Set-II: Past Entrance Papers

Hint

(b) B-DNA is a helical structure with \(20 \AA\) diameter and the distance between the two base pairs is \(3.4 \AA\) and there are 10 base pairs in each turn or pitch (one round). Hence, one turn of the helix is approximately \(34 \AA\) or \(3.4 \mathrm{~nm}(10 \AA=1.0 \mathrm{~nm})\). Z-DNA (in comparison to B-DNA) is a left-handed double-helical structure in which double helix winds to left in a zig-zag pattern (instead of right, like B-DNA).

Hint

(a) Barley seeds are rich in carbohydrates (starch). The starch is hydrolysed by
\(\alpha\)-amylase to monosaccharides unit at the time of germination of seeds.

Hint

(c) \(K_{m}\) value or Michaelis constant is defined as the substrate concentration at which half of the enzyme molecules are forming (ES) complex or concentration of the substrate when the velocity of the enzyme reaction is half the maximal possible. The \(\mathrm{K}_{\mathrm{m}}\) varies from enzyme to enzyme and is used in characterizing the different enzymes. A smaller \(\mathrm{K}_{\mathrm{m}}\) value indicates a greater affinity of the enzyme for its substrate, hence, shows a quicker reaction. \(\mathrm{K}_{\mathrm{m}}\) value is a constant characteristic of an enzyme for its conversion of a substrate.

Hint

(b) A competitive inhibitor competes with substrate molecule for occupying the active site of an enzyme. These inhibitors have structural resemblance with substrate molecules due to which they easily bind with the active site of an enzyme and form an enzyme-inhibitor complex.
\(\underset{\text { (enzyme) }}{E}+\underset{\text { (inhibitor) }} {I} \longrightarrow EI\) complex

Hint

(a) Enzymes control all the life processes. They increase the rate of a biological reaction. The magnitude of the increase may be greater than those affected by other catalysts.

Vitamins are accessory indispensable food factor, organic in nature (organic acid, amino acid esters, alcohols, steroids, etc.) required by an organism in small amounts to maintain normal growth and regulate the metabolic processes.

Hormones are biologically active organic substances that are produced in minute quantities by some specialized organs and exert physiological effects at sites remote from their origin.

Hint

(c) Glycine is considered as the simplest amino acid as it has one amino group, one carboxylic group, and no substituent functional group.

Hint

(a) Each nucleotide consists of three distinct units – a phosphate group derived from phosphoric acid, a pentose sugar, and a ring-shaped nitrogenous base.

Nucleoside \(+\) Phosphoric acid \(\rightarrow\) Nucleotide \(+\mathrm{H}_{2} \mathrm{O}\)

Hint

(a) Carbohydrates are organic compounds synthesized in the chlorophyll-containing cells of some bacteria, algae, and green plant cells, during photosynthesis. Certain photo-autotrophic bacteria e.g., Green sulphur bacteria and purple sulphur bacteria contain pigments like chlorobium chlorophyll and bacteriochlorophyll respectively that help them in photosynthesis.

Hint

(c) DNase (deoxyribonuclease) or simply nuclease is an enzyme which breaks down DNA by hydrolysis of the phosphodiester bonds of its sugar-phosphate backbone. Depending on the position of hydrolysing phosphodiester bonds, nucleases are of two types:
(i) Endonucleases
(ii) Exonucleases
Endonucleases hydrolyse internal phosphodiester bonds in a polynucleotide chain (i.e. DNA). While exonucleases hydrolyse terminal phosphodiester bonds in a polynucleotide chain (i.e. DNA).

Hint

(d) Copper is associated as an activator with tyrosinase. It is widely distributed in plants, animals, and man. It is also known as polyphenol oxidase or catecholase. It oxidizes tyrosine to melanin in mammals and causes the cut surfaces of many fruits and vegetables to darken.

