Blasticidin is an efficient selective antibiotic that acts on both eukaryotic and prokaryotic cells. Blasticidin is a peptidyl nucleoside antibiotic isolated from Streptomyces griseochromogenes that inhibits protein synthesis by interfering with the peptide-bond formation in the ribosomal machinery.
Fig 8.14 Initiation of translation in eukaryotes.
4. The ribosomes have two sites, the A-site (amino acyl site) for the location of new amino acyl-tRNA (except for the fmet-tRNAfmet) and a P-site (peptidyl site) for locating the tRNA attached with newly synthesizing amino acid chain (di and polypeptide). The fmet-tRNAfmet after binding withIF2, is located on the initiation codon AUG at the P-site of the 30S subunit of ribosome which requires energy in the form of GTP. The role of IF1 is not
known. (In eukaryotes more initiation factors i.e., eIF1 to e1F6, are involved in nitiation).
This lecture introduces the cell as the unit of life. Firstly, by the methods we use to see cells and biological structures and what we consider to be "alive". Then by looking at major differences between cell types and their organisation as unicellular or multicellular organisms. Finally, the presence or absence of a nucleus which is the definition of the major 2 classes of cells.
11. The newly synthesized polypeptide is modified by the removal of formyl group of the first amino acid, formyl-methionine. Moreover, a few amino acids from the N-terminal (-NH2 terminal) or from the C-terminal (- COOH terminal) or from both the termini may be removed, The primary structure of the polypeptide gets folded variously to produce three-dimensional secondary and tertiary structures.
9. As the ribosome moves by two codons, next round of protein synthesis is initiated by the attachment of a new ribosome. Thus, at a time, a single mRNA is found to be attached with many ribosomes with their polypeptides of different length, (shortest polypeptide at the 5′ end of the mRNA and longest at the 3′ end), called polysomes.
Fig. 8.15 Peptide bond formation in growing polypeptide.
10. Ultimately, the A-site of ribosome is occupied by the termination codon (UAA,UAG or UGA) at the 3′ end of mRNA, which is not recognized by any tRNA. Thus, the termination of the protein synthesis is helped by the release factors RFl, RF2 and RF3 (in eukaryotes eRF1), which release the newly synthesized polypeptide chain from the P-site (Fig. 8.16).
The process of synthesis of proteins from mRNA (translation of language of nucleic acids into the language of proteins) is called translation. There are 20 different types of amino acids, which constitute various proteins, and these amino acids themselves cannot recognize their respective codons in the mRNA. Different amino acids are carried by their specific tRNA molecules at the
site of protein synthesis (mRNA). There are about 55 types of tRNA molecules available in the cytoplasm, so that one amino acid may have more than one tRNAs.
Fundamentally, most eucaryotes produce energy (ATP) through alcoholfermentation (e.g. yeast), lactic acid fermentation (e.g. muscle cells,neutrophils), aerobic respiration (e.g. molds, protozoa, animals) oroxygenicphotosynthesis (e.g. algae, plants). These modes of energy-generatingmetabolismexist among procaryotes, in addition to all the following types ofenergyproductionwhich are virtually non existent in eucaryotes.
i.RNA polymerase I (or A) found in nucleolus for the synthesis of rRNAs (28S and 18S).
ii.RNA polymerase II (or B) found in nucleoplasm for the synthesis of HnRNA (heterogeneous nuclear RNA) which, after processing gives rise to mRNA.
iii.RNA polymerase III (C) found in nucleoplasm for the synthesis of tRNA and 5S rRNA.
iv.Organeller RNA polymerase found in organelles, mitochondria and chloroplasts, (in photosynthetic organisms) and resembles the prokaryotic RNA polymerase.
The process of synthesis of RNAs (mRNA, tRNA and rRNA) from DNA by the enzyme RNA polymerase is known as transcription. At the time of transcription, the RNA polymerase binds with double stranded DNA (gene) at a particular site (in prokaryotes known as promoter site) and after unwinding of the two strands of DNA by the rotation of the DNA, it starts copying one of
the two strands, known as coding strand (sense strand or template strand). The other strand of the DNA, which is not copied for the RNA synthesis, is known as non-coding strand (antisense strand) (Fig. 8.6).
i.ρ-dependent Termination: In some prokaryotes, the termination of transcription is helped by a ρ (rho) protein that gets attached at the 5′ end of the newly synthesizing mRNA. The ρ then moves along the mRNA and induces the formation of hairpin loop near the 3′ end of mRNA due to the presence of inverted repeated sequences. This helps in the detachment of mRNA from the DNA (Fig. 8.9).