Biochemistry: Unit 5 Nucleic Acids

Which purine nucleotide is synthesized first during de novo purine biosynthesis, serving as the immediate common precursor for both AMP and GMP?

Inosine monophosphate (IMP)

Methotrexate is a folate antagonist. Which enzyme in pyrimidine/purine synthesis is most directly inhibited due to the depletion of tetrahydrofolate (THF)?

Thymidylate synthase

A 3-year-old child presents with self-mutilation, hyperuricemia, and intellectual disability (Lesch-Nyhan syndrome). Which enzyme is deficient?

Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)

What is the fundamental chemical difference between the pentose sugars in RNA and DNA at carbon-2' (C-2')?

RNA contains an -OH group at C-2', whereas DNA contains an -H atom.

Which histone protein binds to the linker DNA region outside the core octamer to stabilize the 30-nm chromatin fiber?

Histone H1

According to Chargaff’s rules, which quantitative base relationship always holds true for double-stranded DNA?

Adenine + Guanine = Cytosine + Thymine

Which DNA conformation is a left-handed helix with a zigzag phosphodiester backbone?

Z-DNA

Severe Combined Immunodeficiency (SCID) with profound B and T cell lymphopenia is caused by a deficiency of which purine enzyme?

Adenosine deaminase (ADA)

5-Fluorouracil (5-FU) is converted to 5-FdUMP, which acts as a suicide inhibitor of which enzyme?

Thymidylate synthase

Which modified nucleoside in the wobble position of tRNA enables base pairing with A, C, or U in mRNA codons?

Inosine

What is the rate-limiting cytosolic enzyme in human de novo pyrimidine biosynthesis?

Carbamoyl phosphate synthetase II (CPS II)

What chemical bond connects the nitrogenous base to C-1' of the pentose sugar in nucleotides?

Beta-N-Glycosidic bond

Which invariant sequence at the 3'-end of mature tRNA serves as the amino acid binding site?

CCA sequence

Allopurinol inhibits xanthine oxidase. Which purine metabolites accumulate in urine as a result?

Xanthine and Hypoxanthine

Which enzyme converting NDPs to dNDPs is selectively inhibited by hydroxyurea?

Ribonucleotide reductase

What is the standard treatment for Hereditary Orotic Aciduria (UMP synthase deficiency)?

Oral Uridine supplementation

What essential post-transcriptional processing function is performed by snRNPs ("snurps")?

Splicing of pre-mRNA introns

MicroRNAs (miRNAs) regulate gene expression post-transcriptionally through which mechanism?

Complementary binding to target 3'-UTR of mRNAs to cause transcript cleavage or translational repression

Which bacterial enzyme relieves torsional supercoiling ahead of the replication fork by making double-stranded cuts?

DNA Gyrase (Topoisomerase II)

Fluoroquinolone antibiotics (e.g., ciprofloxacin) exert their bactericidal effect by targeting which enzymes?

DNA Gyrase and Topoisomerase IV

Which eukaryotic DNA polymerase synthesizes and elongates the lagging strand during nuclear replication?

DNA Polymerase delta

Proofreading capability of E. coli DNA Polymerase III relies on which specific enzymatic activity?

3' to 5' Exonuclease activity

What intrinsic component does telomerase carry to template telomeric repeat synthesis?

An RNA molecule

Rifampin acts against Mycobacterium tuberculosis by inhibiting which enzyme?

Bacterial RNA Polymerase

Which eukaryotic RNA polymerase is responsible for synthesizing messenger RNA (mRNA)?

RNA Polymerase II

Poisoning from Amanita phalloides (death cap mushroom) occurs due to alpha-amanitin inhibition of which enzyme?

RNA Polymerase II

What modification is added to the 5'-end of pre mRNA to protect it from degradation and aid ribosome recognition?

7-Methylguanosine cap

Which conserved prokaryotic promoter sequence at -10 facilitates localized DNA melting?

Pribnow box (TATAAT)

What is the role of the sigma factor subunit in bacterial RNA polymerase?

Recognize and bind specific promoter sequences

Which drug class binds the 30S ribosomal subunit, causing misreading of mRNA genetic code?

Aminoglycosides

What is the mechanism of action of chloramphenicol?

Binding to 50S subunit and inhibiting peptidyl transferase activity

How do tetracyclines inhibit bacterial translation?

Binding to 30S subunit and preventing aminoacyl tRNA attachment to the A-site

Diphtheria toxin halts eukaryotic protein synthesis by ADP-ribosylating which elongation factor?

Elongation Factor 2 (eEF-2)

What triplet sequence acts as the universal START codon in mRNA?

AUG

Which set of mRNA codons functions as stop signals for translation termination?

UAA, UAG, UGA

Sickle cell anemia is caused by a point mutation converting GAG (glutamate) to GTG (valine). What type of mutation is this?

Missense mutation

A mutation that converts an amino acid codon into a premature stop codon is known as a:

Nonsense mutation

Xeroderma pigmentosum (extreme sun sensitivity) results from a defect in which DNA repair pathway?

Nucleotide Excision Repair (NER)

Lynch Syndrome (HNPCC) predisposes to early colorectal cancer due to mutations in which repair pathway?

Mismatch Repair (MMR)

Which repair mechanism uses DNA glycosylases to excise single damaged base lesions?

Base Excision Repair (BER)

Huntington’s disease is caused by an expansion of which unstable trinucleotide repeat sequence in the HTT gene?

CAG

What is the structural consequence of the delta-F508 mutation in Cystic Fibrosis?

In-frame deletion of a single amino acid (phenylalanine at position 508)

What is the correct chronological sequence of steps in each PCR cycle?

Denaturation -> Annealing -> Extension

Which blotting technique is used to analyze RNA expression levels?

Northern Blot

What feature characterizes target sites recognized by restriction endonucleases?

Palindromic sequences with two-fold rotational symmetry

Which CRISPR-Cas9 component guides Cas9 endonuclease to the target DNA locus?

Single guide RNA (sgRNA)

Which chemical group is missing in dideoxynucleotides (ddNTPs) to cause chain termination in Sanger sequencing?

3'-OH group

Tay-Sachs disease (cherry-red macular spot, neurodegeneration) is caused by a deficiency in which enzyme?

Hexosaminidase A

Hemophilia A is an X-linked recessive disorder caused by a deficiency in which coagulation factor?

Factor VIII

What genetic mutation alters the translational reading frame of mRNA from the mutation point onward?

Insertion or deletion of nucleotides not divisible by three