Hint

(a) Minor elements are those which are required in the quantity of less than milligram/gram of dry matter but they are essential for the proper growth and development of an organism e.g., \(\mathrm{Cl}, \mathrm{Mn}, \mathrm{B}, \mathrm{Zn}, \mathrm{Cu}\) Mo, etc. These elements work as non-protein cofactor in enzymes e.g., \(\mathrm{Zn}, \mathrm{Cu}\) etc. They also take part in oxidation-reduction reactions e.g., \(\mathrm{Cu}\), with variable valency. Chloride ion enhances the activity of salivary amylase. Zinc is required for the activity of carbonic anhydrase and alcohol dehydrogenase, etc.

Hint

(b) Lipid molecules are insoluble or sparingly soluble in water but are freely soluble in organic solvents like ether, alcohol, and benzene. The insolubility of lipids in water is due to the fact that the polar groups they contain are much smaller than their nonpolar portions. The nonpolar chains are long complex hydrophobic hydrocarbon chains. If shaken in water lipids often form small droplets or micelles. The complex formed is called emulsions. These non-polar proteins give them water repellent or hydrophobic properties.

Hint

(a) All those sugars which have free aldehyde or ketone group are called reducing sugars. These are able to reduce cupric ions \(\left(\mathrm{Cu}^{+2}\right)\) into cuprous ions \(\left(\mathrm{Cu}^{+}\right)\). Sucrose, and starch are non-reducing sugars.

Hint

(a) Collagen is a major fibrous protein of connective tissue, occurring as white fibres produced by fibroblast.

Hint

(b) Cholesterol forms a major component of animal cell membranes liposomes (artificially created spheres surrounded by a phospholipid bilayer like a membrane) which are used for transformation (transgenics).

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(c) Lysosomes are the reservoirs of acid hydrolases showing optimum activity at
pH \(5.0\) maintained within the lysosome. These include proteases, nucleases, glycosidases, lipase, etc. Among these protease act a very low pH, i.e. 2.

Hint

(a) Nitrogen fixation involves the conversion of atmospheric nitrogen to ammonia. It is done with the help of nitrogenase enzyme which occurs inside thick-walled heterocysts of the blue-green algae. These provide a suitable anaerobic environment for nitrogenase activity even in aerobic conditions.

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(b) Cellulose is fibrous polysaccharide that forms the structural component of the plant cell wall, some primitive fungi, and tunic of ascidians. Cellulose is the most abundant organic substance on earth. It can be digested by only a few microbes present in the gut of ruminants and white ants.

Hint

(d) Erucic acid occurs as glycerides in vegetable oils. It is a monounsaturated
omega-9-fatty acid. It is found in rapseed, mustared seed, and wallflower
seed.

Hint

(b) Cytochrome is the respiratory pigment. It is composed of protein, iron, and
porphyrin ring. It functions as an enzyme in the respiratory chain. Unlike haemoglobin the metal atom in the porphyrin ring must change its valency for the molecule to function.

Cytochromes are basically located in inner mitochondrial membranes and thylakoids of chloroplasts.

Hint

(a) All molecules require certain amount of energy for activation (to overcome the energy barrier) before they can react. This energy is called activation energy. This energy is recovered when products are formed. The essence of an enzyme is its ability to speed up (catalyze) a reaction by making or breaking specific covalent bonds (bonds in which atoms are held together by sharing of electrons). Enzymes act by somehow lowering the temperature at which a given bond is unstable i. e., they speed up a reaction by lowering the activation energy. It is the magnitude of the activation energy which determines how fast the reaction will proceed.

Hint

(c) Feedback inhibition or end product inhibition is the inhibition of the activity of an enzyme catalysing some early reactions of the series by the end product of the metabolic pathway.

For example a substrate \(\mathrm{A}\) is converted into a product \(\mathrm{F}\) through \(\mathrm{B}, \mathrm{C}, \mathrm{D}\) and \(\mathrm{E}\) intermediate products. As the concentration of end product \(F\) increases, it diffuses to allosteric enzyme \(\left(E_{1}\right)\) causing a reduced synthesis of the product B which in turn lowers the rate of enzymatic reactions in rest of the pathway.

Hint

(c) Viruses do not have enzymes so they cannot synthesize proteins. They multiply only inside the living host cell and for multiplication and metabolism they take over the machinery of the host cell. They lack their own cellular machinery and enzymes.

