What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Translation Genetic Code: How Cells Decode mRNA into Proteins

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Introduction to the Genetic Code and Translation

The genetic code is the set of rules by which information encoded in nucleic acid sequences is translated into the amino acid sequences of proteins. It is a triplet code: three consecutive nucleotides in messenger RNA (mRNA), termed a codon, specify a single amino acid. Translation is the cellular process that executes this decoding, converting the linear nucleotide sequence of mRNA into a linear polypeptide chain. This process is universal across all domains of life, with only minor variations in mitochondria and certain ciliates, underscoring its ancient evolutionary origin.

The Central Dogma: DNA to RNA to Protein

The central dogma of molecular biology describes the directional flow of genetic information: DNA is transcribed into RNA, and RNA is translated into protein. While DNA serves as the stable repository of genetic information, it does not directly template protein synthesis. Instead, a gene is transcribed into messenger RNA (mRNA), which carries the genetic message from the nucleus (in eukaryotes) to the cytoplasm, where ribosomes execute translation. The genetic code bridges the language of nucleic acids (four nucleotides) and the language of proteins (twenty standard amino acids). Because four nucleotides cannot specify twenty amino acids in a single-letter code, the code evolved as triplets, yielding 4³ = 64 possible codons—more than enough to encode the twenty amino acids with redundancy.

The Genetic Code Dictionary

The genetic code dictionary is a table that maps each of the 64 possible mRNA codons to its corresponding amino acid or to a translational stop signal. This dictionary is read in the 5′ to 3′ direction along the mRNA. The code is non-overlapping: each nucleotide belongs to exactly one codon, and codons are read sequentially from a fixed starting point. The code is also commaless—there are no spacers or punctuation between codons—which makes the reading frame critically important. A single nucleotide insertion or deletion shifts the reading frame and typically produces a nonfunctional protein downstream of the mutation. For a visual representation of the flow from DNA to protein, see the Translation Biology Diagram.

The Genetic Code: Codons and Amino Acids

The 64 Codons and Their Meanings

Of the 64 codons, 61 are sense codons that specify amino acids, and 3 are stop codons that signal termination of translation. The amino acid methionine is encoded by a single codon (AUG), as are tryptophan (UGG). The remaining 18 amino acids are each encoded by two to six codons. For example, leucine has six codons (UUA, UUG, CUU, CUC, CUA, CUG), while phenylalanine has two (UUU, UUC). This redundancy is termed degeneracy, and it is not random: codons that specify the same amino acid typically share the first two nucleotides and differ only in the third position. This pattern has profound implications for mutation tolerance, as changes in the third codon position often result in silent mutations that do not alter the protein sequence.

Start and Stop Codons

The codon AUG serves a dual role: it encodes methionine and functions as the primary initiation codon. In eukaryotes, the first AUG encountered by the ribosome typically initiates translation, although the surrounding sequence context modulates initiation efficiency. In bacteria, AUG is the most common start codon, but GUG and UUG can also serve this role, encoding formylmethionine when used as start codons. The three stop codons—UAA, UAG, and UGA—do not specify amino acids. They are recognized by release factors rather than transfer RNAs (tRNAs), triggering the hydrolysis of the completed polypeptide from the final tRNA and the dissociation of the ribosomal subunits. UAA is the most frequently used stop codon in many organisms, followed by UGA and UAG.

Degeneracy and Wobble

Degeneracy refers to the fact that multiple codons can specify the same amino acid. This property minimizes the deleterious effects of point mutations: a single nucleotide change in the third codon position often leaves the amino acid unchanged. The wobble hypothesis, proposed by Francis Crick in 1966, explains the molecular basis of degeneracy. The base at the 5′ end of the tRNA anticodon (position 34) can pair non-standardly with the base at the 3′ end of the mRNA codon (position 3). For example, inosine, a modified base found at the wobble position of many tRNAs, can pair with U, C, or A. This allows a single tRNA to recognize multiple codons differing only in their third nucleotide, reducing the number of tRNA species required to decode all 61 sense codons to approximately 31 in bacteria and 48 in humans.

