CBSE BIOCHEM 2025 EXAM QUESTIONS
AND ANSWERS ALL CORRECT
Where does fatty acid synthesis occur primarily? - Answer-Liver, adipose tissue, and
lactating mammary gland tissue.
Where in the cell does fatty acid synthesis occur usually? - Answer-Citrate from the
mitochondrial matrix has to be brought into the cytosol by the citrate shuttle, and this is
where is undergoes conversion to fatty acid.
Where in the cell does fatty acid breakdown occur normally? - Answer-Fatty acyl CoA
goes through the carnitine shuttle into the mitochondrial matrix, which is where beta-
oxidation occurs (fatty acyl CoA --> acetyl CoA-->ketone bodies and TCA cycle). **Acyl
CoA dehydrogenase.
What does long chain fatty acid degradation require? - Answer-Carnitine-dependent
transport into the mitochondrial matrix.
Systemic primary carnitine deficiency: - Answer-Inherited LCFA transport disorder
involving their transport into the mitochondria--> toxic accumulation-->weakness,
hypotonia, and hypoketotic hypoglycemia.
Medium Chain Acyl-CoA dehydrogenase deficiency - Answer-AR disorder of FA
oxidation. Lowered ability to break down FAs into acetyl CoA-->accumulation of 8-10
carbon fatty acyl carnitines in the blood and hypoketotic hypoglycemia. May present in
infancy with vomiting, lethargy, seizures, coma and liver dysfunction. Minor illness can
lead to sudden death -- AVOID FASTING.
Where do Ketone bodies come from and what are they? - Answer-They are made in the
liver from fatty acids and amino acids. They are acetone, acetoacetate and beta-
hydroxybutyrate (Only acetoacetate can be detected in the urine). Ketones are used in
muscle and brain.
What happens during starvation and DKA to oxaloacetate and what does this cause? -
Answer-It becomes depleted because of gluconeogenesis. In alcoholism, excess NADH
shunts oxaloacetate to malate. The result of each of these scenarios is a build-up of
acetyl-CoA --> glucose and FFA are shunted toward production of ketone bodies**.
,What are the metabolic priorities during fasting and starvation? - Answer-To get glucose
to the brain and RBCs and to preserve protein.
Fuel used after a meal? - Answer-Glycolysis and aerobic respiration. Insulin stimulates
storage of fat, glycogen and protein.
Lesch-Nyhan Syndrome: what causes it and what are manifestations? - Answer-
HGPRT deficiency in purine salvage pathway (this enzyme normally converts
hypoxanthine to IMP and guanine to GMP). This deficiency leads to excessive Uric acid
production and de novo purine synthesis. It is X linked recessive. Symptoms:
hyperuricemia, gout, aggression / self mutilation, mental retardation, dystonia.
Treatment for Lesch-Nyhan? - Answer-Allopurinol / febuxostat second line. Both of
these drugs inhibit xanthine oxidase, which converts hypoxanthine to xanthine and
xanthine to uric acid.
Adenosine Deaminase Deficiency: what is it, what effects does it have? - Answer-ADA
is required for degradation of adenosine and deoxyadenosine--dATP builds up and is
toxic to lymphocytes, which is why this disease is a major cause of SCID--severe
combined immunodeficiency (Autosomal recessive).
Why is purine salvage important? - Answer-Because in some tissues, de novo purine
synthesis is not possible.
How are nucleotide monophosphates created? - Answer-Activated ribose-5-phosphate
(PRPP) is added to bases by phosphoribosyltransferases (HGPRT and APRT are the
two types of this enzyme).
tRNA Wobble: what is it and it causes which type of mutation? - Answer-An unusual
pairing of bases in the third position of the codon--causes silent mutation.
Sickle Cell Anemia is caused by which type of mutation? - Answer-Missense mutation--
valine replaces glutamic acid.
Tay-Sachs and Duchenne Muscular Dystrophy are caused by which mutation type? -
Answer-Frameshift
Xeroderma Pigmentosum: is caused by a defect in what specifically? - Answer-
Nucleotide excision repair--pyrimidine dimers are not repaired (caused by UV
exposure). In nucleotide excision repair, bulky helix-disturbing lesions are repaired by
endonucleases. This occurs during G1 of cell cycle.
How is spontaneous / toxic deamination repaired? - Answer-Base excision repair--
involves AP endonucleases throughout the cell cycle
,What is defective in Lynch Syndrome? What is another term for Lynch Syndrome? -
Answer-Lynch syndrome= hereditary nonpolyposis colorectal cancer. Mismatch repair is
defective. This usually occurs in G2 of cell cycle.
What is the underlying pathology in Fanconi Anemia and Ataxia Telangiectasia? -
Answer-Mutation in nonhomologous end joining (repairs double stranded breaks).
