Biomedical Engineering Reference
In-Depth Information
DNA
RNA
5
′
End
5
′
End
O
-
O
-
-
O
-
O
O
O
P
P
O
O
A
U
O
O
5
′
CH
2
5
′
CH
2
H
H
H
H
H
H
H
CH
H
O
H
O
3
′
phospho-
diester
linkages
-
O
-
O
P
O
P
O
O
O
T
C
O
O
5
′
5
′
CH
2
5
′
CH
2
H
H
H
H
H
H
H
H
3
′
O
OH
O
OH
-
O
-
O
P
O
P
O
O
O
C
C
O
O
5
′
CH
2
H
5
′
CH
2
H
H
H
H
H
H
H
O
OH
O
OH
H
H
3
′
End
3
′
End
Figure 1.2
Nucleotide chain seen in DNA and RNA.
base composition remains identical in different tissues of the same species, and quan-
titatively the number of purine residues and pyrimidine residues are the same.
The DNA is too large compared to the cell in which it is packaged. The organi-
zation and structure of DNA is comparatively simple, both structurally as well as
functionally in prokaryotes. The bacterial chromosome is usually a long, single cir-
cular DNA molecule, whereas some organisms may have more than one unit. Apart
from this large, single, circular DNA, some extrachromosomal DNA called plasmids
are also present in numbers ranging from a few thousand to several thousand base
pairs. Plasmids are self-propagating in nature, and daughter plasmids are passed on
to progeny. These plasmids generally carry genes useful to the host; for example,
some confer the host with resistance to antibiotics.
The organization of eukaryotes, however, is complex. Eukaryotic genes are known
to carry nucleotide sequences that are not coding for any biological product inter-
spersed between the coding regions. Such intervening sequences are termed introns,
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