The Machinery of Translation: Ribosomes, tRNA, and mRNA

Ribosome Structure and Function

Ribosomes are large ribonucleoprotein complexes that catalyze peptide bond formation. In prokaryotes, the 70S ribosome consists of a 50S large subunit and a 30S small subunit. The 50S subunit contains the 23S and 5S ribosomal RNAs (rRNAs) and approximately 34 proteins; the 30S subunit contains the 16S rRNA and 21 proteins. In eukaryotes, the 80S ribosome comprises a 60S large subunit (28S, 5.8S, and 5S rRNAs plus ~49 proteins) and a 40S small subunit (18S rRNA plus ~33 proteins). The ribosome has three tRNA binding sites: the A (aminoacyl) site, where the incoming aminoacyl-tRNA binds; the P (peptidyl) site, where the tRNA carrying the growing polypeptide chain resides; and the E (exit) site, through which deacylated tRNAs leave. The peptidyl transferase center, located in the large subunit, is composed entirely of rRNA, making the ribosome a ribozyme. The small subunit contains the decoding center, where codon-anticodon base pairing is monitored for accuracy.

tRNA: The Adapter Molecule

Transfer RNA (tRNA) molecules are the adapters that link codons to amino acids. Each tRNA is typically 73–93 nucleotides long and folds into a cloverleaf secondary structure with three stem-loops and an acceptor stem. The anticodon, a triplet of nucleotides at positions 34–36, is located in the anticodon loop and base-pairs with the mRNA codon. The amino acid is attached covalently to the 3′ end of the tRNA at the conserved CCA sequence, specifically to the 3′ hydroxyl of the terminal adenosine. Aminoacyl-tRNA synthetases catalyze this attachment, charging each tRNA with its cognate amino acid in a two-step reaction requiring ATP. There are at least 20 aminoacyl-tRNA synthetases, one for each amino acid, and they achieve specificity through recognition of both the anticodon and the acceptor stem. The fidelity of this charging step is essential: mischarging a tRNA with the wrong amino acid would result in an incorrect protein despite correct codon-anticodon pairing. For a detailed look at tRNA function, see tRNA Translation.

mRNA: The Messenger

Messenger RNA is the template for translation. In prokaryotes, mRNA is polycistronic, meaning a single mRNA molecule can contain multiple open reading frames (ORFs) encoding several proteins. In eukaryotes, mRNA is monocistronic, encoding a single protein, and is modified at both ends: a 5′ 7-methylguanosine cap and a 3′ poly(A) tail. These modifications stabilize the mRNA and facilitate ribosome binding. The coding sequence of mRNA is flanked by untranslated regions (UTRs): the 5′ UTR contains elements that regulate translation initiation, and the 3′ UTR contains signals for mRNA stability and localization. The sequence between the start codon and the stop codon constitutes the open reading frame, and its length determines the size of the protein product. For an overview of how mRNA is processed and read, see mRNA Translation.

The Process of Translation: Initiation, Elongation, and Termination

Initiation: Assembly of the Ribosome

Translation initiation assembles the ribosome at the start codon with the initiator tRNA in the P site. In bacteria, initiation requires the 30S subunit, mRNA, initiator tRNA (fMet-tRNA^fMet), and three initiation factors: IF1, IF2, and IF3. The 30S subunit binds to the Shine-Dalgarno sequence, a purine-rich sequence (consensus 5′-AGGAGG-3′) located 6–8 nucleotides upstream of the start codon, through complementary base pairing with the anti-Shine-Dalgarno sequence at the 3′ end of 16S rRNA. IF3 prevents premature association of the large subunit, IF1 occupies the A site, and IF2, a GTPase, delivers the initiator tRNA to the P site. GTP hydrolysis by IF2 triggers the dissociation of initiation factors and the association of the 50S subunit, forming the 70S initiation complex.

In eukaryotes, initiation is more complex. The 43S preinitiation complex, consisting of the 40S subunit, eIF2-GTP-Met-tRNA^i, and several eukaryotic initiation factors (eIF1, eIF1A, eIF3, eIF5), binds to the 5′ cap of mRNA via eIF4F (comprising eIF4E, eIF4G, and eIF4A). The complex then scans the 5′ UTR in a 5′ to 3′ direction, unwinding secondary structure with the helicase activity of eIF4A, until it encounters the first AUG codon in a favorable context. AUG recognition triggers GTP hydrolysis by eIF2 and the release of initiation factors, followed by 60S subunit joining to form the 80S initiation complex. The Kozak sequence (gccRccAUGG, where R is a purine) enhances initiation efficiency at the start codon.

