Translation of the Genetic Message

Translation of mRNA

• mRNA contains the information for the primary structure of proteins encoded in the sequence of nucleotides
• Conversion of this information from the nucleotide sequence into an amino acid sequence of as functional protein is referred to as translation
• mRNA also contains a leader sequence, and a trailer sequence

The Genetic Code

• Since the nucleotide alphabet must convey the information for 20 amino acids, nucleotides must function in groups (codons)
• 4¹ = 4 possible combinations (not enough)
• 4² = 16 possible combinations (not enough)
• 4³= 64 possible combinations
• More possible codons than necessary

Deciphering the Code

• H.G. Khorana developed a procedure to synthesis of artificial RNAs
• A polymer of Uracil (poly U) was translated into a polymer Phenylalanines
• Poly A was translated into a polymer of lysine
• Four codons were found that did not encode an amino acid
• AUG serves as a start codon
• UAG,UGA and UAA serve as stop codons

Genetic Code

• The genetic code is:

1. Redundant

-More than one codon per amino acid

2. Non-overlapping

-Codons are distinct blocks of three nucleotides

3. Read in reading frame

-Changes in the nucleotide sequence change the amino acid sequence

4. Universal

Ribosomes

• The translation process occurs on ribosomes
• Ribosomes are composed of a large and small subunit
• Each is composed of rRNA, enzymes and structural proteins
• Large subunit in procaryotes has 2 rRNAs and 35 proteins, the small has 1 rRNA and about 20 proteins
• Eucaryotic ribosomes are somewhat larger

How Ribosomes Function

• In procaryotes the 5' end of the mRNA binds with the small subunit
• This involves a binding sequence (AGGAGGU) within the leader sequence
• This binding positions the initiation codon in the cleft on the ribosome
• The mRNA-small subunit complex then interact with the large subunit
• This forms an active ribosomal complex
• Translation can now begin

Segregation of the Transcriptional and Translational Processes

• Procaryote, which lack a nucleus, do not segregate transcription from translation
• The ribosome can bind to the 5' end of the mRNA and begin translation while the RNA polymerase is still transcribing the gene
• Eucaryotes there are four steps between transcription and translation
• The first three are modifications to the primary transcript (5'capping, 3'polyA tail addition and splicing out of the introns
• The last is passage from nucleus to cytoplasm

Transfer RNA (tRNA)

• tRNAs serve as adapters between the amino acids and the codon which encodes their incorporation
• At least one tRNA is specific for each amino acid
• Attachment of amino acids to the tRNA is conducted by specific enzymes
• Amino acyl synthetases

Structure of tRNAs

• All tRNAs have common structural features
• 73-93 nucleotides in length
• Folded into a cloverleaf shape as a primary strucutre by internal base pairing
• Forming 4 unpaired or loop regions
• Loop 2 is referred to as the anticodon (binding with the codon in the mRNA)
• Other loops serve in ribosome and amino acyl synthetase binding
• 3' end always contains a nucleotide sequence of CAA
• Amino acid attachment site

Translation Cycle

• The translation cycle is comprised of four steps

1. Amino acid activation (attachment of the amino acids to the proper tRNA)

2. Initiation of the translational complex

3. Elongation or extension of the growing polypeptide chain

4. Termination completion of the primary sequence of the amino acids encoded by the message

Sequence of events involved in the translation process

1. mRNA binds with small subunit of the ribosome

2. tRNA (tRNAmet) that is complimentary to the initiation codon (AUG) binds to the mRNA

3. Large ribosome subunit binds to complete the translational complex

4. Translational proceeds by the ribosome moving along the mRNA with portion of the mRNA being translated lying in a groove between the two subunits of the ribosome

• The ribosomal complex has two tRNA binding sites
• P site (peptidyl site )
• A site (amino acyl site)

5. As the cycle begins the tRNAmet occupies the P-site and the A-site is available for the binding of the tRNA with the correct anticodon

6.Once the proper tRNA occupies the A-site with its amino acid; the enzyme peptidyl transferase moves the amino acid from the P-site and bonds it to the amino acid in the A-site and the tRNA vacates the P-site

• Leaving the P-site unoccupied

7. The ribosome moves down one codon along the mRNA; moving the tRNA and its attached amino acid chain from the A-site into the P-site

8. This process repeats itself elongating the polypeptide chain

• The process can add about 15 amino acids per second, with an error rate of 1 mistake/30 chains

9. The translation cycle continues until a termaination codon (UGA,UAG, or UAA) occipies the A-site.

• There are no tRNA for these codons and the A-site is occupied by a release factor
• This causes the release of the polypeptide and dissolution of the ribosomal complex

• This page is maintained by James C. Pushnik: jpushnik@ecst.csuchcio.edu
• Last modified 11/13/96