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8.8 The Central Theme of Protein Synthesis
A protein is a long chain polypeptide. The chain molecule
is formed by joining amino-acid molecules with peptide linkages. The sequence
of amino acids in each type of protein is highly specific. The functional
proteins form enzymes and hormones and control all metabolic and biochemical
reactions and processes in cells. Any alteration in the sequence of amino
acids in protein molecules can affect the function of that protien. Hence,
bio-synthesis of proteins is a highly specific process.
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The main principle of protein synthesis. According to Crick (1958), DNA determines the sequence of amino acids in a polypeptide (protein) through mRNA. This is the main principle (central dogma) of protein synthesis. This involves transcription and translation.
Components involved in protein synthesis : The process requires
The 20 types of amino acids that are specified by the genetic code.
DNA (to provide the base sequence).
The non-genetic RNAs (m-RNA, t-RNA and r-RNA).
Ribosomes (site of protein synthesis) and
Various enzymes (factors).
Mechanism of protein synthesis
As stated earlier, the mechanism of protein synthesis involves two main events namely (A) Transcription (B) Translation.
Transcription is the process of formation of mRNA on the DNA strand. It takes place in the presence of enzyme RNA Polymerase.
One of the strands of DNA acts as a template for the formation of the same m-RNA in a series, one by one. Such ribosomes bound to the same mRNA strand are called polyribosomes or polysomes. The polypeptide chain formation in each ribosome is independent and the same as described above.
In Translation one particular group gets transmitted from one Band of DNA to another Band of DNA. In this procedure the DNA polymer (enzyme) is used. Normally, 40% of protein synthesis occurs in this manner.
SUMMARY : MOLECULAR GENETICS AND GENE EXPRESSION
(1) DNA is the genetic material in almost all organisms. (2) Chromosomes carry genes. (3) The chromatin consists of DNA and two types of proteins (histones and non-histones). (4) During prophase of the cell division, DNA of the chromatin undergoes coiling, super coiling and folding and along with the associated proteins, forms highly a condensed and prominent chromosome (as seen during metaphase). (5) This is explained with the help of necleosome solenoid model (Komberg and Thomas, 1974). (6) DNA molecule has a double stranded, helical structure in which the two antiparallel polynucleotide strands are helically coiled round each other, and joined by hydrogen bonds (Watson and Crick). (7) DNA has the ability of self-replication. (8) RNA molecule is single stranded. It can not replicate. It is produced on DNA strand. (9) Non-genetic RNA is of 3 types and helps in protein synthesis. (10) In prokaryotes, the single circular chromosome has a circular DNA capable of replication. (11) Prokaryotes also possess plasmids and episomes. These are the extra chromosomal circular DNA which carry genes and are capable of self-replication. (12) Genetic information required to produce a protein is called the genetic code. It is present on the DNA in the form of the base sequence. It is transcribed on m-RNA and is then used for synthesis of protein in the ribosomes. (13) Genetic code is triplet. (14) Each triplet or codon on m-RNA specifies one amino acid. (15) In all, there are 64 codons in the genetic code of which 61 codons specify 20 different amino acids. (16) Three codons (UAA, UAG and UGA) do not specify any acid (stop codons). (17) AUG is the starting or initiation codon. (18) In protein synthesis, m-RNA provides the sequence of the arrangement of amino acids in form of the codon sequence. These are carried by the t-RNA and are arranged in the sequence with the help of matching anticodons on t-RNA. (19) After formation, the peptide chain is released with the help of release factors.
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