Biochemistry Solved Question Papers Singi Yatiraj
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MBBS Phase I Examination October 2005: (REVISED SCHEME 2)

 
LONG ESSAYS
 
1. Name the ketone bodies. Give two conditions characterized by excessive production of ketone bodies. Explain the metabolic derangements and consequences of ketosis.
Ketone bodies are water soluble and energy yielding compounds. The three 3 ketone bodies are
  • Acetone
  • Acetoacetate
  • β - hydroxybutyrate
Excessive production of ketone bodies occur in:
  1. Starvation.
  2. Alkalosis.
  3. Pregnancy toxemia.
  4. Prolonged either anesthesia.
  5. High fat feeding.
  6. After severe exercises.
  7. Injection of anterior pituitary extracts.
 
Ketosis
Ketosis is the condition where there is accumulation of abnormal amount of ketone bodies in tissues and body fluids and the urinary excretion of β - hydroxybutyric acid exceeds 200 mg daily.
 
Metabolic derangements
When the dietary supply of the glucose is deprived as in starvation, any available oxaloacetate is diverted for gluconeogenesis to meet the obligatory glucose requirements of the brain. To provide the alternate source of fuel, the lipolysis is increased thus increasing the mitochondrial level of acetyl CoA but oxaloacetate is deficient. Hence, citric acid cycle can not utilised the excess acetyl CoA which become substrate for ketogenesis.
When there is deficiency of insulin, like diabetes mellitus, the lipolysis is accelerated producing more acetyl CoA. Similarly in deficiency of insulin which has inhibitory effect on gluconeogenesis is absent thus decreasing the availability of oxaloacetate for citric acid cycle.2
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Consequences
The ketone bodies namely acetoacetate and β - hydroxybutyric acid are acids and when they accumulate, metabolic acidosis results. The plasma bicarbonate is used up for buffering these acids and hence, its level falls. Patients have typical acidotic breathing or Kussmaul's breathing due to compensatory hyperventilation. The smell of the breath of ketosis patient is acetone like. There is loss of important electrolytes like Na+ and K+ as they are excreted in urine as salts of ketone bodies. There is dehydration due to osmotic diuresis induced by ketonuria, Na+ loss in urine and associated vomiting.
Thus the consquences of ketosis are hypokalemia, dehydration and acidosis which are contributory for the lethal effects of ketosis.
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2. Explain the steps of activation, initiation, elongation and termination of protein biosynthesis.
Translation is a process by which the genetic information present in the form of a sequence of bases in mRNA is expressed to produce a sequence of amino acids in a protein. This occurs in cytoplasm. Translation consists of steps like
  1. Activation of amino acids.
  2. Initiation of protein synthesis.
  3. Elongation of polypeptide chain.
  4. Termination of protein synthesis.
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a. Activation of amino acids to form amino acyl - tRNA
Components required are; all the 20 amino acids, their corresponding tRNAs, their corresponding activating enzymes called amino acyl - tRNA synthetases and ATP.
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Each amino acyl - tRNA synthetase is specific for the amino acid and the corresponding tRNA. This specificity is responsible for correct incorporation of amino acid in the protein.
 
b. Initiation of protein synthesis
This requires mRNA, 40S and 60S ribosomal units, initiation factors (proteins) 1, 2 and 3, methionyl - tRNA and GTP. To start with, the 2 units of ribosome are separate and they assemble during initiation step to form the complete 80S ribosome. It is on ribosome the synthesis of protein occurs.
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Fig. 1.1:
Steps are
  1. Binding of mRNA to the 40S ribosome in the presence of initiation factor (IF) 3 to form a complex 1.
  2. Binding of methionyl - tRNA with GTP and IF2 to form complex II.
  3. Complex I and II in the presence of IF 1 form a complex III. In this step the anticodon (3’ - UAC-5’) of methionyl - tRNA attaches to the initiation codon AUG. Anticodon of tRNA of each amino acyl - tRNA recognises its codon in mRNA. The TYC arm is involved in binding of amino acyl - tRNA to the ribosome.
  4. Upon release of IF 1, 2 and 3, the 60S ribosome attaches to complex III and GTP provides energy by hydrolysing to GDP and Pi.
At this point of end of initiation step the formation of 80S ribosome is complete with mRNA attached. Ribosome has 2 sites called P (Peptidyl) site and A (Amino acyl) site. Now the P site contains methionyl - tRNA and A site is vacant.
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Fig. 1.2:
 
