Translation

Start at the first AUG (Met) and read in threes: CCU Pro, UGG Trp, AAG Lys, UGC Cys, then UAG, a stop codon, ends the chain.

• Correct: Met-Pro-Trp-Lys-Cys: Correct: AUG to the codon before UAG.
• Thr-Met-Pro-Trp-Lys-Cys: ACG lies before the start codon, so it is not read.
• Met-Pro-Trp-Lys-Cys-His: UAG is a stop codon; CAU after it is not read.
• Pro-Trp-Lys-Cys: The start codon AUG is read too, and puts methionine first.

Nucleotides 10-12 are the third codon read from AUG, UGG (Trp). With A at position 12 it reads UGA, a stop codon, so the chain ends after Met-Pro.

• Correct: Met-Pro, because the third codon read becomes UGA, a stop codon: Correct: UGG → UGA turns tryptophan's codon into a stop.
• Met-Pro-Trp-Lys-Cys, because a single change does not alter the protein: Some single changes leave the amino acid the same, but UGA is a stop codon, so this one shortens the chain.
• Met-Pro-Lys-Cys, because the changed codon is skipped by the ribosome: Ribosomes do not skip codons; a stop codon ends the chain there.
• Met-Pro-Glu-Lys-Cys, because the third codon read now codes for a new amino acid: UGA is not a codon for glutamic acid (GAA or GAG); it is a stop codon.

The anticodon pairs with the codon antiparallel: codon 5′-UGG-3′ pairs with anticodon 3′-ACC-5′ (U with A, G with C, G with C).

• Correct: 3′-ACC-5′: Correct: complementary and antiparallel.
• 3′-UGG-5′: This repeats the codon; an anticodon is complementary to it.
• 5′-ACC-3′: The bases are complementary, but written this way the anticodon would run parallel to the codon.
• 3′-TCC-5′: tRNA is RNA, so it carries U, not T; and T does not pair with U.

UCUCUC… read in non-overlapping triplets is UCU-CUC-UCU-CUC…, two different codons in turn, matching the alternating serine and leucine. A two-letter code would read UC-UC-UC…, one codon repeated, giving one amino acid.

• Correct: Read in threes, the RNA gives UCU and CUC in turn, so two amino acids alternate.: Correct: an odd-length codon makes the two codons alternate.
• Read in twos, the RNA gives UC and UC, so it would code for serine and leucine in turn.: Read in twos, every codon would be UC, giving one amino acid repeated, not two alternating.
• Read in threes, the RNA gives one codon repeated, which is why just two amino acids appear.: Triplets from UCUC… alternate between UCU and CUC; they are not one repeated codon.
• Read in ones, each U and C codes for its own amino acid, so two amino acids alternate.: Four bases read singly could code for only four amino acids, and the poly-U, poly-A and poly-C results each give one amino acid, not a mixture.

A secreted protein starts with a signal peptide that directs the ribosome to the ER. Once the chain is threaded into the ER, an enzyme there cuts the signal peptide off, so the finished protein is shorter.

• Correct: Its signal peptide was cut off after the chain entered the ER vesicles.: Correct: the first ~20 amino acids are removed inside the ER.
• Fewer amino acids were available, so the ribosome stopped early.: Every tube had all twenty amino acids; the difference depends on the ER being present.
• Inside the vesicles, the protease removed part of the protein.: The protease could not reach protein inside the sealed vesicles; that is why tube 2's protein was protected.
• ER membranes speed up translation, so the chain had less time to grow.: Speed does not change the length; a ribosome reads the mRNA to its stop codon.

In tube 2 the protein entered the vesicles; in tube 3 the same protein, finished first, did not. So entry happens during translation: the signal peptide emerges first and brings the ribosome to the ER, and the chain is threaded in as it grows.

• Correct: The protein enters the ER while it is being made, as the ribosome builds it.: Correct: targeting happens as the chain is made.
• The protein enters the ER after it is finished, but just when the cell is intact.: Nothing in the data points to a need for an intact cell; the timing of adding the vesicles is the only difference from tube 2.
• The ER vesicles in tube 3 were broken, so they could not take up any protein.: The same kind of vesicles took in the protein in tube 2; nothing suggests tube 3's were damaged.
• The finished protein is too small to pass through the ER membrane.: The protein is the same size as in tube 1, and the same protein entered in tube 2, so size is not the reason.