Elongation: Adding Amino Acids

Elongation proceeds in three repeating steps: decoding, peptide bond formation, and translocation. In bacteria, elongation factor Tu (EF-Tu) delivers aminoacyl-tRNA to the A site as a ternary complex with GTP. Correct codon-anticodon pairing induces a conformational change in the decoding center of the 30S subunit, triggering GTP hydrolysis by EF-Tu and release of EF-Tu-GDP. The ribosome then catalyzes peptide bond formation: the peptidyl transferase center in the 23S rRNA transfers the polypeptide from the P-site tRNA to the amino group of the A-site aminoacyl-tRNA, extending the chain by one amino acid. The reaction is spontaneous and does not require an external energy source beyond the prior activation of the amino acid. Following peptide bond formation, the ribosome translocates by one codon: elongation factor G (EF-G), another GTPase, catalyzes the movement of the mRNA and tRNAs such that the deacylated tRNA moves to the E site and the peptidyl-tRNA moves from the A site to the P site. The vacated A site is then ready for the next aminoacyl-tRNA. Elongation in eukaryotes uses the analogous factors eEF1A (delivery) and eEF2 (translocation), with eEF1B acting as a guanine nucleotide exchange factor for eEF1A. The rate of elongation in bacteria is approximately 15–20 amino acids per second at 37°C, while eukaryotic elongation proceeds at roughly 5–10 amino acids per second.

Termination: Stop Codons and Release

Termination occurs when a stop codon (UAA, UAG, or UGA) enters the A site. No tRNA recognizes these codons; instead, class I release factors bind. In bacteria, RF1 recognizes UAA and UAG, while RF2 recognizes UAA and UGA. These release factors mimic tRNAs in shape and bind in the A site, where they catalyze the hydrolysis of the ester bond between the completed polypeptide and the P-site tRNA, releasing the protein. RF3, a GTPase, then promotes dissociation of RF1 or RF2. In eukaryotes, a single class I release factor, eRF1, recognizes all three stop codons, and eRF3, a GTPase, stimulates the termination reaction. Following peptide release, the ribosome dissociates into subunits, aided by ribosome recycling factor (RRF) and EF-G in bacteria, or by ABCE1 in eukaryotes. The mRNA is then available for further rounds of translation, and the ribosomal subunits can participate in new initiation events.

The Role of the Genetic Code in Translation

Codon-Anticodon Pairing

The genetic code is physically realized through codon-anticodon base pairing. The anticodon of tRNA, positioned at nucleotides 34–36, pairs antiparallel with the mRNA codon. The first two positions of the codon (positions 1 and 2) form standard Watson-Crick base pairs with anticodon positions 3 and 2, respectively, and this pairing is strictly monitored by the ribosome. The third codon position (position 3) pairs with anticodon position 34, the wobble position, where non-standard pairing is permitted. This arrangement ensures that the specificity of the genetic code resides primarily in the first two codon nucleotides, while the third position tolerates flexibility. The ribosome's decoding center checks the geometry of the first two base pairs, rejecting mismatches through a kinetic proofreading mechanism: incorrect aminoacyl-tRNAs bind with lower affinity and are released before GTP hydrolysis by EF-Tu can occur. The overall error rate of translation is approximately 10⁻⁴ per codon, meaning roughly one error per 10,000 amino acids incorporated.

Maintaining the Reading Frame

The reading frame is the grouping of nucleotides into consecutive triplets, and it is established at initiation and maintained throughout elongation. Because the code is non-overlapping and commaless, any shift in the reading frame changes the identity of every subsequent codon. The ribosome maintains the reading frame through the precise translocation step: after peptide bond formation, the mRNA advances exactly three nucleotides relative to the ribosome. The 16S rRNA and ribosomal proteins form a channel that grips the mRNA, and the translocation reaction is coupled to GTP hydrolysis by EF-G, ensuring processivity. Errors in translocation can lead to frameshifts, which typically produce truncated or nonfunctional proteins. Programmed frameshifting, where the ribosome shifts frame at a specific sequence, is used by some viruses (e.g., HIV) to produce polyproteins from overlapping reading frames. For more on how reading frames are established and maintained, see Reading Frame Translation.

