WATSON AND CRICKS MODEL OF DNA

Modern concept of Gene

            “The terms “muton,” “cistron,” and “recon” were proposed by the American geneticist S. Benzer in 1957 to describe a definite gene function.”

CISTRON-

1. A segment of DNA that is involved in producing a polypeptide chain; it can include regions preceding and following the coding DNA as well as introns between the exons.
2. It is considered a unit of heredity; "genes were formerly called factors".
3. It is the smallest unit functioning as a transmitter of genetic information.
4. In modern molecular genetics the cistron is essentially synonymous with the gene. It can include regions preceding and following the coding DNA as well as introns.
5.  A DNA segment corresponding to one polypeptide chain plus the start-and-stop codon.
6. The smallest unit of genetic material that must be intact to function as a transmitter of genetic information; as traditionaly construed, approximately synonymous with gene.
7. The smallest unit of a chromosome in which alteration can be effective in causing a mutation

MUTON –

1. A gene when specified as the smallest hereditary element that can be altered by mutation.
2. The smallest section of a gene, mutation is a change in a muton, leading to the emergence of a new, mutant form of a given organism.
3. The muton corresponds to one pair of nucleotides in the DNA molecule and to a single nucleotide in viruses, which contain only single-stranded DNA or RNA.

RECON-

1. Gene as a unit of recombination is called recon.
2. It is smallest segment of DNA which can undergo recombination is called recon.
3. There may be more than one recon in a cistron.

NUCLEIC ACID

1.  The nucleic acid were first discovered by FRIEDRICH MEISCHER (1869) and named as nuclein.
2. It was renamed as nucleic acid by ALTMANN (1889) due to it’s acidic natures.
3. Nucleic acid are long chain polymer of nucleotides of very high molecular weight.
4. They are complex macromolecules composed of C, H, O, N and P.
5. These are polymers because they are formed from monomeric units called nucleotides.
6. They contain strong phosphoric acid (H₃PO₄), hence called as nucleic acid.
7. Nucleic acids are of two types-DNA (Deoxyribose nucleic acid) and RNA (Ribose nucleic acid ).

Deoxyribosenucleic acid (DNA)

“DNA is a double stranded, helical, high molecular weight, polymer compound, made up of from Deoxyribose sugar, phosphoric acid, and nitrogen bases. It contains genetic information in triplet codes”.

• DNA is the most important chemical of the living organisms.
• DNA is the genetic material in all organism except in some viruses.
• It is located mainly at chromosomes in nucleus but sometimes occurs in mitochondria and chloroplast.
• It is master molecule of cell which carries  hereditary information from generation to generation.
• Each chromosomes contain one molecule of DNA i.e. number of DNA molecules is equal to the number of chromosomes.
• DNA is linear, double stranded helical structure in eukaryotes and circular in prokaryotes.
• DNA is composed of three chemical components-

·         Deoxyribose sugar.

·         Phosphoric acid.

·         Nitrogen bases.

Deoxyribose sugar :

• The sugar in DNA is the Deoxyribose sugar (C₅H₁₀O₄).
• It is pentose sugar cantaining 5-carbon atom.
• It has 5 membered pentagonal ring formed by four carbon atoms and one oxygen atom.
• The four carbon atoms of the ring are numbered as 1, 2, 3, 4 and 5^th carbon atom is outside the ring from a part of CH₂ group.
• There are three –OH groups at 1, 3 and 5. And -H atoms at 1, 2, 3, 4^th numbers of carbon atoms.
• The sugar in DNA contains are oxygen atom less than ribose sugar (C₅H₁₀O₅) ,hence known as Deoxyribose sugar.

Phosphoric acid :

• It is an inorganic molecule present in the form of phosphoric acid (H₃PO₄).
• It has three reactive –OH groups of which two are involved in the formation of the sugar phosphate backbone of each DNA strand.
• It is joined with 3’ carbon of one sugar molecule and 5’ carbon of adjacent sugar molecule.

Nitrogen bases :

1.     These are the complex organic compound containing in their ring, so known as nitrogen base.

2.     They are of two types- 1.Purine and 2. Pyrimidine.

3.    Purine are larger and double ring compound e.g.  Adenine and Guanine.                        

4.     Pyrimidine are smaller and single ring compound e.g. Cytosine, Thymine and Uracil (Uracil is present in place of Thymine in RNA).                                                  

5.     In all, four nitrogen bases are present in DNA and Purine and Pyrimidine are present in the ratio 1:1.

                               

                                        

NUCLEOSIDES:

1.     A combination of Deoxyribose sugar and nitrogen base is known as nucleosides.

2.     The nitrogen base is attached to carbon number first of sugar to form nucleoside.

3.     As there are four types of nitrogen bases, the nucleosides are of four types-

a.     Deoxyribose sugar + Adenine= Deoxyadenosine.

b.    Deoxyribose sugar + Guanine=Deoxyguanosine.

c.     Deoxyribose sugar + Cytosine=Deoxycytidine.

d.    Deoxyribose sugar + Thymine=Deoxythymidine.

NUCLEOTIDE:

1.     The basic unit of DNA molecule consisting of a nucleoside and phosphoric acid is known as Nucleotide.

2.     The nucleotide of DNA are known as Deoxyribonucleotides.

3.     In a nucleotide, sugar molecule is in the center and nitrogen base is attached to  its first carbon atom and phosphate group at its fifth carbon atom.

4.    There are four types of nucleotide-                                         

Ø  Adenine nucleotide.

Ø  Guanine nucleotide.

Ø  Cytosine nucleotide.

Ø  Thymine nucleotide.