Translation: mRNA to Protein

Overview of Translation

Translation is the process by which messenger RNA (mRNA) is decoded by ribosomes to produce a specific polypeptide (protein). This process is fundamental to gene expression and occurs in the cytoplasm of cells. - Key Point 1: Translation converts the nucleotide sequence of mRNA into the amino acid sequence of a protein. - Key Point 2: The process follows the central dogma: DNA → RNA → Protein.

Genetic Code and Codons

The genetic code consists of triplet codons, each made up of three nucleotides in mRNA. Each codon specifies an amino acid or a stop signal. - Key Point 1: There are 64 possible codons (43), but only 20 amino acids, resulting in redundancy. - Key Point 2: The start codon (AUG) codes for methionine and signals the beginning of translation. - Key Point 3: Stop codons (UAA, UAG, UGA) signal the end of translation.

Example: Decoding a DNA Sequence

- The non-template DNA strand: 5' ATG GCC AAT GAC TTT CAA TAA 3' - Template strand: 3' TAC CGG TTA CTG AAA GTT ATT 5' - Transcribed mRNA: 5' AUG GCC AAU GAC UUU CAA UAA 3' - Translated protein: Met (start), Ala, Asn, Asp, Phe, Gln, (stop)

Components Required for Translation

Several molecular components are essential for translation: - Messenger RNA (mRNA): Serves as the template for protein synthesis. - Transfer RNA (tRNA): Brings amino acids to the ribosome and matches them to the mRNA codon via its anticodon. - Ribosome: Composed of ribosomal RNA (rRNA) and proteins; catalyzes peptide bond formation. - Aminoacyl tRNA synthetases: Enzymes that charge tRNAs with their specific amino acids. - Initiation, elongation, and release factors: Proteins that facilitate the steps of translation.

Structure and Function of tRNA

Transfer RNA (tRNA) is a small RNA molecule that carries a specific amino acid and recognizes the corresponding mRNA codon through its anticodon. - Key Point 1: The anticodon is antiparallel and complementary to the mRNA codon. - Key Point 2: tRNA is charged with an amino acid by aminoacyl tRNA synthetase.

Steps of Translation

Initiation

Translation begins when the small ribosomal subunit, initiation factors, and tRNA carrying methionine (tRNAMet) assemble at the mRNA's 5' cap and scan for the start codon (AUG). - Key Point: The large ribosomal subunit joins, and the tRNA is positioned in the P site.

Elongation

During elongation, the ribosome moves along the mRNA, matching codons with tRNA anticodons and catalyzing peptide bond formation. - Key Point 1: The ribosome has three sites: E (exit), P (peptidyl), and A (aminoacyl). - Key Point 2: Peptide bonds are formed between amino acids, and the polypeptide grows in the N → C direction.

Termination

Translation ends when a stop codon enters the A site. A release factor binds, causing the polypeptide to be released from the tRNA and the ribosome to dissociate.

Polyribosomes and Protein Synthesis Locations

Multiple ribosomes can translate a single mRNA simultaneously, forming a polyribosome (polysome). Proteins are synthesized either in the cytoplasm or on the rough endoplasmic reticulum (RER), depending on their destination. - Key Point 1: Cytoplasmic ribosomes synthesize proteins for use within the cell. - Key Point 2: RER-bound ribosomes synthesize proteins for secretion or membrane insertion.

Transmembrane and Secretory Protein Synthesis

Proteins destined for membranes or secretion are synthesized on the RER and processed through the secretory pathway (ER → Golgi → destination). - Key Point: Transmembrane domains are typically hydrophobic and embed into the lipid bilayer.

Summary Table: Codons and Amino Acids

Key Definitions

- Codon: A sequence of three mRNA nucleotides that codes for an amino acid or stop signal. - Anticodon: A sequence of three nucleotides in tRNA that is complementary to the mRNA codon. - Polyribosome: A group of ribosomes simultaneously translating a single mRNA. - Aminoacyl tRNA synthetase: An enzyme that attaches the correct amino acid to its tRNA.

Important Equations

- Number of possible codons: - Peptide bond formation:

Practice Questions

1. Each of the 20 common amino acids is specified by: C. one or more codons 2. The tRNA anticodon: D. all of the above (antiparallel, hydrogen bonds, base pairs with codon) 3. The mRNA is: D. A and B are true (read and made in 5' to 3' direction) 4. If translation begins with the first start codon in the mRNA below, the second codon would be: A. 5'-CAC-3' 5. Which amino acid will be attached to this tRNA (anticodon 3' CGU 5', codon 5' GCA 3'): Alanine

Summary

Translation decodes mRNA's triplet code (codons) to amino acids, producing proteins essential for cellular function. Nucleic acid → Protein