Start Codon Selection and Kozak Sequence

Accurate start codon selection is critical because it defines the reading frame. In eukaryotes, the scanning mechanism ensures that the ribosome initiates at the first AUG in a favorable context. The Kozak consensus sequence, gccRccAUGG, where R is a purine (A or G) at position −3 and G at position +4, strongly enhances initiation. Mutation of these key positions can reduce translation efficiency by an order of magnitude. If the first AUG is in a poor context, the ribosome may skip it and initiate at a downstream AUG (leaky scanning), producing multiple protein isoforms from a single mRNA. In bacteria, the Shine-Dalgarno sequence positions the start codon in the P site, and the distance between the Shine-Dalgarno sequence and the start codon (typically 6–8 nucleotides) is critical for accurate initiation. The initiator tRNA, which carries methionine (formylmethionine in bacteria), is unique in that it binds directly to the P site, not the A site, establishing the correct reading frame from the first codon.

Evidence for the Genetic Code: Experimental Discoveries

Nirenberg and Matthaei's Poly-U Experiment

In 1961, Marshall Nirenberg and Heinrich Matthaei performed the landmark experiment that cracked the first codon. They prepared a cell-free translation system from E. coli by lysing cells and isolating the ribosomes, tRNAs, and translation factors. They then added a synthetic mRNA consisting solely of uracil residues (poly-U) along with 20 amino acids, one of which was radiolabeled in each reaction. The only reaction that produced a polypeptide was the one containing radiolabeled phenylalanine, yielding polyphenylalanine. This demonstrated that the codon UUU specifies phenylalanine. The experiment established that the genetic code could be deciphered using synthetic RNAs, and it provided the first direct evidence that RNA sequence determines protein sequence.

Khorana's Repetitive RNA Polymers

H. Gobind Khorana extended Nirenberg's approach by synthesizing repetitive RNA polymers of known sequence. For example, the polymer (UC)ₙ, consisting of alternating uracil and cytosine, produces a reading frame of UCU CUC UCU CUC... which encodes a polypeptide with alternating serine and leucine. By analyzing the amino acid composition of the resulting peptides, Khorana could deduce the codons for specific amino acids. Similarly, (UUC)ₙ produces a repeating sequence of UUC UUC UUC..., encoding a polypeptide of phenylalanine, serine, and leucine in a repeating pattern. These experiments, combined with Nirenberg's ribosome binding assays, allowed the assignment of all 64 codons by 1966. Khorana also synthesized the first complete gene in vitro, a yeast tRNA gene, demonstrating the feasibility of chemical gene synthesis.

The Wobble Hypothesis

The wobble hypothesis, proposed by Francis Crick in 1966, explained the observed degeneracy of the genetic code and the fact that fewer than 61 tRNAs are needed to decode all sense codons. Crick noted that the first two bases of the codon form standard base pairs with the anticodon, while the third base can form non-standard "wobble" pairs. He proposed that the 5′ base of the anticodon (position 34) can pair with multiple bases at the 3′ position of the codon. Specifically, inosine (I), a deaminated derivative of adenosine, can pair with U, C, or A. The rules are: U at the wobble position pairs with A or G; G pairs with U or C; I pairs with U, C, or A. This hypothesis was confirmed experimentally when the sequences of tRNA genes revealed the presence of inosine at position 34 in many tRNAs. The wobble hypothesis elegantly explains how a minimal set of tRNAs can decode all 61 sense codons while maintaining the specificity of the first two codon positions.

Methods to Study Translation and the Genetic Code

Ribosome Profiling (Ribo-seq)

Ribosome profiling, or Ribo-seq, is a high-throughput technique that provides a genome-wide snapshot of translation. The method involves treating cells with a translation inhibitor such as cycloheximide, which freezes ribosomes on mRNA. The mRNA is then digested with nuclease, leaving ribosome-protected fragments (RPFs) of approximately 28–30 nucleotides. These fragments are purified, converted to cDNA, and sequenced. The resulting reads map to the transcriptome, revealing the positions of ribosomes at codon resolution. Ribo-seq data can identify translated open reading frames, quantify translation efficiency, and detect ribosome pausing at specific codons. This technique has revealed widespread translation of upstream open reading frames (uORFs) and non-canonical translation events, expanding our understanding of the translatome.