c. Elongation step
This requires elongation factors (EF1 and EF2), GTP, the various amino acyl - tRNAs and the enzyme peptidyl transferase in addition to mRNA, 40S and 60S ribosomal units.
  1. Each amino acyl - tRNA complexes with EF1 and GTP.
  2. Depending on the second codon, appropriate amino acyl - tRNA EF1-GTP complex binds to A site with the release of EF1, GDP and Pi. The anticodon of this tRNA attaches to the 2nd codon.
  3. Peptidyl transferase forms the peptide bond between the carboxyl group of methionine and the NH2 group of the 2nd amino acid. This results in the attachment of dipeptide to the tRNA in the A site. Protein synthesis starts from the NH2 terminal with the NH2 group of methionine as free amino end.
  4. P site now has only tRNA (no amino acid). The discharged tRNA vacates the P site.
  5. EF 2 and GTP are responsible for the translocation of the dipeptidyl - tRNA from A site to P site. GTP is split to GDP and Pi. At the same time the ribosome moves along the mRNA in 5’ α ’3 direction by one codon. EF 2 is also released. At this stage the first codon AUG is outside the ribosome, the 2nd codon is directly opposite the P site and the 3rd codon is opposite the A site.
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    Fig. 1.3:
    5
    The above cycle repeats a number of times till a ‘chain termination’ codon is approached. At this stage P site contains tRNA with its attached polypeptide.
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    Fig. 1.4:
 
d. Termination step
This requires releasing factor, GTP, and peptidyl transferase.
Releasing factor with GTP and peptidyl transferase brings about hydrolysis of the bond between the polypeptide and tRNA. Upon hydrolysis the entire protein synthesizing machinery dissociates to polypeptide, tRNA, 40S and 60S ribosome units, mRNA, releasing factor, GDP and Pi. At a time, many ribosomes may be attached to a single mRNA at intervals and as each ribosome moves along the mRNA strand in 5’ α 3’ direction, it weaves a copy of the same protein. An assembly of many ribosomes on a single mRNA is called ‘Polysome.’
Energy requirement of one peptide bond: Formation of amino acyl - tRNA requires 2 high energy phosphate bonds from ATP, entry of amino acyl - tRNA into a site requires one GTP and translocation of peptidyl - tRNA from A site to P site requires one GTP. Thus, for the formation of one peptide bond requires 4~P bonds.
 
SHORT ESSAYS
 
3. Explain the role of hypoglycemic hormones.
Insulin is the only hypoglycemic hormone. It is a polypeptide hormone secreted by the β cells of the islets of Langerhans of pancreas.
 
Actions of insulin
Insulin is an important hormone regulating the metabolism of carbohydrates, proteins and fats. It plays a vital role in regulating the blood sugar level.
 
a. Carbohydrate metabolism
Insulin is the only anti diabetic hormone in the body.
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  1. Insulin increases the transport and uptake of the glucose.
  2. By stimulating the enzymes participating in glycolysis, insulin increases the peripheral utilization of the glucose.
  3. Insulin inhibits the neoglucogenesis and glycogenolysis and thus prevent rise in blood glucose level.
  4. Insulin promotes glycogenesis thus stores excess glucose in form glycogen.
 
b. Protein metabolism
Insulin prevents cellular utilization of proteins and causes synthesis and storage of proteins.
  1. Insulin facilitates entry of amino acids in the cell.
  2. It accelerates protein synthesis by influencing the DNA transcription and translation of mRNA.
  3. By inhibiting proteolytic enzymes, it inhibits protein catabolism.
 
c. Fat metabolism
Insulin causes synthesis and storage of fat.
  1. Insulin promotes synthesis of fatty acids and triglycerides.
  2. It inhibits lipolysis by inhibiting lipase.
  3. It incorporates free fatty acids into neutral fats.
  4. It facilitates entry of free fatty acid into adipocytes.
 
d. Nucleic acid metabolism
  1. Insulin promotes synthesis of DNA and RNA and thus help in tissue growth.
 
e. Mineral metabolism
  1. Insulin influences K+, Na+ and Ca++ metabolism.
 
f. Growth
By virtue of its anabolic action on the protein metabolism, insulin along with growth hormone promotes growth.
 
