Chargaff's rule of DNA composition

Erwin Chargaff, an American biochemist who who emigrated to the United States during the Nazi era .  He discovered 2 rules that help to study the structure of double helix DNA. According to Chargaff's rule:

• DNA has equal number of adenine and thymine residues (A=T) and equal numbers of guanine and cytosine residues(G=C)
• This is the Chargaff”s rule of molar equivalence between the purines and pyrimidines in DNA structure.
• The double helical structure of DNA derives its strength from Chargaff’s rule.
• Single stranded DNA and RNAs which are usually single stranded do not obey the rule.
• Two possible helical forms of DNA are mirror images of each other.

Major bases in nucleic acid

As mentioned, nucleic acid is divided into DNA and RNA

Bases in DNA:

1. PURINES: adenine and guanine
2. PYRIMIDINES: thymine and cytosine

Bases in RNA:

1. PURINES: adenine and guanine
2. PYRIMIDINES: uracil and cytosine

Organization in the prokaryotic and eukaryotic

Organization of prokaryotic DNA:

• The DNA is organized as a single chromosome in the form of a double stranded circle.
• These chromosomes are packed in the form of nucleoids, by interaction with proteins and certain cations.

Organization of eukaryotic DNA:

• The DNA is associated with various proteins to form chromatin which then gets organized into compact structures, the chromosomes.
• The DNA double helix is wrapped around the core proteins, histones - basic in nature.
• The core is composed of two molecules of histones(H2A,H2B,H3& H4)
• Each core with two turns of DNA wrapped round it into nucleosome-basic unit of chromatic.
• The length of DNA is considerably reduced by formation of 10nm fiber.
• The continuous string of nucleosome represents – beads on a string form of chromatin.
• The 10nm fiber is further coiled to produce 30 nm fiber with 6 nucleosomes in every turns.
• These 30 nm fibers are further organized into loops by anchoring the fiber at A/T rich regions namely scafloldassociated regions(SARS) to a protein scafold.

Nucleotides

Nucleotides are molecules that make up the structure of either DNA and RNA. It plays an important role in metabolism too as it act as the source of chemical energy like ATP(adenosine triphosphate) or GTP(guanosine triphosphate), involve in cellular signaling and forms important cofactors in enzymatic reactions. It composed of :

1. A nitrogenous base
2. A pentose sugar (5C)
3. A phosphate

The functions of nucleotides included:

1. Building blocks or monomeric units in the nucleic acid structure.
2. Structural components of several coenzymes of B-complex vitamins e.g. FAD,NAD+
3. Carriers of high energy intermediates in the biosynthesis of carbohydrates, lipids & proteins e.g. UDP-glucose.
4. Involved in energy reactions of the cell e.g ATP is the energy currency of the cell.
5. Control several metabolic reactions by their action as allosteric regulators.

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Salient Features

The structure of DNA is complex and complicated. And now we are looking at its intrinsic.

1. The DNA is a right handed double helix. It consist of two polyribonucleotide chains (strands) twisted around each other on a common axis.
2. The two strands are antiparallel i.e., one strand runs in the 5’ to 3’direction while the other in 3’ to 5’ direction.
3. The width(or diameter) of a double helix is 20 A ˚(2nm).
4. Each turn (pitch)of the helix is 34A˚(3.4nm) with 10 pairs of nucleotides, each pair placed at a distance of about 3.4 A ˚(0.34nm).
5. Each strand of DNA has a hydrophillic deoxyribose phosphate backbone on the outside(periphery) of the molecule while the hydrophobic bases are stacked inside(core).
6. The two polynucleotide chains are not identical but complementary to each other due to base pairing.
7. The two strands are held together by hydrogen bonds formed by complementary base pairs. The A-T pair has 2 hydrogen bonds while the C-G pair has 3 hydrogen bonds. The G-C is stronger by about 50% than A-T.
8. The hydrogen bonds are formed between a purine and pyrimidine only. The only base arrangement possible in DNA structure is A-T, T-A , G-C, C-G.
9. The complementary base pairing in DNA helix proves Chargaff’s rule. The content of adenine equals to that of thymine and guanine equals cytosine.
10. The genetic information resides on one of the two strands known as template strand or sense strand. The opposite strand is antisense strand. 
11. The double helix has (wide) major grooves and (narrow) minor grooves along the phosphodiester backbone.

DNA replication

DNA replication is the basis of biological inheritance and it is a fundamental process that occur in every living organisms. In replication, each strand of original double-stranded DNA serves as template for the reproduction of the the complementary strand. Therefore, following DNA replication, two identical DNA molecules have been produced from a single double-stranded DNA molecule.

DNA replication inside the cell begins at a specific location called 'origins'. Unwinding of DNA at the origin, and synthesis of new strands, forms a replication fork. In addition to DNA polymerase, the enzyme that synthesizes the new DNA by adding nucleotides that matched to the template strand, a number of other proteins are associated with the fork and assist in the initiation and continuation of DNA synthesis.  Whereas for DNA replication that perform replication outside the cell, DNA polymerase isolated from cells and artificial primers are used to initiate the DNA synthesis at known sequences in a template molecule. The polymerase chain reaction (PCR), a common laboratory technique, employs such artificial synthesis in a cyclic manner to amplify a specific target DNA fragment from a pool of DNA.

Nucleic acids

The nucleic acids are divided into two types which is:

• deoxyribonucleic acid (DNA)
• ribonucleic acid (RNA)

Functions of nucleic acid:

 DNA

1.  Chemical basis of heredity-bank of genetic information.
2. Maintain the identity of different species over millions of years.
3. Every aspect of cellular function is under the control of DNA.
4. DNA is organized into genes, the fundamental units of  genetic information.
5. Genes control the synthesis of protein through the mediation of RNA.

RNA

1. Messenger RNA(mRNA) - Specifies the sequence of amino acids in the protein synthesis, the   translation.
2. Ribosomal RNA(rRNA) - It is found in combination with proteins and associated with the structure and function of ribosomes, the site of protein synthesis.
3. Transfer RNA(tRNA) - To deliver amino acids to ribosomes for protein synthesis.
4. RNA is the genetic material that carrying hereditary information in many viruses.