Hint

(a) ATP is a nucleotide as it is composed of adenine, ribose sugar, and phosphoric acid. There are two additional phosphate groups attached to the phosphate group of AMP. The last two phosphate molecules are connected by high-energy bonds.

Hint

(a) Glycoproteins are proteins that contain sugars like carbohydrates as prosthetic group. In most glycoproteins, the linkage is between asparagine and N-acetyl-D-glucosamine. Some glycoproteins are immunoglobulins, membrane proteins, and muscle proteins. Lipoproteins are proteins complexed with lipids like triglycerides, phospholipids, etc. Nucleoproteins are proteins associated with nucleic acids and chromoproteins are proteins associated with pigments e. g ., cytochrome, and phytochrome.

Hint

(d) For human beings, eight amino acids are essential: Leucine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. So, these are known as essential amino acids.Cannot be synthesised in an animal body.

Hint

(b) Glycoproteins (mucoproteins) are conjugated proteins having a simple or complex sugar (galactose) residue at their N-terminal end. Glycoprotein is found in egg white, mucin, antibody lgG, cell membrane, saliva, synovial fluid, and heparin. Lipoproteins contain lipids and nucleoproteins contain nucleic acid as the prosthetic group.

Hint

(b) For human beings, eight amino acids are essential. These are leucine, isoleucine, Lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

Hint

(d) Enzymes enhance the rate of a reaction by just lowering the activation energy (the energy required for substances to react and get converted into product) of a reaction.

Hint

(a) In feedback inhibition, the product of an enzyme-catalyzed reaction accumulates and acts as an inhibitor of the reaction.

Hint

(b) Cellulose is the most abundant carbohydrate. Cellulose molecule is composed of 1600 to 6000 glucose molecules joined together. Those polymers form long-twisting macromolecules of cellulose. The chains are unbranched and linear. The successive glucose residues are joined together by \(\beta-1-4\), linkages.

Hint

(a) Lactose is popularly known as milk sugar. It is a disaccharide composed of one molecule of glucose and one molecule of galactose. The covalent bond that joins these two monosaccharide units is called glycosidic bond or glycosidic linkage. It is a reducing sugar.

Hint

(b) In the beginning of the S-phase, DNA replication occurs. DNA replication can occur in diffuse/less tightly coiled euchromatin. So active DNA stains light in colour when stained with acetocarmin and feulgen reagent in comparison to heterochromatin.

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(c) Conjugate enzymes (eg dehydrogenase enzymes) have an additional non-protein cofactor attached to the protein part. The cofactor can be organic or inorganic. Loosely attached organic cofactor are called coenzymes example NAD, FAD, etc. while firmly attached one is prosthetic group example heme, biotin, etc. The protein part of the conjugate enzyme is known as apoenzyme while the whole enzyme is called as holoenzyme.

Hint

(a) All three are proteins enzymes amylase, rennin and trypsin acts respectively on the substrates starch, lactose and protein. Amylase acts at an optimum \(\mathrm{pH}-6.8-7\), rennin at \(\mathrm{pH}-2\) and trypsin at an optimum \(\mathrm{pH}-8.5\).

Hint

(c) DNA consists of deoxyribose sugar, phosphate molecules, and nitrogenous bases-adenine, guanine, cytosine, and thymine, whereas RNA consists of ribose sugar, phosphate molecules, and nitrogenous bases-adenine, guanine, cytosine, and uracil.
Thus, estimating the incorporation of radiolabelled thymine can measure DNA synthesis and radiolabelled uracil can measure RNA synthesis, as all other nitrogenous bases are similar in both DNA and RNA.

Hint

(c) Feedback inhibition or end product inhibition occurs when the end product of a metabolic pathway inhibits the activity of an enzyme catalysing some early reactions of the series. The end product is the inhibitor and the enzyme inactivated is called an allosteric enzyme. The enzyme is regulated by modulators that bind noncovalently at site other than the active site. An example of feedback inhibition is the inhibition of the activity of the enzyme hexokinase by glucose 6-phosphate in glycolysis. This enzyme catalyses the conversion of glucose into glucose 6-phosphate but as the reaction proceeds, an increase in the concentration of glucose 6-phosphate inhibits the activity of hexokinase.