4. Metabolic functions of cysteine.
  1. Cysteine is used for synthesis of body proteins.
  2. Cysteine is a particularly prominent amino acid in the proteins of nails, hairs, hoofs and keratin of the skin (sclero - proteins).
  3. Cysteine is also a constituent of many other polypeptide hormones like insulin and vasopressin, etc. where it is of great importance in maintaining the secondary and tertiary structures.
 
iv. Glucogenesis
Cysteine is catabolised to pyruvic acid which is glucogenic.7
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v. Glutathione synthesis
Glutathione is a tripeptide containing glycine, cysteine and glutamic acid.
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vi. Taurine synthesis
This taurine combines with cholic acid to form taurocholic acid a bile acid.
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vii. Mercaptoethanolamine synthesis
Mercaptoethanolamine is an important constituent of coenzyme A
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viii. Detoxification
Cysteine couples with certain aromatic compounds which then undergo acetylation to form mercapturic acid which are excreted in urine.
  • Bromobenzene + cysteine + Acetic acid α Bromophenyl mercapturic acid
  • Naphthalene + cysteine + Acetic acid α Naphthyl mercapturic acid.
 
5. What is oxidative phosphorylation? Explain chemiosmotic theory.
Oxidative phosphorylation is production of ATP by combining ADP and Pi using energy produced by flow of electron from NADPH (or FAD) to molecular oxygen in electron transport chain of mitochondria.
The synthesis of a molecule of ATP requires about of 7.3 Kcal/mol. There are such 3 sites where energy produced is sufficient to bind a phosphate to ADP.
 
Sites of ATP synthesis
  1. Site I - When there is transfer of electron from NADH to CoQ through flavoprotein. It releases about 12 Kcal/ mol of energy.
  2. Site II - Where there is transfer of electron from Cyt b to Cyt c which liberates about 10 Kcal/ mol of energy.
  3. Site III - When electron is transferred from Cyt aa3 to molecular oxygen the energy produced is 24 Kcal/mol.
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The P/O ratio is an index of oxidative phosphorylation. It is defined as the number of inorganic phosphate molecules incorporated into ATP for every atom of oxygen consumed. It is 3 for NADH and 2 for FAD (since FAD by passes the site I and directly gives it electron to CoQ thus producing only 2ATPs).
 
Chemiosmotic theory
Of different theories put forward the most accepted is Mitchell's chemiosmotic theory which explains this mechanism.
According to it the hydrogen ions produced by oxidation are driven out of inner membrane. Each of the respiratory cahin complexes I, III and IV acts as a proton pump. The inner membrane is impermeable to ions in general but particularly to the H+, which accumulates outside the membrane creating an electrochemical potential difference across the membrane. This consists of chemical potential and an electric potential. This electrochemical potential difference drives an inner membrane located ATP synthase to synthesise ATP from ADP and Pi.9
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Fig. 1.5: Diagrammatic representation of chemiosmotic hypothesis for oxidative phosphorylation(I, III, IV and V-Respiratory chain complexes; F0, F1 - Protein subunits for phosphorylation)
Membrane located ATP synthase consists of i) several proteins collectively known as Fi subunit which project into the matrix and contain the ATP synthase and ii) a membrane protein subunit known as Fo which serves as a stalk for attachment of F. H+ pass through the Fo-Fi complex, leading to the formation of ATP from ADP and Pi in the following sequence
  • Subunit Fo translocates H+ into the matrix.
  • Fi subunit catalyses the formation of ATP by conformational change in itself.
  • Fo-Fi subunits co-ordinates with each other and couple the disappearance of proton gradient with ATP synthesis.
 
