CBSE Biochem Exam Questions With
All Correct Answers A+ Solution
Guide 2025 New Update
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).
COPYRIGHT © 2025 BY OLIVIA GREENWAYS, ALL RIGHTS RESERVED 1
,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
COPYRIGHT © 2025 BY OLIVIA GREENWAYS, ALL RIGHTS RESERVED 2
,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
COPYRIGHT © 2025 BY OLIVIA GREENWAYS, ALL RIGHTS RESERVED 3
, 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'
COPYRIGHT © 2025 BY OLIVIA GREENWAYS, ALL RIGHTS RESERVED 4
All Correct Answers A+ Solution
Guide 2025 New Update
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).
COPYRIGHT © 2025 BY OLIVIA GREENWAYS, ALL RIGHTS RESERVED 1
,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
COPYRIGHT © 2025 BY OLIVIA GREENWAYS, ALL RIGHTS RESERVED 2
,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
COPYRIGHT © 2025 BY OLIVIA GREENWAYS, ALL RIGHTS RESERVED 3
, 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'
COPYRIGHT © 2025 BY OLIVIA GREENWAYS, ALL RIGHTS RESERVED 4
