CUET UG Biology Genetics Molecular Basis of Inheritance DNA Replication Transcription Translation

Must-Know (15–20 detailed bullets)

• DNA replication is semi-conservative, proven by Meselson and Stahl in 1958 using E. coli grown in ^15N and ^14N media; density gradient centrifugation showed hybrid DNA after one generation.
• DNA polymerase III is the primary enzyme for DNA elongation in prokaryotes; it adds nucleotides in the 5' → 3' direction only.
• The leading strand is synthesized continuously, while the lagging strand is synthesized discontinuously as Okazaki fragments (1000–2000 nucleotides long in prokaryotes).
• DNA ligase joins Okazaki fragments on the lagging strand by catalyzing phosphodiester bond formation.
• Origin of replication (oriC in E. coli) is the specific site where DNA replication begins; it is AT-rich for easier strand separation.
• Helicase unwinds the DNA double helix at the replication fork; topoisomerase (e.g., DNA gyrase) prevents supercoiling ahead of the fork.
• Single-strand binding proteins (SSBs) stabilize single-stranded DNA during replication.
• RNA primase synthesizes a short RNA primer (10–12 nucleotides) to provide a 3'-OH group for DNA polymerase to begin synthesis.
• In eukaryotes, DNA replication occurs during the S phase of the cell cycle; multiple origins of replication (replicons) are present per chromosome.
• Transcription is the process of copying genetic information from DNA to RNA; it occurs in the 5' → 3' direction, with RNA polymerase reading the template strand 3' → 5'.
• In prokaryotes, a single RNA polymerase synthesizes all RNA types; in eukaryotes, RNA polymerase II transcribes mRNA.
• The promoter region (e.g., -10 Pribnow box: TATAAT and -35 box: TTGACA in E. coli) is where RNA polymerase binds to initiate transcription.
• Transcription has three stages: initiation, elongation, and termination; termination occurs via rho-dependent or rho-independent (hairpin loop) mechanisms in prokaryotes.
• In eukaryotes, primary transcript (hnRNA) undergoes processing: 5' capping (7-methylguanosine), 3' polyadenylation (poly-A tail ~200 adenines), and splicing to remove introns.
• Splicing is carried out by the spliceosome (composed of snRNPs like U1, U2, etc.) which recognizes GU-AG rule at intron-exon boundaries.
• Translation is the synthesis of polypeptides on ribosomes using mRNA; it occurs in the cytoplasm in prokaryotes and on rough ER in eukaryotes.
• The ribosome has three sites: A (aminoacyl), P (peptidyl), and E (exit); the initiator tRNA carries methionine (formylmethionine in prokaryotes).
• Genetic code is universal, degenerate, non-overlapping, and commaless; AUG codes for methionine and is the start codon.
• Wobble hypothesis explains flexibility in base pairing at the third position of the codon (e.g., tRNA anticodon G can pair with U or C).
• Operon model (lac operon in E. coli) was given by Jacob and Monod; it includes structural genes (z, y, a), operator, promoter, and regulator gene (i).

Difficulty Level

Intermediate — requires understanding of processes, enzymes, and directionality; includes diagrams-based reasoning but no advanced exceptions.

Common CUET Traps (3 bullets)

• Trap: Assuming DNA polymerase can initiate synthesis without a primer.
  Avoid: DNA polymerase requires a free 3'-OH end; RNA primase provides the primer.
• Trap: Believing introns are translated into protein.
  Avoid: Introns are non-coding sequences removed during RNA splicing; only exons are expressed.
• Trap: Confusing the template strand with the coding strand in transcription.
  Avoid: Template strand is 3'→5', transcribed into RNA; coding strand is 5'→3' and matches RNA sequence (except T/U).

Practice MCQs (5 questions)

Q1. Which enzyme is responsible for joining Okazaki fragments during DNA replication?
A. Helicase
B. DNA polymerase I
C. DNA ligase
D. Primase
Answer: C
Explanation: DNA ligase catalyzes the formation of phosphodiester bonds between Okazaki fragments.
Why others fail: DNA polymerase I removes RNA primers but does not join fragments; ligase does the final sealing.

Q2. In eukaryotic transcription, the 5' end of hnRNA is capped with:
A. Adenine triphosphate
B. Methylated guanine
C. Uracil nucleotide
D. Phosphate group
Answer: B
Explanation: 5' capping involves addition of 7-methylguanosine to protect mRNA and aid ribosome binding.
Why others fail: Adenine is added at 3' end as poly-A tail, not at 5'; uracil is not used in capping.

Q3. Which of the following correctly represents the direction of RNA synthesis during transcription?
A. 3' → 5' on the template strand
B. 5' → 3' on the RNA molecule
C. 5' → 3' on the template strand
D. 3' → 5' on the RNA molecule
Answer: B
Explanation: RNA polymerase synthesizes RNA in the 5' → 3' direction, complementary to the template strand.
Why others fail: Option A describes template strand reading direction, not RNA synthesis direction.

Q4. In the lac operon, what happens when lactose is present?
A. Repressor binds to operator
B. Repressor is inactivated
C. RNA polymerase is blocked
D. Structural genes are not transcribed
Answer: B
Explanation: Lactose acts as an inducer, binding to repressor and causing conformational change that prevents operator binding.
Why others fail: Repressor binds operator only in absence of lactose; presence leads to transcription.

Q5. During translation, the amino acid attached to tRNA in the P site forms a peptide bond with the amino acid in which site?
A. E site
B. A site
C. P site
D. Both A and P sites
Answer: B
Explanation: Peptide bond formation occurs between the polypeptide in P site and new amino acid in A site.
Why others fail: E site holds deacylated tRNA exiting; no bond forms there.

Last-Minute Revision (15–20 one-liners)

• ⚠️ Meselson-Stahl experiment used ^15N and ^14N to prove semi-conservative replication.
• ⚠️ DNA polymerase adds nucleotides only in 5' → 3' direction; cannot initiate synthesis.
• ⚠️ Leading strand: continuous; lagging strand: discontinuous (Okazaki fragments).
• ⚠️ RNA primer is synthesized by primase; removed later by DNA polymerase I.
• ⚠️ Helicase unwinds DNA; topoisomerase prevents supercoiling.
• ⚠️ SSBs stabilize single-stranded DNA during replication.
• ⚠️ Eukaryotic replication has multiple origins per chromosome; prokaryotes have single origin.
• ⚠️ Transcription: DNA → RNA; template strand read 3'→5'; RNA made 5'→3'.
• ⚠️ Prokaryotic promoter: -10 (TATAAT), -35 (TTGACA); eukaryotic: TATA box.
• ⚠️ Rho-dependent termination uses rho protein; rho-independent uses hairpin loop.
• ⚠️ hnRNA processing: 5' cap, poly-A tail (~200 A), splicing.
• ⚠️ Spliceosome removes introns based on GU-AG rule.
• ⚠️ Genetic code: 64 codons, 61 code for amino acids, 3 are stop codons (UAA, UAG, UGA).
• ⚠️ AUG is start codon; codes for methionine (f-Met in prokaryotes).
• ⚠️ Wobble position is third base of codon; allows flexible pairing.
• ⚠️ Ribosomal sites: A (incoming tRNA), P (growing chain), E (exit).
• ⚠️ Lac operon: z (β-galactosidase), y (permease), a (transacetylase).
• ⚠️ Inducer in lac operon is allolactose (derived from lactose).
• ⚠️ Jacob and Monod proposed operon model; verify from NCERT.
• ⚠️ tRNA has anticodon loop; amino acid attaches at 3' end (CCA sequence).