Chemistry Reference
In-Depth Information
NH
2
O
O
H
H
N
N
N
N
N
N
N
N
N
N
H
2
N
O
H
H
H
uracil
adenine
guanine
The naming of nucleosides depends upon the sugars. Thus, adenine attached to ribose is called
adenosine. When it is attached to deoxyribose, it is called deoxyadenosine.
Hydrolysis of nucleoproteins separates the acids from the proteins. Further hydrolysis yields the
components of nucleic acids, namely sugars, bases, and phosphoric acid. The nucleic acids differ
from each other, depending upon the source, in chain lengths, sequences, and distributions of bases.
Just like in the proteins, the primary structure of nucleic acids is determined by partial and sequential
hydrolysis.
8.6.1 DNA and RNA
Deoxynucleic acids have been isolated from all types of living cells and it was established that their
main function is to carry genetic information [
57
]. These are very high molecular weight polymeric
materials. Some were found to be as high as 100 million. Analyses of DNA structures show that the
numbers of adenine bases are always the same as the number of thymine bases. Also, the numbers of
guanine bases always equal the numbers of cytosines. Based on the information from various analyses
and an X-ray investigation of the structure, Watson and Crick concluded that the secondary structure
of DNA must be a double helix [
58
]. Two separate right-handed helical chains wind around each
other and are held together by hydrogen bonding between base pairs. The bases that are paired off are
adenine with thymine and guanine with cytosine:
H
H
N
O
H
N
H
O
thymine
cyiosine
NN
NN
H
N
N
sugar
H
N
sugar
N
adenine
guanine
O
O
H
N
N
N
sugar
N
N
sugar
H
10.7 A
10.7
o
The base pairs are extended perpendicularly toward the center and the deoxyribose-phosphate
ester chains are located on the outside of the helix. The two strands are antiparallel to each other. One
turn of the helix corresponds to ten nucleotide pairs, 34
˚
in length. The width of the helix is 20
˚
.
Evidence was presented that some DNAs in their native forms are cyclic [
59
] and may even occur as
two interlocking rings. While most known DNA molecules form a right-handed helix, a left-handed
helix can be prepared synthetically in the laboratory [
52
]. It was speculated that in some instances
left-handed helixes may exist
in nature and have a biological
function [
53
]. These DNA
conformations were named
Z
-DNAs because the backbones zigzag down the molecule.
