Molecular and cellular biology is roughly one-third of the CLEP exam, and the flow of genetic information (DNA → RNA → protein) is among its most heavily tested ideas. Expect terse "which of the following is generally true" stems, especially about bacteriophages.
DNA is a double helix of two nucleotide strands. Each nucleotide has a sugar (deoxyribose), a phosphate, and one of four bases: adenine (A), thymine (T), guanine (G), cytosine (C).
RNA differs from DNA: it uses ribose (not deoxyribose), uracil (U) instead of thymine, and is typically single-stranded.
When DNA copies itself, the two strands separate and each serves as a template for a new complementary strand. Each daughter molecule ends up with one original strand and one new strand — this is why replication is called semiconservative.
Transcription copies a gene's DNA into messenger RNA (mRNA). RNA polymerase reads the DNA template strand and builds an mRNA that is complementary and antiparallel to it (with U replacing T).
In eukaryotes, the initial transcript (pre-mRNA) is processed before it leaves the nucleus: non-coding introns are spliced out and coding exons are joined. A cap and tail are added. Because introns are removed, they never appear in the final protein — even though they were transcribed.
The mRNA sequence is read in three-base units called codons. Each codon specifies one amino acid (or a stop signal). Translation occurs at the ribosome:
[GRAPH: Ribosome on an mRNA strand — tRNAs deliver amino acids, each anticodon base-pairing with the next mRNA codon; a growing polypeptide chain extends from the ribosome.]
Reading a template strand 3′-TAC-5′ yields mRNA 5′-AUG-3′ (the common "start" codon, methionine).
Cells do not express every gene at once. By transcribing only the genes whose products are currently needed, a cell conserves resources and responds to its environment.
A virus is a nucleic-acid genome (DNA or RNA) in a protein coat; it is not a cell and cannot reproduce on its own. A bacteriophage is a virus that infects bacteria by injecting its nucleic acid and hijacking the host's machinery.
1. B — Thymine. Fix: In DNA the purine adenine pairs with the pyrimidine thymine (A–T). - A: Cytosine pairs with guanine, not adenine. - C: Guanine pairs with cytosine. - D: Uracil replaces thymine in RNA, not DNA. - E: A base does not pair with itself. - Fix rule: "A–T and G–C" in DNA; swap in U for T only in RNA.
2. E — Opposite 5′→3′ directions. Fix: Antiparallel means the strands point in reverse orientations relative to each other. - A: Complementary, not identical, sequences. - B: Bases are held by hydrogen bonds, not covalent bonds. - C: Same direction would be "parallel," which is wrong. - D: DNA is not made of RNA. - Fix rule: Antiparallel = "head-to-tail" 5′/3′ orientation.
3. E — One original and one new strand. Fix: Each template strand is conserved in a daughter molecule alongside a newly built partner. - A: Two new strands would be conservative-copy thinking, which is incorrect. - B: Two old strands describes the parent, not a product. - C: DNA replication uses DNA nucleotides, not RNA. - D: Half the nucleotides are original, so "none" is wrong. - Fix rule: "Semi" = half old, half new in each new helix.
4. D — 5′-AUG-3′. Fix: mRNA is complementary and antiparallel to the template; 3′-TAC-5′ → 5′-AUG-3′ (with U for T). - A: Retains T and wrong orientation; mRNA has no thymine. - B: Simply rewrites the template with T; ignores complementarity. - C: Uses U correctly but keeps the template bases uncomplemented. - E: Keeps thymine, which RNA never uses. - Fix rule: Complement the template and replace every T with U.
5. C — Transfer RNA (tRNA). Fix: tRNA anticodons pair with mRNA codons and deliver the matching amino acids. - A: DNA polymerase copies DNA, not a translation role. - B: rRNA forms the ribosome but does not ferry amino acids. - D: mRNA carries the message but cannot deliver amino acids. - E: Helicase unwinds DNA during replication. - Fix rule: tRNA = the "delivery truck" that reads codons and drops off amino acids.
