Double-stranded DNA takes the shape of a double helix, like a ladder that has been twisted
- sides of ladder are deoxyribose sugar and phosphate backbone
- rungs of ladder are made of pairs of nitrogenous bases
- there are only four bases:
- A = adenine
- C = cytosine
- T = thymine (in RNA, the base U = uracil is used instead of T)
- G = guanine
C, T, and U are called pyrimidines and each has a single
nitrogen-containing ring. A and G are called purines and each has
two nitrogen-containing rings.
A always pairs with T (or in RNA, it pairs with U) G always pairs with
C.
So if you know the sequence of bases on one side of the 'ladder,' you
can automatically determine the sequence of bases on the other side. The strands are therefore complementary to each other.
Each side of the ladder is called a strand. Sometimes the two strands are referred to as the plus and minus strand, positive and negative, sense and antisense, or Watson and Crick.
The two strands are antiparallel; they are parallel to each other but are oriented in opposite directions.
DNA is 'read' in a specific direction, just like letters and words in the English language are read from left to right. Each end of DNA molecule has a number. One end is referred to as 5' (five prime) and the other end is referred to as 3' (three prime). The 5' and 3' designations refer to the number of carbon atom in a deoxyribose sugar molecule to which a phosphate group bonds.
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