What is the mRNA start codon and what does it code for in prokaryotes, eukaryotes? -
Answer-AUG--codes for methionine in eukaryotes, N-formylmethionine in prokaryotes
What are the mRNA stop codons? - Answer-UGA, UAA, UAG
What sequences are common in gene promoters? What binds at the promoter? -
Answer-AT-rich sequences and TATA and CAAT boxes. RNA pol II and other
transcription factors bind to promoter.
What does promoter mutation commonly result in? - Answer-Decrease in gene
transcription
Alpha-aminitin - Answer-Found in death cap mushrooms. Inhibits RNA pol II--severe
hepatotoxicity if ingested
Rifampin - Answer-Inhibits RNA pol in prokaryotes
Actinomycin D (Dactinomycin) - Answer-Inhibits RNA pol in eukaryotes and prokaryotes
What happens to form mRNA? - Answer-HnRNA is immature, and is what is first
transcribed by RNA pol II. 5' capping with methylguanosine, 3' adenylation (poly A tail),
and splicing out of introns all occur in order to make mature mRNA.
How does splicing occur and what diseases are effected by splicing abnormalities? -
Answer-1. Primary transcript joins snRNPs to make spliceosome. 2. Lariat loop is
formed and 3. Lariat (intron) is cut out to join 2 exons together. In SLE, anti-Smith
antibodies are antibodies to spliceosomal snRNPs. In MCTD (mixed connective tissue
disease), antibodies against U1-RNPs are found.
What do microRNAs (miRNAs) do? - Answer-Post-transcriptional regulation of gene
expression. They target mRNA and degrade / inactivate certain sequences identified by
complimentary base pairing. This is a mechanism by which cancer begins! MiRNAs
inactivate tumor suppressor genes.
tRNA (transfer RNA) has a defining sequence at one of its ends- what is that sequence
and what is the significance of that end? - Answer-The sequence is CCA at the 3' end,
and this end is where the amino acid is covalently bound. The amino acid acceptor site
is 5'-CCA-3'
, What are the three arms of tRNA? What are their functions and what are they
composed of? - Answer-T arm, D arm and acceptor stem. T arm: site of tRNA ribosomal
binding. Contains ribothymidine, pseudouridine, and cytidine. D arm: site of recognition
by aminoacyl synthetase. Contains dihydrouridine. Acceptor stem: 5'-CAA-3'.
What is "charging" in the context of tRNA, how does it occur, and what is the
significance of the energy exchanged? - Answer-Charging is the process of loading an
amino acid onto the acceptor stem of tRNA. The enzyme that does this is aminoacyl
tRNA synthetase and an ATP is used to create this bond. The energy of this bond will
be used to create the peptide bond*
What are the ribosomal subunits involved in prokaryotic and eukaryotic protein
synthesis? - Answer-Eukaryotes-- 40s + 60s--> 80s. Prokaryotes: 30s + 50s --> 70s
What is translocation and what is its energy source? - Answer-"Initiation" or
Translocation is the process whereby initiation factors (IFs) assemble the 40s ribosomal
subunit and initiator tRNA. IFs are released when this complex joins with the 60s /
mRNA. This process uses GTP for energy. **Remember, charging uses ATP for
energy. Charging is the loading of the tRNA with the amino acid.
What are the three phases of protein translation? - Answer-Initiation (translocation),
elongation, termination
Describe elongation (protein synthesis) - Answer-60s subunit has 3 sites: A, P, E.
Aminoacyl-tRNA enters A site. Ribozyme (rRNA) catalyzes formation of peptide bonds,
and growing polypeptide is transferred to aa in A site. The ribosome advances 3
nucleotides towards 3' end of the mRNA, which moves the next peptidyl tRNA to the P
site. E site is where empty tRNA is held before being released.
Describe the process of termination - Answer-The stop codon is recognized by release
factor and protein synthesis stops.
What are the common post-translational modifications and what happens in each? -
Answer-Trimming and covalent alterations. Trimming: N or C termini are cleaved from
zymogen to yield mature protein (example is trypsinogen --> trypsin). Covalent
alterations include: acetylation, hydroxylation, ubiquitination, methylation,
glyocosylation, phosphorylation.
What is heat shock protein (hsp60) and what is it an example of? - Answer-In yeast,
hsp60 prevents desaturation of proteins at high temps. This is an example of a
CHAPERONE protein--which facilitations or maintains folding of proteins intracellularly.
What are the major cell cycle regulators? How do they work and interact with one
another? - Answer-Cyclins, CDKs, tumor suppressors. Cyclins activate CDKs, which are
constitutive and inactive by default. These cyclin-CDK complexes phosphorylate other
proteins to regulate the cell cycle and control its progression. At times these complexes
AND ANSWERS ALL CORRECT
Where does fatty acid synthesis occur primarily? - Answer-Liver, adipose tissue, and
lactating mammary gland tissue.
