Purpose: Self-check on DNA structure, replication, transcription, translation, mutations, and biotech tools.
Score: 0 / 12
Topic 3.1 — DNA Structure & Replication
Question 1
Which base-pairing rule and bond type are correct for DNA?
Solution: A=T (2 H-bonds), G≡C (3 H-bonds). DNA strands are antiparallel (5'→3' opposite 3'→5'). U is in RNA (replacing T).
Question 2
DNA replication is described as "semiconservative" because:
Solution: Meselson–Stahl (1958) demonstrated semiconservative replication: original strands serve as templates; each daughter duplex has one old, one new strand.
Question 3
Match each enzyme with its job in DNA replication: helicase, primase, DNA polymerase III, ligase. Which seals Okazaki fragments?
Solution: Helicase unwinds; primase lays RNA primers; polymerase III adds nucleotides 5'→3'; pol I removes primers, replacing with DNA; ligase forms phosphodiester bonds sealing Okazaki fragments on the lagging strand.
Question 4
If one DNA strand reads 5'-ATG CCG TTA-3', the complementary strand reads:
Solution: Antiparallel and complementary: 3'-TAC GGC AAT-5'. (Same sequence reversed in direction.)
Topic 3.2 — Transcription & Translation
Question 5
During eukaryotic mRNA processing in the nucleus, which step removes non-coding sequences?
Solution: Pre-mRNA is processed: 5' cap, 3' poly-A tail, and splicing — introns removed, exons joined by the spliceosome (snRNPs).
Question 6
Translate the mRNA codon 5'-AUG-3'. Which amino acid is added, and what is the role of this codon?
Solution: AUG is the START codon — it specifies methionine and signals where the ribosome begins translation. Stop codons are UAA, UAG, UGA.
Question 7
If a DNA template strand is 3'-TAC GGT AAA-5', the resulting mRNA is:
Solution: RNA is synthesized 5'→3' complementary to template (T→A, A→U, C→G, G→C). Template 3'-TAC GGT AAA-5' → mRNA 5'-AUG CCA UUU-3'.
Topic 3.3 — Mutations
Question 8
A single nucleotide change converts a codon into a STOP codon. What kind of mutation is this?
Solution: Nonsense mutations create a premature stop codon → truncated, usually non-functional protein. Silent: same amino acid. Missense: different amino acid. Frameshift: insertion/deletion shifts reading frame.
Question 9
An insertion of one base into the middle of a coding sequence is most likely to:
Solution: Insertion or deletion of a number not divisible by 3 shifts the reading frame, scrambling all subsequent codons; usually severely disruptive.
Topic 3.4 — Biotechnology Tools
Question 10
Which biotech tool amplifies a small DNA sample to billions of copies in a few hours?
Solution: PCR cycles: denature (94 °C, strands separate) → anneal (50–65 °C, primers bind) → extend (72 °C, Taq polymerase synthesizes). After n cycles → 2ⁿ copies.
Question 11
Restriction enzymes (e.g., EcoRI) cut DNA at:
Solution: Restriction enzymes recognize specific (typically 4–8 bp) palindromic sequences and produce sticky or blunt ends. EcoRI: 5'-G^AATTC-3'. Used in cloning to ligate compatible fragments.
Question 12
In gel electrophoresis, DNA fragments migrate from:
Solution: The phosphate backbone gives DNA a net negative charge, so it migrates to the positive electrode (anode). The agarose gel matrix retards larger fragments more, so smaller ones travel farther — used to separate by size and compare to a ladder.