NUCLEIC ACIDS

 

NUCLEIC ACIDS

PREPARED BY MR. ABHIJIT DAS

Nucleic acids are organic compounds that play a crucial role in the storage and transmission of genetic information in living organisms. They are composed of nucleotide units, which consist of a sugar molecule (either ribose or deoxyribose), a phosphate group, and a nitrogenous base (purin or pyrimidine). There are two main types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

NITROGENOUS BASES:

Nitrogenous bases in DNA and RNA are divided into two main types: purines and pyrimidines.

1.    PURINES: Adenine (A) and Guanine (G) are purines. They have a double-ring structure.

2.    PYRIMIDINES: Thymine (T) and Cytosine (C) in DNA, and Uracil (U) in RNA, are pyrimidines. They have a single-ring structure.

These bases pair up in a specific manner (A with T/U and G with C) to form DNA or RNA.

NUCLEOTIDE FORMATION:

A nucleotide is the basic structural unit of nucleic acids like DNA and RNA. It is composed of three main components:

1.    Sugar: The sugar in DNA is deoxyribose, while in RNA, it's ribose.

2.    Phosphate Group (H₃PO₄): It provides the negative charge in the DNA or RNA molecule.

3.    Nitrogenous Base: There are four possible nitrogenous bases in DNA: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). In RNA, Thymine is replaced by Uracil (U).

A nucleoside is formed by combining a nitrogenous base with a sugar molecule. The process involves a glycosidic bond formation between the nitrogenous base and the sugar.

A nucleotide is composed of a nucleoside (nitrogenous base + sugar) and a phosphate group. The phosphate group is added to the 5' carbon of the sugar molecule.

STRUCTURE OF DNA (WATSON AND CRICK MODEL):

The Watson-Crick model, proposed by James Watson and Francis Crick in 1953, describes the structure of DNA, the molecule that carries genetic information. The key features of the Watson-Crick model are:

1.    Double Helix Structure:

·        DNA is a double-stranded molecule, forming a helical structure resembling a twisted ladder or spiral staircase.

·        The two strands run in opposite directions, a configuration referred to as antiparallel.

2.    Complementary Base Pairing:

·        Adenine (A) always pairs with Thymine (T) through two hydrogen bonds.

·        Cytosine (C) always pairs with Guanine (G) through three hydrogen bonds.

·        This complementary base pairing ensures that the two strands are held together by hydrogen bonds between the paired bases.

3.    Base Pairing Specificity:

·        The specificity of base pairing is such that the amount of adenine is equal to the amount of thymine, and the amount of cytosine is equal to the amount of guanine. This is known as Chargaff's rules.

4.    Sugar-Phosphate Backbone:

·        The backbone of the DNA double helix is composed of alternating sugar (deoxyribose) and phosphate groups.

·        The sugar-phosphate backbone runs along the outer edges of the helical structure.

SOME BASIC INFO:

1.    Base Pairs in One Turn:

·        In one complete turn of the DNA helix, there are approximately 10 base pairs.

2.    Pitch:

·        The pitch is the distance along the helical axis required for one complete turn of the helix. In DNA, the pitch is about 34 angstroms (or 3.4 nanometers).

3.    Angle:

·        The helical twist of DNA results in an angle between adjacent base pairs. This angle is approximately 36 degrees.

4.    Distance Between Two Base Pairs:

·        The distance between the centers of two adjacent base pairs in the DNA helix is approximately 3.4 angstroms (or 0.34 nanometers).

5.    Total Angle of One Turn:

·        The total angle of one complete turn of the DNA helix is 360 degrees.

RNA AND THEIR FUNCTIONS:

RNA, or ribonucleic acid, is an organic molecule with various functions in the cell

1.    Messenger RNA (mRNA):

·        Function: Carries genetic information from the DNA in the cell nucleus to the ribosomes in the cytoplasm. The information is then used to synthesize proteins through a process called translation.

2.    Ribosomal RNA (rRNA):

·        Function: Forms an integral part of ribosomes, the cellular structures where protein synthesis occurs. Ribosomes are composed of rRNA and proteins, and they facilitate the assembly of amino acids into proteins.

3.    Transfer RNA (tRNA):

·        Function: Transfers specific amino acids to the ribosome during protein synthesis. Each type of tRNA is linked to a particular amino acid and has an anticodon that pairs with the complementary codon on mRNA.