CBSE Biochem Exam Review 2024| Questions and
Answers
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 must be activated and at others they must be inactivated for proper cell
cycle progression. The general tumor suppressor pathway: p53 induces p21, which
inhibits CDKs, leading to the hypophosphorylation of Rb and its activation. Rb then
inhibits E2F, thereby inhibiting the G1--> S transition. E2F is a transcription factor.
**INHIBITION OF THESE GENES RESULTS IN UNCONTROLLED CELL GROWTH.
THIS IS THE CASE IN LI FRAUMENI SYNDROME FOR EXAMPLE.
Permanent cell types: what are they and what are some examples? ** Answer ** These
remain in G0 and can only regenerate from stem cells. Examples include neurons,
skeletal and cardiac muscle, and RBCs.
Quiescent (stable) cell types: what are they and what are some examples? ** Answer
** These remain in G0 until stimulated to enter G1. Examples include lymphocytes and
hepatocytes.
Labile cell types: what are they and what are some examples? ** Answer ** These are
continuously dividing--they never go to G0, and progress straight to a short G1. Skin,
gut epithelium, germ cells, hair follicles, bone marrow are all examples.
Which cell type is most affected by chemotherapy? ** Answer ** Labile cells!
What does RER do? ** Answer ** Synthesizes secretory proteins and performs N-
linked oligosaccharide addition.
What is a Nissl body and what does it do? ** Answer ** Nissl bodies are RER in
neurons. They secrete peptide neurotransmitters for secretion.
What do free ribosomes make? ** Answer ** Proteins for cytosol and organelles.
What cell types are rich in RER? ** Answer ** Mucous-secreting goblet cells of the
small intestine and antibody-secreting plasma cells.
What does SER do and where is SER heavily concentrated? ** Answer ** SER
synthesizes steroids and detoxes poisons and drugs. It is common in liver hepatocytes
and adrenals / gonads (these last two make steroid hormones!)
What does the Golgi apparatus do? ** Answer ** It modifies N-linked oligosaccharides
on asparagine and adds O-linked oligosaccharides to serine and threonine. It also adds
mannose-6-phosphates to proteins destined for lysosomes.
What do endosomes do? ** Answer ** They are the organizing centers of the cell for
things that are brought in from extracellular environment and also for vesicles from the
golgi. It packages and sends things either to lysosomes, back to golgi, or to membrane,
etc.
Answers
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 must be activated and at others they must be inactivated for proper cell
cycle progression. The general tumor suppressor pathway: p53 induces p21, which
inhibits CDKs, leading to the hypophosphorylation of Rb and its activation. Rb then
inhibits E2F, thereby inhibiting the G1--> S transition. E2F is a transcription factor.
**INHIBITION OF THESE GENES RESULTS IN UNCONTROLLED CELL GROWTH.
THIS IS THE CASE IN LI FRAUMENI SYNDROME FOR EXAMPLE.
Permanent cell types: what are they and what are some examples? ** Answer ** These
remain in G0 and can only regenerate from stem cells. Examples include neurons,
skeletal and cardiac muscle, and RBCs.
Quiescent (stable) cell types: what are they and what are some examples? ** Answer
** These remain in G0 until stimulated to enter G1. Examples include lymphocytes and
hepatocytes.
Labile cell types: what are they and what are some examples? ** Answer ** These are
continuously dividing--they never go to G0, and progress straight to a short G1. Skin,
gut epithelium, germ cells, hair follicles, bone marrow are all examples.
Which cell type is most affected by chemotherapy? ** Answer ** Labile cells!
What does RER do? ** Answer ** Synthesizes secretory proteins and performs N-
linked oligosaccharide addition.
What is a Nissl body and what does it do? ** Answer ** Nissl bodies are RER in
neurons. They secrete peptide neurotransmitters for secretion.
What do free ribosomes make? ** Answer ** Proteins for cytosol and organelles.
What cell types are rich in RER? ** Answer ** Mucous-secreting goblet cells of the
small intestine and antibody-secreting plasma cells.
What does SER do and where is SER heavily concentrated? ** Answer ** SER
synthesizes steroids and detoxes poisons and drugs. It is common in liver hepatocytes
and adrenals / gonads (these last two make steroid hormones!)
What does the Golgi apparatus do? ** Answer ** It modifies N-linked oligosaccharides
on asparagine and adds O-linked oligosaccharides to serine and threonine. It also adds
mannose-6-phosphates to proteins destined for lysosomes.
What do endosomes do? ** Answer ** They are the organizing centers of the cell for
things that are brought in from extracellular environment and also for vesicles from the
golgi. It packages and sends things either to lysosomes, back to golgi, or to membrane,
etc.