Hint

55. (d) : Polysaccharides are complex long chain carbohydrates which are formed by dehydrate synthesis or polymerisation of more than 10 but generally very large number of units called monosacccharides. Starch, glycogen, and cellulose are all polysaccharides. Starch is a glucosan homopolysaccharide which is the main reserve food of plants. Glycogen is also a glucosan homopoly-saccharide which is the major reserve food of fungi, animals, and some bacteria. It is also called animal starch. Cellulose is the structural polysaccharide of plant cell walls, some fungi, and protists. It is a fibrous glucosan homopolysaccharide of high tensile strength.

Hint

(c) Proteins show enormous diversity because of different proportions and sequences of twenty amino acid within the protein molecule. A large number of permutations and combinations of these amino acids are responsible for the unlimited variety of proteins. Proteins are the most abundant and most varied of the macromolecules having one or more polypeptides (chains of amino acids). The proteins constitute almost $50 \%$ of the total dry weight of the cell. Proteins may be simple or conjugated. Among conjugated, proteins may be phosphoprotein, glycoprotein, nucleoprotein, chromoprotein, lipoprotein, flavoprotein, metalloprotein, etc. Functionally proteins may be structural protein, enzymes, hormones, respiratory pigment, etc.

Hint

(d) The bases are of two types-purines and pyrimidines. The purine derivatives adenine (A) and guanine (G) are double-ring structures whereas pyrimidine derivatives thymine, cytosine, and uracil are single-ring structures. Thymine (T) and cytosine (C) are found in DNA and cytosine (C) and uracil (U) is found in RNA.

Hint

(a) Proteins are the most diverse among organic compounds. Among the biomolecules, proteins constitute \(9-12 \%\), fat \(1-3 \%\), carbohydrates 1-2\%, minerals \(1-3 \%\), nucleic acids \(2 \%\) and water \(60-75 \%\).

Hint

(c) Purines are 9-membered double-ring nitrogen bases which possess nitrogen at 1,3,7 and 9 positions, e.g. adenine (A), guanine (G). These purines are present in both DNA and RNA.

Hint

(b) Pyrenoids consist of dense proteinaceous areas surrounded by a starchy sheath. These are unique to the chloroplast of algae and are associated in polymerising sugars into reserve polymers(starch).

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(d) The bonds that exist between the phosphate group of one nucleotide and the hydroxyl group of sugar (ribose or deoxyribose) of the adjacent nucleotide is known as the phosphodiester bond.

Hint

(a) In the given question there are 4 pyrimidines as 2 cytosine and 2 thymine in option ‘ \(a\) ‘.

Hint

(c) Carbon, hydrogen, oxygen, and nitrogen are called four big elements of the living body they make up about \(99 \%\) of the mass of most cells. As C, H, O, and N are the lightest elements so the bonds they form are the strongest covalent bonds. So that the compounds formed are stable, varied in size and shapes. Carbon constitutes more than \(50 \%\) of the dry matter. It has been observed that human body contains \(0.5 \%\) hydrogen, \(18.5 \%\) carbon, \(65 \%\) oxygen and \(3.3 \%\) nitrogen. Other elements are present in a very lesser amount.

Hint

(c) One complete turn of DNA is \(34 \AA\) long and has 10 base pairs.

Hint

(b) Glycogen (animal starch) is a polysaccharide consisting of a highly branched polymer of glucose occurring in animal tissues, especially in liver and muscle cells. It is the major store of carbohydrate energy in animal cells.

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(d) Enzymes act by reducing the amount of activation energy. The binding energy is the source of energy used by enzyme to lower the activation energy of the reaction.
Activation energy is the minimum energy required from outside to overcome the energy barrier of reactants. Enzymes lower the energy of activation by two ways-bringing reactant molecules together, and developing strain in bonds of reactants.

Hint

(d) In RNA and DNA, purines are the same. The two purines are adenine and guanine. The two pyrimidines are cytosine and thymine in DNA but in RNA thymine is replaced by uracil.

