Nucleic Acid – Discovery , Significance & Types

Nucleic acid is polymer of nucleotides.

Discovery

A German Chemist. Friedrich Miescher. discovered Nucleic in 1869. Ile discovered DNA only four years after the publication of Menders work. Miescher extracted a white substance from the nuclei of human cells and fish sperm. He called this substance nuclein as it was associated with the nucleus. The nuclein was acidic in character. So the nuclei]] was called nucleic acid.

Research revealed its significance in the. last half of the 20th cent. RNA was first made by laboratory synthesis in 1955. In 1965 the nucleotide sequence of tRNA was determined. In 1967 the synthesis of biologically active DNA was achieved. The -amount of RNA varies from cell to cell. But the amount of DNA is normally constant for all typical cells of a given species of plant or animal.

Significance

Nucleic acids are Ibund in the chromosomes of living cells and

,viruses. They play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis. In most organisms, nucleic acids occur in combination with proteins. The combined substances are called nucleoproteins. Nucleic acid molecules are complex chains of varying length. The two chief types of nucleic acids are DNA (deoxyribonucleic acid). DNA carries the hereditary information from generation to generation. RNA (ribonucleic acid) delivers the instructions to the cell’s protein manufacturing sites (ribosome).

Chemical composition

P.A. Levene determined the basic structure of nucleus acid in 1920’s. Nucleic acid molecules are made of repeating units called nucleotide. Ile found that nucleotide contains three main components:

(a)    Phosphate (PO₄ groups)

(b)    Five carbon sugars: ribose and deoxyribose

(c)    Nitrogen containing bases. There are two types of bases:

• Purines: These are adenine. A. and guanine. G
• Pyrimidines: These are thymine. T and cytosine, C. RNA contains uracil, U instead of T.

Erwin Chargaff proved that the amount of adenine in DNA always equals the amount of thymine. Similarly amount of guanine always equals the amount of cytosine. Thus there is always equal proportion of purine (A+G) and pyrimidine

(C+T).

Bonding in Nucleic acid

Nitrogen base is attached to carbon number 1 of a pentose sugar in nucleotide. Phosphate group is attached to carbon number 5 of the sugar. A free hydroxyl (OH) group is attached to the 3′ carbon atom. The 5′ phosphate and 3 hydroxyl groups react chemically with each other. A covalent bond is formed between them. It links tv.o nucleotides. This reaction releases a water molecule. So it is called dehydration reaction. It allows DNA and RNA to form long chains of nucleotides. The phosphate group is linked to the two sugars by a pair of ester (P-O-C) bonds. Thus the linkage between two nucleotides is called a phosphodiester bond. The two unit polymer of this reaction still has a free 5′ phosphate group at one end and a free 3′ hydroxyl group at the other. Thus it can link to other nucleotides. In this way. many thousands of nucleotides join together

in long chains. Linear strands of DNA or RNA always have a free 5′ phosphate group at one end and a free 3′ hydroxyl group at the other.

Types of nucleic acids

There are two types of nucleic acids: DNA and RNA

DNA (DEOXYRIBONUCLEIC ACID)

The nucleic acid polymer of deoxyribonucleotides is called DNA. DNA is hereditary material. It is present in nucleus. A small amount of DNA is also present in mitochondria and chloroplasts. DNA has follov,ing tbur important characteristics:

I. It is a genetic material. It codes for the sequence of amino acids in proteins and control protein snnthesis.

2. It replicates itself before cell division.
3. It is present in the nucleus of cukar)ode cells.
4. It can change over time for evolutional) changes.

Chemical composition of DNA

DNA is polymer of deoxy ribonucleotides. It contains four nitrogen bases: Adenine. guanine. thn mine and c>tosine. Uracil is absent in it.

Similarly it contains deoxyribose. Deoxyribose is deficient in one oxygen atom at carbon number 2. DNA is composed of two polynucleotide chains. DNA contains genes. Each gene controls specific characteristics. DNA is composed of four types of nucleotides:

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

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

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

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

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

Function of DNA: The intbrmation carried by a given gene is coded in the sequence of nucleotides. These nucleotide sequences determine the sequences of amino acids in the polypeptide chain of the protein. There are long portions of DNA called satellite DNA between the genes of DNA of higher organisms. Satellite DNA code no proteins. Sometimes junk DNA occurs within a gene. In this case. the coding portions are called exons and the noncoding (junk) portions are called introns. Junk DNA makes up 97% of the DNA in the human genome. Little is known of its purpose.

Double helix model of DNA (Watson and Crick model)

It was discovered in 1950s that covalent bonds are present in a nucleic acid. The scientists started work on discovery of the three-dimensional structure of DNA. Watson and Crick proposed the model of DNA:

I. The DNA is helical in shape. It is made up of two strands.

2. The helix has a uniform width of 2 nut
3. Its nitrogenous bases are 0.34 mn apart. Ten layers of base pairs are present on each turn of the helix. –
4. The phosphate groups were present outside the helix. But the nitrogenous bases are present in the interior of the double helix.
5. The double helix is a ladder like. It has rigid rungs. Its ladder twists in a spiral fashion. The side ropes are the equivalent sugar—phosphate backbones. The rungs are pairs of nitrogenous
   2. Franklin’s X-ray data indicate that the helix makes one full turn after every 3.4 nm of its length.
   3. The pairing of the nitrogenous bases is complement. Adenine pair with thymine (7) and guanine (C) with cytosine. Adenine and Guanine are larger bases. They have two ringed structures. They are called purine base. On the other hand. the cytosine and thymine are pyrimidine bases. They have single ring. In this way one purine and one pyrimidine combination keep the diameter of the DNA uniform.
   4. Both strands of the DNA afe anti parallel. One strand is in 5 —3 direction. The other strand is in 3 — 5 strands.
   5. The helix of DNA has two grooves. One is major groove and the other minor groove. Both These grooves alternate with each other.

   

GUANINE (G)             Cti °SINE (C)

Fig: Hydrogen bonding between base pairs

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 (1,1) 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 sy nthesized from DNA in nucleus. After their synthesis they are transferred to cytoplasm. All three RNA (mRNA. tRNA and rRNA ) interact %slat each other. They synthesize

proteins from the genetic in                (gene).

(a)        Messenger RNA (mRNA)

The mRNA brings genetic. message from nucleus to the ribosome. The rriRNA 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 niRNA.

Now. this iii RNA has genetic information for the synthesis of specific protein. ‘Ibis mRNA. attaches with the ribosome for sAnthesis of protein. The int2 \ A consist; of single strand of variable length. Its size depends on size of gene (on DNA) for the specific protein. For example. the inRNA has 3.000 nucleotides for a protein of 1.000 amino acids.

(b)Transfer RNA (tRNA)

The tlINA reads message (code) on ml2NA and transfer specific – amino acid to the ribosome. It has C 10 \ CF leaf like structure. It has icodes.1 hese amino acids are linked to form poi) peptide chain of

icprotein. I here H one [RNA or each amino acid. So the HI contains    t pes of tRN V It Iiinth about 10 to 20% of the total Hilidar RNA, lhe t RNA has snail ,i/e. Its chain is composed of 75 1111C ICH idcs,

C)Ribosomal RNA (1-12NA)

Flie rRNA combines with ribosomal proteins and forms ribosome. Ribosome is inade up of rR \ A and protein. I he r12N A idrins 50 ‘Hi of the ribosome. It forith, a large part. about 80 ‘t/o. of iota! RNA. It act; as machine for the N‘ ‘,thesis of protein.

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