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| genetics | |
| nucleic acids | |
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: A base-paired DNA sequence: ATCGAT TAGCTA : The corresponding base-paired RNA sequence: AUCGAU UAGCUA The following abbreviations are commonly used to describe the length of a DNA/RNA molecule:
In case of single stranded DNA/RNA we talk about Nucleotide s, abbreviated nt (or knt, Mnt, Gnt), rather than base pairs, as they are not paired. The larger nucleic acids, adenine and guanine, are members of a class of doubly-ringed chemical structures called Purine s; the smaller nucleic acids, cytosine and thymine (and uracil), are members of a class of singly-ringed chemical structures called Pyrimidine s. Purines are only complementary with pyrimidines: pyrimidine-pyrimidine pairings are energetically unfavourable because the molecules are too far apart for hydrogen bonding to be established; purine-purine pairings are energetically unfavourable because the molecules are too close, leading to electrostatic repulsion. The only other possible pairings are GT and AC; these pairings are mismatches because the pattern of hydrogen donors and acceptors do not correspond. Thymine and adenine bond together through two hydrogen bonds, while cytosine and guanine bond together through three hydrogen bonds. As Hydrogen Bond s are not very strong, the two nucleotide strands will separate on temperatures higher than 94 °C . Chemical analogs of nucleotides can take the place of proper nucleotides and establish non-canonical base-pairing, leading to errors in DNA Replication and DNA Transcription . Some analogs are Carcinogen s; others are Chemotherapy drugs. Guanine and cytosine form 3 hydrogen bonds while adenine and thymine form only 2 hydrogen bonds. Consequently A-T pairs are less stable. SEE ALSO REFERENCES |
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