cx-ncert-theory-class12-chap05-molecular-basis-of-inheritance-part-5-en

<h6 id="successmolecular-basis-of-inheritancesuccess"><Success>Molecular basis of inheritance</Success></h6>
<h6 id="primarygenetic-codeprimary"><Primary>Genetic code</Primary></h6>
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<p>During Replication and Transcription a nucleic acid was copied to form another nucleic acid. Hence, these processes are easy to conceptualise on the basis of complementarity. The process of Translation requires transfer of genetic information from a polymer of nucleotides to synthesise a polymer of amino acids.</p>
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<p>Neither does any complementarity exist between nucleotides and amino acids, nor could any be drawn theoretically.</p>
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<p>There existed ample evidences, though, to support the notion that change in nucleic acids (genetic material) were responsible for change in amino acids in proteins.</p>
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<footer style="font-size:18px">Introduction$\rightarrow$ The DNA$\rightarrow$ The Search for Genetic Material$\rightarrow$ RNA World$\rightarrow$ Replication$\rightarrow$ Transcription$\rightarrow$ <span style="color:blue">Genetic code</span>$\rightarrow$ Translation$\rightarrow$ Regulation of gene expression$\rightarrow$ Human genome project$\rightarrow$ DNA fingerprinting</footer>

<h6 id="successmolecular-basis-of-inheritancesuccess-1"><Success>Molecular basis of inheritance</Success></h6>
<h6 id="primarygenetic-codeprimary-1"><Primary>Genetic code</Primary></h6>
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<p>This led to the proposition of a Genetic code that could direct the sequence of amino acids during synthesis of proteins.If determining the biochemical nature of genetic material and the structure of DNA was very exciting, the proposition and deciphering of Genetic code were most challenging.</p>
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<li>In a very true sense, it required involvement of scientists from several disciplines – physicists, organic chemists, biochemists and geneticists.</li>
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<footer style="font-size:18px">Introduction$\rightarrow$ The DNA$\rightarrow$ The Search for Genetic Material$\rightarrow$ RNA World$\rightarrow$ Replication$\rightarrow$ Transcription$\rightarrow$ <span style="color:blue">Genetic code</span>$\rightarrow$ Translation$\rightarrow$ Regulation of gene expression$\rightarrow$ Human genome project$\rightarrow$ DNA fingerprinting</footer>

<h6 id="successmolecular-basis-of-inheritancesuccess-2"><Success>Molecular basis of inheritance</Success></h6>
<h6 id="primarygenetic-codeprimary-2"><Primary>Genetic code</Primary></h6>
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<p>It was George Gamow, a physicist, who argued that since there are only 4 bases and if they have to code for 20 amino acids, the code should constitute a combination of bases.</p>
<p>He suggested that in order to code for all the 20 amino acids, the code should be made up of three nucleotides. This was a very bold proposition, because a permutation combination of $4^3$ (4 × 4 × 4) would generate 64 codons; generating many more codons than required.</p>
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<footer style="font-size:18px">Introduction$\rightarrow$ The DNA$\rightarrow$ The Search for Genetic Material$\rightarrow$ RNA World$\rightarrow$ Replication$\rightarrow$ Transcription$\rightarrow$ <span style="color:blue">Genetic code</span>$\rightarrow$ Translation$\rightarrow$ Regulation of gene expression$\rightarrow$ Human genome project$\rightarrow$ DNA fingerprinting</footer>

<h6 id="successmolecular-basis-of-inheritancesuccess-3"><Success>Molecular basis of inheritance</Success></h6>
<h6 id="primarygenetic-codeprimary-3"><Primary>Genetic code</Primary></h6>
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<p>Providing proof that the codon was a triplet, was a more daunting task. The chemical method developed by Har Gobind Khorana was instrumental in synthesising RNA molecules with defined combinations of bases (homopolymers and copolymers). Marshall Nirenberg’s cell-free system for protein synthesis finally helped the code to be deciphered.</p>
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<li>Severo Ochoa enzyme (polynucleotide phosphorylase) was also helpful in polymerising RNA with defined sequences in a template independent manner (enzymatic synthesis of RNA). Finally a checker-board for Genetic code was prepared which is given in Table 5.1.</li>
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<footer style="font-size:18px">Introduction$\rightarrow$ The DNA$\rightarrow$ The Search for Genetic Material$\rightarrow$ RNA World$\rightarrow$ Replication$\rightarrow$ Transcription$\rightarrow$ <span style="color:blue">Genetic code</span>$\rightarrow$ Translation$\rightarrow$ Regulation of gene expression$\rightarrow$ Human genome project$\rightarrow$ DNA fingerprinting</footer>