Inhibitors and uncouplers
  • Uncouplers are compounds which the electron transport but blocks the establishment of proton gradient across the inner mitochondrial membrane. 2,4-dinitrophenol, Dinitricresol, pentochlorophenol, oligomycin and atractyloside act as uncouplers of oxidative phosphorylation. Tyroxine and long chain fatty acids acts as physiological uncouplers at high concentration.
  • Atractyloside inhibits the translocase and oligomyicn inhibits flow of protons acting through a protein present in Fo-Fi stalk.
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Fig. 1.6: Inhibitors of respiratory chain and oxidative phosphorylation
 
6. Classify lipoproteins. Mention their functions.
Lipoprotein are conjugated protein in which the lipid is the prosthetic part and apolipoprotein is the protein. It makes the insoluble lipids soluble and helps in their transport.
They are composed of triglycerides and cholesterol ester core surrounded by apolipoprotein shell, phospholipids and free cholesterol.
They are classified into 4 distinct groups depending on their densities which can be separated by electrophoresis.
 
Classification
  1. Chylomicrons.
  2. Very low density lipoprotein (VLDL).
  3. Low density lipoprotein (LDL).
  4. High density lipoprotein (HDL).
 
Composition
Protein
Lipid
Chylomicrons
2%
98%
Very low density lipoprotein (VLDL)
9%
91%
Low density lipoprotein (LDL)
21%
79%
High density lipoprotein (HDL)
50%
50%
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Metabolism of Lipoproteins
  1. Chylomicrons
    Initially nascent chylomicrons are assembled in the intestine form triglycerides, phospholipids cholesterol and apo A, apo B 48 and transported to blood via lymphatics. In the blood apo C and apo E are added to this chylomicrons from HDL to form complete chylomicron.
    In extrahepatic tissues capillaries, lipoprotein lipase hydrolyses TG to glycerol and free fatty acids. The free fatty acids are esterified back to triglycerides and stored by adipose tissue. Glycerol is circulated to liver. Chylomicrons depleted of triglycerides lose apo A and apo C to form chylomicron remnant which is degraded in liver. Apo A and apo C return to HDL.
  2. VLDL
    Liver synhesises nascent VLDL of triglycerides, phospholipids, cholesterol esters, cholesterol, apo B100, apo C and apo E. It becomes complete VLDL after addition of extra apo C and apo E from HDL.
    VLDL is hydrolysed to release its triglycerides. Then it loses its apo C and becomes intermittent density lipoprotein called VLDL remnant. This it loses apo E to become LDL.
  3. LDL
    It is obtained from VLDL. It is consists of triglycerides, phospholipids, cholesterol and its esters and apo B100. About half of the circulating LDL is taken up by extrahepatic tissues and half is degraded in liver.
  4. HDL
    It is synthesised in liver and intestine. It consists of triglycerides, phospholipids, cholesterol and its esters, apo A, apo C and apo E. It serves as reservoir of apo C and apo E for formation of chylomicrons and VLDL. It is finally degraded in liver.
 
Functions
  1. Chylomicrons
    Transport of dietary triglycerides and cholesterol esters from intestine to pheripheral tissues.
  2. VLDL
    Transport of endogenous triglycerides from liver to extrahepatic tissues.
  3. LDL
    Transport of cholesterol from liver to extrahepatic tissues.
  4. HDL
    Transport of cholesterol from extrahepatic tissues to liver in esterified form.
 
7. Therapeutic uses of enzymes.
Enzymes have found a role in treatment of certain conditions besides their very important diagnostic roles in numerous diseases
The enzymes used therapeutically are:
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Enzymes
Therapeutic uses
i.
Streptokinase
As a thrombolytic agent in myocardial infarction,
ii.
Urokinase
unstable angina, ischemic stroke, pulmonary embolism,
iii.
Alteplase
deep vein thrombosis and acute peripheral arterial
iv.
Arvin
occlusion
v.
L - Asparaginase
As a anticancer drug in some cases of lymphoblastic leukemia and reticulum cell sarcoma
vi.
Pancreatin
As a replacement therapy in chronic pancreatitis, obstruction caused by the cancer of the head of pancreas, cystic fibrosis and after total gastrectomy and pancreatectomy
vii.
Diastase
As a digestant
viii.
Pepsin and Papain
As a digestant in gastric achylia
ix.
Pancreatic dornase
As a mucolytic agent in cough
x.
Hyaluronidase
To promote the rapid absorption of drugs and fluids given subcutenously or intramuscularly
xi.
Streptodornase
xii.
Trypsin
For debridement of chronic ulcers along with streptokinase
xiii.
Chymotrypsin
For traumatically induced inflammation and edema of soft tissues
xiv.
Alpha chymotrypsin
For dissolving the suspensory ligament of lens to facilitate dissection of lens during intracapsular extraction of cataract
xv.
Collagenase
Debridement of dermal ulcers and severe burns
 
