MLT GlobeChemistry: Biochemistry basics & molecular biology
58 Chemistry MCQs on Biochemistry basics & molecular biology with answers and explanations.
Methane (MW 16) is a gas at room temperature, while water (MW 18) is a liquid. What mainly accounts for this difference?
Each water molecule can hydrogen-bond with up to four neighbours, holding them together as a liquid. Methane has only weak van der Waals forces.
Which is the most basic (first) level of DNA packaging in eukaryotic chromatin?
DNA wraps about 1.7 turns around a histone octamer to form a nucleosome; nucleosomes then coil into the 30-nm fibre and higher-order loops.
Electron micrographs of relaxed chromatin show a 'beads-on-a-string' appearance. What are the beads?
Each bead is a histone octamer wrapped with DNA; linker DNA forms the string between them (the 10-nm fibre).
As the replication fork moves, strain builds in the DNA ahead of it. Which enzymes relieve this supercoiling?
Topoisomerases (e.g., DNA gyrase) cut and reseal strands to remove positive supercoils. Helicase unwinds the helix, which creates the strain.
HIV copies its RNA genome into DNA using an enzyme best described as:
Reverse transcriptase reads an RNA template and makes complementary DNA, the basis of RT-PCR and antiretroviral targeting.
Non-standard base pairing between codon and anticodon is allowed at which codon position?
Crick's wobble hypothesis: the 3′ codon base pairs loosely with the 5′ anticodon base, so one tRNA can read several codons.
Human mitochondrial DNA is best described as a:
mtDNA is a closed circular double-stranded genome of about 16.6 kb, resembling bacterial DNA; nuclear chromosomes are linear double-stranded DNA.
The double-helix model of DNA, with antiparallel strands and complementary base pairing, was proposed in 1953 by:
Watson and Crick built the model using X-ray data from Franklin and Wilkins. Avery showed DNA carries genetic information; Sanger developed sequencing methods; Mendel described inheritance patterns.
During differential centrifugation of a tissue homogenate, which component sediments first at the lowest speed?
Nuclei are the largest, densest particles and pellet at about 600–1,000 g. Mitochondria need higher speed, while microsomes and ribosomes require ultracentrifugation.
Breaking down glycogen to carbon dioxide and water with release of energy is an example of:
Catabolic pathways degrade complex molecules and release energy. Anabolism, including gluconeogenesis and other biosynthesis, consumes energy to build molecules.
Translation of messenger RNA into a polypeptide chain takes place on the:
Ribosomes, free or bound to the rough ER, carry out protein synthesis. The nucleolus assembles ribosomal subunits but does not translate mRNA.
Which organelle, nicknamed the cell's 'suicide bag', contains acid hydrolases that can digest the cell itself?
Lysosomes hold acid hydrolases working at about pH 5; their rupture causes autolysis. Peroxisomes contain oxidases and catalase rather than digestive hydrolases.
Which function is carried out chiefly by the Golgi apparatus?
The Golgi completes glycosylation, packages proteins into vesicles and directs them to their destinations. ATP synthesis is mitochondrial, translation ribosomal, and autophagic digestion lysosomal.
Actin microfilaments are directly involved in:
Actin filaments interact with myosin in contraction and drive cell crawling and cytokinesis. The spindle is made of microtubules, gap junctions of connexins, and calcium is stored in the ER.
Which of the following is an organelle rather than a cell inclusion?
Inclusions are stored or pigment material that is not metabolically organised, such as glycogen, lipid and melanin. The centrosome is an organelle organising microtubules.
Which technique provided the diffraction evidence for the helical structure of DNA?
Franklin's X-ray diffraction images revealed the helix and its dimensions. Density-gradient ultracentrifugation (Meselson–Stahl) showed semiconservative replication, not helical structure.
Which RNA carries the genetic message from nuclear DNA to the ribosomes?
mRNA carries codons transcribed from DNA. tRNA brings amino acids, rRNA forms the ribosome, and snRNA takes part in splicing. Polysomes are several ribosomes on one mRNA, not the carrier itself.
In mammalian cells, most ribosomal RNA is transcribed in the:
RNA polymerase I transcribes rRNA genes in the nucleolus, where subunits are assembled before export. Ribosomes use rRNA but do not make it.
Cyanide stops aerobic ATP production by blocking the respiratory chain in which organelle?
Mitochondria, the cell's powerhouse, produce most ATP by oxidative phosphorylation; cyanide inhibits cytochrome c oxidase (complex IV) there.
Mammalian cell lines, including transformed cells, are usually cultured at:
Human and other mammalian cells grow best at body temperature, usually with 5% CO2. Lower temperatures slow growth; 42 °C causes heat stress.