31 Years NEET Previous Year Questions: Biomolecules - 3 - NEET MCQ

Ribosomes are the robust organelles, present in the cytoplasm, composed of rRNA and a set of specific proteins. They are present free in cytosol or on the surface of the endoplasmic reticulum. The spatial separation depends on the presence of ER-targeting signal sequence on the protein being synthesized, thus, a particular ribosome might be ER-bound while synthesizing one protein and free in the cytosol when translating another protein

So, the correct answer is 'On ribosomes present in cytoplasm as well as in mitochondria'

Thymidine is used to assess the rate of  DNA synthesis, since thymine is unique to DNA. 

mRNA or messenger RNA brings coded information  from DNA to form polypeptides. rRNA or ribosomal RNA occurs inside ribosomes and is involved in protein synthesis. 

Heterochromatin is a specialized chromosome material which remains tightly coiled and stains darkly in interphase. Euchromatin is that portion of the chromosome that stains with low intensity and uncoils during interphase. In the beginning of S-sphase, DNA replication occurs. DNA replication can occur in diffuse/ less tightly coiled euchromatin.

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

Cellulose is the structural polysaccharide of plant cell walls. They are long chain polymers of b-glucose units. The adjacent b-glucose units are joined by  linkages. 

Lactose is a disaccharide that consists of b-D-galactose and b-D-glucose molecules bonded through a b1-4 glycosidic linkage. Lactose makes up around 2-8% of the solids in milk. The name comes from the Latin word for milk, plus the -ose ending used to name sugars.

Gene mutation or point mutation is the change in expression of a gene caused by change in number sequence and  types of nucleotides. A mutation from a wild gene type to a new type is called forward mutation. Reversal of mutated gene to wild type is reverse mutation. Chromosomal mutations are changes in the morphology of chromosomes.

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

Length of one turn of DNA helix is 34Å.

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 having at their n- terminal and a simple or complex sugar residue. 

Kim et al (1973) suggested L shape model of t- RNA by X-Ray diffraction while studying  phenyl alanine t - RNA of  yeast.

DNA and RNA are both polymers of nucleotides . DNA has deoxyribose sugar while RNA  has ribose sugar. DNA has thymine while RNA has uracil in place of thymine.

Feedback inhibition is the process by where the end product terminates the reaction. The accumulation of the end product interferes with the enzymatic activity by changing the shape of its active site. On changing the shape of the active site, the substrate does not attach to the active site and thus the reaction terminates. During glycolysis, the glucose changes to glucose 6 phosphates in presence of hexokinase. When the amount of glucose 6 phosphate exceeds it deactivate hexokinase.
So, the correct answer is option A.

Activation energy is the energy required to over come energy barrier of the reactants and make them reactive to start a chemical reaction. Enzymes lower the requirement of activation energy.

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.

Ribozymes catalytically active RNA molecule discovered in 1980’s are self splicing introns indicating their possible role as intermediates in the evolution of biological systems from abiotic substances.

Cellulose is the most abundant organic compound, most abundant polysaccharide and most abundant bipolymer found on earth.

Eukaryotes have three types of  RNA polymerases :
(i)  RNA polymerase I - It is found in nucleus and is responsible for r-RNA synthesis.
(ii)  RNA polymerase II - It is found in nucleoplasm and is responsible for m-RNA synthesis.
(iii) RNA polymerase III-It is found in nucleoplasm and is responsbile for t-RNA and 5-S RNA synthesis.

Erucic acid is a monounsaturated omega-9 fatty acid, denoted 22 : 1 w-9. It is prevalent in rapeseed, wallflower seed, and mustard seed, making up 40 to 50 percent of their oils. Erucic acid is also known as cis-13-docosenoic acid and the trans isomer is known as brassidic acid.

Water attracting molecules are called hydrophilic. Water repelling molecules are called hydrophobic. Amino acids carry simultaneously positive and negative charges. Such substances are called zwitter ions.  Lipids are compounds of C, H, O but the ratio of H and O is more than 2 : 1 that is the ratio of oxygen is lesser than  carbohydrates. Lipids are insoluble in  water but soluble in non-polar solvents such as benzene, chloroform etc. Commonest lipid found in a cell is phospholipid. It contains a hydrophilic (polar) head and a hydrophobic (non- polar tail). 

Reducing sugar is any sugar that has an aldehyde group or is capable  of forming one in solution through isomerisation. Reducing monosaccharides include glucose, fructose, glyceraldehyde  and galactose. Many disaccharides,  like lactose and maltose also have a reducing form, as one of the two units may have an open- chain form with an aldehyde group.
However, sucrose and trehalose in which the anomeric carbons of the two units are linked together, are non reducing disaccharides since neither of the rings is capable of opening.

Collagen is a major fibrous protein of connective tissue occuring as white fibres produced by fibroblast.

Malignant cancer is caused by loss of control over cell’s reproduction capacity. The cells undergo rapid division. Therefore, they are more easily damaged by radiations than normal cells.

Tyrosinase (polyphenol oxidase, catecholase) is a copper-containing oxidase which is widely distributed in plants, animals and man. It oxidizes tyrosine to the pigment melanin in mammals and causes the cut surfaces of many fruits and vegetables darken (browning reactions).