Most of the vital activities of the cell are controlled by the two master molecules namely proteins and nucleic acids. Proteins are important both from the point of view of structure and function of the cell.
Structurally proteins constitute in integral part of membranes, pigments etc. Functionally in the form of enzymes, proteins play such a vital role in cell physiology, that without proteins (enzymes) no metabolic reaction is possible.
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Even the synthesis of DNA is regulated by proteins. In the form of Chromoproteins (Chlorophyll, Cytochromes etc), proteins mediate all energy transformation reaction.
It is because proteins are so important that the nucleic acid directly regulates the protein synthesis.
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Protein structure is highly complex (as has been pointed out earlier) as well as specific. The primary structure is determined by the quality, quantity and the sequence of amino acids.
Even an alteration of the position of one amino acid would alter the structure of the protein, and if it happens to be an enzyme regulating an important reaction, the entire reaction is disturbed leading to several abnormalities.
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Hence it is but natural that there is direct control of DNA in the synthesis of proteins. DNA, RNA and ribosomes of the cell are involved in the synthesis of proteins.
The central dogma:
The basic mechanism of protein synthesis is that the nuclear DNA regulates the assembly of amino acids in the ribosomes to produce a polypeptide chain.
Since DNA cannot move out of the nucleus it sends a message – mRNA which carries the message in the form of sequence of nucleotides.
This mRNA forms a template on the ribosomes. Another type of RNA, tRNA found in cytoplasm (whose nucleotide sequence is complementary to that of mRNA), picks up the amino acids and assembles them on the ribosomes to produce a peptide chain. This may be briefly expressed as follows:
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DNA Replication —> DNA Transcription —» RNA Translation —» Proteins translation Transcription is the copying of a complementary messenger RNA strand on a DNA strand.
In translation, the genetic information present in mRNA (in the form of sequence of nucleotides) directs the sequence of amino acids to be assembled. We shall study these events in detail.
Transcription:
Copying of a complementary strand of mRNA requires the following – a template, activated precursors, a divalent metal ion and RNA polymerase.
DNA molecules serve as a template for the production of mRNA. Of the two strands of DNA only one strand called the ‘sense’ or the ‘coding’ strand transcribes mRNA. Single stranded DNA also can serve as a template.
The activated precursors required for mRNA synthesis are – Adenine triphosphate, Guanine triphosphate, Uracil triphosphate and Cytosine triphosphate (ATP, GTP, UTP and CTP) Mg++ or Mn are the divalent metal ions required for mRNA synthesis. The enzyme RNA polymerase is necessary for transcription. The nucleotides of mRNA are assembled together with the help of RNA polymerase.
The enzyme consists of two parts – a core enzyme and a sigma factor. The sigma factor initiates transcription of mRNA on the DNA template and the core enzyme continues transcription. The DNA base sequence is thus transcribed into mRNA base sequence.