6. A — Silent (synonymous) mutation. Fix: A base change that leaves the amino acid unchanged is silent, thanks to the redundant genetic code. - B: A frameshift comes from insertions/deletions, not a substitution. - C: A nonsense mutation creates a stop codon, changing the product. - D: A whole-gene deletion is far larger than one base. - E: Duplication is a chromosomal-level change, not a single substitution. - Fix rule: Same amino acid after a swap = silent mutation.
7. B — Shifts the reading frame, altering all subsequent codons. Fix: Inserting one base displaces every downstream triplet, usually scrambling the protein. - A: Mutations do not self-repair before translation here. - C: A frameshift affects many codons, not just one. - D: Insertions do not convert DNA into RNA. - E: Early frameshifts typically have major effects. - Fix rule: Insert/delete a non-multiple of three → frameshift downstream.
8. B — Lysogenic cycle. Fix: Integration of phage DNA into the host chromosome without immediate lysis defines the lysogenic cycle. - A: The lytic cycle bursts the cell right away. - C: The Calvin cycle is photosynthesis. - D: The Krebs cycle is respiration. - E: "Transcription cycle" is not a viral life cycle. - Fix rule: Integrate and lie low = lysogenic; burst quickly = lytic.
9. D — Inject nucleic acid and use the host's machinery. Fix: Phages are non-cellular; they hijack host ribosomes and enzymes to replicate. - A: A virus carries either DNA or RNA, not both at once. - B: Viruses lack their own ribosomes. - C: Viruses cannot reproduce without a host. - E: Viruses do not photosynthesize. - Fix rule: Viruses are obligate hijackers — no host, no replication.
10. C — Transcribing only currently needed genes. Fix: Selective gene expression conserves energy and materials. - A: Continuous replication of all genes wastes resources and is not how regulation works. - B: Translating everything at once is exactly what regulation avoids. - D: Cells regulate expression without permanently deleting genes. - E: Proteins are not reverse-converted into DNA. - Fix rule: Regulation = "make only what you need, when you need it."
11. A — RNA has ribose + uracil; DNA has deoxyribose + thymine. Fix: These sugar and base differences distinguish the two nucleic acids. - B: They are not identical. - C: Reversed — DNA uses thymine, RNA uses uracil. - D: RNA is usually single-stranded and DNA double-stranded, and it is not the only difference. - E: DNA is readily copied during replication. - Fix rule: RNA = Ribose + Uracil; DNA = Deoxyribose + Thymine.
12. A — Introns are spliced out before translation. Fix: Splicing removes introns from pre-mRNA so only exon-coded sequence is translated. - B: Introns are removed, not translated into stops. - C: Introns are not copied back into DNA. - D: Introns are discarded, not turned into tRNA. - E: Introns are transcribed, then removed — that is the whole point. - Fix rule: Introns are transcribed but spliced out; only exons reach the protein.
1. B — Thymine. Fix: In DNA the purine adenine pairs with the pyrimidine thymine (A–T). - A: Cytosine pairs with guanine, not adenine. - C: Guanine pairs with cytosine. - D: Uracil replaces thymine in RNA, not DNA. - E: A base does not pair with itself. - Fix rule: "A–T and G–C" in DNA; swap in U for T only in RNA.
2. E — Opposite 5′→3′ directions. Fix: Antiparallel means the strands point in reverse orientations relative to each other. - A: Complementary, not identical, sequences. - B: Bases are held by hydrogen bonds, not covalent bonds. - C: Same direction would be "parallel," which is wrong. - D: DNA is not made of RNA. - Fix rule: Antiparallel = "head-to-tail" 5′/3′ orientation.
3. E — One original and one new strand. Fix: Each template strand is conserved in a daughter molecule alongside a newly built partner. - A: Two new strands would be conservative-copy thinking, which is incorrect. - B: Two old strands describes the parent, not a product. - C: DNA replication uses DNA nucleotides, not RNA. - D: Half the nucleotides are original, so "none" is wrong. - Fix rule: "Semi" = half old, half new in each new helix.