<h6 id="successmolecular-basis-of-inheritancesuccess-5"><Success>Molecular basis of inheritance</Success></h6>
<h6 id="primarygenetic-codeprimary-5"><Primary>Genetic code</Primary></h6>
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<p>The salient features of Genetic code are as follows:</p>
<p>(i) The codon is triplet. 61 codons code for amino acids and 3 codons do not code for any amino acids, hence they function as stop codons.</p>
<p>(ii) Some amino acids are coded by more than one codon, hence the code is degenerate.</p>
<p>(iii) The codon is read in mRNA in a contiguous fashion. There are no punctuations.</p>
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<footer style="font-size:18px">Introduction$\rightarrow$ The DNA$\rightarrow$ The Search for Genetic Material$\rightarrow$ RNA World$\rightarrow$ Replication$\rightarrow$ Transcription$\rightarrow$ <span style="color:blue">Genetic code</span>$\rightarrow$ Translation$\rightarrow$ Regulation of gene expression$\rightarrow$ Human genome project$\rightarrow$ DNA fingerprinting</footer>

<h6 id="successmolecular-basis-of-inheritancesuccess-6"><Success>Molecular basis of inheritance</Success></h6>
<h6 id="primarygenetic-codeprimary-6"><Primary>Genetic code</Primary></h6>
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<p>(iv) The code is nearly universal: for example, from bacteria to human UUU would code for Phenylalanine (phe). Some exceptions to this rule have been found in mitochondrial codons, and in some protozoans.</p>
<p>(v) AUG has dual functions. It codes for Methionine (met) , and it also act as initiator codon.</p>
<p>vi) UAA, UAG, UGA are stop terminator codons. If following is the sequence of nucleotides in mRNA, predict the sequence of amino acid coded by it (take help of the checkerboard):</p>
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<footer style="font-size:18px">Introduction$\rightarrow$ The DNA$\rightarrow$ The Search for Genetic Material$\rightarrow$ RNA World$\rightarrow$ Replication$\rightarrow$ Transcription$\rightarrow$ <span style="color:blue">Genetic code</span>$\rightarrow$ Translation$\rightarrow$ Regulation of gene expression$\rightarrow$ Human genome project$\rightarrow$ DNA fingerprinting</footer>

<h6 id="successmolecular-basis-of-inheritancesuccess-7"><Success>Molecular basis of inheritance</Success></h6>
<h6 id="primarygenetic-codeprimary-7"><Primary>Genetic code</Primary></h6>
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<p>-AUG UUU UUC UUC UUU UUU UUC</p>
<p>Now try the opposite. Following is the sequence of amino acids coded by an mRNA. Predict the nucleotide sequence in the RNA:</p>
<p>Met-Phe-Phe-Phe-Phe-Phe-Phe</p>
<p>Do you face any difficulty in predicting the opposite? Can you now correlate which two properties of Genetic code you have learnt?</p>
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<footer style="font-size:18px">Introduction$\rightarrow$ The DNA$\rightarrow$ The Search for Genetic Material$\rightarrow$ RNA World$\rightarrow$ Replication$\rightarrow$ Transcription$\rightarrow$ <span style="color:blue">Genetic code</span>$\rightarrow$ Translation$\rightarrow$ Regulation of gene expression$\rightarrow$ Human genome project$\rightarrow$ DNA fingerprinting</footer>

