In 1869 F.Miecher isolated unidentified substance from pus cells. He gave the name nuclein, later Altmann introduced the term 'nucleic acid'. All living organisms contain two types of nucleic acids namely DNA (Deoxyribo Nucleic Acid) & RNA (Ribose Nucleic Acid). James Watson & Francis Crick (1953) proposed the model of DNA.
The model of DNA
they proposed was based mainly on X-ray diffraction pictures taken by Maurice
Wilkins & Rosalind Franklin.
 |
| X-ray diffraction |
Biochemical
evidences from the work of Chargaff also helped in proposing the three
dimensional model of DNA. James Watson & Francis Crick shared Nobel Prize
with Maurice Wilkins in 1962.
Genetic material of living organisms is either DNA or RNA
i. DNA (Deoxyribo Nucleic Acid)
Almost all the DNA of eukaryotic cells is found in the linear
chromosomes of the nucleus. Small amounts of circular DNA are found in
cytoplasmic organelles such as mitochondria & chloroplast.
The bases in DNA have carbon-nitrogen ring structures (nitrogenous
bases)
Adenine & Guanine (double ring nitrogenous bases) are purines.
Thymine and Cytosine (single ring nitrogenous bases) are pyramidines
- Adenine (A)-6-amino purine,
- Cytosine (C)-2 -oxy-4-aminopyrimidine,
- Guanine (G)-2-amino-6-oxopurine,
- Thymine (T)-2, 4-dioxy-5-methylpyrimidine
- Sugar + base=Nucleoside (The combination of a base with the sugar constitutes a nucleoside)
- Sugar + base (Nucleoside) + phosphate=Nucleotide
- In DNA, the sugar is a form of ribose called deoxyribose (it is missing the oxygen in the 2’ carbon atom)
He found that in DNA the concentration of adenine always equalled
the concentration of thymine and the concentration of guanine always equalled
the concentration of cytosine ie. the amount of purine=the amount of pyramidine
in a given DNA molecule.
A/T or G/C=1, A=T, G=C
A+T=C+G (No. of AT pairs need not equal
to the number of GC pairs)
Bonds in DNA : Hydrogen Bonds and Phosphodiester Bonds
- Adenine is paired to Thymine by 2
hydrogen bonds
- Cytosine is paired to Guanine by 3
hydrogen bonds
DNA contains many mono deoxyribo
nucleotides covalently linked by 3’5’-phosphodiester bonds. The resulting long, unbranched chain has polarity, with both a 5’end
and a 3’end that are not attached to other nucleotides.
The sequence of nucleotides is read as 5’-3’. DNA exists as a double stranded molecule (A few viruses ØX174 contain
single stranded DNA as genetic material), in which the two strands wind around
each other forming a double helix. The two chains are paired in an
anti-parallel manner that is the 5’ end of one strand is paired with 3’ end of
the other strand.
Most DNA molecules are right handed
double helices. The stacking of the paired bases by their flat surfaces in the
center of the molecules to forms a hydrophobic core. Together with the hydrogen
bonds between the base pairs, these hydrophobic interactions contribute
to the stability of the molecule.
DNA is negatively charged, why?
At neutral pH,there is no charge
on any of the bases, but the phosphates on the surface of the DNA molecules are ionized, giving the molecule
a net negative charge.
A complete turn of DNA (pitch) has a
length of about 34A0
The spacing between adjacent base pairs
is 10
A0 and the helix is 20
Ao in diameter
The ds DNA has 2 different
grooves: a major groove and a minor groove, where interactions between specific nucleotide sequence and specific
DNA binding enzymes (like DNA polymerase) may occur inorder to activate or
repress a gene action.
ii. RNA (Ribonucleic acids)
All RNA molecules are synthesised from
DNA templates. Most RNA molecules that are synthesised & processed in the
nucleus soon find their way into the cytoplasm, there they function in the
synthesis of proteins.
RNA is a polymer chain of
ribonucleotides joined by 3’5’ phosphodiester bonds.
How RNA is different from DNA?
The covalent structure is very similar
to that for DNA except that uracil replaces thymine and ribose replace deoxyribose.
RNA are single stranded, though most RNA assume double stranded nature by folding back on
themselves and forming hydrogen bonds (RNA secondary structure)
3 major classes of RNA: messenger RNA (m RNA), transfer RNA (t RNA) and ribosomal RNA
r RNA is the most abundant RNA and constituents 70-80% of the
total RNA. All kinds of RNA are formed from a template strand of DNA, catalyzed
by enzymes called RNA polymerases.
How to determine the concentration of
nucleic acids?
Nucleic acids absorb UV light due to
the conjugated aromatic nature of the bases. The absorption properties of
nucleic acids can be used for detection, quantitation and assessment of
purity.
Purity of DNA
- The approximate purity of ds DNA preparations
may be estimated by determination of the ratio of absorbance at 260nm and
280 nm (A260/A280).
- For pure DNA, the value is 1.8. Values above
1.8 suggest RNA contamination and those below 1.8 suggest protein
contamination.
- DNA can be melted (to form single strands) in
solution by increasing temperature (Tm) or
decreasing salt concentration.
- Heating DNA to 1000C lead to the
breaking & separation of the two poynucleotide chain, called denaturation whereas, slow cooling of heated
DNA solution will lead to pairing & formation of double helix,
called renaturation or annealing.
- Renaturation kinetics is used to assess the
complexity of genomes.
ds or ss DNA has more absorbance?
- The absorbance of isolated nucleotides is >
ss RNA or ss DNA>ds DNA.
- The increse in absorbance of ss DNA over ds
DNA is termed as hyperchromic effect. In
otherwords dsDNA is hypochromic with respect to ss DNA. Hyperchromic
effect is due to the presence of exposed nitrogenous bases in ss DNA.
Higher the GC content higher the Tm
will be. This is due to the presence of 3 H bonds between G&C compared to 2
H bonds between A&T.
Tags:
Chargaff’s rule
Denaturation
DNA
DNA molecules are much more stable
Forms of DNA
hyperchromic effect
Purity of DNA
Renaturation
RNA
Stability of DNA