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PROTEIN SYNTHESIS APPARATUS

For protein synthesis tRNA, mRNA, ribosomes, certain enzymes and proteins are required.

1.tRNA: Amino acids themselves cannot directly recognize the base sequences (codons) present in mRNA, but the anticodon of tRNA (that is complementary to codon in mRNA) helps in the recognition of the codons in mRNA. Thus, a tRNA (transfer RNA) carries a
specific amino acid to its corresponding codon. The nucleotide sequence of tRNA for
the first time was given by Robert Holley (1964) for alanyl-tRNA (tRNNla) of yeast
and the gene for this tRNAala was artificially synthesized by Hargobind Khorana.
Several models of tRNA have been proposed and according to clover leaf model, the
single-stranded tRNA molecule is folded like a clover leaf having three double helical
stems (secondary structure due to intrastrand base pairing as A = U and G = C) each
terminating into a single stranded loop, All tRNA molecules have G (guanosine) at 5′ end
and for the attachment of specific amino acids CCA sequence at the 3′ end. The anticodon loop contains anticodon (sequence of 3 nucleotides) that recognizes its complementary codon (sequence of 3 nucleoli des) in mRNA. Not only the anticodon, but the 4 nucleotides present on the two sides of the anticodon and the base pairs of the attached stem are also important for the correct recognition of the respective codon, and this is known as extended anticodon hypothesis. The amino acyl tRNA synthetase binding loop recognizes the specific amino acyl tRNA synthetase that in turn recognizes the specific amino acid. The ribosomal binding loop binds with the ribosome during protein synthesis (Fig. 8.3 and 8.4).

Fig.8.3

Fig. 8.3 Three dimensional structure of tR.NA.

Fig.8.4

Fig. 8.4 Clover-leaf model of tRNA.

2.mRNA: After transcription, the information of protein present in DNA is transferred to mRNA (messenger RNA), which contains various codons (set of three nucleotides) for different amino acids constituting a particular protein. The initiation codon lies at (in prokaryotes and many mRNAs of eukaryotes) or near (many mRNAs of eukaryotes).

5′ end ofmRNA and termination codon at or near the 3′ end. These codons (except for the termination codons) are successively read by the anticodons present in their respective tRNAs (Fig. 8.5).

Fig.8.5 Fig. 8.5 mRNA with ribosomes and growing polypeptide chains.

3.Ribosomes: They are made up of rRNAs (ribosomal RNAs) and proteins and have about 23 nm average diameters. They have two subunits, one larger and the other smaller and at the time of translation process of protein synthesis, the two subunits bind with the mRNA The mRNA attached with a number of ribosomes that are involved in the synthesis of the same protein constitutes a polysome (polyribosome).

In prokaryotes, the ribosomes have 66% rRNAs and 34% proteins and are of 70S (S is the Swedberg unit indicating the sedimentation coefficient) type with 30S smaller (consisting of 16S rRNA and about 21 proteins) and 50S larger (consisting of 23S rRNA, 5S rRNA and about 31 proteins) subunits.

In eukaryotes, the ribosomes in the cytoplasm contain about 60% rRNA and 40% proteins and are of 80S type with smaller 40S (made up of 18S rRNA and about 33 proteins) and larger 60S (consisting of 28S rRNA, 5.8S rRNA, 5S rRNA and about 45 proteins) subunits.

These ribosomes are found either freely in the cytoplasm or attached with the endoplasmic reticulum (rough endoplasmic reticulum). In the organelles, mitochondria and chloroplasts (in photosynthetic organisms), the ribosomes are generally of 70S type, as in prokaryotes.