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Summary A Level Biology
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Both DNA and RNA are known as nucleic acids. They have been given this name for thesimple reason that they are made up of structures called nucleotides. Those nucleotides,
themselves comprising a number of components, bond together to form the double-helix first
discovered by the scientists James Watson and Francis Crick in 1956. This discovery won
the two scientists the Nobel Prize. For now, when we discuss nucleic acids you should
assume we are discussing DNA rather than RNA, unless otherwise specified.
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Nucleotides
A nucleotide consists of three things:
● A nitrogenous base, which can be either adenine, guanine, cytosine, or thymine (in
the case of RNA, thymine is replaced by uracil).
● A five-carbon sugar, called deoxyribose because it is lacking an oxygen group on one
of its carbons.
● One or more phosphate groups.
The nitrogen bases are pyrimidine in structure and form a bond between their 1' nitrogen and
the 1' -OH group of the deoxyribose. This type of bond is called a glycosidic bond. The
phosphate group forms a bond with the deoxyribose sugar through an ester bond between
one of its negatively charged oxygen groups and the 5' -OH of the sugar ().
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,Nucleic Acids:
Nucleotides join together through phosphodiester linkages between the 5' and 3' carbon
atoms to form nucleic acids. The 3' -OH of the sugar group forms a bond with one of the
negatively charged oxygens of the phosphate group attached to the 5' carbon of another
sugar. When many of these nucleotide subunits combine, the result is the large
single-stranded polynucleotide or nucleic acid, DNA ()
Two sides of the nucleic acid strand shown above are different, resulting in polarity. At one
end of the large molecule, the carbon group is unbound and at the other end, the -OH is
unbound. These different ends are called the 5'- and 3'-ends, respectively.
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, The Helical Structure of DNA:
One strand is oriented in the 5' to 3' direction while the complementary strand runs in the 3'
to 5' direction. Because the two strands are oppositely oriented, they are said to be
antiparallel to each other. The two strands bond through their nitrogen bases (marked A, C,
G, or T for adenine, cytosine, and guanine). Adenine only bonds with thymine, and cytosine
only bonds with guanine. The nitrogen bases are held together by hydrogen bonds: adenine
and thymine form two hydrogen bonds; cytosine and guanine form three hydrogen bonds.
As a result of anti-parallel pairing, the nitrogen base groups face the inside of the helix while
the sugar and phosphate groups face outward. The sugar and phosphate groups in the helix
therefore make up the phosphate backbone of DNA. The backbone is highly negatively
charged as a result of the phosphate groups.
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The Importance of the Hydrogen Bond:
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