Reporter Gene Assays

Reporter gene assays are used to measure translation efficiency of specific mRNA sequences. A reporter gene, such as firefly luciferase or green fluorescent protein (GFP), is fused to a sequence of interest. The reporter's activity or fluorescence is proportional to the amount of protein produced, providing a quantitative readout of translation. By mutating codons in the reporter mRNA and measuring changes in expression, researchers can assess the effects of codon usage, mRNA secondary structure, and upstream regulatory elements on translation. Dual-luciferase assays, which use both firefly and Renilla luciferases, are commonly used to normalize for transfection efficiency and cell viability. For example, replacing rare codons with frequent codons in the coding sequence of a recombinant protein can increase expression levels by 2- to 5-fold in E. coli.

Site-Directed Mutagenesis of Codons

Site-directed mutagenesis allows precise alteration of specific codons to study their function. Using PCR-based methods such as overlap extension or the QuikChange protocol, researchers introduce point mutations, insertions, or deletions into a cloned gene. The mutant gene is then expressed, and the protein product is analyzed for changes in expression level, folding, or activity. This approach has been used to define the minimal requirements for a start codon, to test the effects of synonymous codon changes on protein expression, and to create codon-optimized genes for heterologous expression. For instance, replacing the rare arginine codons AGA and AGG with the more common CGT and CGC in a gene expressed in E. coli can increase protein yield by preventing ribosome stalling and tRNA depletion.

Common Pitfalls and Misconceptions in Understanding the Genetic Code

Confusing Codons and Anticodons

A frequent error is confusing codons with anticodons. A codon is a triplet of nucleotides on mRNA that specifies an amino acid; it is read in the 5′ to 3′ direction. An anticodon is a triplet of nucleotides on tRNA that base-pairs with the codon; it is also written 5′ to 3′ but pairs antiparallel to the codon. For example, the codon 5′-AUG-3′ pairs with the anticodon 3′-UAC-5′, which is written as 5′-CAU-3′. Students often mistakenly write the anticodon as the reverse complement without accounting for antiparallel pairing. A useful check: the anticodon should be the reverse complement of the codon. For AUG, the complement is UAC, and reversed it is CAU, which is the correct 5′ to 3′ anticodon sequence.

Misunderstanding Degeneracy

Degeneracy is often misinterpreted as meaning the code is ambiguous or that a single codon can specify multiple amino acids. In fact, degeneracy means the opposite: multiple codons specify the same amino acid, but each codon specifies only one amino acid. The code is unambiguous—no codon codes for more than one amino acid. Degeneracy provides robustness against mutations, particularly at the third codon position. However, synonymous mutations are not always neutral: codon usage bias can affect translation speed, protein folding, and mRNA stability. For example, codons that match abundant tRNAs are translated faster than rare codons, and clusters of rare codons can cause ribosome stalling and protein misfolding.

Ignoring the Reading Frame

Another common error is overlooking the critical importance of the reading frame. Because the code is read in non-overlapping triplets, the same mRNA sequence can encode three different proteins depending on the starting point. A single nucleotide insertion or deletion shifts the reading frame, altering every subsequent codon and typically producing a premature stop codon. Students sometimes assume that a mutation in one codon only affects that amino acid, but a frameshift mutation affects the entire downstream sequence. When analyzing mutations, always determine whether the mutation is a substitution (affects one codon) or an insertion/deletion (affects the reading frame). For a deeper explanation of how reading frames are defined, see Read Genetic Code.

Summary and Practical Takeaways

Key Concepts to Remember

The genetic code is a triplet, non-overlapping, commaless code that is read from a fixed starting point. It consists of 64 codons: 61 sense codons specifying amino acids and 3 stop codons. The code is degenerate, with most amino acids encoded by multiple codons, and it is nearly universal. Translation requires three main components: ribosomes (rRNA and proteins), tRNAs (adapters with anticodons), and mRNA (the template). The process proceeds through initiation, elongation, and termination, each requiring specific protein factors and GTP hydrolysis. The wobble hypothesis explains how a reduced set of tRNAs can decode all codons through flexible base pairing at the third codon position. The reading frame is established at initiation and maintained by the ribosome's translocation machinery.

