Nucleic Acids -Bio-Molecules | Organic Chemistry PDF Download

Nucleic Acids

Nucleic acids are long-chain polymeric molecules, the monomer (the repeating unit) is known as the nucleotides and hence sometimes nucleic acids are referred to as polynucleotides.

• Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are two major types of nucleic acids. 
• DNA and RNA are responsible for the inheritance and transmission of specific characteristics from one generation to the other. There are prominently two types of nucleic acids known to us. 

Functions of Nucleic Acids

1. Nucleic acids are responsible for the transmission of inherent characters from parent to offspring.
2. They are responsible for the synthesis of protein in our body
3. DNA fingerprinting is a method used by forensic experts to determine paternity. It is also used for the identification of criminals. It has also played a major role in studies regarding biological evolution and genetics.

Two types of Nucleic Acids- DNA & RNA

Deoxyribonucleic Acid (DNA) 

Chemically, DNA is composed of a pentose sugar, phosphoric acid and some cyclic bases containing nitrogen.

• The sugar moiety present in DNA molecules is β-D-2-deoxyribose. The cyclic bases that have nitrogen in them are adenine (A), guanine (G), cytosine(C), and thymine (T). 
• These bases and their arrangement in the molecules of DNA play an important role in the storage of information from one generation to the next one. 
• DNA has a double-strand helical structure in which the strands are complementary to each other.

Ribonucleic Acid (RNA)

The RNA molecule is also composed of phosphoric acid, a pentose sugar and some cyclic bases containing nitrogen.

• RNA has β-D-ribose in it as the sugar moiety. The heterocyclic bases present in RNA are adenine (A), guanine (G), cytosine(C) and uracil (U). In RNA the fourth base is different from that of DNA. 
• The RNA generally consists of a single strand which sometimes folds back; that results in a double helix structure. There are three types of RNA molecules, each having a specific function:

1. messenger RNA (m-RNA)
2. ribosomal RNA (r-RNA)
3. transfer RNA (t-RNA)

Nucleotides and Nucleosides

• Nucleotide = Nitrogeneous base, Pentose sugar, Phosphate
• Nucleoside = Nitrogeneous base, Pentose sugar

Bases (Purine and Pyrimidine)

• Pyrimidine contains two pyridine-like nitrogens in a six-membered aromatic ring.
• Purine has 4 N’s in a fused-ring structure. Three are basic like pyridine-like and one is like that in pyrrole.

Pentose Sugars

There are two related pentose sugars:

• RNA contains ribose
• DNA contains deoxyribose

“The sugars have their carbon atoms numbered with primes to distinguish them from the nitrogen bases”

Structures of DNA

The Double Helical Structure of DNA

Two polynucleotide strands, running in opposite directions (anti-parallel) and coiled around each other in a double helix.

The strands are held together by complementary hydrogen- bonding between specific pairs of bases. Weak forces stabilize DNA.

• Stacking interactions (Van Der Waals forces)
• Hydrophobic effect
• Charge-charge
• Hydrogen bonding

Little contribution to stability

Large contribution to selectivity

Tertiary Structure of DNA

It is supercoiled. Each cell contains about two meters of DNA. DNA is “packaged” by coiling around a core of proteins known as histones.

The DNA-histone assembly is called a nucleosome. Histones are rich in lysine and arginine residues.

Processes in the Transfer of Genetic Information

Replication:

• Identical copies of DNA are made. DNA is replicated by the coordinated efforts of a number of proteins and enzymes. 
• For replication, DNA must be unknotted, uncoiled and the double helix unwound. Topoisomerase is an enzyme that unknots and uncoils DNA. Helicase is a protein that unwinds the DNA double helix. 
• DNA polymerase is an enzyme that replicates DNA using each strand as a template for the newly synthesized strand. DNA ligase: enzyme that catalyzes the formation of the phosphodiester bond between pieces of DNA. 
• Replication process is semi-conservative: Each new strand of DNA contains one parental (old, template) strand and one daughter (newly synthesized) strand. Unwinding of DNA by helicases is exposed by the DNA bases so that replication can take place. 
• Helicase hydrolyzes ATP in order to break the hydrogen bonds between DNA strands.

Transcription:

Stages of Transcription are

• Promoter Recognition
• Chain Initiation
• Chain Elongation
• Chain Termination

Translation: 

Genetic messages are decoded to make proteins. Translation, or protein synthesis, is directed in eukayotic cells by an mRNA molecule. Translation can be seen to occur in two phases: 

(1) information transfer, in which RNA base sequence of the mRNA determines the sequence of amino acids and 

(2) chemical processes, in which the peptide bonds between the adjacent amino acids are formed. The components required for translation include mRNA, ribosomes, tRNA, aminoacyl tRNA synthetases, and accessory proteins involved in initiation, elongation, and termination.

Components required for translation:

1. mRNA

2. Ribosomes

3. tRNA

Bonding in DNA

Why does DNA not contain Uracil?

• DNA damage from UV light, hydrolysis, oxidation
• If DNA contained Uracil, it would be unable to recognize a hydrolyzed cytosine
• In RNA, damage not as important, and T production is costly.