MCAT Biological and Biochemical Foundations of Living Systems Quiz: 1a Peptide Bonds Primary Structure

What this quiz covers

This quiz focuses on 1a Peptide Bonds Primary Structure, giving you a quick way to practice the rules, question types, and explanations that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Because peptide bonds are identical in all proteins, primary structure cannot influence susceptibility to backbone cleavage; the result must reflect differences in disulfide bonding.
2. Sequence order can change local chemical environment around specific peptide bonds, altering their likelihood of acid-catalyzed hydrolysis even when composition is the same. (correct answer)
3. Sequence order only affects secondary structure; peptide bonds determine α-helices directly, which prevents any primary-structure-dependent cleavage differences.
4. Greater resistance implies more peptide bonds were formed during translation, indicating the stable protein must be longer despite identical mass.

Explanation: This question tests understanding of how primary structure sequence affects peptide bond stability despite identical composition. While all peptide bonds have the same chemical structure (C-N linkage), their susceptibility to hydrolysis depends on the local sequence context created by neighboring amino acids. The passage describes different acid stability between proteins with identical composition but different sequences, indicating that sequence order creates different chemical environments around specific peptide bonds, altering their hydrolysis rates (choice B). Choice A incorrectly claims all peptide bonds are identical in susceptibility, ignoring that neighboring side chains create different local pH, steric, and electronic environments. A key insight: peptide bond chemistry is uniform, but peptide bond stability varies with sequence context because neighboring residues influence the local chemical environment affecting hydrolysis rates.

Question 2

1. Insertion adds one additional peptide bond and shifts the register of all downstream residues, altering the primary sequence order presented to the receptor. (correct answer)
2. Insertion does not change peptide bond number because the peptide bond is a noncovalent interaction; only side chains determine primary structure.
3. Insertion primarily changes β-sheet hydrogen bonding, which defines primary structure and therefore directly determines receptor recognition.
4. Insertion decreases the number of peptide bonds by one because adding a residue replaces two peptide bonds with one longer backbone linkage.

Explanation: This question tests understanding of how amino acid insertion affects peptide bond number and primary structure. Primary structure is the linear sequence of amino acids connected by peptide bonds, with N amino acids requiring N-1 peptide bonds. The passage describes insertion of one residue between positions 5 and 6, which adds one additional peptide bond and shifts all downstream residues by one position, changing the sequence presented to the receptor (choice A). Choice D incorrectly suggests insertion decreases peptide bonds, but mathematically, adding a residue must add one peptide bond to connect it to its neighbors. A key calculation principle: for a peptide with N amino acids, there are always N-1 peptide bonds, so insertion increases both amino acid count and peptide bond count by one.

Question 3

1. Peptide bonds cannot form at all without proline because proline catalyzes peptidyl transferase activity on the ribosome.
2. Ribosomes require proline specifically to create disulfide bonds that stabilize the primary sequence, so translation stops at proline codons.
3. Without prolyl-tRNA, the ribosome cannot add the next amino acid via peptide bond formation, so elongation stalls at codons requiring proline incorporation. (correct answer)
4. Without proline, the peptide forms normally but cannot fold into helices, causing apparent truncation during electrophoresis rather than true termination.

Explanation: This question tests understanding of how missing amino acids prevent peptide bond formation at specific codons. Peptide bonds form when aminoacyl-tRNA delivers its amino acid to the growing chain, but without prolyl-tRNA, the ribosome cannot add proline via peptide bond formation. The passage describes termination immediately before proline codons when proline is absent, indicating that elongation stalls because the ribosome cannot incorporate the missing amino acid (choice C). Choice A incorrectly suggests proline catalyzes peptide bond formation for all amino acids, but peptidyl transferase activity is an intrinsic ribosomal function that doesn't require any specific amino acid. A key principle: each codon requires its corresponding aminoacyl-tRNA to continue elongation, and missing any amino acid causes termination at codons specifying that amino acid.