<h6 id="successmolecular-basis-of-inheritancesuccess-8"><Success>Molecular basis of inheritance</Success></h6>
<h6 id="primarygenetic-codeprimary-8"><Primary>Genetic code</Primary></h6>
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<p><strong>Mutations and Genetic code</strong></p>
<p>The relationships between genes and DNA are best understood by mutation studies. You have studied about mutation and its effect in Chapter 4. Effects of large deletions and rearrangements in a segment of DNA are easy to comprehend. It may result in loss or gain of a gene and so a function.</p>
<ul>
<li>The effect of point mutations will be explained here. A classical example of point mutation is a change of single base pair in the gene for beta globin chain that results in the change of amino acid residue glutamate to valine.</li>
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<footer style="font-size:18px">Introduction$\rightarrow$ The DNA$\rightarrow$ The Search for Genetic Material$\rightarrow$ RNA World$\rightarrow$ Replication$\rightarrow$ Transcription$\rightarrow$ <span style="color:blue">Genetic code</span>$\rightarrow$ Translation$\rightarrow$ Regulation of gene expression$\rightarrow$ Human genome project$\rightarrow$ DNA fingerprinting</footer>

<h6 id="successmolecular-basis-of-inheritancesuccess-9"><Success>Molecular basis of inheritance</Success></h6>
<h6 id="primarygenetic-codeprimary-9"><Primary>Genetic code</Primary></h6>
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<p>It results into a diseased condition called as sickle cell anemia. Effect of point mutations that inserts or deletes a base in structural gene can be better understood by following simple example.</p>
<p>Consider a statement that is made up of the following words each having three letters like Genetic code.</p>
<p><strong>RAM HAS RED CAP</strong></p>
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<footer style="font-size:18px">Introduction$\rightarrow$ The DNA$\rightarrow$ The Search for Genetic Material$\rightarrow$ RNA World$\rightarrow$ Replication$\rightarrow$ Transcription$\rightarrow$ <span style="color:blue">Genetic code</span>$\rightarrow$ Translation$\rightarrow$ Regulation of gene expression$\rightarrow$ Human genome project$\rightarrow$ DNA fingerprinting</footer>

<h6 id="successmolecular-basis-of-inheritancesuccess-10"><Success>Molecular basis of inheritance</Success></h6>
<h6 id="primarygenetic-codeprimary-10"><Primary>Genetic code</Primary></h6>
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<p>If we insert a letter B in between HAS and RED and rearrange the statement, it would read as follows:</p>
<p><strong>RAM HAS BRE DCA P</strong></p>
<p>Similarly, if we now insert two letters at the same place, say BI’. Now it would read,</p>
<p><strong>RAM HAS BIR EDC AP</strong></p>
<p>Now we insert three letters together, say BIG, the statement would read</p>
<p><strong>RAM HAS BIG RED CAP</strong></p>
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<footer style="font-size:18px">Introduction$\rightarrow$ The DNA$\rightarrow$ The Search for Genetic Material$\rightarrow$ RNA World$\rightarrow$ Replication$\rightarrow$ Transcription$\rightarrow$ <span style="color:blue">Genetic code</span>$\rightarrow$ Translation$\rightarrow$ Regulation of gene expression$\rightarrow$ Human genome project$\rightarrow$ DNA fingerprinting</footer>

<h6 id="successmolecular-basis-of-inheritancesuccess-12"><Success>Molecular basis of inheritance</Success></h6>
<h6 id="primarygenetic-codeprimary-12"><Primary>Genetic code</Primary></h6>
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<p>The conclusion from the above exercise is very obvious. Insertion or deletion of one or two bases changes the reading frame from the point of insertion or deletion. However, such mutations are referred to as frameshift insertion or deletion mutations.</p>
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<li>Insertion or deletion of three or its multiple bases insert or delete in one or multiple codon hence one or multiple amino acids, and reading frame remains unaltered from that point onwards.</li>
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<footer style="font-size:18px">Introduction$\rightarrow$ The DNA$\rightarrow$ The Search for Genetic Material$\rightarrow$ RNA World$\rightarrow$ Replication$\rightarrow$ Transcription$\rightarrow$ <span style="color:blue">Genetic code</span>$\rightarrow$ Translation$\rightarrow$ Regulation of gene expression$\rightarrow$ Human genome project$\rightarrow$ DNA fingerprinting</footer>

