5.04:_Base_Pairing_in_DNA_and_RNA

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<span id="Rules_of_Base_Pairing"></span><span id="Rules_of_Base_Pairing"></span><h2 class="lt-bio-4742">Rules of Base Pairing</h2> <figure><img style="width: 255px; height: 274px;" class="internal" alt="Diagram showing chemical structures of DNA base pairs: thymine with adenine, and cytosine with guanine. Thymine and adenine are connected by two hydrogen bonds; cytosine and guanine by three." loading="lazy" width="255px" height="274px" src="https://bio.libretexts.org/@api/deki/files/6523/BasePairing.gif?revision=1&amp;size=bestfit&amp;width=255&amp;height=274" /><figcaption>Figure 5.4.1: Base Pairing</figcaption></figure> <p class="lt-bio-4742">The rules of base pairing (or nucleotide pairing) are:</p> <ul> <li class="lt-bio-4742">A with <strong>T</strong>: the purine <strong>adenine</strong> (A) always pairs with the pyrimidine <strong>thymine</strong> (T)</li> <li class="lt-bio-4742"><strong>C</strong> with <strong>G</strong>: the pyrimidine <strong>cytosine</strong> (C) always pairs with the purine <strong>guanine</strong> (G)</li>
</ul> <p class="lt-bio-4742">This is consistent with there not being enough space (20 &Aring;) for two purines to fit within the helix and too much space for two pyrimidines to get close enough to each other to form hydrogen bonds between them. But why not A with C and G with T? The answer: only with A &amp; T and with C &amp; G are there opportunities to establish <strong>hydrogen bonds</strong> (shown here as dotted lines) between them (two between A &amp; T; three between C &amp; G). These relationships are often called the rules of <strong>Watson-Crick</strong> base pairing, named after the two scientists who discovered their structural basis.</p> <table class="mt-responsive-table" align="center"> <caption>Table 5.4.1: Relative Proportions (%) of Bases in DNA</caption> <thead> <tr> <th scope="col" class="mt-align-center"><strong>Organism</strong></th> <th scope="col" class="mt-align-center"><strong>A</strong></th> <th scope="col" class="mt-align-center"><strong>T</strong></th> <th scope="col" class="mt-align-center"><strong>G</strong></th> <th scope="col" class="mt-align-center"><strong>C</strong></th> </tr> </thead> <tbody> <tr> <td class="mt-align-center lt-bio-4742">Human</td> <td style="width: 41px; text-align: center;" class="lt-bio-4742">30.9</td> <td style="width: 44px; text-align: center;" class="lt-bio-4742">29.4</td> <td class="mt-align-center lt-bio-4742">19.9</td> <td class="mt-align-center lt-bio-4742">19.8</td> </tr> <tr> <td class="mt-align-center lt-bio-4742">Chicken</td> <td style="width: 41px; text-align: center;" class="lt-bio-4742">28.8</td> <td style="width: 44px; text-align: center;" class="lt-bio-4742">29.2</td> <td class="mt-align-center lt-bio-4742">20.5</td> <td class="mt-align-center lt-bio-4742">21.5</td> </tr> <tr> <td class="mt-align-center lt-bio-4742">Grasshopper</td> <td style="width: 41px; text-align: center;" class="lt-bio-4742">29.3</td> <td style="width: 44px; text-align: center;" class="lt-bio-4742">29.3</td> <td class="mt-align-center lt-bio-4742">20.5</td> <td class="mt-align-center lt-bio-4742">20.7</td> </tr> <tr> <td class="mt-align-center lt-bio-4742">Sea Urchin</td> <td style="width: 41px; text-align: center;" class="lt-bio-4742">32.8</td> <td style="width: 44px; text-align: center;" class="lt-bio-4742">32.1</td> <td class="mt-align-center lt-bio-4742">17.7</td> <td class="mt-align-center lt-bio-4742">17.3</td> </tr> <tr> <td class="mt-align-center lt-bio-4742">Wheat</td> <td style="width: 41px; text-align: center;" class="lt-bio-4742">27.3</td> <td style="width: 44px; text-align: center;" class="lt-bio-4742">27.1</td> <td class="mt-align-center lt-bio-4742">22.7</td> <td class="mt-align-center lt-bio-4742">22.8</td> </tr> <tr> <td class="mt-align-center lt-bio-4742">Yeast</td> <td style="width: 41px; text-align: center;" class="lt-bio-4742">31.3</td> <td style="width: 44px; text-align: center;" class="lt-bio-4742">32.9</td> <td class="mt-align-center lt-bio-4742">18.7</td> <td class="mt-align-center lt-bio-4742">17.1</td> </tr> <tr> <td class="mt-align-center lt-bio-4742">E. coli</td> <td style="width: 41px; text-align: center;" class="lt-bio-4742">24.7</td> <td style="width: 44px; text-align: center;" class="lt-bio-4742">23.6</td> <td class="mt-align-center lt-bio-4742">26.0</td> <td class="mt-align-center lt-bio-4742">25.7</td> </tr> </tbody>
</table> <p class="lt-bio-4742">The rules of base pairing tell us that if we can &quot;read&quot; the sequence of nucleotides on one strand of DNA, we can immediately deduce the complementary sequence on the other strand. The rules of base pairing explain the phenomenon that whatever the amount of adenine (A) in the DNA of an organism, the amount of thymine (T) is the same (called <strong>Chargaff's rule</strong>). Similarly, whatever the amount of guanine (G), the amount of cytosine (C) is the same. The C+G:A+T ratio varies from organism to organism, particularly among the bacteria, but within the limits of the experimental error, A=T and C=G.</p>

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<hr class="autoattribution-divider" /><div class="autoattribution"><p>This page titled <a target="_blank" class="internal mt-self-link" href="/Sandboxes/johnnyphung/biology/05:_DNA/5.04:_Base_Pairing_in_DNA_and_RNA">5.4: Base Pairing in DNA and RNA</a> is shared under a <a rel="nofollow" href="https://creativecommons.org/licenses/by/3.0" target="_blank">CC BY  3.0</a>  license and was authored, remixed, and/or curated by <a rel="nofollow" target="_blank" href="http://www.biology-pages.info/">John W. Kimball</a> via <a rel="nofollow" href="https://www.biology-pages.info/" target="_blank">source content</a> that was edited to the style and standards of the LibreTexts platform.</p></div>

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