Translation: Decoding mRNA into Protein

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Translation: mRNA to Protein

Overview of Translation

Translation is the process by which messenger RNA (mRNA) is decoded by ribosomes to synthesize a polypeptide (protein). This process occurs in the cytoplasm and is a fundamental step in gene expression, following transcription. The genetic code, composed of triplet codons, specifies the sequence of amino acids in the resulting protein.

Translation direction: mRNA is read from 5' to 3' by the ribosome.
Polypeptide direction: Proteins are synthesized from the N-terminus to the C-terminus.
Codons: Each codon consists of three nucleotides and codes for a specific amino acid or a stop signal.
Start codon: AUG (methionine) marks the beginning of translation.
Stop codons: UAA, UAG, and UGA signal termination.

Central dogma: DNA to RNA to Protein

The Genetic Code and Codon Tables

The genetic code is universal and redundant, meaning multiple codons can code for the same amino acid. Codon tables are used to determine which amino acid corresponds to each mRNA codon.

64 codons: 61 code for amino acids, 3 are stop codons.
Redundancy: Some amino acids are specified by more than one codon.
Example: CAU and CAC both code for histidine; CAA and CAG code for glutamine.

Explanation of codon chart usage Color-coded mRNA codon table

Transcription and Translation: From DNA to Protein

Transcription produces mRNA from a DNA template, which is then translated into protein. The non-template strand of DNA matches the mRNA sequence (except T is replaced by U), while the template strand is used for mRNA synthesis.

Transcription: DNA → mRNA
Translation: mRNA → Protein
Example: DNA sequence 5'-ATG GCC AAT GAC TTT CAA TAA-3' is transcribed to mRNA 5'-AUG GCC AAU GAC UUU CAA UAA-3' and translated to Met-Ala-Asn-Asp-Phe-Gln-(stop).

DNA to mRNA to protein translation example

Components Required for Translation

Several molecular components are essential for translation, including three main types of RNA and various protein factors.

mRNA: Carries the genetic code from DNA.
tRNA: Brings amino acids to the ribosome; each tRNA has an anticodon complementary to the mRNA codon.
rRNA: Structural and catalytic component of ribosomes.
Initiation, elongation, and release factors: Proteins that facilitate the steps of translation.
Aminoacyl-tRNA synthetases: Enzymes that charge tRNAs with their specific amino acids.

Three types of RNA and translation factors

Structure and Function of tRNA

Transfer RNA (tRNA) is a key adaptor molecule in translation. Each tRNA has a specific anticodon that pairs with the mRNA codon and an amino acid attachment site.

Anticodon: Three-nucleotide sequence complementary and antiparallel to the mRNA codon.
Amino acid attachment: Each tRNA is charged with a specific amino acid by its corresponding aminoacyl-tRNA synthetase.
Hydrogen bonding: Anticodon pairs with codon via hydrogen bonds.

tRNA structure and codon-anticodon pairing tRNA structure with labeled regions

Ribosome Structure and Sites

The ribosome is a complex of rRNA and proteins, consisting of a large and small subunit. It has three binding sites for tRNA: the A (aminoacyl), P (peptidyl), and E (exit) sites.

A site: Binds incoming charged tRNA.
P site: Holds tRNA with growing polypeptide chain.
E site: Releases uncharged tRNA.

Ribosome structure with A, P, E sites

Steps of Translation

Initiation

Translation initiation involves the assembly of the ribosome on the mRNA at the start codon (AUG).

Initiation complex: Includes initiation factors, small ribosomal subunit, and tRNAMet.
Start codon recognition: The complex scans for AUG, where the large subunit joins and the tRNAMet occupies the P site.

Translation initiation complex formation Translation initiation steps

Elongation

During elongation, the ribosome moves along the mRNA, adding amino acids to the growing polypeptide chain.

Codon recognition: tRNA anticodon pairs with mRNA codon in the A site.
Peptide bond formation: Catalyzed by the ribosome, transferring the polypeptide to the tRNA in the A site.
Translocation: Ribosome shifts, moving tRNAs through the E, P, and A sites.

Ribosome elongation cycle

Termination

Translation ends when a stop codon enters the A site. Release factors bind, causing the polypeptide to be released and the ribosome to dissociate.

Stop codons: UAA, UAG, UGA.
Release factor: Not a tRNA, but recognizes stop codon and triggers release.

Polyribosomes (Polysomes)

Multiple ribosomes can translate a single mRNA simultaneously, forming a polyribosome. This increases the efficiency of protein synthesis.

Polyribosome: Several ribosomes attached to one mRNA, each synthesizing a polypeptide.

Polyribosome structure and function

Summary Table: Codon to Amino Acid Mapping

The following table summarizes the relationship between mRNA codons and their corresponding amino acids:

First Base	Second Base	Third Base	Amino Acid
A	U	G	Methionine (Start)
U	A	A	Stop
C	A	U	Histidine
C	A	A	Glutamine
G	G	C	Glycine
U	U	U	Phenylalanine
A	A	G	Lysine
G	A	A	Glutamic acid

Color-coded mRNA codon table

Key Concepts and Exam Questions

Each amino acid is specified by: One or more codons.
tRNA anticodon: Is antiparallel and base pairs with the mRNA codon via hydrogen bonds.
mRNA: Is read and made in the 5' to 3' direction; translation starts at the 5' cap.
Translation direction: Always 5' to 3' along the mRNA.
Polypeptide synthesis: N-terminus to C-terminus.

Summary

Translation is the process of decoding mRNA's triplet code (codons) into amino acids, resulting in the synthesis of proteins. This process is central to gene expression and the flow of genetic information from nucleic acids to functional proteins.