<h6 id="successmolecular-basis-of-inheritancesuccess-13"><Success>Molecular basis of inheritance</Success></h6>
<h6 id="primarygenetic-codeprimary-13"><Primary>Genetic code</Primary></h6>
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<p><strong>tRNA– the Adapter Molecule</strong></p>
<p>From the very beginning of the proposition of code, it was clear to Francis Crick that there has to be a mechanism to read the code and also to link it to the amino acids, because amino acids have no structural specialities to read the code uniquely.</p>
<ul>
<li>He postulated the presence of an adapter molecule that would on one hand read the code and on other hand would bind to specific amino acids. The tRNA, then called sRNA (soluble RNA), was known before the Genetic code was postulated. However, its role as an adapter molecule was assigned much later.</li>
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<footer style="font-size:18px">Introduction$\rightarrow$ The DNA$\rightarrow$ The Search for Genetic Material$\rightarrow$ RNA World$\rightarrow$ Replication$\rightarrow$ Transcription$\rightarrow$ <span style="color:blue">Genetic code</span>$\rightarrow$ Translation$\rightarrow$ Regulation of gene expression$\rightarrow$ Human genome project$\rightarrow$ DNA fingerprinting</footer>

<h6 id="successmolecular-basis-of-inheritancesuccess-14"><Success>Molecular basis of inheritance</Success></h6>
<h6 id="primarygenetic-codeprimary-14"><Primary>Genetic code</Primary></h6>
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<p>tRNA has an anticodon loop that has bases complementary to the code, and it also has an amino acid acceptor end to which it binds to amino acids. tRNAs are specific for each amino acid (Figure 5.12). For initiation, there is another specific tRNA that is referred to as initiator tRNA.</p>
<ul>
<li>There are no tRNAs for stop codons. In figure 5.12, the secondary structure of tRNA has been depicted that looks like a clover-leaf. In actual structure, the tRNA is a compact molecule which looks like inverted L.</li>
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<footer style="font-size:18px">Introduction$\rightarrow$ The DNA$\rightarrow$ The Search for Genetic Material$\rightarrow$ RNA World$\rightarrow$ Replication$\rightarrow$ Transcription$\rightarrow$ <span style="color:blue">Genetic code</span>$\rightarrow$ Translation$\rightarrow$ Regulation of gene expression$\rightarrow$ Human genome project$\rightarrow$ DNA fingerprinting</footer>

<h6 id="successmolecular-basis-of-inheritancesuccess-16"><Success>Molecular basis of inheritance</Success></h6>
<h6 id="primarytranslationprimary"><Primary>Translation</Primary></h6>
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<p>Translation refers to the process of polymerisation of amino acids to form a polypeptide (Figure 5.13). The order and sequence of amino acids are defined by the sequence of bases in the mRNA. The amino acids are joined by a bond which is known as a peptide bond.</p>
<ul>
<li>Formation of a peptide bond requires energy. Therefore, in the first phase itself amino acids are activated in the presence of ATP and linked to their cognate tRNA – a process commonly called as charging of tRNA or aminoacylation of tRNA to be more specific.</li>
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<footer style="font-size:18px">Introduction$\rightarrow$ The DNA$\rightarrow$ The Search for Genetic Material$\rightarrow$ RNA World$\rightarrow$ Replication$\rightarrow$ Transcription$\rightarrow$ Genetic code$\rightarrow$ <span style="color:blue">Translation</span>$\rightarrow$ Regulation of gene expression$\rightarrow$ Human genome project$\rightarrow$ DNA fingerprinting</footer>