Question 4

1. Synonymous mutations alter peptide bond resonance, making all peptide bonds in the protein spontaneously hydrolyze during translation.
2. Rare codons change the N-to-C directionality of chain growth, producing reversed primary sequences that cannot be released.
3. Rare codons prevent formation of disulfide bonds in the matrix, causing the protein to remain ribosome-bound.
4. Rare codons slow tRNA delivery, increasing dwell time and promoting stalling before the next peptide bond forms, yielding peptidyl-tRNA intermediates. (correct answer)

Explanation: This question evaluates understanding of how codon usage affects peptide bond formation rates and primary structure synthesis in mitochondria. Peptide bonds are formed by the ribosome's peptidyl transferase, linking amino acids into the primary sequence, with efficiency influenced by tRNA availability. Here, synonymous mutations to rare Lys codons in the mitochondrial enzyme's mRNA reduce translation speed, leading to ribosome-associated intermediates. The correct answer, A, follows because rare codons delay tRNA delivery, prolonging dwell time and stalling before peptide bond formation, yielding peptidyl-tRNA species. A common distractor, like choice B, is wrong as it incorrectly suggests codon changes reverse synthesis directionality, a misconception ignoring ribosomal mechanics. In similar scenarios, check if codon rarity correlates with translational pausing. Confirm stalling results from impaired elongation rather than post-translational effects like disulfide bonding.

Question 5

1. Polyproline regions pause because the ribosome temporarily switches to C-to-N synthesis to accommodate proline residues.
2. Polyproline regions pause because disulfide bonds cannot form between prolines, preventing completion of the primary structure.
3. Polyproline regions pause because ribosomes must form α-helices before each peptide bond, and proline prevents helix formation.
4. Certain sequences (e.g., polyproline) can slow peptide bond formation, causing ribosomal pausing that can be mitigated by factors enhancing peptidyl transfer efficiency. (correct answer)

Explanation: This question tests the understanding of how peptide bonds contribute to the primary structure of proteins, particularly in the context of translational challenges posed by specific amino acid sequences. Peptide bonds form the backbone of the primary structure by linking amino acids in a specific sequence during protein synthesis on the ribosome. In this scenario, the expression of a peptide with multiple consecutive prolines in mammalian cells leads to ribosomal pauses at the polyproline region, which are alleviated by overexpressing a specialized elongation factor. The correct answer, choice D, logically follows because polyproline sequences are known to hinder efficient peptide bond formation, causing pauses that can be resolved by factors improving peptidyl transferase activity, directly tying into primary structure extension. A common distractor, choice C, is incorrect because it confuses secondary structure formation with the ribosomal mechanism of peptide bond synthesis, as ribosomes do not require α-helix formation for bond creation and proline's effect is on bond kinetics rather than helix prevention. To check similar questions, verify if the explanation aligns with known ribosomal stalling mechanisms, such as those involving poor substrates for peptidyl transferase. Additionally, ensure distractors do not conflate primary structure processes with post-translational modifications or secondary structure elements.

Question 6

1. Primary structure can still be produced because codon–anticodon pairing determines amino acid order independently of peptide bond formation.
2. Blocking peptide bond formation prevents extension of the amino acid sequence, so the ordered primary structure cannot be synthesized beyond short peptidyl intermediates. (correct answer)
3. Blocking peptide bond formation primarily disrupts tertiary structure by preventing disulfide crosslinks, while primary structure remains intact.
4. Blocking peptide bond formation increases protein stability because fewer backbone bonds reduces the number of sites available for hydrolysis.

Explanation: This question tests understanding of peptide bond formation as the fundamental requirement for primary structure synthesis. Peptide bonds covalently link amino acids into the linear sequence that defines primary structure, and without their formation, the polypeptide chain cannot extend. The passage describes cessation of elongation with ribosomes still on mRNA, indicating that translation initiation occurred but peptide bond formation is blocked, preventing synthesis beyond short peptidyl intermediates (choice B). Choice A incorrectly suggests primary structure can form without peptide bonds, but codon-anticodon pairing only determines which amino acid to add - the peptide bond is still required to actually connect it to the growing chain. A fundamental principle: primary structure IS the peptide-bonded sequence of amino acids, so blocking peptide bond formation prevents primary structure synthesis entirely.