8. Explain metabolic role of thiamine and its deficiency manifestations.
Refer Question No. 6, April 2002.
 
9. What is anion gap? Explain normal anion gap acidosis and high anion gap acidosis with example.
The anion gap is a mathematical approximation of the difference between the anions and cations routinely measured in serum, i.e. the unmeasured anions in serum constitute the anion gap which is due to the presence of protein anions, sulphate, phosphate and organic acids.
 
Normal anion gap acidosis
Normal anion gap results when there is a loss of both anions and cations thus though the anion gap is normal but acidosis prevails.13
 
Causes
  • Loss of intestinal secretions as in diarrhea.
 
Treatment
  • Restoration of the ECF volume.
 
High anion gap acidosis
In acidosis resulting from the renal failure and due to accumulation of acids like ketoacids or lactic acids, there is increase in the anion gap. The anion gap increases due to accumulation of other buffer anions. But there is no change in chloride level.
Anion gap is also increased in diabetic ketosis and accumulation of lactic acids in hypoxic states.
Compensation occurs through elimination of CO2 the rate and depth of respiration is increased. The renal compensation is not very effective unless GFR increases. In lactic acidosis and ketoacidosis, HCO3 is used up but new bicarbonate is generated by activation of carbonic anhydrase. The H+ excretion is increased.
 
10. Lesch-Nyhan syndrome and orotic aciduria.
 
Lesch-Nyhan syndrome
Lesch-Nyhan syndrome is a sex linked metabolic disorder in the purine salvage pathway due deficiency of enzyme hypoxanthine guanine phosphoribosyltransferase (HGPRT).
It is characterized by excessive uric acid production causing gouty arthritis and neurological abnormalities such as mental retardation, aggressive behavior and learning disabilities. The patients have an urge for self mutilation by biting their own fingers and lips.
 
Orotic aciduria
Orotic aciduria is rare metabolic disorder produced due to the deficiency of the enzymes orotate phosphoribosyltransferase and OMP decarboxylase of pyramidine synthesis.
It is characterized by excretion of orotic acid in urine, severe anemia and retarded growth.
It is treated by feeding diet rich in uridine and / or cytidine because these compounds through phosphorylation provide pyramidine nucleotides required for DNA and RNA synthesis and UTP inhibits carbomoyl phosphate synthetase II and blocks synthesis of orotic acid.
 
11. What is mutation? Give an account on point mutation.
Mutation is a random undetected heritable variation caused by an alteration in the nucleotide sequence at some point of the DNA of the cell, which may be in form of addition, deletion or substitution of one or more bases.
Mutation is a natural event, taking place all the time at its particular frequency. It ranges from 10−2 to 10−10 per bacterium per division. Though mutation occurs spontaneously several agents called as mutagens can increase its frequency they are UV rays, alkylating agents, acridine dyes, 5-bromouracil and 2-aminopurine. Mutation may affect any gene and hence 14may modify any characters of the bacterium. Most mutants, however go unrecognized as the mutation may be lethal or it may affect some minor function that may not be expressed. Mutation is best appreciated when it involves a function, which can be readily observed such as nutritional requirements, biochemical reaction, antigenic structure, morphological features, colony form, drug susceptibility, virulence and host range.
 
Mechanism of mutation
Molecular basis of mutation is that during DNA replication some error creeps in while the progeny strands are copied. For example, while replication guanine may bind with the thymine instead of adenine due to tautomerism.
 
