-
2 long polynucleotide chains composed of
four types of nucleotides. A nucleotide is composed of a 5 carbon sugar,
a phosphate group and a nitrogenous base (of which there are 4 types). The sugar
in DNA is deoxyribose (a ribose sugar without an O attached to the H on
the #2 carbon sugar). The term "nucleoside" is sometimes used to simply refer to
the sugar and nitrogenous base without the phosphate group.
-
Each strand of DNA contains a sequence of
nucleotides that is exactly complementary to the nucleotide sequence of its
partner strand. During DNA replication as when a cell divides, each such
complementary strand will serve as a template for a new strand.
-
the 2 polynucleotide strands run in
opposite direction; they are anti-parallel in that one strand runs from 5' to 3'
while the opposite strands runs in the 3' to 5' direction.
-
the DNA strands do not exist in a straight
line, but rather rap around each other to form a "double helix". Each turn of
DNA on this double helix makes up 10.4 nucleotide pairs. This coiling also
creates 2 grooves; a wider major groove and a smaller minor groove.
Grooves facilitate interactions of proteins and DNA through H bonding.
-
The nucleotides are covalently linked
together in the chain through the sugars and phosphates (i.e., alternating
sugar-phosphate-sugar-phosphate) which forms the outside or "backbone" of the
chain.
-
the 5' end of each polynucleotide strand
carries an unlinked phosphate group attached to the 5' position on the sugar
ring and, the 3' end of each strand carries a free hydroxyl (-OH) group attached
to the 3' position on the sugar ring.
-
the nitrogenous bases are stacked like
pennies pointing toward the center of the helix and the 2 strands are held
together by hydrogen bonding between the bases; adenine (A) pairs with thymine
(T) which forms 2 H bonds and guanine (G) pairs with cytosine (C) which forms 3
H bonds.
-
because purines (A or G) are larger
bases than pyrimidines (T or C), all base pairs are the same size which
gives the DNA molecule its uniform size
-
The N-9 of a purine and N-1 of a
pyrimidine is attached to the C-1 of deoxyribose
