from oxyhaemoglobin of vertebrates show a relationship amongst the vertebrates. Crystals of closely related species have the same pattern or configuration while those of distantly related species have different configuration.

For example, crystals of birds have a basic resemblance and exhibit a basic similarity with the oxyhaemoglobin crystals of mammals, reptiles and amphibians.

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(i) Regulatory gene in this operon is 'i' gene.

Role of regulatory gene Its role in switching off operon is as follows

(a) It codes for the represser protein of the operon, which is synthesised constitutively,

(b) The represser has the affinity for the operator gene. It binds to the operator and prevents the RNA polymerase from transcribing the structural gene (a)

(ii) 'Lac' operon is called an inducible operon system because it is a regulated unit of genetic material, which is switched on or off in response to the presence or absence of a chemical.

(iii) Lactose is an inducer molecule. Gene 'z' codes for \[\beta -\] galactosidase, which is responsible for the hydrolysis of lactose or galactoside 'y' gene codes for lactose or galactoside permease. It allows the entry of lactose from outside.