Question 7

1. N-methylation increases α-helix content, which directly increases peptide bond hydrolysis by the proteasome.
2. N-methylation creates a disulfide bond that blocks proteasomal cleavage at the modified site.
3. N-methylation prevents ribosomal peptide bond formation, so peptide Y cannot be synthesized as full length.
4. N-methylation can reduce hydrogen-bonding and alter backbone geometry at a specific linkage, decreasing protease access/catalysis at nearby peptide bonds. (correct answer)

Explanation: This question explores how peptide bond modifications affect primary structure processing by proteasomes. Peptide bonds' features like hydrogen-bonding influence protease recognition, with N-methylation altering geometry. In this antigen study, N-methylation reduces cleavage near the site in peptide Y. The correct answer, A, follows because it decreases protease access without global changes. A common distractor, like choice C, errs by claiming synthesis prevention, ignoring assay context. In similar comparisons, link modifications to local stability. Differentiate backbone effects from synthesis or folding impacts.

Question 8

1. The Gly→Val substitution increases peptide bond resonance, making the backbone unbreakable and therefore targeted for degradation.
2. The Gly→Val substitution prevents peptide bond formation at that site, so translation stops and triggers degradation.
3. The Gly→Val substitution removes a disulfide bond in the collagen triple helix, destabilizing the primary structure directly.
4. The Gly→Val substitution changes the amino acid sequence while preserving peptide bond connectivity; altered sequence can impair proper folding/processing and increase degradation. (correct answer)

Explanation: This question assesses how primary structure mutations affect stability via folding, distinct from peptide bonds themselves. Peptide bonds link the sequence, with mutations altering residues but not connectivity. In these procollagen variants, Gly to Val increases degradation despite full length. The correct answer, A, follows because sequence change impairs folding, targeting degradation. A common distractor, like choice B, incorrectly posits synthesis stop, contradicting length data. In biosynthesis studies, link sequence to downstream fate. Avoid confusing with disulfide or resonance effects.

Question 9

1. Acetylation increases α-helix content, which directly prevents formation of peptide bonds during synthesis.
2. Acetylation removes the peptide bond adjacent to Lys, so trypsin has no bond to cleave and the peptide becomes shorter before digestion.
3. Acetylation introduces a new disulfide bond at Lys that blocks trypsin by covalently sealing the backbone.
4. Trypsin cleavage depends on side-chain identity/charge near a peptide bond; acetylating Lys alters recognition while leaving the peptide bonds themselves present. (correct answer)

Explanation: This question distinguishes peptide bonds in primary structure from side-chain features recognized by proteases like trypsin. Peptide bonds form the backbone, but cleavage specificity relies on adjacent side chains. In this peptide, Lys acetylation reduces cleavage at that site, preserving length. The correct answer, A, follows because acetylation alters recognition without backbone change. A common distractor, like choice B, wrongly suggests bond removal, contradicting LC-MS data. In digestion assays, link modifications to specificity. Confirm backbone integrity versus side-chain effects.

Question 10

1. The mutation changes the direction of ribosomal synthesis, producing a C-to-N polypeptide that cannot form peptide bonds correctly.
2. The Cys→Ser mutation prevents peptide bond formation at that position, truncating the enzyme and causing loss of secretion.
3. Peptide bonds primarily stabilize secondary structure through side-chain crosslinks, so changing Cys to Ser directly breaks peptide bonds.
4. Peptide bonds still define the same backbone connectivity (primary structure length), while loss of a Cys can disrupt disulfide bonding that stabilizes the folded enzyme. (correct answer)

Explanation: This question distinguishes peptide bonds in primary structure from side-chain effects like disulfide bonding in enzyme function. Peptide bonds define the linear sequence and length of primary structure, independent of side-chain modifications. In this genetic variant, Cys to Ser mutation preserves full-length secretion but loses activity, with disulfides in wild-type. The correct answer, A, follows because peptide bonds maintain backbone connectivity, while lost disulfides destabilize folding. A common distractor, like choice B, wrongly suggests truncation from prevented bond formation, ignoring ribosomal tolerance. In similar cases, verify if mutations alter sequence without blocking synthesis. Differentiate backbone integrity from side-chain stabilization.

Question 11

1. Replacing Pro with Ala can increase backbone flexibility and expose adjacent peptide bonds to proteolysis, accelerating degradation. (correct answer)
2. Replacing Pro with Ala eliminates a disulfide bridge, destabilizing the hormone and accelerating degradation.
3. Replacing Pro with Ala prevents peptide bond formation at position 8, truncating the hormone during synthesis.
4. Replacing Pro with Ala primarily disrupts α-helix hydrogen bonding patterns, which directly breaks peptide bonds in plasma.

