Which of the following statements about translation is INCORRECT? | NeetCrackers

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Which of the following statements about translation is INCORRECT?

Options

1

mRNA is read in the 5' to 3' direction

2

The ribosome moves along mRNA in the 3' to 5' direction

3

The first amino acid corresponds to the N-terminal end of the polypeptide

4

Peptide bond formation occurs at the peptidyl transferase centre of the large ribosomal subunit

Correct Answer

Ribosome moves 3' to 5' -- INCORRECT (moves 5' to 3')

Solution

1

A: mRNA read 5' to 3' = TRUE

B: Ribosome moves 3' to 5' = FALSE (moves 5' to 3')

2

C: First amino acid = N-terminal = TRUE

D: Peptide bond at peptidyl transferase in large subunit = TRUE

Answer: B is INCORRECT

Ribosome moves 5' to 3' along mRNA
Statement B says 3' to 5' = INCORRECT

Theory: Molecular Biology

1. Translation Overview

Translation: mRNA decoded into protein at ribosomes. Direction: mRNA read 5' to 3'. Ribosome moves 5' to 3'. Polypeptide grows N-terminus to C-terminus. Three stages: Initiation (AUG start codon), Elongation (peptide bonds formed), Termination (stop codon). Genetic code: 64 codons. 61 sense + 3 stop (UAA, UAG, UGA). AUG = start, encodes Met (eukaryotes) or fMet (prokaryotes). Code is universal, degenerate, non-overlapping, unambiguous.

2. Ribosome Structure

Prokaryotic 70S: 30S subunit (16S rRNA + 21 proteins) + 50S subunit (23S + 5S rRNA + 31 proteins). Eukaryotic 80S: 40S (18S rRNA) + 60S (28S + 5.8S + 5S rRNA). Three functional sites: A site (incoming aminoacyl-tRNA), P site (growing chain), E site (deacylated tRNA exits). Peptidyl transferase: in large subunit. 23S rRNA is the actual catalyst (ribozyme). Nobel Prize 2009 for ribosome crystal structure.

3. tRNA Structure and Charging

tRNA: adaptor linking codon to amino acid. Cloverleaf 2D, L-shaped 3D. 3'-CCA end: amino acid attaches. Anticodon loop: 3 nt complementary to codon. Aminoacyl-tRNA synthetases charge tRNA with correct amino acid (uses 2 ATP equivalent). Wobble hypothesis: 5' base of anticodon pairs flexibly with 3' codon position, allowing one tRNA to recognise multiple codons. Inosine pairs with U, C, A.

4. Initiation

Prokaryotic: 30S binds Shine-Dalgarno sequence (AGGAGG, ~10 nt before AUG). fMet-tRNA at P site. IF1, IF2, IF3. 50S joins. Eukaryotic: 43S complex scans from 5' cap along mRNA until Kozak AUG. eIF4F cap complex. 60S joins. ~12 initiation factors. eIF2 brings Met-tRNA. Key difference: prokaryotes use SD sequence for direct loading; eukaryotes scan from 5' cap.

5. Elongation

Three steps per codon: (1) Decoding: EF-Tu/eEF1A delivers aa-tRNA with GTP to A site. Correct pairing means GTP hydrolysis, tRNA accommodated. (2) Peptide bond: peptidyl transferase (rRNA ribozyme) transfers chain from P-site tRNA to A-site amino acid. (3) Translocation: EF-G/eEF2 + GTP moves ribosome 3 nt in 5 to 3 direction. A becomes P, P becomes E, E exits. 2 GTP per cycle total.

6. Termination

Stop codon at A site: no cognate tRNA. Release factors (RF1, RF2 in prokaryotes; eRF1 in eukaryotes) enter A site. Peptidyl transferase hydrolyses ester bond: polypeptide released. Ribosome disassembles. Post-translational modifications: signal peptide removal, glycosylation, phosphorylation, disulfide bond formation, ubiquitination (for degradation).

7. Antibiotic Inhibitors of Translation

Prokaryotic 30S inhibitors: Tetracyclines (block A site), Aminoglycosides like streptomycin (misreading). Prokaryotic 50S inhibitors: Chloramphenicol (blocks peptidyl transferase at 23S rRNA), Erythromycin (blocks exit tunnel), Linezolid (blocks initiation). Eukaryotic 80S inhibitors: Cycloheximide, ricin (research tools). Selectivity: because 70S and 80S rRNA sequences differ at antibiotic binding sites.

8. Mutations Affecting Translation

Silent: same amino acid, no effect. Missense: different amino acid (sickle cell: Glu to Val, GAG to GTG). Nonsense: amino acid codon to stop codon, truncated protein. Frameshift: insertion/deletion not multiple of 3, reading frame shifts downstream, usually most severe. Splice site mutations: intron not removed or exon skipped, aberrant protein. Start codon mutations: no or very little protein made.

Frequently Asked Questions

1. Why does the ribosome move 5' to 3' not 3' to 5'? ⌄

The AUG start codon is near the 5' end of mRNA. Reading must proceed from start (5') toward stop codon (3'). Translocation by EF-G moves ribosome exactly 3 nt toward the 3' end each cycle. All known RNA polymerases and ribosomes process nucleic acids in this universal 5' to 3' direction.

2. What is the significance of the peptidyl transferase being RNA? ⌄

Peptidyl transferase activity resides in 23S/28S rRNA (Nobel Prize 2009, Ramakrishnan/Steitz/Yonath). This means the ribosome is a ribozyme. It supports the RNA World hypothesis: RNA evolved catalytic function before proteins existed. The machine that makes all proteins is itself RNA-based - a molecular fossil of early life.

3. What are A, P and E sites? ⌄

A (aminoacyl) site: incoming charged tRNA with new amino acid. P (peptidyl) site: tRNA carrying the growing polypeptide. E (exit) site: deacylated tRNA leaves from here. During elongation: new aa-tRNA enters A, peptide transferred A to A-site amino acid, translocation moves A-tRNA to P, P-tRNA to E.

4. What is the genetic code and what does degenerate mean? ⌄

64 codons encode 20 amino acids + 3 stop codons. Degenerate: multiple codons specify the same amino acid (e.g., Leu has 6 codons: UUA, UUG, CUU, CUC, CUA, CUG). Degeneracy is mostly at the 3rd codon position (wobble position). This means many mutations at position 3 are silent (synonymous). Universal: same code in all organisms (minor exceptions in mitochondria).

5. What distinguishes prokaryotic from eukaryotic translation? ⌄

Prokaryotic: 70S ribosome, Shine-Dalgarno sequence for initiation, fMet start amino acid, 3 initiation factors, can be coupled with transcription. Eukaryotic: 80S ribosome, 5-cap scanning mechanism, Met start amino acid, ~12 initiation factors, separated from transcription (nuclear/cytoplasmic). These differences explain antibiotic selectivity and allow recombinant protein production strategies.

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