Ch 9 art 2 for exam 2

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Ch. 9 – The Molecular Biology of Translation

Introduction to Translation

Translation is the process by which the genetic information encoded in messenger RNA (mRNA) is used to assemble a specific sequence of amino acids, forming a polypeptide (protein). This process is fundamental to gene expression and is highly conserved across all domains of life.

The Relationship Between Genes and Proteins

Gene and Protein Sequence Correspondence

Genes are composed of DNA sequences that are transcribed into mRNA.
The coding strand of DNA has the same sequence as the mRNA (except T is replaced by U in RNA).
The template strand is used by RNA polymerase to synthesize the complementary mRNA.
Each group of three nucleotides (codon) in mRNA specifies one amino acid in the protein.

Example: For the DNA coding sequence ATG GGG CCC AAT GAA, the mRNA is AUG GGG CCC AAU GAA, which translates to the protein sequence: Met-Gly-Pro-Asn-Glu.

The Genetic Code

Features of the Genetic Code

The genetic code is universal with few exceptions, meaning it is used by almost all organisms.
It is composed of triplet codons, each specifying one amino acid.
The code is degenerate: multiple codons can code for the same amino acid.
There are start (AUG) and stop (UAA, UAG, UGA) codons that signal initiation and termination of translation.

The genetic code table

Organization of the Genetic Code

The code can be visualized as a table with the first, second, and third bases of the codon determining the amino acid.
Codons are read in the 5' to 3' direction on the mRNA.

Organization of the genetic code

Chemical Properties of Amino Acids

Amino acids can be classified as nonpolar, polar uncharged, polar positively charged, or polar negatively charged.
The chemical properties of amino acids influence protein structure and function.

The 20 common amino acids

tRNA and Aminoacyl-tRNA Synthetases

tRNA Structure and Function

Transfer RNA (tRNA) molecules serve as adaptors that match amino acids to their corresponding codons in mRNA.
Each tRNA has an anticodon that base-pairs with a codon on the mRNA and an acceptor stem that attaches to a specific amino acid.

tRNA and aminoacyl-tRNA synthetase structure

Aminoacyl-tRNA Synthetases

These enzymes attach the correct amino acid to its corresponding tRNA, a process called charging the tRNA.
Proofreading by these enzymes ensures high fidelity in protein synthesis.

Aminoacyl-tRNA synthetases attach amino acids to tRNAs

Codon-Anticodon Interaction and the Wobble Hypothesis

Base Pairing Rules

Codon-anticodon pairing follows standard Watson-Crick base pairing (A-U, G-C) but is antiparallel (mRNA 5'→3', tRNA 3'→5').
The third position of the codon (the "wobble" position) allows for non-standard base pairing, increasing the efficiency of translation.

Wobble base pairing allows a tRNA to recognize two or more codons

The Wobble Position

Inosine (I) in the tRNA anticodon can pair with U, C, or A in the mRNA codon.
This flexibility allows one tRNA to recognize multiple codons for the same amino acid.

Wobble position in tRNA and codon recognition

Wobble Base-Pairing Rules Table

3′ Nucleotide of Codon	5′ Nucleotide of Anticodon
A or G	U
G	C
U	A
U or C	G
U, C, or A	I

Examples of Wobble Pairing

Some tRNAs can pair with more than one codon due to wobble at the third position.

Wobble position: examples

Post-Translational Modifications

Types and Importance

After translation, proteins often undergo post-translational modifications such as phosphorylation, methylation, and adenylation.
These modifications are critical for protein function, localization, and regulation.
Proteins may also be cleaved to become active or to remove targeting sequences.

Cleavage of N-terminal amino acids Chemical modification of internal amino acids Polypeptide cleavage and insulin maturation

Translation in Eukaryotes: Protein Targeting and Processing

Targeting to the Rough Endoplasmic Reticulum (ER)

Proteins destined for secretion or for certain organelles are targeted to the rough ER by a signal sequence.
During translation, the ribosome-mRNA complex binds to the ER, and the growing polypeptide is translocated into the ER lumen.
Proteins may then be transported to the Golgi apparatus for further modification and sorting.

Translation in eukaryotes: targeting protein to the Rough ER

Summary Table: Key Concepts in Translation

Concept	Description
Genetic Code	Triplet codons in mRNA specify amino acids
tRNA	Adaptor molecule with anticodon and amino acid attachment site
Aminoacyl-tRNA Synthetase	Enzyme that charges tRNA with correct amino acid
Wobble Hypothesis	Flexibility in third codon position allows one tRNA to recognize multiple codons
Post-Translational Modification	Chemical changes to proteins after synthesis, essential for function
Protein Targeting	Signal sequences direct proteins to correct cellular locations

Review and Concept Integration

Translation is tightly linked to transcription and DNA replication in the central dogma of molecular biology.
Understanding the genetic code and translation mechanisms is essential for genetic engineering, biotechnology, and medicine.