Explanation: This question tests knowledge of how primary structure influences peptide bond stability and susceptibility to proteolysis in biological fluids. Peptide bonds form the backbone of the primary structure, connecting amino acids in a sequence that can affect local conformation and accessibility to enzymes. In this scenario, the Pro8 to Ala8 substitution in the peptide hormone alters the primary sequence, impacting its stability in human plasma without affecting disulfide content. The correct answer, A, logically follows because replacing Pro with Ala increases backbone flexibility, exposing adjacent peptide bonds to proteases and accelerating degradation. A common distractor, like choice B, is incorrect as it mistakenly attributes stability to disulfide bridges, which are absent here, confusing side-chain interactions with backbone effects. For similar questions, assess if the amino acid change alters conformational rigidity around peptide bonds. Also, differentiate between backbone accessibility and unrelated modifications like disulfide bonds.

Question 12

1. The labeled material is likely peptidyl-tRNA because peptide bond formation is slowed, trapping nascent primary structures covalently linked to tRNA. (correct answer)
2. The labeled material is likely mispaired β-sheets because peptide bonds determine tertiary packing in reducing conditions.
3. The labeled material is likely disulfide-linked oligomers because reducing agents do not change primary sequence.
4. The labeled material is likely full-length toxin because mRNA binding alone is sufficient for peptide bond formation.

Explanation: This question assesses the impact of impaired peptide bond formation on primary structure synthesis in bacterial systems. Peptide bonds covalently join amino acids, defining the primary structure, and their formation requires functional peptidyl transferase activity on the ribosome. In this study, the ribosome mutation reduces peptidyl transferase efficiency, causing accumulation of RNase-sensitive, high-molecular-weight labeled material. The correct answer, A, is logical because slowed peptide bond formation traps nascent chains as peptidyl-tRNAs, which are RNase-sensitive but not affected by reducing agents. A common distractor, like choice C, errs by linking the material to disulfide oligomers, misunderstanding that primary structure synthesis precedes disulfide formation. For analogous questions, identify if impairments lead to tRNA-bound intermediates. Differentiate between covalent tRNA linkages and post-synthetic modifications like disulfides.

Question 13

1. Changing Pro to Val increases α-helix hydrogen bonding, which converts peptide bonds into ester bonds that resist proteolysis.
2. Changing Pro to Val prevents peptide bond formation during translation, so the protease has no substrate bond to cleave.
3. Changing Pro to Val introduces a new disulfide bond that directly protects the Asp-Pro peptide bond from hydrolysis.
4. Changing Pro to Val alters the immediate sequence context and backbone conformation, reducing recognition and hydrolysis of that specific peptide bond by the protease. (correct answer)

Explanation: This question explores how local primary structure affects specific peptide bond cleavage by proteases. Peptide bonds' susceptibility depends on surrounding sequence context influencing recognition and conformation. In this cytosolic protein, Pro to Val mutation reduces cleavage at the Asp-Pro motif site. The correct answer, A, follows because Val alters context and geometry, decreasing protease access. A common distractor, like choice B, errs by claiming prevented synthesis, ignoring unchanged abundance. In analogous questions, assess sequence motifs for protease specificity. Distinguish contextual effects from synthesis blocks or unrelated bonds.

Question 14

1. Peptide bonds can continue forming until ribosomes stall at the mRNA end, leaving peptidyl-tRNA species that remain RNase-sensitive without proper termination. (correct answer)
2. Without a stop codon, peptide bonds cannot form at all, so only free amino acids accumulate.
3. Without a stop codon, ribosomes release full-length protein normally because disulfide bonds trigger termination.
4. Without a stop codon, the ribosome reverses direction and removes peptide bonds one by one, causing heterogeneous lengths.

Explanation: This question evaluates peptide bond formation in primary structure synthesis without proper termination. Peptide bonds continue forming until ribosomal stalling at mRNA ends, yielding heterogeneous peptidyl-tRNAs. In this system, stop codon absence produces RNase-sensitive, variable-length polypeptides. The correct answer, A, is consistent because bonds form, but termination fails, causing stalling. A common distractor, like choice B, errs by claiming no bonds form, ignoring elongation. For termination defects, check for stalled intermediates. Differentiate from reversal or disulfide-triggered release.