Where in the cell does fatty acid synthesis occur usually? - Answer-Citrate from the
mitochondrial matrix has to be brought into the cytosol by the citrate shuttle, and this is
where is undergoes conversion to fatty acid.
Where in the cell does fatty acid breakdown occur normally? - Answer-Fatty acyl CoA
goes through the carnitine shuttle into the mitochondrial matrix, which is where beta-
oxidation occurs (fatty acyl CoA --> acetyl CoA-->ketone bodies and TCA cycle). **Acyl
CoA dehydrogenase.
What does long chain fatty acid degradation require? - Answer-Carnitine-dependent
transport into the mitochondrial matrix.
Systemic primary carnitine deficiency: - Answer-Inherited LCFA transport disorder
involving their transport into the mitochondria--> toxic accumulation-->weakness,
hypotonia, and hypoketotic hypoglycemia.
Medium Chain Acyl-CoA dehydrogenase deficiency - Answer-AR disorder of FA
oxidation. Lowered ability to break down FAs into acetyl CoA-->accumulation of 8-10
carbon fatty acyl carnitines in the blood and hypoketotic hypoglycemia. May present in
infancy with vomiting, lethargy, seizures, coma and liver dysfunction. Minor illness can
lead to sudden death -- AVOID FASTING.
Where do Ketone bodies come from and what are they? - Answer-They are made in the
liver from fatty acids and amino acids. They are acetone, acetoacetate and beta-
hydroxybutyrate (Only acetoacetate can be detected in the urine). Ketones are used in
muscle and brain.
What happens during starvation and DKA to oxaloacetate and what does this cause? -
Answer-It becomes depleted because of gluconeogenesis. In alcoholism, excess NADH
shunts oxaloacetate to malate. The result of each of these scenarios is a build-up of
acetyl-CoA --> glucose and FFA are shunted toward production of ketone bodies**.
,What are the metabolic priorities during fasting and starvation? - Answer-To get glucose
to the brain and RBCs and to preserve protein.
Fuel used after a meal? - Answer-Glycolysis and aerobic respiration. Insulin stimulates
storage of fat, glycogen and protein.
Lesch-Nyhan Syndrome: what causes it and what are manifestations? - Answer-
HGPRT deficiency in purine salvage pathway (this enzyme normally converts
hypoxanthine to IMP and guanine to GMP). This deficiency leads to excessive Uric acid
production and de novo purine synthesis. It is X linked recessive. Symptoms:
hyperuricemia, gout, aggression / self mutilation, mental retardation, dystonia.
Treatment for Lesch-Nyhan? - Answer-Allopurinol / febuxostat second line. Both of
these drugs inhibit xanthine oxidase, which converts hypoxanthine to xanthine and
xanthine to uric acid.
Adenosine Deaminase Deficiency: what is it, what effects does it have? - Answer-ADA
is required for degradation of adenosine and deoxyadenosine--dATP builds up and is
toxic to lymphocytes, which is why this disease is a major cause of SCID--severe
combined immunodeficiency (Autosomal recessive).
Why is purine salvage important? - Answer-Because in some tissues, de novo purine
synthesis is not possible.
How are nucleotide monophosphates created? - Answer-Activated ribose-5-phosphate
(PRPP) is added to bases by phosphoribosyltransferases (HGPRT and APRT are the
two types of this enzyme).
tRNA Wobble: what is it and it causes which type of mutation? - Answer-An unusual
pairing of bases in the third position of the codon--causes silent mutation.
Sickle Cell Anemia is caused by which type of mutation? - Answer-Missense mutation--
valine replaces glutamic acid.
Tay-Sachs and Duchenne Muscular Dystrophy are caused by which mutation type? -
Answer-Frameshift
Xeroderma Pigmentosum: is caused by a defect in what specifically? - Answer-
Nucleotide excision repair--pyrimidine dimers are not repaired (caused by UV
exposure). In nucleotide excision repair, bulky helix-disturbing lesions are repaired by
endonucleases. This occurs during G1 of cell cycle.
How is spontaneous / toxic deamination repaired? - Answer-Base excision repair--
involves AP endonucleases throughout the cell cycle
,What is defective in Lynch Syndrome? What is another term for Lynch Syndrome? -
Answer-Lynch syndrome= hereditary nonpolyposis colorectal cancer. Mismatch repair is
defective. This usually occurs in G2 of cell cycle.
What is the underlying pathology in Fanconi Anemia and Ataxia Telangiectasia? -
Answer-Mutation in nonhomologous end joining (repairs double stranded breaks).
