1. DNA: The Chemical Nature of the Gene
Genetics

2. Genetic Material Possesses Several Key Characteristics
Life is characterized by tremendous diversity, but the coding instructions of all living organisms are written in the same genetic language—that of nucleic acids.
Surprisingly, the idea that genes are made of nucleic acids was not widely accepted until after 1950.
This doubt was due in part to a lack of knowledge about the structure of deoxyribonucleic acid (DNA).
Until the structure of DNA was understood, how DNA could store and transmit genetic information was unclear.
Even before nucleic acids were identified as the genetic material, biologists recognized that, whatever the nature of the genetic material, it must possess three important characteristics.

3. Genetic material must contain complex information.
The genetic material must be capable of storing large amounts of information—instructions for all the traits and functions of an organism.
This information must have the capacity to vary, because different species and even individual members of a species differ in their genetic makeup.
At the same time, the genetic material must be stable, because alterations to the genetic instructions (mutations) are frequently detrimental.

4. 2. Genetic material must replicate faithfully.
A second necessary feature is that genetic material must have the capacity to be copied accurately.
Every organism begins life as a single cell, which must undergo billions of cell divisions to produce a complex, multicellular creature like us.
At each cell division, the genetic instructions must be transmitted to descendant cells with great accuracy.
When organisms reproduce and pass genes on to their progeny, the coding instructions must be copied with fidelity.

5. 3. Genetic material must encode the phenotype.
The genetic material (the genotype) must have the capacity to “code for” (determine) traits (the phenotype).
The product of a gene is often a protein; so there must be a mechanism for genetic instructions to be translated into the amino acid sequence of a protein.

6. All Genetic Information Is Encoded in the Structure of DNA or RNA
DNA consists of a large number of linked, repeating units, called nucleotides; each nucleotide contains a sugar, a phosphate, and a base.

7. These findings became known as Chargaff ’s rules.
They discovered that, within each species, there is some regularity in the ratios of the bases: the amount of adenine is always equal to the amount of thymine (A = T), and the amount of guanine is always equal to the amount of cytosine (G = C).
These findings became known as Chargaff ’s rules.

8. Watson and Crick’s Discovery of the Three-Dimensional Structure of DNA
Watson and Crick investigated the structure of DNA, not by collecting new data but by using all available information about the chemistry of DNA to construct molecular models.
By applying the laws of structural chemistry, they were able to limit the number of possible structures that DNA could assume.
They tested various structures by building models made of wire and metal plates.

9. The model developed by Watson and Crick showed that DNA consists of two strands of nucleotides wound around each other to form a right-handed helix, with the sugars and phosphates on the outside and the bases in the interior.
They recognized that the double-stranded structure of DNA with its specific base pairing provided an elegant means by which the DNA can be replicated.

11. DNA Consists of Two Complementary and Antiparallel Nucleotide Strands That Form a Double Helix
The structure of DNA at three levels of increasing complexity, known as the primary, secondary, and tertiary structures of DNA.
The primary structure of DNA refers to its nucleotide structure and how the nucleotides are joined together.
The secondary structure refers to DNA’s stable three-dimensional configuration, the helical structure worked out by Watson and Crick.
DNA’s tertiary structures, which are the complex packing arrangements of double-stranded DNA in chromosomes.

12. The Primary Structure of DNA
The primary structure of DNA consists of a string of nucleotides joined
together by phospho-diester linkages.
Nucleotides :
DNA is typically a very long molecule and is therefore termed a macromolecule.
For example, within each human chromosome is a single DNA molecule that, if stretched out straight, would be several centimeters in length, thousands of times longer than the cell itself.
In spite of its large size, DNA has a quite simple structure: it is a polymer— that is, a chain made up of many repeating units linked together.

13. The repeating units of DNA are nucleotides, each comprising three parts: (1) a sugar, (2) a phosphate, and (3) a nitrogen-containing base.
a Sugar:
The sugars of nucleic acids—called pentose sugars— have five carbon atoms, numbered 1′, 2′, 3′, and so forth.
The sugars of DNA and RNA are slightly different in structure.
RNA’s sugar, called ribose, has a hydroxyl group (–OH) attached to the 2′-carbon atom, whereas DNA’s sugar, or deoxyribose, has a hydrogen atom (–H) at this position and therefore contains one oxygen atom fewer overall.
This difference gives rise to the names ribonucleic acid (RNA) and deoxyribonucleic acid (DNA).

14. This minor chemical difference is recognized by all the cellular enzymes that interact with DNA or RNA, thus yielding specific functions for each nucleic acid.
Furthermore, the additional oxygen atom in the RNA nucleotide makes it more reactive and less chemically stable than DNA. For this reason, DNA is better suited to serve as the long-term repository of genetic information.

16. 2- Nitrogenous base :-
Which may be of two types—a purine or a pyrimidine
Each purine consists of a six-sided ring attached to a five-sided ring, whereas each pyrimidine consists of a six-sided ring only.
Both DNA and RNA contain two purines, adenine and guanine (A and G), which differ in the positions of their double bonds and in the groups attached to the six-sided ring.
Three pyrimidines are common in nucleic acids: cytosine (C), thymine (T), and uracil (U).
Cytosine is present in both DNA and RNA; however, thymine is restricted to DNA, and uracil is found only in RNA.
A deoxyribose or a ribose sugar and a base together are referred to as a nucleoside.

18. 3- Phosphate group :-
The third component of a nucleotide is the phosphate group, which consists of a phosphorus atom bonded to four oxygen atoms.
Phosphate groups are found in every nucleotide and frequently carry a negative charge, which makes DNA acidic.
The phosphate group is always bonded to the 5′-carbon atom of the sugar in a nucleotide.

20. Secondary Structures of DNA
The secondary structure of DNA refers to its three-dimensional configuration—its fundamental helical structure.
DNA’s secondary structure can assume a variety of configurations, depending on its base sequence and the conditions in which it is placed.
The double helix A fundamental characteristic of DNA’s secondary structure is that it consists of two polynucleotide strands wound around each other—it’s a double helix.
The sugar–phosphate linkages are on the outside of the helix, and the bases are stacked in the interior of the molecule.
The two polynucleotide strands run in opposite directions—they are antiparallel, which means that the 5′ end of one strand is opposite the 3′ end of the other strand.

22. The strands are held together by two types of molecular forces:
1) Hydrogen bonds link the bases on opposite strands. These bonds are relatively weak compared with the
2) Covalent phospho-diester bonds that connect the sugar and phosphate groups of adjoining nucleotides on the same strand.

24. Different secondary structures :-
The three-dimensional structure of DNA described by Watson and Crick is termed the B-DNA structure.
This structure exists when plenty of water surrounds the molecule and there is no unusual base sequence in the DNA—conditions that are likely to be present in cells.
The B-DNA structure is the most stable configuration for a random sequence of nucleotides under physiological conditions, and most evidence suggests that it is the predominate structure in the cell.

25. Another secondary structure that DNA can assume is the A-DNA structure, which exists if less water is present.
Like B-DNA, A-DNA is an alpha (right-handed) helix, but it is shorter and wider than B-DNA.and its bases are tilted away from the main axis of the molecule.
A radically different secondary structure, called Z-DNA, forms a left-handed helix.

26. References
Benjamin A. Pierce, Genetics: A Conceptual Approach, 4th Edition. 4th Edition. W. H. Freeman.

27. The End