Principle of Spectrophotometer

The purpose of spectrophotometer is to detect the quantity of the samples such as DNA, RNA and proteins. It involves passing of the monochromatic light (light of single wavelength) through the sample. The reason for this is Nucleic acids and proteins absorb light at specific wavelength alone. For example

DNA - 260nm
RNA - 260nm
Proteins - 280nm

There are certain other compounds that absorb light of 230 nm. They are Guanidine and Phenol. 

In order to calculate the concentration of nucleic acids and proteins, a formula is deduced from the law called Beer lamberts law. According to this law...

The amount of radiation absorbed may be measured in a number of ways:

Transmittance, T = P / P₀

% Transmittance, %T = 100 T
 
Absorbance,

A = log₁₀ P₀ / P
A = log₁₀ 1 / T
A = log₁₀ 100 / %T
A = 2 - log₁₀ %T

The absorbance can also be formulated as below 

A=ebc

Where A is absorbance (no units, since A = log₁₀ P₀ / P )
e is the molar absorbtivity with units of L mol^-1 cm^{-1
b is the path length of the sample - that is, the path length of the cuvette in which the sample is contained. We will express this measurement in centimetres.
c is the concentration of the compound in solution, expressed in mol L-1}

The reason why we prefer to express the law with this equation is because absorbance is directly proportional to the other parameters, as long as the law is obeyed.

Why is DNA, the genetic material. Why not RNA?

No, both DNA and RNA are considered to be genetic material. However DNA is the genetic material in almost all the organisms, except few viruses which have RNA as their genetic material. 

The reason for DNA being the genetic material comes from its stability. DNA is more stable than RNA.
DNA as we know has deoxyribose sugar, it lack the 2' -OH group, whereas RNA posses the 2' -OH group.
Because DNA lacks the OH group it is less reactive, it doesnt easily react. However RNA reacts easily due to the presence of -OH group.

What's the difference between DNA and RNA?

DNA and RNA, both are nucleic acids. They are composed of repeating units of nucleotides. Again each nucleotide consists of a sugar, a phosphate and a nucleic acid base. The main difference comes from the sugar present in DNA and RNA. The sugar in DNA is deoxyribose whereas in RNA it is ribose. The deoxyribose sugar lacks 2' hydroxyl (oxygen-hydrogen atom combination called a hydroxyl).






Another difference comes from the composition of the nucleotides. DNA has repeating units of Adenine, Guanine, Thymine and Cytosine. However RNA has all the bases except Thymine, instead of thymine it has Uracil.