In this tutorial we will consider each of the above building blocks and how they are linked together to form both DNA and RNA
The Nitrogenous Bases
The tables below show the five nitrogenous bases found in DNA and RNA. Note the similarities between the bases in each class. The bases are linked to the sugars through the nitrogen atoms shown in blue.
Sugars
There are two sugars found in nucleic acids
In nucleotides, the sugar is linked (esterified) to a phosphate group at the 3 and/or the 5 position (shown in green). The nitrogenous base is attached at the 1 position (shown in blue).
If you are wondering, the carbons in the sugar are numbered 1 to 5 so as not to be confused with the numbering of the carbons in the bases.
Nucleic Acids - Base Pairs
The unique, information-carrying ability of DNA and RNA is closely tied to the specific hydrogen bonding interactions that occur between the bases.
In DNA, strong hydrogen bonding interactions occur selectively between guanine and cytosine (G-C) and between adenine and thymine (A-T). In RNA, G-C and A-U interactions are found. These interactions are shown in the figures below.
Notice how there are three hydrogen bonds between guanine and cytosine and only two between adenine and thymine. This makes the G-C interaction slightly stronger than the A-T interaction
Now what happens if we try to match up any combination other than A-T or G-C? As shown below, other combinations such as guanine-thymine do not lead to favorable hydrogen-bonding interactions.
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guanine |
thymine |
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Nucleic Acids - Nucleosides and Nucleotides
Nucleosides
The combination of a base and a sugar is called a nucleoside. Two examples are shown below.
guanine + ribose =
guanosine
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cytosine + deoxyribose =
deoxycytidine
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Nucleotides
Nucleotides are comprised of three portions, a base, a sugar, and a phosphate. Addition of a phosphate group at the 5 (or 3 ) position of the sugar turns the nucleoside into a nucelotide. A familiar example, adenosine monophosphate or AMP, is shown below
adenine + ribose + phosphate = adenosine 5 -monophosphate (AMP)
Nucleic Acids - Polynucleotides
If the carbon atoms in positions three and five are esterified with phosphates, we can link nucleotides together to form polynucleotides. Shown below is a short segment of a (deoxy) polynucleotide with the bases thymine, adenine, and guanine.
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We can describe the structure of polynucleotides in much the same way we described the structure of proteins.
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The primary structure is the order in which the bases occur. The convention is to list them left to right from the 5 end to the 3 end, though when drawing two strands it is necessary to specify the ends of both strands. The above figure has the primary structure 5 -TAG-3 .
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The backbone consists of the phosphate-sugar chain that runs the length of the polynucleotide.
Finally, the secondary structure is the three-dimensional structure of the polynucleotide strand or strands. For DNA the most important secondary structure consists of a double helix (two strands) held together by hydrogen bonding between the bases
The most unique and interesting aspect of DNA is its three dimensional helical structure, the double helix. The double helix consists of two strands of complementary DNA running antiparallel, that is, 5 to 3 on one strand, and 3 to 5 on the other strand.
The primary structure of an example sequence of double-stranded DNA is given below. Notice how every guanine is matched with a cytosine and every adenine is matched with a thymine.
A second important form of nucleic acid is called transfer RNA (tRNA) and is used in the process of translation. In translation, a sequence of bases in DNA (a gene) is translated into a sequence of amino acids and used to synthesize a protein. There are different tRNA molecules for each of the twenty naturally occuring amino acids.
The tRNA molecule is a single strand of RNA that folds onto itself to form several loops. The function of the tRNA molecule is to translate from DNA code into protein code and therefore has two important regions. The anticodon loop contains a sequence of three bases which complement the DNA code (the codon) for one amino acid. The CCA-terminus serves as a handle to which an enzyme can attach the amino acid specific for that tRNA molecule