NUCLEIC ACIDS

1. Nucleic Acids

Nucleic acids can be defined as biopolymers composed of nucleotides.

A nitrogenous base + pentose sugar + phosphate together constitute a single nucleotide.

If the pentose sugar is ribose, then the polymer is regarded as RNA (Ribonucleic acid) and if the pentose sugar is derivative of ribose i.e. Deoxyribose then the polymer is known as DNA (Deoxyribonucleic acid).

We can say that DNA is composed of repeating units of deoxyribonucleotides and RNA is composed of repeating units of ribonucleotides.

Nitrogenous base + pentose sugar (deoxy ribose) = Nucleosides

Nitrogenous base + pentose sugar (deoxy ribose) + phosphate group = Nucleotides.

Nucleotide is the basic unit of nucleic acid. Example of nucleotide is ATP.

2. DNA

Zacharis termed the term "DNA".

The landmark paper in the field of study of structure of DNA was given by Watson and Crick's that included the details about double helical structure of DNA.

According to Watson and Crick Model:

Watson and Crick proposed a double helix model for DNA.

According to this model, DNA is composed of two polynucleotide chains that are complementary and antiparallel to each other, helically coiled; like a revolving ladder.

Both strands of DNA held together by hydrogen bonds present between nitrogenous bases of both strands.

Adenine binds to thymine by two hydrogen bonds and cytosine binds to guanine by three hydrogen bonds.

The direction of phosphodiester bond is opposite to each other in both the strands of DNA i.e. if the direction of phosphodiester bond in one of the strands is 3' - 5' then it is 5'-3' in another strand.

Chargaff's Equivalency Rule:

In ds DNA total amount of purine = pyrimidine.

[A] + [G] = [T] + [C]

[A]+[G] / [T] +[C] = 1

Base ratio can be defined as the amount of: [A]+[T] / ([G] +[C] ) = constant for a given species

Helix length is the distance between two successive steps (each step comprises of a pair of nitrogen bases linked by hydrogen bonds) i.e. 3.4 A0 .

In one complete turn of DNA molecule there are such 10 steps (10 pairs of nitrogen bases). Thus, the length of one complete turn is 34 A0.

Diameter of the DNA molecule is the distance between phosphates of two strands is 20A0.

Distance between sugar of two strands is 11.1 A0.

In eukaryotic nucleus the DNA is associated with histone protein to form nucleoprotein.

Histone occupies major groove of DNA at 300 angle. Bond between DNA and histone is a salt linkage (Mg + 2).

[A]+[T] / ([G] +[C]) ratio is not consistent with double helical structure of DNA.

Based on the direction of twisting, DNA are of two types:

3. RNA

RNA can be defined as a polymer made up of one or more nucleotides.

A strand of RNA can be best described as a chain with a nucleotide at each chain link.

RNA plays a central role in “ Central Dogma" of molecular biology.

An organism's genetic information is encoded as a linear sequence of bases in the cell's DNA.

During the process known as transcription, an RNA copy of a segment of DNA, or messenger RNA (mRNA), is made.

This strand of RNA can then be read by a ribosome in order to form a protein.

RNAs also plays an essential roles in protein synthesis as well as in gene regulation.

Genomic RNA

Sometimes RNA works as a genetic material and transfers information from one generation to another.Such type of RNA is known as genomic RNA.

Non-Genetic RNA

Ribosomal RNA (r-RNA)- Constitute 80% of the total RNA in a cell.

Molecules of rRNA are synthesized in nucleolus (a specialized region of nucleus) which appears as a dense area within the nucleus and contains the genes that encode rRNA.

At least one large rRNA and one small rRNA is present in each ribosome. These large and small rRNAs combine with ribosomal proteins to form the large and small subunits of the ribosome (e.g., 50S and 30S, respectively, in bacteria) in the nucleolus. (These subunits generally are named according to their rate of sedimentation, measured in Svedberg units [S], in a centrifugal field. However, since these are not measures of molecular weight, the coefficients cannot be directly added.)

rRNAs play an active role in recognizing conserved portions of mRNAs and tRNAs.

Type Type of Ribosome Large Subunit Type of rRNAs found in large sub units Small Subunit Type of rRNAs found in small sub units
Prokaryotic 70S 50S 5s and 23s 30S 16s
Eukaryotic 80S 60S 5s, 5.8s, 28s 40S 18s

During protein synthesis rRNA provides attachment site to t-RNA and m-RNA and attaches them to ribosomes.The bonds formed between them are known as salt linkages. It attaches t-RNA to the larger subunit of ribosome and mRNA to smaller subunit of ribosome.

tRNA

It constitutes almost 10-15% of the total RNA.

It is a single stranded molecule, synthesised in the nucleus and is also known as soluble RNA or Adapter RNA.

It is the smallest RNA (4s).

The structure of tRNA can be best explained by Clover leaf Model presented by Holley.

This model explains that the 2-D structure of the tRNA appears clover leaf like while as in 3D structure it appears L-shaped.

The 5'end consists of guanine and at 3'end a conserved sequence of three nucleotides is present i.e. CCA, the 3'end is also known as the acceptor end.

The tRNA accepts amino acids at acceptor point.

Amino acid binds to 3'end by its -COOH group.

The structure of tRNA is a folded one and as a result some complementary nitrogenous bases come close to each other and are linked by hydrogen bonds.

Some regions where hydrogen bonds are not formed such regions are known as loops.

There are some unusual nitrogenous bases present in these regions like inosine, pseudouracil, dihydrouridine.

mRNA

mRNA constitutes 1 - 5% of the cell's total RNA.

mRNA is the transcript of genetic DNA in the nucleus and the process is known as Transcription.

It the most least stable form of RNA.