Study Strategies for Exams

To master the genetic code, practice translating mRNA sequences into protein sequences using the codon table. Work through examples that require you to identify the reading frame, determine the anticodon for a given codon, and predict the effects of mutations. Memorize the start codon (AUG) and the three stop codons (UAA, UAG, UGA). Understand the wobble rules and be able to predict how many tRNAs are needed to decode a given set of codons. When studying translation, draw the ribosome with its A, P, and E sites and trace the movement of tRNAs and mRNA through each elongation cycle. Compare prokaryotic and eukaryotic initiation, noting the key differences in ribosome binding and start codon recognition. Finally, be able to explain the experimental evidence that deciphered the code, including the poly-U experiment and Khorana's repetitive polymers. For a concise overview of the entire process, see RNA Translation and Translation a Level.

Frequently Asked Questions

What is the genetic code in translation?

The genetic code is the set of rules by which nucleotide triplets (codons) in mRNA specify amino acids during protein synthesis. It consists of 64 codons: 61 encode the 20 standard amino acids, and 3 are stop signals. The code is read in the 5′ to 3′ direction, is non-overlapping, and is degenerate, meaning most amino acids are specified by more than one codon.

How does the genetic code work during translation?

During translation, the ribosome reads the mRNA sequence codon by codon. Each codon base-pairs with the anticodon of a specific tRNA carrying the corresponding amino acid. The ribosome catalyzes peptide bond formation between successive amino acids, building a polypeptide chain in the N-terminal to C-terminal direction. The process continues until a stop codon is encountered, which triggers termination and release of the completed protein.

What is the role of the genetic code in translation?

The genetic code provides the mapping between nucleic acid sequence and protein sequence. It determines which amino acid is added for each codon, ensures the correct reading frame is maintained, and defines the start and stop signals for translation. Without the genetic code, the ribosome would have no way to interpret the mRNA sequence and synthesize a functional protein.

Why is the genetic code described as degenerate?

The genetic code is degenerate because most amino acids are encoded by more than one codon. For example, leucine has six codons, while serine has six as well. This redundancy arises because the third nucleotide of a codon can often vary without changing the amino acid. Degeneracy provides protection against mutations and allows organisms to optimize codon usage for efficient translation.

What are start and stop codons?

Start codons initiate translation. The primary start codon is AUG, which encodes methionine in eukaryotes and formylmethionine in bacteria. Stop codons terminate translation: UAA, UAG, and UGA do not encode amino acids but are recognized by release factors that trigger polypeptide release and ribosome dissociation.

What is the wobble hypothesis?

The wobble hypothesis, proposed by Francis Crick, explains how a single tRNA can recognize multiple codons. The base at the 5′ end of the anticodon (position 34) can form non-standard base pairs with the 3′ base of the codon (position 3). For example, inosine can pair with U, C, or A. This flexibility reduces the number of tRNAs needed to decode all codons.

How was the genetic code deciphered?

The genetic code was deciphered through experiments by Marshall Nirenberg, Heinrich Matthaei, and H. Gobind Khorana in the 1960s. Nirenberg used synthetic poly-U RNA to show that UUU encodes phenylalanine. Khorana used repetitive RNA polymers to deduce additional codons. The complete code was established by 1966 using ribosome binding assays with trinucleotide codons.

What is the difference between a codon and an anticodon?

A codon is a triplet of nucleotides on mRNA that specifies an amino acid, read in the 5′ to 3′ direction. An anticodon is a triplet of nucleotides on tRNA that base-pairs with the codon, also written 5′ to 3′ but pairing antiparallel. For the codon 5′-AUG-3′, the anticodon is 3′-UAC-5′, written as 5′-CAU-3′. The codon determines the amino acid; the anticodon ensures the correct tRNA delivers that amino acid.

Key Takeaways

The genetic code is a triplet, non-overlapping code: three nucleotides (a codon) specify one amino acid, and codons are read sequentially from a fixed start point.
There are 64 codons: 61 sense codons encoding amino acids and 3 stop codons (UAA, UAG, UGA); AUG is the primary start codon and encodes methionine.
The code is degenerate and unambiguous: multiple codons can specify the same amino acid, but each codon specifies only one amino acid.
Translation requires ribosomes (rRNA and proteins), tRNAs with anticodons, and mRNA as the template; the process has three stages: initiation, elongation, and termination.
The wobble hypothesis explains how flexible base pairing at the third codon position allows fewer tRNAs to decode all codons.
The reading frame is established at initiation and maintained by precise translocation; frameshift mutations alter every downstream codon and typically destroy protein function.
The genetic code was deciphered through Nirenberg's poly-U experiment and Khorana's repetitive RNA polymers, with the wobble hypothesis completing the theoretical framework.