20-Bio-A3 Biomechanics · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2018 — 04-Bio-A3, Cellular and Molecular Biology and Biochemistry. Three-hour, CLOSED-BOOK exam; only a Casio or Sharp approved calculator permitted. The paper carries six questions of equal value (20 marks each): FIVE questions constitute a complete paper and only the first five as they appear in the answer book are marked (100 marks total), with candidates urged to state any interpretive assumptions in writing. All SIX questions are worked below as a complete study resource. Question 6 is a 30-item True/False set marked +0.67 for a correct answer, 0 for a blank, and −0.67 for an incorrect answer.
Reference texts: Alberts et al., Molecular Biology of the Cell (6th ed.) — cell structure, membranes, transport, DNA/RNA/protein synthesis; Nelson & Cox, Lehninger Principles of Biochemistry (7th ed.) — protein structure, enzyme kinetics, membrane transport; Sambrook & Russell, Molecular Cloning: A Laboratory Manual (4th ed.) — recombinant DNA, PCR, cloning; Murphy & Weaver, Janeway's Immunobiology (9th ed.) — antibody structure and therapeutic antibodies; Webster (ed.), Medical Instrumentation: Application and Design (5th ed.) — imaging techniques.
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Approach. Each statement is evaluated against the underlying molecular/cell biology fact it tests, with a one-line justification; several items are deliberately constructed as a plausible but SWAPPED or inverted version of a true fact (e.g. purine/pyrimidine identity, H-bond counts, enzyme mechanism) and must be checked against the precise definition rather than pattern-matched.
| # | Statement (paraphrased) | Verdict | Rationale |
|---|---|---|---|
| 1 | Pyrimidine bases in DNA include guanine and adenine. | False | Guanine and adenine are purines (double-ring); the DNA pyrimidines are cytosine and thymine (single-ring). |
| 2 | RNA nucleotides carry –OH groups at the 2' and 3' carbons of the sugar. | True | Ribose has hydroxyls at both 2' and 3'; deoxyribose (DNA) lacks the 2'-OH. |
| 3 | RNA is less stable than DNA because of the –OH groups on its sugar. | True | The 2'-OH makes the phosphodiester backbone susceptible to base- and enzyme-catalyzed hydrolysis/transesterification that DNA (no 2'-OH) resists. |
| 4 | Okazaki fragments are short DNA strands made discontinuously during replication. | True | Definition of Okazaki fragments — discontinuous synthesis in the 5'→3' direction away from the fork. |
| 5 | Okazaki fragments occur on the lagging strand. | True | The lagging strand is synthesized discontinuously (away from fork movement); the leading strand is continuous. |
| 6 | DNA polymerase is involved in DNA replication. | True | Direct definition — DNA polymerase synthesizes new DNA strands during replication. |
| 7 | RNA polymerase is involved in translation. | False | RNA polymerase carries out transcription (DNA → RNA); translation (mRNA → protein) is performed by ribosomes/tRNA. |
| 8 | Eukaryotes have at least three different RNA polymerases. | True | Eukaryotes have Pol I (rRNA), Pol II (mRNA/most), Pol III (tRNA, 5S rRNA) — at least three. |
| 9 | An operon is a set of prokaryotic genes transcribed together into one mRNA. | True | Definition of a prokaryotic operon (e.g. lac operon) — a polycistronic transcription unit. |
| 10 | An operator site (distinct from the operon) is where a repressor or activator binds. | True | The operator is a specific DNA sequence, separate from the structural genes it regulates, bound by regulatory proteins. |
| 11 | The Pribnow box and TATA box are prokaryotic and eukaryotic promoter elements respectively. | True | Pribnow box (−10 element) is bacterial; TATA box is the eukaryotic Pol II core-promoter analogue. |
| 12 | A Shine–Dalgarno sequence is a ribosome-binding site, generally upstream of the start codon. | True | Bacterial mRNA ribosome-binding element, base-pairs with 16S rRNA, positioned upstream of the AUG start codon. |
| 13 | Both ribosome types have two subunits, but eukaryotic ribosomes have four rRNA types vs. three for prokaryotic. | True | Eukaryotic: 28S/18S/5.8S/5S rRNA (4 types); prokaryotic: 23S/16S/5S rRNA (3 types); both assemble as large+small subunits. |
| 14 | Ribosome subunits/rRNA are characterized by how they separate under centrifugal force. | True | Sedimentation coefficient (Svedberg, S units) from ultracentrifugation is exactly how ribosomal components are classified (e.g. 30S, 50S, 70S). |
| 15 | Of the three major RNA types, tRNA has the fewest nucleotides. | True | tRNA is ~76–90 nt, much smaller than typical mRNA or rRNA molecules. |
| 16 | Aerobic cells produce significantly more ATP via the electron transport chain than via glycolysis. | True | Oxidative phosphorylation yields ~30–34 ATP/glucose vs. glycolysis's net 2 ATP/glucose. |
| 17 | Enzymes increase the activation energy required for a reaction. | False | Enzymes LOWER activation energy (stabilizing the transition state), which is how they accelerate reaction rate. |
| 18 | Kinases rearrange atom positions so the molecular formula is unchanged but the molecule differs. | False | That description defines an isomerase, not a kinase; kinases transfer a phosphate group onto a substrate, changing its molecular formula. |
| 19 | Ribosomes are composed of two DNA subunits. | False | Ribosomes are built from ribosomal RNA (rRNA) and protein, not DNA. |
| 20 | Proteins can be made up of multiple polypeptide chains. | True | Quaternary structure — e.g. hemoglobin's four subunits. |
| 21 | Methionine can form a disulfide bond with another methionine. | False | Disulfide bonds form between the thiol (–SH) side chains of two CYSTEINE residues, not methionine (which has a thioether, not a free thiol). |
| 22 | More hydrogen bonds form between A–T than between G–C in DNA. | False | G–C pairs form 3 hydrogen bonds; A–T pairs form only 2 — G–C has more, not A–T. |
| 23 | Restriction enzymes cut DNA at particular nucleotide sequences. | True | Definition of a restriction endonuclease — recognizes and cleaves specific palindromic recognition sequences. |
| 24 | Apoptosis is a "programmed" cell death driven by environmental cues leading to controlled destruction. | True | Standard definition of apoptosis as a regulated, signal-driven cell-death program, distinct from necrosis. |
| 25 | Introns are segments of DNA that code for a gene. | False | Introns are the NON-coding intervening sequences removed by splicing; exons are the coding segments retained in mature mRNA. |
| 26 | Prokaryotes have their genomic material surrounded by a nuclear membrane distinct from the cytoplasmic membrane. | False | Prokaryotes lack a nucleus/nuclear membrane; DNA resides in the unbound nucleoid region of the cytoplasm. |
| 27 | There are different nucleotides for prokaryotic vs. eukaryotic organisms. | False | The four DNA bases (A, T, G, C) and RNA base U are chemically identical/universal across prokaryotes and eukaryotes. |
| 28 | Some eukaryotic organelles are on the same order of size as prokaryotic organisms. | True | Mitochondria/chloroplasts (~1–5 µm) are comparable in size to free-living bacteria (~1–2 µm), consistent with their endosymbiotic bacterial origin. |
| 29 | Viruses cannot be seen using a light microscope. | True | Most viruses (~20–300 nm) are below the ~200 nm diffraction-limited resolution of light microscopy; electron microscopy is required. |
| 30 | Codons are three-nucleotide sequences in the coding strand of DNA (or in mRNA). | True | Definition of a codon; the coding (sense) strand of DNA has the same sequence as mRNA (with T in place of U). |