<h6 id="successmolecular-basis-of-inheritancesuccess-18"><Success>Molecular basis of inheritance</Success></h6>
<h6 id="primarytranslationprimary-2"><Primary>Translation</Primary></h6>
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<p>tRNA - the adapter molecule would be favoured energetically. The presence of a catalyst would enhance the rate of peptide bond formation. The cellular factory responsible for synthesising proteins is the ribosome.</p>
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<li>The ribosome consists of structural RNAs and about 80 different proteins. In its inactive state, it exists as two subunits; a large subunit and a small subunit.</li>
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<footer style="font-size:18px">Introduction$\rightarrow$ The DNA$\rightarrow$ The Search for Genetic Material$\rightarrow$ RNA World$\rightarrow$ Replication$\rightarrow$ Transcription$\rightarrow$ Genetic code$\rightarrow$ <span style="color:blue">Translation</span>$\rightarrow$ Regulation of gene expression$\rightarrow$ Human genome project$\rightarrow$ DNA fingerprinting</footer>

<h6 id="successmolecular-basis-of-inheritancesuccess-19"><Success>Molecular basis of inheritance</Success></h6>
<h6 id="primarytranslationprimary-3"><Primary>Translation</Primary></h6>
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<p>When the small subunit encounters an mRNA, the process of Translation of the mRNA to protein begins. There are two sites in the large subunit, for subsequent amino acids to bind to and thus, be close enough to each other for the formation of a peptide bond.</p>
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<li>The ribosome also acts as a catalyst (23S rRNA in bacteria is the enzyme- ribozyme) for the formation of peptide bond</li>
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<footer style="font-size:18px">Introduction$\rightarrow$ The DNA$\rightarrow$ The Search for Genetic Material$\rightarrow$ RNA World$\rightarrow$ Replication$\rightarrow$ Transcription$\rightarrow$ Genetic code$\rightarrow$ <span style="color:blue">Translation</span>$\rightarrow$ Regulation of gene expression$\rightarrow$ Human genome project$\rightarrow$ DNA fingerprinting</footer>

<h6 id="successmolecular-basis-of-inheritancesuccess-20"><Success>Molecular basis of inheritance</Success></h6>
<h6 id="primarytranslationprimary-4"><Primary>Translation</Primary></h6>
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<p>A Translational unit in mRNA is the sequence of RNA that is flanked by the start codon (AUG) and the stop codon and codes for a polypeptide. An mRNA also has some additional sequences that are not translated and are referred as untranslated regions (UTR). The UTRs are present at both 5’ -end (before start codon) and at 3’ -end (after stop codon). They are required for efficient Translation process.</p>
<p>For initiation, the ribosome binds to the mRNA at the start codon (AUG) that is recognised only by the initiator tRNA. The ribosome proceeds to the elongation phase of protein synthesis</p>
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<footer style="font-size:18px">Introduction$\rightarrow$ The DNA$\rightarrow$ The Search for Genetic Material$\rightarrow$ RNA World$\rightarrow$ Replication$\rightarrow$ Transcription$\rightarrow$ Genetic code$\rightarrow$ <span style="color:blue">Translation</span>$\rightarrow$ Regulation of gene expression$\rightarrow$ Human genome project$\rightarrow$ DNA fingerprinting</footer>

<h6 id="successmolecular-basis-of-inheritancesuccess-21"><Success>Molecular basis of inheritance</Success></h6>
<h6 id="primarytranslationprimary-5"><Primary>Translation</Primary></h6>
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<p>During this stage, complexes composed of an amino acid linked to tRNA, sequentially bind to the appropriate codon in mRNA by forming complementary base pairs with the tRNA anticodon. The ribosome moves from codon to codon along the mRNA.</p>
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<li>Amino acids are added one by one, translated into Polypeptide sequences dictated by DNA and represented by mRNA. At the end, a release factor binds to the stop codon, terminating Translation and releasing the complete polypeptide from the ribosome.</li>
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<footer style="font-size:18px">Introduction$\rightarrow$ The DNA$\rightarrow$ The Search for Genetic Material$\rightarrow$ RNA World$\rightarrow$ Replication$\rightarrow$ Transcription$\rightarrow$ Genetic code$\rightarrow$ <span style="color:blue">Translation</span>$\rightarrow$ Regulation of gene expression$\rightarrow$ Human genome project$\rightarrow$ DNA fingerprinting</footer>