Information on DNA from- indstate.edu
As a class, the nucleotides may be considered one of the most important metabolites of the cell.
Nucleotides are found primarily as the monomeric units comprising the major nucleic acids of the
cell, RNA and DNA. However, they also are required for numerous other important functions
within the cell. These functions include:
1. serving as energy stores for future use in phosphate transfer reactions. These reactions are
predominantly carried out by ATP.
2. forming a portion of several important coenzymes such as NAD+, NADP+, FAD and coenzyme
A.
3. serving as mediators of numerous important cellular processes such as second messengers in
signal transduction events. The predominant second messenger is cyclic-AMP (cAMP), a cyclic
derivative of AMP formed from ATP.
4. controlling numerous enzymatic reactions through allosteric effects on enzyme activity.
5. serving as activated intermediates in numerous biosynthetic reactions. These activated
intermediates include S-adenosylmethionine (S-AdoMet or SAM) involved in methyl transfer
reactions as well as the many sugar coupled nucleotides involved in glycogen and glycoprotein
synthesis.
Structure of DNA
Utilizing X-ray diffraction data, obtained from crystals of DNA, James Watson and Francis Crick
proposed a model for the structure of DNA. This model (subsequently verified by additional data)
predicted that DNA would exist as a helix of two complementary antiparallel strands, wound around
each other in a rightward direction and stabilized by H-bonding between bases in adjacent strands.
In the Watson-Crick model, the bases are in the interior of the helix aligned at a nearly 90 degree
angle relative to the axis of the helix. Purine bases form hydrogen bonds with pyrimidines, in the
crucial phenomenon of base pairing. Experimental determination has shown that, in any given
molecule of DNA, the concentration of adenine (A) is equal to thymine (T) and the concentration of
cytidine (C) is equal to guanine (G). This means that A will only base-pair with T, and C with G.
According to this pattern, known as Watson-Crick base-pairing, the base-pairs composed of G and
C contain three H-bonds, whereas those of A and T contain two H-bonds. This makes G-C basepairs more stable than A-T base-pairs.
A-T Base Pair
G-C Base Pair
The antiparallel nature of the helix stems from the orientation of the individual strands. From any
fixed position in the helix, one strand is oriented in the 5'—>3' direction and the other in the 3'—>5'
direction. On its exterior surface, the double helix of DNA contains two deep grooves between the
ribose-phosphate chains. These two grooves are of unequal size and termed the major and minor
grooves. The difference in their size is due to the asymmetry of the deoxyribose rings and the
structurally distinct nature of the upper surface of a base-pair relative to the bottom surface.
DNA Replication
Replication of DNA occurs during the process of normal cell division cycles. Because the genetic
complement of the resultant daughter cells must be the same as the parental cell, DNA replication
must possess a very high degree of fidelity. The entire process of DNA replication is complex and
involves multiple enzymatic activities.
The mechanics of DNA replication was originally characterized in the bacterium, E. coli which
contains 3 distinct enzymes capable of catalyzing the replication of DNA. These have been
identified as DNA polymerase (pol) I, II, and III. Pol I is the most abundant replicating activity in E.
coli but has as its primary role to ensure the fidelity of replication through the repair of damaged
and mismatched DNA. Replication of the E. coli genome is the job of pol III. This enzyme is much
less abundant than pol I, however, its activity is nearly 100 times that of pol I.
My opinion
Overall I found the information on this site slightly confusing. A lot of the
information contained on the website was very in-dept and I think would be more
suited for a student in university. Yet it did include a diagram of the chemical
arrangement of the base pairs which I think is useful. Also I think the information on
the function of DNA is also very useful to our course.