4. D — 5′-AUG-3′. Fix: mRNA is complementary and antiparallel to the template; 3′-TAC-5′ → 5′-AUG-3′ (with U for T). - A: Retains T and wrong orientation; mRNA has no thymine. - B: Simply rewrites the template with T; ignores complementarity. - C: Uses U correctly but keeps the template bases uncomplemented. - E: Keeps thymine, which RNA never uses. - Fix rule: Complement the template and replace every T with U.
5. C — Transfer RNA (tRNA). Fix: tRNA anticodons pair with mRNA codons and deliver the matching amino acids. - A: DNA polymerase copies DNA, not a translation role. - B: rRNA forms the ribosome but does not ferry amino acids. - D: mRNA carries the message but cannot deliver amino acids. - E: Helicase unwinds DNA during replication. - Fix rule: tRNA = the "delivery truck" that reads codons and drops off amino acids.
6. A — Silent (synonymous) mutation. Fix: A base change that leaves the amino acid unchanged is silent, thanks to the redundant genetic code. - B: A frameshift comes from insertions/deletions, not a substitution. - C: A nonsense mutation creates a stop codon, changing the product. - D: A whole-gene deletion is far larger than one base. - E: Duplication is a chromosomal-level change, not a single substitution. - Fix rule: Same amino acid after a swap = silent mutation.
7. B — Shifts the reading frame, altering all subsequent codons. Fix: Inserting one base displaces every downstream triplet, usually scrambling the protein. - A: Mutations do not self-repair before translation here. - C: A frameshift affects many codons, not just one. - D: Insertions do not convert DNA into RNA. - E: Early frameshifts typically have major effects. - Fix rule: Insert/delete a non-multiple of three → frameshift downstream.
8. B — Lysogenic cycle. Fix: Integration of phage DNA into the host chromosome without immediate lysis defines the lysogenic cycle. - A: The lytic cycle bursts the cell right away. - C: The Calvin cycle is photosynthesis. - D: The Krebs cycle is respiration. - E: "Transcription cycle" is not a viral life cycle. - Fix rule: Integrate and lie low = lysogenic; burst quickly = lytic.
9. D — Inject nucleic acid and use the host's machinery. Fix: Phages are non-cellular; they hijack host ribosomes and enzymes to replicate. - A: A virus carries either DNA or RNA, not both at once. - B: Viruses lack their own ribosomes. - C: Viruses cannot reproduce without a host. - E: Viruses do not photosynthesize. - Fix rule: Viruses are obligate hijackers — no host, no replication.
10. C — Transcribing only currently needed genes. Fix: Selective gene expression conserves energy and materials. - A: Continuous replication of all genes wastes resources and is not how regulation works. - B: Translating everything at once is exactly what regulation avoids. - D: Cells regulate expression without permanently deleting genes. - E: Proteins are not reverse-converted into DNA. - Fix rule: Regulation = "make only what you need, when you need it."
11. A — RNA has ribose + uracil; DNA has deoxyribose + thymine. Fix: These sugar and base differences distinguish the two nucleic acids. - B: They are not identical. - C: Reversed — DNA uses thymine, RNA uses uracil. - D: RNA is usually single-stranded and DNA double-stranded, and it is not the only difference. - E: DNA is readily copied during replication. - Fix rule: RNA = Ribose + Uracil; DNA = Deoxyribose + Thymine.
12. A — Introns are spliced out before translation. Fix: Splicing removes introns from pre-mRNA so only exon-coded sequence is translated. - B: Introns are removed, not translated into stops. - C: Introns are not copied back into DNA. - D: Introns are discarded, not turned into tRNA. - E: Introns are transcribed, then removed — that is the whole point. - Fix rule: Introns are transcribed but spliced out; only exons reach the protein.