Hint

(d) Amino acids are mostly synthesized from \(\alpha\)-ketoglutaric acid. These are the precursors of amino acids. A five-carbon compound formed during Krebs’ cycle is a \(\alpha\)-ketoglutaric acid which is the first dicarboxylic acid formed. Pyruvic acid converted into alanine, \(\alpha\)-ketoglutaric acid into glutamic acid, OAA into aspartic acid, and polymerization of such amino acids results in the formation of proteins.

Hint

(b) RNA occurs in the nucleus as well as in the cytoplasm of the eukaryotic cells and in prokaryotic cell, it is found in the cytoplasm. DNA is found in the nucleus, mitochondria, and chloroplast. Chloroplast and sphaerosomes are found only in cytoplasm.

Hint

(d) Water is the most abundant substance of living beings. The water content of actively living cells varies between \(60-95 \%\). In human beings maximum, water content is found in the embryo \(90-95 \%\). Water content decreases thereafter in adults and the aged where it is 65 \(-70 \%\)

Hint

(a) The bases are of two types-purines and pyrimidines. The purine derivatives adenine (A) and guanine (G) are double-ring structures whereas pyrimidine derivatives thymine, cytosine, and uracil are single-ring structures. Thymine (T) and cytosine (C) are found in DNA and cytosine (C) and uracil (U) is found in RNA.

Hint

(d) Amino acids are the smallest structural units of proteins. Plants and some
microorganisms can synthesise these amino acids from inorganic nitrogen/ \(\alpha\)-keto acids by reductive amination and transamination. Amino
acids besides bearing a carboxyl group, possess an amino group attached to
\(\alpha\)-carbon, hence, called as \(\alpha\)-amino
acids.

Hint

(b) A glycogen molecule is a long highly branched chain of about 30000 to \(100000 \alpha\)-D glucose units joined by glycosidic bonds. It is storage form of glucose, popularly called animal starch. It provides energy to animals, fungi, and bacteria.

Hint

(b) Enzymes having slightly different molecular structures but performing identical activities are called isoenzymes. Over 100 enzymes are known to have isoenzymes. Thus \(\alpha\)-amylase of wheat endosperm has 16 isozymes, lactic dehydrogenase has 5 isoenzymes in man.

Hint

(c) Nucleotide is an organic compound consisting of a nitrogen-containing purine or pyrimidine base linked to a sugar (ribose or deoxyribose) and a phosphate group.

Hint

(d) DNA is the largest macromolecule in organisms. It is a long, double chain of deoxyribonucleotide, or deoxyribotide units. The two deoxyribonucleotide chains are twisted around a common axis to form a right-handed double helix (spiral) that encloses a cylindrical space in it. Each deoxyribonucleotide unit, in turn, consists of three different molecules : phosphate, \(\left(\mathrm{PO}_{4}^{3}-\right)\), a 5 -carbon deoxyribose sugar \(\left(\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}_{4}\right)\) and a nitrogenous base.

Hint

(d) According to Chargaff’s rules, the amount of adenine is always equal to that of thymine, and the amount of guanine is always equal to that of cytosine i.e., \(\mathrm{A}=\mathrm{T}(120)\) and \(\mathrm{G}=\mathrm{C}(120)\), therefore, the total no. of nucleotides would be \(120 \times 4=480\).

Hint

(d) The nucleic acids (DNA and RNA) are molecules having complex structure and very high molecular weights. The nucleic acid is composed of a large number of nucleotide molecules joined into a linear, unbranched chain. Nucleotide is an organic compound consisting of a nitrogen-containing purine or pyrimidine base linked to a sugar (ribose or deoxyribose) and a phosphate group.

Hint

(d) Cytochromes are generally membrane-bound hemoproteins that contain heme groups and carry out electron transport. The heme group is a highly conjugated ring system (which means its electrons are very mobile) surrounding a metal ion, which readily interconverts between the oxidation states. For many cytochromes the metal ion present is that of iron, which interconverts between \(\mathrm{Fe}^{2+}\) (reduced) and \(\mathrm{Fe}^{3+}\) (oxidized) states (electron-transfer processes) or between \(\mathrm{Fe}^{2+}\) (reduced) and \(\mathrm{Fe}^{3+}\) (formal, oxidized) states (oxidative processes). Cytochromes are thus capable of performing oxidation and reduction.