Point mutation
Point mutation is a type of mutation where one base pair is replaced by another.
The two types of point mutations.
  1. Transition
    Here a base, either purine or pyramidine is replaced by another base of same type.
  2. Transversion
    Here a purine base is replaced by a pyramidine base and vice versa.
 
Consequences of point mutation
  1. Silent mutation
    Some mutations may go unnoticed because of the degeneracy of the codon. For example, codon UCA codes for serine and any point mutation causing change in the third base, ie UCO may not cause biochemical abnormality because codon UCO also codes for amino acid serine.
  2. Missense mutation
    In this case, the mutated codon codes for different amino acid. This change may be acceptable, partially acceptable or unacceptable with reqared to function of protein molecule. Sickel cell anemia is an example of mis-sense mutation.
  3. Nonsense mutation
    Sometimes a mutation may make a codon one of the terminating codon which acts as a stop signal for protein synthesis.
 
12. What is nitrogen balance? Explain factors affecting nitrogen balance.
An adult healthy individual maintaining constant weight is said to be in nitrogen balance or nitrogenous equilibrium if his amount of intake of N in food as dietary proteins is equal to the excretion of N in urine and sweat as urea, uric acid, creatine, creatinine, amino acids and in feces as unabsorbed N.
Similarly a person can be in positive nitrogen balance or negative nitrogen balance:
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Fig. 1.7:
 
Positive nitrogen balance
This is a state in which the nitrogen intake is higher than the output. It is usually seen growing children, pregnant women or during recovery after serious illness where some amount of nitrogen is retained in the body causing a net increase in the body protein.
 
Negative nitrogen balance
This is a condition where the nitrogen output is higher than the input. It may occur due to inadequate dietary intake of protein or destruction of tissues or serious illness. However, the body adopts itself and increases the break down of tissue protein. This results in loss of nitrogen from the body depleting the body protein prolonged negative balance may even lead to death.
 
Factors influencing nitrogen balance
Besides growth, pregnancy, protein deficiency, illness, following factors influence nitrogen balance.
 
Hormones
Growth hormone and insulin promotes positive nitrogen balance while corticosteroids results in negative nitrogen balance.
 
Diseases
Cancer and uncontrolled diabetes cause negative nitrogen balance.
 
SHORT ANSWERS
 
13. Define oncogene. Give examples of oncogene.
Oncogenes are genes capable of causing cancer. They are located in the human chromosomes.
Examples
  1. abl
  2. erb-A
  3. erb-B
  4. myc
    16
  5. sis
  6. src
  7. ras
 
14. Name mucopolysaccharides. Mention their function.
Refer Question No. 3, Oct. 2004.
 
15. Give brief account on Quarternary structure of proteins.
Proteins with more than two polypeptides aggregate to form one functional unit. This is referred to as quarternary structure of protein.
Here each polypeptide is called monomer, protomer or subunit and the protein is called oligomer. Thus a protein with two polypeptide chain is called is dimer, three polypeptides as trimer and so on.
These monomeric subunits are held together by non-covalant bonds namely hydrogen bonds, hydrophobic interactions and ionic bonds.
Heamoglobin is a tetramer of 2 α chains and 2 β chains and similarly immunoglobulin G is also a tetramer of 2 heavy chains and 2 light chains.
 
16. Define free radicals. How free radicals are generated in the body?
Free radicals are the chemical species that possess one or are usually generated by the partial reduction of O2.
Biologically important free radicals
  1. Superoxide radical (O2).
  2. Hydrogen peroxide (H2O2).
  3. Hydroxyl peroxide (OH).
  4. Lipid peroxide radical (ROO).
  5. Nitric oxide (NO).
  6. Peroxy nitrite (ONOO).
  7. Hypochlorous acid (HOCl).
 
Formation of free radicals
The free radicals are produced by partial reduction of O2 within mitochondrial inner membrane catalysed by cytochrome oxidase.
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17. Name subcellular organelles. Which are metabolic functions of cytosol?
 
Subcellular organelles
  1. Nucleus
  2. Endoplasmic reticulum
  3. Golgi apparatus
  4. Mitochondria
  5. Lysozomes
  6. Centrosomes
  7. Microtubules and microfilaments
  8. Centrioles
  9. Secretory vesicles
  10. Ribosomes
 
Function of cytosol
  • Supporting subcellular organelles.
 
