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NUCLEIC ACIDS (DNA AND RNA)

The polymers of nucleotides are called nucleic acids. The unit of nucleic acid.

1. Discovery of nucleic acids

F. Miescher isolated the nucleic acid from the nucleus of pus cells in 1870. As they were isolated from nucleus and they were acidic in nature, so hey were named as nucleic acid.

2. Types of nucleic acids

There are two types of nucleic acids

➤DNA or deoxyribonucleic acid:

DNA is present in chromosomes, in the nucleus of the cells. A small amount of DNA is also present in mitochondria and chloroplast

➤RNA or ribonucleic acid:

RNA: It is present in the nucleolus, ribosome and cytosol of cytoplasm. Small amount of RNA is also present in other part of the cell.

3. Structure of Nucleic acid:

Nucleic acids are complex substances. They are polymers of units called nucleotides. DNA is composed of deoxyribonucleotides and RNA is composed of ribonucleotides.

Nucleotides (Nitrogen base + Pentose sugar + phosphoric acid)

Each nucleotide is made up of three subunits pentose sugar, nitrogen base and phosphoric acid.

(a) Pentose sugar:

It is five-carbon monosaccharide. The nucleotides of RNA contain ribose sugar. On the other hand, the nucleotides of DNA contain deoxyribose sugar.

(b) Nitrogen bases:

Nitrogen bases are of two types.

➤Pyrimidines: These are composed of single ring. These are Cytosine (C), Thymine (T) and Uracil (U).

➤Purines:These are composed of double rings. These are Adenine (A) and Guanine (G)

(c) Phosphoric acid ( H₃PO_4 ): It has the ability to form ester linkage with OH group of Pentose sugar.

The nitrogen base is attached to the carbon 1 of the pentose sugar in a typical nucleotide. The phosphoric acid is attached with the carbon 3 of the pentose sugar. ATP (adenosine triphosphate) is also an important nucleotide. The cells use ATP as energy currency.

Nucleoside (Nitrogen base + pentose sugar)

The compound formed by the nitrogenous base and pentose sugar is called nucleoside. A nucleoside and phosphoric acid combine to form a nucleotide.

DNA (Deoxyribonucleic acid)

DNA is the hereditary material. It controls the properties and activities of a cell.

1. Components of DNA

DNA is made up of four kinds of nucleotides. These are

(i) d-adenosine monophosphate (d-AMP).

(ii) d-guanosine monophosphate (d-GMP).

(iii) d-cytidine monophosphate (d-CMP).

(iv) d-thymidine monophosphate (d-TMP).

The nucleotides are linked with one another by phosphodiester bonds and form a long chain. This chain is called Polynucleotide chain. This chain has a specific sequence. There may be different lengths of nucleotide chain.

።Dinucleotide: Two nucleotides join to form dinucleotide. Nicotinamide adenine dinucleotide (NAD) is a dinucleotide. It is important coenzyme. It takes part in several oxidation-reduction reactions in the cell.

።Trinucleotides: Three nucleotides join to form trinucleotide.

።Polynucleotides: Many nucleotides join to form polynucleotide.

2. Ratio of bases in DNA

Erwin Chargaff provided a data about the ratio of different bases of DNA

molecule. It shows that adenine and Thymine have equal ratio and guanine and

cytosine have equal ratio.

3. Watson & Crick model of DNA

Maurice Wilkins and Rosalind Franklin determined the structure of DNA by X-ray diffraction technique. Finally, James D. Watson and Francis Crickproposed the model of DNA. It suggests:

(i) The DNA is made up of two polynucleotide chains or strands.

(ii) These two strands coil around each other and form a double helix.

(iii) The coiling of the two strands is opposite (antiparallel to each other i. e. one strand is 3- 5 the other is 5-3).
(iv) The two strands are attached with each other by weak hydrogen bonds. Two hydrogen bonds are formed between A and T and three hydrogen bonds are formed between G and C pairs.
(v) Adenine (A) is always opposite to Thymine (T) and Guanine (G) is always opposite to Cytosine.
(vi) The two strands coil around each other. Each turn of the strands is about 34 Angstrom. This turn contains 10 base pairs. (One Angstrom = 10^-¹⁰ m or one 100 millionth of a centimetre).

4. Amount of DNA in different Species

The amount of DNA depends on the number of chromosomes in a species. Each species has fix number of chromosomes. So the amount of DNA is also fixed for a particular species. The germ cells (sperm and ova) contain one half the amounts of the somatic cells (body cells).

Amount of DNA in different types of cells of chicken (bird) and a Carp
Type of cell

(fish) in picograms (one pictogram = 10^-¹⁰ gram)

5. Importance of DNA

All the information for the structure and the functioning of a cell are stored in DNA. For example, E. coli (bacteria) contains single chromosome. This chromosome contains 5 million bases. These bases are arranged in a particular linear order. These bases contain information. The information are present in the form of units called "gene". A gene is a small part of DNA that controls a specific character. Gene is unit of biological inheritance.

(d) Haemophilus influenzae (a bacteria that cause influenza is the first microbe that genome have been completely sequenced. This sequencing was published in 1995.

RNA (RIBONUCLEIC ACID)

The polymers of ribonucleotides are called RNA. The RNA molecule has single strand. Sometimes, this strand may fold back to give double helical characteristics. The nitrogenous bases form complementary pairings. RNA have nitrogenous base Uracil in place of Thymine. Cytosine (C) forms pair with Guanine (G) and Uracil (U) forms pair with Adenine (A). DNA synthesizes RNA. The process of synthesis of RNA from DNA is called transcription.

Types of RNA

There are three main types of RNA. These are messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA). All three types of RNA are synthesized from DNA in nucleus. After their synthesis they are transferred to cytoplasm. All three RNA (mRNA, tRNA and rRNA) with each other. They synthesize proteins from the genetic information (gene).

(a) Messenger RNA (mRNA)

The mRNA brings genetic message from nucleus to the ribosome. The mRNA is about 3 to 4% of the total RNA in the cell. The ribosomes are present in the cytoplasm for the synthesis of the particular protein. DNA transfers its genetic information to mRNA. Now, this mRNA has genetic information for the synthesis of specific protein. This mRNA, attaches with the ribosome for synthesis of protein. The mRNA consists of single strand of variable length. Its size depends on size of gene (on DNA) for the specific protein. For example, the mRNA has 3,000 nucleotides for a protein of 1000 amino acids.

(b) Transfer RNA (tRNA)

The tRNA reads message (code) on mRNA and transfer specific amino acid to the ribosome. These amino acids are linked to form polypeptide chain of the protein. There is one specific tRNA for each amino acid. So the cell contains 20 types of tRNA. It forms about 10 to 20% of the total cellular RNA. The tRNA has small size. Its chain is composed of 75 to 90 nucleotides.

(c) Ribosomal RNA (rRNA)

The rRNA combines with ribosomal proteins and forms ribosome. Ribosome is made up of rRNA and protein. The rRNA forms 50 % of the ribosome. It forms a large part, about 80 %, of total RNA. It acts as machine for the synthesis of protein.

Nuclear Acid | DNA Or RNA |