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| Translation-Synthesis of Proteins |
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| Translation is the mechanism by which the triplet base sequence of an mRNA guides the linking of a specific sequence of amino acids to form a polypeptide (protein) on ribosomes. All the proteins a cell needs are synthesized by the cell within itself. |
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| Protein synthesis requires amino acids, DNA, RNAs, ribosomes and enzymes, enzyme activators and ATP molecules. |
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| Proteins are the polymers of amino acids. Therefore, amino acids form the raw material for protein synthesis. The proteins found in living organisms need about 20 amino acids as building blocks or monomers. These are available in the cytoplasmic matrix as an amino acid pool. |
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| In order to maintain its own special characteristics a cell must manufacture proteins exactly similar to those already present in it. Thus, protein synthesis requires specificity control to provide instructions about the exact sequence in which the given numbers and kinds of amino acids should be linked to form the desired polypeptides. The specificity control is exercised by DNA through mRNA. Sequences of 3 consecutive nitrogenous bases in the DNA double helix form the biochemical or genetic code. Each base triplet codes for a specific amino acid. Since the DNA is more or less stable, the proteins formed in a cell are exactly like the preexisting proteins. |
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| RNA molecule is a long, unbranched, single-stranded polymer of ribonucleotides. Each nucleotide unit is composed of three smaller molecules, a phosphate group, a 5-carbon ribose sugar, and a nitrogen-containing base. The bases in RNA are adenine, guanine, uracil and cytosine. The various components are linked up as in DNA. There are three types of RNA in every cell: messenger RNA or mRNA, ribosomal RNA or rRNA and transfer RNA or tRNA. The three types of RNAs are transcribed from different regions of DNA template. RNA chain is complementary to the DNA strand, which produces it. All the three kinds of RNAs play a role in protein synthesis. |
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| Differences Between RNA Types |
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| mRNA |
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| The DNA, that controls protein synthesis, is located in the chromosomes within the nucleus, whereas the ribosomes, on which the protein synthesis actually occurs, are placed in the cytoplasm. Therefore, some sort of agency must exist to carry instructions from the DNA to the ribosomes. This agency does exist in the form of mRNA. The mRNA molecule carries the message (information) from DNA about the sequence of particular amino acids to be form a polypeptide, hence its name. It is also called informational RNA or template RNA. The mRNA forms about 5% of the total RNA of a cell. Its molecule is linear and the longest of all the three RNA types. Its length is related to the size of the polypeptide to be synthesized with its information. There is a specific mRNA for each polypeptide. Because of the variation is size in mRNA population in a cell; the mRNA is often called heterogeneous nuclear RNA, or hnRNA. |
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| It has at its 5 end a cap of methylated guanine followed successively by an initiation codon (AUG or GUG), a long coding region, a termination codon (UAA or UAG or UGA) and a poly-A tail of many adenine-containing nucleotides at 3 end. A small non-coding region may be present after the head and before tail. |
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| In eukaryotes, mRNA carries information for one polypeptide only. It is monocistronic (monogenic) because it is transcribed from a single cistron (gene) and has a single terminator codon. |
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| Bacterial mRNA often carries information for more than one polypeptide chain. Such an mRNA is said to be polycistronic (polygenic) because it is transcribed from many continuous (adjacent) genes. A polycistronic mRNA has an initiator codon and a terminator codon for each polypeptide to be formed by it. |
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| tRNA |
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| The tRNA carries a specific amino acid from the amino acid pool to the mRNA on the ribosomes to form a polypeptide, hence its name. The tRNAs form about 15% of the total RNA of a cell. Its molecule is the smallest and has the form of a cloverleaf. It has four regions: |
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 Carrier End: This is the 3 end of the molecule. Here a specific amino acid becomes attached. The tRNA molecule has a base triplet CCA with OH group at the tip. The COOH of amino acid joins the OH group. |
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 Recognition End: It is the opposite end of the molecule. It has 3 unpaired ribonucleotides. The bases of these ribonucleotides are complementary bases of the triplet found on mRNA chain called a codon. This triplet base sequence in tRNA is called as an anticodon. The anticodon binds with the codon at the time of translation. |
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 Enzyme Site: It is on one lateral side of the molecule. It is meant for a specific charging enzyme which catalyses the binding of a specific amino acid to tRNA molecule. |
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 Ribosome Site: It is on the other lateral side of the molecule. It is meant for attachment to a ribosome. |
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| fig. 22.3 A tRNA Molecule |
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| rRNA |
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| The rRNA molecule is greatly coiled. In combination with proteins, it forms the small and large subunits of the ribosomes, hence its name. It forms about 80% of the total RNA of a cell. A eukaryotic ribosome is 80S; with a large 60S subunit consists of 28S, 5.8S and 5S rRNAs and over 45 different basic proteins, the smaller 40S subunit comprises 18S RNA and about 33 different basic proteins. A prokaryotic ribosome is 70S; its large 50S subunit consists of 23S and 5S rRNAs and about 34 different basic proteins; its small 30S subunit comprises 16S rRNA and about 21 different basic proteins. The 3 end of 18S rRNA (16S rRNA in prokaryotes) has a binding site for the mRNA cap. The 5S rRNA has a binding site for tRNA. The rRNA also seems to play some general role in protein synthesis. It is involved in assembling the amino acid molecules brought by tRNA, into a polypeptide chain. |
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| There are two more types of RNA, recognised in the cell namely, |
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 Small nuclear RNA (snRNA) that helps in processing of rRNA and mRNA; and |
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 Small cytoplasmic RNA (scRNA) which helps in binding the ribosome to ER. |
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| Ribosomes are tiny ribonucleoprotein particles without a covering membrane. They serve as the site for protein synthesis. Hence, they are called protein factories of the cell. Each ribosome consists of larger and smaller subunits. The subunits of ribosome occur separately when ribosomes are not involved in protein synthesis. The two subunits join when protein synthesis starts, and undergo dissociation (separate) when protein synthesis stops. Many ribosomes line up on the mRNA chain during protein synthesis. Such a group of active ribosomes is called a polyribosome, or a polysome. In a polysome, the adjacent ribosomes are about 340 Ao apart. The number of ribosomes in a polysome is related to the length of the mRNA molecule, which reflects the length of the polypeptides to be synthesized. It is now known that polypeptide synthesis occurs at the polysomes and not at the single free ribosomes, in both prokaryotes and eukaryotes. |
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| fig. 22.4 Ultrastructure of a Ribosome |
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| A ribosome has two binding sites for tRNA molecules. One is called A site (acceptor or aminoacyl) and the other is termed P site (peptidyl). These sites span across the larger and smaller subunits of the ribosome. The A site receives the tRNA amino acid complex. The tRNA leaves from P site, after releasing its amino acid. However, the first tRNAamino acid complex directly enters the P site of the ribosome. |
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| A eukaryotic ribosome has a groove at the junction of the two subunits. From this groove, a tunnel extends through the large subunit and opens into a canal of the endoplasmic reticulum. The polypeptides are synthesized in the groove between the two ribosomal subunits and pass through the tunnel of the large subunit into the endoplasmic reticulum. While in the groove, the developing polypeptide is protected from the cellular enzymes. The smaller subunit forms a cap over the larger subunit. The larger subunit attaches to the endoplasmic reticulum by two glycoproteins named ribophorin I and II. |
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| The function of the ribosome is to hold the mRNA, tRNA and the associated enzymes controlling the process in position, until a peptide bond is formed between the adjacent amino acids. |
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