Translation

Translation is the next step in using information from the DNA gene to produce a functional protein. This process changes the genetic information from a nucleic-acid-based language into an amino-acid-based language of polypep-tides and proteins. For these reasons, the term "translation" describes this complex cascade of events.

Following transportation of the mRNA into the cytoplasm, translation begins with the mRNA binding to a ribo-some and requires additional nucleic acids, specifically protein-associated RNA molecules (Figure 1-6). A ribo-some is a complex of about 50 different proteins associated

Figure 1-6. RNA translation. RNA is translated through binding events between the mRNA, a ribosome, tRNA, and amino acids, resulting in the production of a protein polypeptide chain. (Reprinted from Leonard D. Diagnostic Molecular Pathology, copyright 2003, with permission from Elsevier.)

Table 1-6. The Human Genetic Code

S AS

SECOND BASE OF CODON

UUU

UCU

UAU

UGU

Phenylalanine (Phe/F)

Serine (Ser/S)

Tyrosine (Tyr/Y)

Cysteine (Cys/C)

UUA

UCA

UAA

UGA

Phenylalanine (Phe/F)

Serine (Ser/S)

Tyrosine (Tyr/Y)

Cysteine (Cys/C)

UUC

UCC

UAC

UGC

Leucine (Leu/L)

Serine (Ser/S)

STOP

STOP

UUG

UCG

UAG

UGG

Leucine (Leu/L)

Serine (Ser/S)

STOP

Tryptophan (Trp/W)

CUU

CCU

CAU

CGU

Leucine (Leu/L)

Proline (Pro/P)

Histidine (His/H)

Arginine (Arg/R)

CUA

CCA

CAA

CGA

Leucine (Leu/L)

Proline (Pro/P)

Histidine (His/H)

Arginine (Arg/R)

CUC

CCC

CAC

CGC

Leucine (Leu/L)

Proline (Pro/P)

Glutamine (Gln/Q)

Arginine (Arg/R)

CUG

CCG

CAG

CGG

Leucine (Leu/L)

Proline (Pro/P)

Glutamine (Gln/Q)

Arginine (Arg/R)

AUU

ACU

AAU

AGU

Isoleucine (Ile/I)

Threonine (Thr/T)

Asparagine (Asn/N)

Serine (Ser/S)

AUA

ACA

AAA

AGA

Isoleucine (Ile/I)

Threonine (Thr/T)

Asparagine (Asn/N)

Serine(Ser/S)

AUC

ACC

AAC

AGC

Isoleucine (Ile/I)

Threonine (Thr/T)

Lysine (Lys/K)

Arginine (Arg/R)

AUG START

ACG

AAG

AGG

Methionine (Met/M)

Threonine (Thr/T)

Lysine (Lys/K)

Arginine (Arg/R)

GUU

GCU

GAU

GGU

Valine (Val/V)

Alanine (Ala/A)

Aspartic Acid (Asp/D)

Glycine (Gly/G)

GUA

GCA

GAA

GGA

Valine (Val/V)

Alanine (Ala/A)

Aspartic Acid (Asp/D)

Glycine (Gly/G)

GUC

GCC

GAC

GGC

Valine (Val/V)

Alanine (Ala/A)

Glutamic Acid (Glu/E)

Glycine (Gly/G)

GUG

GCG

GAG

GGG

Valine (Val/V)

Alanine (Ala/A)

Glutamic Acid (Glu/E)

Glycine (Gly/G)

with several ribosomal RNA (rRNA) molecules. Prokary-otic ribosomes consist of 30S and 50S subunits. Svedberg (S) units are the sedimentation rate of a particle. In eukary-otes, rRNA molecules associate with proteins in the nucle-olus to form 40S and 60S subunits. Recognition of the 5' cap of the eukaryotic mRNA by a ribosome initiates the process of translation.33

Each amino acid is encoded by one or more 3-nucleotide sequences, which are collectively known as the genetic code (Table 1-6). Each set of 3 nucleotides of an mRNA that encodes an amino acid is called a codon. As is seen in Table 1-6, the first and second nucleotide positions largely determine which amino acid is encoded by the mRNA codon, while the third base has less effect on which amino acid will be incorporated. In addition to encoding amino acids,certain mRNA codons are used to initiate (START) or terminate (STOP) translation. The genetic code differs slightly between organisms and between mitochon-drial DNA and eukaryotic DNA (Table 1-7). Thus, while one mRNA encodes only one protein sequence, a protein sequence can be encoded by several different mRNA sequences. This is referred to as the degeneracy of the genetic code.

Synthesis of the encoded protein begins at the initiation codon of the mRNA, the first AUG codon after the promoter and encodes a methionine amino acid. This methio-nine codon establishes the reading frame of the mRNA. The next step in the translation process uses RNA molecules to bridge the information from the sequential mRNA codons to the encoded amino acid in the growing polypeptide chain of the protein. Another set of RNA molecules, tRNA, contain a sequence complementary to each mRNA codon known as the anticodon. The 3' end of each type of tRNA binds the specific amino acid corresponding to its anti-codon sequence. Base pairing of codons with complementary anticodons permits sequential alignment of new amino acids of the polypeptide chain and occurs in the

Table 1-7. Exceptions to the Universal Code in Mammals

Codon

Nuclear Code

Mitochondrial Code

UGA

Stop

Trp

AUA

Ile

Met

AGA

Arg

Stop

AGG

Arg

Stop

small subunit of the ribosome. The large subunit of the ribosome catalyzes the covalent bonds linking each sequential amino acid to the growing polypeptide chain.

Translation ceases when the ribosome encounters a stop codon (UAA, UAG, or UGA). Release factors bound to the stop codon catalyze the addition of a water molecule rather than an amino acid, thus resulting in a COOH terminus to the completed polypeptide chain.34 Some factors bound to the 3' untranslated portion of the gene also affect termination.

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