All Free Biology MCQs with Answers

441. In double stranded DNA, the total amount of purines always equals the total amount of pyrimidines because

• A. the sugars bond only to purines
• B. phosphate groups pair with pyrimidines
• C. hydrogen bonds form only between bases of equal size
• D. each base pair contains one purine and one pyrimidine

Explanation: Adenine and guanine are the purines, thymine and cytosine the pyrimidines, and every pair across the helix matches one of each. This is why Chargaff found A plus G equals T plus C, and it keeps the two strands a constant distance apart.

442. The backbone of a single strand of DNA consists of

• A. alternating bases and phosphate groups
• B. bases only, stacked one above another
• C. alternating deoxyribose sugars and phosphate groups
• D. alternating sugars and bases

Explanation: Phosphodiester bonds link the sugar of one nucleotide to the phosphate of the next, making a repeating sugar phosphate rail with the bases hanging off it as side groups. The bases face inward to pair with the opposite strand rather than forming the backbone.

443. DNA replication is described as semi conservative because each daughter molecule

• A. contains one old strand and one newly synthesised strand
• B. is built entirely from new nucleotides
• C. conserves only half of its original bases
• D. is rebuilt only in every second cell cycle

Explanation: When the two parental strands separate, each serves as a template for a new partner, so both daughter helices retain one original strand. This was proved by Meselson and Stahl using nitrogen isotopes, ruling out models in which old DNA was chopped up or fully conserved.

444. Chargaff's rules state that in any DNA molecule

• A. adenine equals guanine and cytosine equals thymine
• B. the amount of adenine is always greater than that of thymine
• C. the amount of adenine equals thymine and guanine equals cytosine
• D. purines are always more numerous than pyrimidines

Explanation: Because A pairs with T and G pairs with C across the helix, their amounts must match in any double stranded DNA. Pairing A with G instead would ignore the actual hydrogen bonding partners, and purines equal rather than outnumber pyrimidines.

445. Biotechnology is best defined as

• A. the practical use of living organisms or their products for human benefit
• B. the study of fossils
• C. the chemistry of synthetic drugs
• D. the physics of living cells

Explanation: From baking and brewing to genetic engineering, biotechnology harnesses cells and enzymes to make products and services. It is defined by application to human needs, not by any single technique.

446. Which of the following is an example of conventional biotechnology?

• A. Gene splicing in the laboratory
• B. Baking bread with yeast
• C. DNA fingerprinting
• D. Somatic cell cloning

Explanation: Humans have used microorganisms for bread, cheese and vinegar for thousands of years, long before DNA was understood. Gene splicing and fingerprinting belong to modern, molecular biotechnology.

447. Yoghurt is produced when bacteria convert milk sugar into lactic acid. This process is called

• A. photosynthesis
• B. aerobic respiration
• C. fermentation
• D. nitrogen fixation

Explanation: The bacteria ferment lactose anaerobically, and the acid they produce thickens the milk proteins. The same principle preserves many foods, from pickles to cheese.

448. A plasmid is

• A. part of the main bacterial chromosome
• B. a viral protein coat
• C. a type of ribosome
• D. a small circular DNA molecule that replicates independently in bacteria

Explanation: Plasmids carry extra genes such as antibiotic resistance and replicate separately from the chromosome. Their small size and independence make them the standard vectors of genetic engineering.

449. The single stranded ends produced by some restriction enzymes, which can base pair with matching ends on other DNA, are called

• A. telomeres
• B. sticky ends
• C. Okazaki fragments
• D. primers

Explanation: Staggered cuts leave short overhangs that anneal with complementary overhangs from DNA cut by the same enzyme, easing the joining of gene and vector. Enzymes that cut straight across leave blunt ends instead.

450. In gel electrophoresis, the smaller DNA fragments

• A. move faster and travel farther through the gel
• B. remain near the wells
• C. dissolve in the buffer
• D. become positively charged

Explanation: The gel acts as a molecular sieve, so small fragments slip through its pores more easily than large ones. Fragment size can therefore be read directly from the distance travelled.