What is the mRNA start codon and what does it code for in prokaryotes, eukaryotes? -
Answer-AUG--codes for methionine in eukaryotes, N-formylmethionine in prokaryotes
What are the mRNA stop codons? - Answer-UGA, UAA, UAG
What sequences are common in gene promoters? What binds at the promoter? -
Answer-AT-rich sequences and TATA and CAAT boxes. RNA pol II and other
transcription factors bind to promoter.
What does promoter mutation commonly result in? - Answer-Decrease in gene
transcription
Alpha-aminitin - Answer-Found in death cap mushrooms. Inhibits RNA pol II--severe
hepatotoxicity if ingested
Rifampin - Answer-Inhibits RNA pol in prokaryotes
Actinomycin D (Dactinomycin) - Answer-Inhibits RNA pol in eukaryotes and prokaryotes
What happens to form mRNA? - Answer-HnRNA is immature, and is what is first
transcribed by RNA pol II. 5' capping with methylguanosine, 3' adenylation (poly A tail),
and splicing out of introns all occur in order to make mature mRNA.
How does splicing occur and what diseases are effected by splicing abnormalities? -
Answer-1. Primary transcript joins snRNPs to make spliceosome. 2. Lariat loop is
formed and 3. Lariat (intron) is cut out to join 2 exons together. In SLE, anti-Smith
antibodies are antibodies to spliceosomal snRNPs. In MCTD (mixed connective tissue
disease), antibodies against U1-RNPs are found.
What do microRNAs (miRNAs) do? - Answer-Post-transcriptional regulation of gene
expression. They target mRNA and degrade / inactivate certain sequences identified by
complimentary base pairing. This is a mechanism by which cancer begins! MiRNAs
inactivate tumor suppressor genes.
tRNA (transfer RNA) has a defining sequence at one of its ends- what is that sequence
and what is the significance of that end? - Answer-The sequence is CCA at the 3' end,
and this end is where the amino acid is covalently bound. The amino acid acceptor site
is 5'-CCA-3'
, What are the three arms of tRNA? What are their functions and what are they
composed of? - Answer-T arm, D arm and acceptor stem. T arm: site of tRNA ribosomal
binding. Contains ribothymidine, pseudouridine, and cytidine. D arm: site of recognition
by aminoacyl synthetase. Contains dihydrouridine. Acceptor stem: 5'-CAA-3'.
What is "charging" in the context of tRNA, how does it occur, and what is the
significance of the energy exchanged? - Answer-Charging is the process of loading an
amino acid onto the acceptor stem of tRNA. The enzyme that does this is aminoacyl
tRNA synthetase and an ATP is used to create this bond. The energy of this bond will
be used to create the peptide bond*
What are the ribosomal subunits involved in prokaryotic and eukaryotic protein
synthesis? - Answer-Eukaryotes-- 40s + 60s--> 80s. Prokaryotes: 30s + 50s --> 70s
What is translocation and what is its energy source? - Answer-"Initiation" or
Translocation is the process whereby initiation factors (IFs) assemble the 40s ribosomal
subunit and initiator tRNA. IFs are released when this complex joins with the 60s /
mRNA. This process uses GTP for energy. **Remember, charging uses ATP for
energy. Charging is the loading of the tRNA with the amino acid.
What are the three phases of protein translation? - Answer-Initiation (translocation),
elongation, termination
Describe elongation (protein synthesis) - Answer-60s subunit has 3 sites: A, P, E.
Aminoacyl-tRNA enters A site. Ribozyme (rRNA) catalyzes formation of peptide bonds,
and growing polypeptide is transferred to aa in A site. The ribosome advances 3
nucleotides towards 3' end of the mRNA, which moves the next peptidyl tRNA to the P
site. E site is where empty tRNA is held before being released.
Describe the process of termination - Answer-The stop codon is recognized by release
factor and protein synthesis stops.
What are the common post-translational modifications and what happens in each? -
Answer-Trimming and covalent alterations. Trimming: N or C termini are cleaved from
zymogen to yield mature protein (example is trypsinogen --> trypsin). Covalent
alterations include: acetylation, hydroxylation, ubiquitination, methylation,
glyocosylation, phosphorylation.
What is heat shock protein (hsp60) and what is it an example of? - Answer-In yeast,
hsp60 prevents desaturation of proteins at high temps. This is an example of a
CHAPERONE protein--which facilitations or maintains folding of proteins intracellularly.
What are the major cell cycle regulators? How do they work and interact with one
another? - Answer-Cyclins, CDKs, tumor suppressors. Cyclins activate CDKs, which are
constitutive and inactive by default. These cyclin-CDK complexes phosphorylate other
proteins to regulate the cell cycle and control its progression. At times these complexes
