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NUCLEIC ACIDS, DNA REPLICATION, MUTAGENESIS, DNA REPAIR
SECTION OVERVIEW:
I
...
DNA Nucleotide
B
...
Functions of Nucleotides

II
...


IV
...

VI
...


Nucleic Acid Structure

B
...


Different Classes of Nucleic Acids

D
...


Complementarity of base pairing

F
...


Replication

B
...

2
...

4
...


Structure of Nucleotides
DNA is a double strand consisting of nucleotide bases – T (thymine), C (cytosine), A (adenine) and G
(guanine)
...

A
...
Nitrogenous base (A, T, C, or G)
These are aromatic and hydrophobic
...
Structure
i
...
Purine - basic structure:

Common purine bases include adenine and guanine in the tautomeric forms that
predominant at pH 7

o

Adenine

Guanine

b
...

In aqueous solution: HA +H2 O ⇌A- +H3 O+
HA is a generic acid that dissociates into A- (conjugate base) which combines
with a water molecule to make an hydronium ion
The dissociation constant is written as a quotient of the equilibrium
concentrations (mol/L), denoted by [HA], [A−] and [H3O+]
[A- ][H3 O+ ]
Ka =
[HA]⌈H2 O⌉
- In general, the concentration of water can be taken as a constant and
ignored: HA ⇌ A- + H+ &
[A- ][H+ ]
Ka =
[HA]
it is easier to use the log of Ka (pKa) – the larger the pKa, the smaller the
extent of dissociation pKa = -log10 Ka
o Keto-enol tautomerism
The pKa value will specify whether protons are associated with ring Nitrogen at
a neutral pH; determining whether these nitrogens will serve as H acceptors or
donors during hydrogen bonding (between the bases)
-

Tautomerism of Uracil at pH 7
Keto

pKa = 9
...
4

⇌
pKa < 5
o Strong absorbance of UV light
The UV absorption spectra of common ribonucleotides

2
...
Nucleoside – compound formed when a nitrogenous base is linked to a pentose sugar
RNA contains D-ribose and DNA contains 2-deoxy-D-ribose
...
This difference affects the secondary
structure and stability

o Furanose structures
D-ribose
β-D-ribofuranose

i
...
The carbon of this bond is anomeric

o

Anomeric carbon – carbohydrate can exist in an acyclic or a cyclic form
...
This is referred
to as the anomeric carbon

⇌

⇌

⇌

Anomeric
carbon

-

-

Locating the anomeric carbon
▪

First identify the O within the ring

▪

There is a carbon on either side
...
It is
this group that is the anomeric carbon

The difference between α and β – the OH group attached to the anomeric
carbon (remember this is the stereocentre) has the ability to change
direction
...
Nucleoside naming
Adding -idine to the root of a pyrimidine or adding -osine to the root of a purine
Cytidine

Guanosine

Uridine

Adenosine

iii
...
Pyrimidines are
generally in the anti conformation as the O atom sterically hinders the position of
the ring structure above the ribose
...
Attached to a phosphate group
Nucleotides are nucleoside phosphates (a nucleoside is a nitrogenous base attached to a
pentose sugar) and polyprotic acids
...
The
phosphates are negative
...


B
...
Monophosphates
There are four common ribonucleotides – AMP, GMP, CMP and UMP
These are the individual nucleosides attached to only one
phosphate group forming a nucleotide
...
Remember that the nitrogenous base is attached
to the 1st carbon on the sugar)
GMP – guanosine 5’-monophosphate
CMP – cytidine 5’-monophosphate
UMP – uridine 5’-monophosphate

2
...
This is the
primary source of energy
...
These bonds are high in energy
...


3
...
Functions of Nucleotides
1
...

a
...

c
...


ATP – central to energy metabolism
GTP – drives the synthesis of proteins
CTP – drives the synthesis of lipids
UTP – drives the metabolism of carbohydrates

2
...
The nitrogenous bases serve as units of recognition



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