Question 15

1. The presence of N-formylmethionine changes the peptide bond pattern throughout the chain, producing different primary structures despite identical residue order.
2. Peptide bonds are covalent amide linkages that define the residue order; an N-terminal formyl group modifies the terminus without changing the internal peptide bond connectivity of the primary structure. (correct answer)
3. Because peptide bonds are stabilized mainly by disulfide bridges, identical peptide fragments imply identical disulfide connectivity rather than identical primary structure.
4. Base-catalyzed hydrolysis selectively cleaves disulfide bonds, so identical fragment sets indicate the same disulfide pattern even if peptide bond connectivity differs.

Explanation: This question tests understanding of how post-translational modifications relate to peptide bonds and primary structure. Primary structure is defined by the sequence of amino acids connected through peptide bonds from N-terminus to C-terminus. The N-formylmethionine modification in bacterial proteins is a post-translational modification of the N-terminal amino group that doesn't alter the peptide bond connectivity throughout the chain. The correct answer (B) properly distinguishes between terminal modifications and the internal peptide bond network that defines primary structure. Choice A incorrectly suggests that N-terminal modifications change peptide bond patterns throughout the chain. The key insight is that primary structure is determined by the covalent peptide bond linkages between amino acids, while terminal modifications are separate chemical alterations that don't disrupt this fundamental connectivity.

Question 16

1. Proteins will be synthesized with the correct primary sequence but will fail to form alpha helices due to loss of backbone hydrogen bonding.
2. Translation will proceed normally, but disulfide bonds will not form because peptide bond formation is required to oxidize cysteine residues.
3. Nascent chains will remain short because elongation requires repeated peptide bond formation to covalently extend the polypeptide in the N-to-C direction. (correct answer)
4. Ribosomes will skip codons to compensate for reduced peptidyl transferase activity, producing full-length proteins with altered secondary structure but unchanged length.

Explanation: This question tests understanding of how peptide bond formation is essential for polypeptide elongation and primary structure assembly. Primary structure is built sequentially through repeated peptide bond formation as the ribosome moves along the mRNA. The antibiotic inhibits peptidyl transferase activity, which catalyzes peptide bond formation between the growing chain and incoming amino acids. The correct answer (C) recognizes that without peptide bond formation, the polypeptide cannot be extended beyond short fragments. Choice A incorrectly focuses on secondary structure, while peptide bond formation is required for any chain elongation regardless of later folding. The fundamental concept is that primary structure assembly requires sequential peptide bond formation to covalently link amino acids in the order specified by the mRNA.

Question 17

1. Deamidation increases disulfide bond formation, which compensates for any peptide bond loss and preserves primary sequence length.
2. Asn deamidation breaks peptide bonds adjacent to Asn, shortening the backbone; therefore unchanged length distributions indicate that peptide bonds are not part of primary structure.
3. Asn\u2192Asp conversions primarily alter secondary structure hydrogen bonding; primary structure is unaffected because the backbone peptide bonds remain identical.
4. Because peptide bonds link amino acids in a defined order, changing Asn to Asp changes primary structure at specific positions while leaving peptide-bond count and chain length essentially unchanged. (correct answer)

Explanation: This question tests understanding of how post-translational modifications affect primary structure without altering peptide bond formation. Primary structure consists of the specific sequence of amino acids linked by peptide bonds, and deamidation of asparagine to aspartate changes the chemical identity of residues at specific positions. Deamidation is a spontaneous chemical reaction that converts the amide side chain of asparagine to the carboxyl side chain of aspartate, changing the primary structure at those positions while leaving all peptide bonds intact. The unchanged protein length distributions confirm that peptide bonds are not broken during deamidation - only the side chain chemistry changes. Answer B incorrectly claims that deamidation breaks peptide bonds, but this modification only affects the asparagine side chain, not the backbone. The essential concept is that primary structure includes both the peptide bond framework and the specific amino acid identities, and either can be modified independently.