18. Degradation of heme.
Refer Question No. 12, April 1999.
 
19. Gout.
Refer Question No. 11, April 2000.
 
20. What is polymerase chain reaction? Mention its application.
Refer Question No. 11, Oct. 2006 (RS2).
 
21. What are metabolic roles of zinc and selenium?
 
Selenium
  1. Selenium acts as an antioxidant complementary to vitamin E.
  2. It is present in glutathione peroxidase which decomposes organic peroxides and hydrogen peroxides.
  3. It is present in 5 deiodinase which converts thyroxin to T3.
 
Zinc
Refer Question No. 10, Oct. 2006 (RS2).
 
22. Which are biologically important nucleotides? Mention their functions.
Nucleotides are the nitrogenous bases attached to pentose to which is attached a phosphate, i.e. Nucleotide = base + pentose + phosphate = Nucleoside + phosphate
Thus the nucleotides are nucleosides phosphates. The purine nucleotides contain either adenine or guanine as the nitrogenous base. The pyramidine nucleotides contain either uridine or cytosine as the nitrogenous base.
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Some of the important nucleotides and their function
Function
ADENOSINE NUCLEOTIDES
a. Adenosine triphosphate (ATP)
-
Providing energy for many endergonic processes.
-
Synthesis of many biological important compounds like creatinine phosphate, fatty acids, proteins and glucose.
-
Donating the phosphate group in phosphotransferase reactions.
-
Muscle contraction, sperm motility, nerve impulse transmission etc.
-
Formation of active methionine and active sulphate.
b. Adenosine monophosphate.(AMP)
-
It is the constituent of RNAs.
-
It activates the enzyme phosphofructokinase and inhibits fructose 1-6 diphosphatase and adenylsuccinate synthetase
c. Deoxyadenosine monophosphate (dAMP)
-
It is the constituent of DNA.
d. Cyclic AMP (cAMP)
-
It acts as second messenger for various hormones.
GAUNOSINE NUCLEOTIDES
a. Guanosine triphosphate (GTP)
-
It is required for protein synthesis.
-
It is involved in oxidation of succinyl CoA in TCA cycle.
b. Guanosine monophosphate (GMP) and Deoxyguanosine monophosphate (dGMP)
-
They are the integral part of RNA and DNA respectively.
URIDINE NUCLEOTIDES
a. Uridine triphosphate (UTP)
-
It can be converted to CTP by glutamine.
-
It forms UDPG on reacting with glucose 6 phosphate.
b. Uridine monophosphate (UMP)
-
It is the constituent of RNAs.
-
It can be converted to UDP and UTP.
c. Deoxyuridine monophosphate (dUMP)
-
It is the constituent of DNA.
d. Uridine diphospho glucose (UDPG)
-
It acts as intermediate between the glucose and galactose interconversion.
-
It transfers the glucose molecule to the glycogen primer in glycogenesis.
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Function
CYTIDINE NUCLEOTIDES
a. Deoxycytidine monophosphate (dCMP)
-
It is the constituent of DNA.
b. Cytidine monophosphate (CMP)
-
It is the integral part of RNA.
-
CMP-acetyl neuraminic acid is important precursor of cell wall polysaccharides in bacteria.
-
CMP-Sialic acid is present in the salivary glands.
c. Cytidine diphosphate (CDP)
-
CDP-choline, CDP-glycerol and CDP-ethanolamine are involved in biosynthesis of phospholipids.
MISCELLANEOUS NUCLEOTIDES
a. Phospho adenosine phospho sulphate (PAPS)
-
It donates the SO4 group to various molecules in)
* Heparin biosynthesis.
* Chondroitin sulfate biosynthesis.
* Keratosulphate synthesis.
* Formation of sulfalipids.
b. S-adenosyl methionine (SAM)
-
It donates the methyl group to various molecules in methylation reactions.
c. NAD+, NADP+, FAD and FMN
-
They are involved in the oxidation-reduction reactions helping in the transfer of H+
Thus nucleotides play important role in biochemical functions of the body besides being an integral part of the nucleic acids.