Hint

(a) Nucleotides are the building blocks of nucleic acids (DNA and RNA). A single nucleotide is composed of a phosphate molecule, a five-carbon sugar (either ribose or deoxyribose) and a purine (adenine or guanine), or a pyrimidine (thymine or cytosine or uracil) nitrogenous base.

Hint

(d) Uracil forms nucleoside with only ribose sugar while thymine forms the same with only deoxyribose sugar. Other nitrogen bases (i.e. adenine, guanine, cytosine) produce nucleosides with both sugars.

Hint

(c) The density of DNA decreases on heating as hydrogen bonds break down. According to Chargaff’s rules, the amount of adenine is always equal to that of thymine, and the amount of guanine is always equal to that of cytosine i.e \(\mathrm{A}=\mathrm{T}\) and \(\mathrm{G}=\mathrm{C}\). The base ratio \(\mathrm{A}+\) \(\mathrm{T} / \mathrm{G}+\mathrm{C}\) may vary form species to species, but is constant for a given species.

Hint

(b) In RNA and DNA, purines are the same. The two purines are adenine and guanine. The two pyrimidines are cytosine and thymine in DNA but in RNA thymine is replaced by uracil.

Hint

(a) According to the Watson-Crick model, the DNA molecule consists of two long, parallel chains which are joined together by short crossbars at regular intervals. The two chains are spirally coiled around a common axis in a regular manner to form a right-handed double helix.

Hint

(a) Potassium \(\left(K^{+}\right)\)is an essential element, loosely held to the
apoenzyme part of the enzyme. \(\mathrm{K}^{+}\)is an inorganic cofactor (metal activator) of enzyme pyruvate kinase.

Hint

(d) Agrobacterium tumefaciens introduces t-DNA into the plant.

Agrobacterium tumefaciens is a soil bacterium that has the ability to infect plant cells and transfer a defined sequence of their DNA to the plant cell by infection and a causative agent of crown gall disease. Agrobacterium tumefaciens cell contains a plasmid known as the Ti (tumor-inducing) plasmid (140–235 kb).

Hint

(a) Length of one turn of DNA helix is \(34 \AA\).
\(
\begin{aligned}
&1 \AA=10^{-1} \mathrm{~nm} \\
&\therefore 34^{-}=10^{-1} \times 34=\frac{34 \times 1}{10}=3.4 \mathrm{~nm}
\end{aligned}
\)

Hint

(b) Besides polypeptides, conjugated proteins posses additional groups, metals, or ions. Nucleoproteins are proteins associated with nucleic acids. Lipoproteins are proteins associated with lipids.

Chromoproteins are proteins associated with pigments. Glycoproteins are proteins with a simple or complex sugar residue at their n-terminal.

Hint

(d) Essential amino acids are those which cannot be synthesized in the body from precursors. These amino acids must be present in our diet. They are valine, leucine, isoleucine, lysine, methionine, phenylalanine, tryptophan, and threonine.

Hint

(b) Reduced protein synthesis causes thinning of hairs.

Any reduction in protein synthesis leads to a reduction of hair shaft diameter which is easily discovered on the hair shaft, The second property is linear growth which enables the bulb date of any point along a hair to be estimated by dividing the distance of the point from the hair bulb by the rate of hair growth.

Hint

(d) Secondary structure is the development of new stearic relationships amongst the amino acids through the formation of intra and inter-polypeptide hydrogen bonds.

Secondary structure refers to regular, recurring arrangements in space of adjacent amino acid residues in a polypeptide chain. It is maintained by hydrogen bonds between amide hydrogens and carbonyl oxygens of the peptide backbone. The major secondary structures are \(\alpha\)-helices and \(\alpha\)-structures.

Hint

(c) There are only 20 amino acids. These can be arranged in different order in the polypeptide chain to form a diverse array of proteins.

Hint

(b) In bacteria the chromosome or nucleoid is a naked double-stranded DNA. The bacterial DNA is folded with the help of RNAs forming a supercoiled circular complex. The coiling is stabilized with the help of non-histonic polyamines and a number of positively charged ions. Eukaryotic DNA is associated with histone proteins that are helpful in its coiling to form chromatin fibres.