Question 18

1. Chelating Mg$^{2+}$ primarily prevents disulfide bond formation in the cytosol, which shortens primary structure by removing covalent crosslinks needed for elongation.
2. Reduced Mg$^{2+}$ can destabilize ribosomal structure and impair elongation, decreasing the frequency of peptide bond formation and yielding shorter primary structures despite intact mRNA. (correct answer)
3. Chelating Mg$^{2+}$ strengthens peptide bonds by increasing backbone planarity, which causes ribosomes to pause but still complete full-length primary structures.
4. Mg$^{2+}$ chelation selectively disrupts hydrogen bonds in \u03b2-sheets, so primary structure length decreases as secondary structure fails to form co-translationally.

Explanation: This question tests understanding of how cofactor availability affects peptide bond formation during translation. Primary structure formation requires sequential peptide bond formation catalyzed by the ribosome, and this process depends on proper ribosomal structure and function. Mg²⁺ ions are essential cofactors that stabilize ribosomal RNA structure and facilitate the peptidyl transferase reaction that forms peptide bonds. When Mg²⁺ is chelated and becomes unavailable, ribosomal structure is destabilized and elongation is impaired, leading to ribosome stalling (increased occupancy) and premature termination that produces shorter polypeptides. Answer A incorrectly invokes disulfide bonds, which form post-translationally between cysteines and are not involved in elongation or affected by Mg²⁺. The key concept is that peptide bond formation requires a functional ribosome with proper cofactors - disrupting these requirements leads to incomplete primary structures despite intact mRNA templates.

Question 19

1. Translation can continue to produce full-length polypeptide as long as disulfide bonds can form between cysteines, because these covalent links substitute for peptide bonds in the backbone.
2. Inhibiting peptidyl transferase prevents covalent linkage of amino acids into a continuous backbone, so the linear sequence (primary structure) cannot be extended even if mRNA and tRNAs are present. (correct answer)
3. Blocking peptide bond formation primarily disrupts \u03b1-helix and \u03b2-sheet hydrogen bonding patterns, while leaving the amino acid sequence length largely unchanged.
4. Peptide bond inhibition increases primary-structure stability by preventing backbone rotation, thereby favoring accumulation of longer polypeptides on the ribosome.

Explanation: This question tests understanding of how peptide bonds are essential for forming the primary structure of proteins. Peptide bonds are the covalent linkages between the carboxyl group of one amino acid and the amino group of the next, creating the continuous polypeptide backbone that defines primary structure. Chloramphenicol inhibits peptidyl transferase, the ribosomal enzyme that catalyzes peptide bond formation during translation. Without functional peptidyl transferase, amino acids cannot be covalently linked together, preventing extension of the growing polypeptide chain even though mRNA and tRNAs remain available. The incorrect answer A suggests disulfide bonds could substitute for peptide bonds in the backbone, but disulfide bonds form between cysteine side chains and do not contribute to the main chain connectivity. The key concept is that peptide bonds are absolutely required for primary structure formation - without them, translation cannot produce a continuous polypeptide sequence.

Question 20

1. A nonsense mutation prevents formation of peptide bonds only between cysteines, so the N-terminus remains while the C-terminus fails due to absent disulfide bonds.
2. A nonsense mutation introduces a stop codon that halts peptide bond formation beyond residue 120, producing a shorter primary structure containing the N-terminus but lacking the C-terminal sequence. (correct answer)
3. The ribosome can bypass stop codons by forming additional peptide bonds through the side chains, so the full-length primary structure should still be produced but misfolded.
4. The negative C-terminal assay indicates altered \u03b1-helix hydrogen bonding; peptide bonds and primary sequence length are expected to be unchanged by a nonsense mutation.

Explanation: This question tests understanding of how nonsense mutations affect primary structure by terminating peptide bond formation. Primary structure is the linear sequence of amino acids linked by peptide bonds from N-terminus to C-terminus, and translation continues only until a stop codon is encountered. A nonsense mutation at codon 120 introduces a premature stop codon that causes the ribosome to terminate translation, preventing peptide bond formation beyond residue 119. This produces a truncated protein containing amino acids 1-119 (including the N-terminus) but lacking residues 120-210 (including the C-terminus), explaining why the N-terminal antibody is positive but the C-terminal antibody is negative. Answer C incorrectly suggests ribosomes can bypass stop codons through side-chain bonds, but peptide bonds only form between backbone groups, and stop codons cause ribosome release. The fundamental principle is that peptide bond formation is directional and sequential - once translation stops, no further residues can be added to extend the primary structure.