Watson and Crick's model of DNA

The structure of DNA, as represented in Watson and Crick's model, is a double-stranded, antiparallel, right-handed helix. The sugar-phosphate backbones of the DNA strands make up the outside of the helix, while the nitrogenous bases are found on the inside and form hydrogen-bonded pairs that hold the DNA strands together.

In the model below, the orange and red atoms mark the phosphates of the sugar-phosphate backbones, while the blue atoms on the interior of the helix belong to the nitrogenous bases.

Antiparallel orientation

Double-stranded DNA is an antiparallel molecule, meaning that it's composed of two strands that run alongside each other but point in opposite directions. In a double-stranded DNA molecule, the 5' end (phosphate-bearing end) of one strand aligns with the 3' end (hydroxyl-bearing end) of its partner, and vice versa.

Right-handed helix

In Watson and Crick's model, the two strands of DNA twist around each other to form a right-handed helix. All helices have a handedness, which is a property that describes how their grooves are oriented in space.

Base pairing

In Watson and Crick's model, the two strands of the DNA double helix are held together by hydrogen bonds between nitrogenous bases on opposite strands. Each pair of bases lies flat, forming a "rung" on the ladder of the DNA molecule.

Base pairs aren't made up of just any combination of bases. Instead, if there is an A found on one strand, it must be paired with a T on the other (and vice versa). Similarly, a G found on one strand must always have a C for a partner on the opposite strand. These A-T and G-C associations are known as complementary base pairs.

Divergent evolution is the evolution of a number of different forms of animals or plants froms of a common ancestral form. The driving force behind, it is adaptations to newly involved habitat and the prevailing environmental conditions there. As the original population increases in size, it spreads out from its centre of origin to exploit new habitas and food resources. In this results in a number of populations each adapted to its particular habitat eventually these populations will differ from each other sufficiently to become new species. A good example of this process is the evolution of the Australian marsupials into species adapted as carnivores, herbivores, burrowers, fliers, etc. Another example is that of peritadactyl limb in mammals.  The flipper of a seal, wing of a bat, forelimb of a male, front legs of horse and the arm of a man perform different functions, but exhibit the same structural plan including same pentadactyl pattern of bones.         

Pollen grains represent the male portion of the reproductive process in plants and trees. These tiny bodies are swirling in the air and on the legs of insects so that they can join the female part of the plant to create a new seed. This important process is known as fertilization. Each pollen grain contains vegetative (non-reproductive) cells (only a single cell in most flowering plants but several in other seed plants) and a generative (reproductive) cell. In flowering plants the vegetative tube cell produces the pollen tube, and the generative cell divides to form the two sperm cells.

Every individual acquires genetic information from its parents. Sexual mode of reproduction involves two parents of opposite ***. An offspring receives 50% of genetic information from it father and remaining 50% from its mother. Offspring produced are not the exact copies of the parents. Minor differences between the offspring and parent are called as variations. Variations in offspring are due o genetic variations.
Maximum variations occur during sexual reproduction. This is because of the recombination of genetic material of two different individuals. There are more chances for variations to occur.

Colour blindness is a common hereditary (inherited) condition which means it is usually passed down from your parents.

Red/green colour blindness is passed from mother to son on the 23rd chromosome, which is known as the *** chromosome because it also determines ***. Chromosomes are structures which contain genes – these contain the instructions for the development of cells, tissues and organs. If you are colour blind it means the instructions for the development of your cone cells are faulty and the cone cells might be missing, or less sensitive to light or it may be that the pathway from your cone cells to your brain has not developed correctly.

The 23rd chromosome is made up of two parts – either two X chromosomes if you are female or an X and a Y chromosome if you are male. The faulty ‘gene’ for colour blindness is found only on the X chromosome. So, for a male to be colour blind the faulty colour blindness ‘gene’ only has to appear on his X chromosome. For a female to be colour blind it must be present on both of her X chromosomes.

If a woman has only one colour-blind ‘gene’ she is known as a ‘carrier’ but she won’t be colour blind. When she has a child she will give one of her X chromosomes to the child. If she gives the X chromosome with the faulty ‘gene’ to her son he will be colour blind, but if he receives the ‘good’ chromosome he won’t be colour blind.

A colour blind boy can’t receive a colour blind ‘gene’ from his father, even if his father is colour blind because his father can only pass an X chromosome to his daughters.

A colour blind daughter, therefore, must have a father who is colour blind and a mother who is a carrier (who has also passed the faulty ‘gene’ to her daughter). If her father is not colour blind, a ‘carrier’ daughter won’t be colour blind. A daughter can become a carrier in one of two ways – she can acquire the ‘gene’ from a carrier mother or from a colour blind father.

This is why red/green colour blindness is far more common in men than women.

Blue colour blindness affects both men and women equally because it is carried on a non-*** chromosome.

For the sake of the following explanation, a normal X chromosome is shown as (X) whilst a colour blind carrying X chromosome is shown in bold (X).

The colour-blind ‘gene’ is carried on one of the X chromosomes. Since men have only one X chromosome, if his X chromosome carries the colour blind ‘gene’ (X) he will be colour blind (XY). A woman can have either:-
(i) two normal X chromosomes, so that she will not be colour blind or be a carrier (XX),
(ii) or, one normal X and one colour blind carrying X chromosome, in which case she will be a carrier (XX), or rarely
(iii) she will inherit a colour blind X from her father and a colour blind X from her mother and be colour blind herself (XX). She will pass on colour blindness to all of her sons if this is the case.

the habitual taking of illegal drugs. i.e Drug Addiction. It is defined as psychological dependence on drug for the feeling of well-being.  The person becomes so habituated to drugs and body become tolerant to drugs that only high doses are needed for response.

NITROGEN CYCLE

• Nitrogen is a constituent of amino acids, proteins, hormones, chlorophylls and many of the vitamins.
• Plants compete with microbes for the limited nitrogen that is available in soil; thus, nitrogen is a limiting nutrient for both natural and agricultural eco-systems.
• Nitrogen exists as two nitrogen atoms joined by a very strong triple covalent bond (N ≡ N).
• The process of conversion of nitrogen (N2) to ammonia is termed as nitrogen fixation.
• Decomposition of organic nitrogen of dead plants and animals into ammonia is called ammonification.
• Some of this ammonia volatilises and re-enters the atmosphere but most of it is converted into nitrate by soil bacteria in the following steps:

2NH₃+ 3O₂ à 2NO₂^- + 2H⁺ + 2H₂0

2N0₂^- + O₂ à 2N0₃^-

• Ammonia is first oxidised to nitrite by the bacteria Nitrosomonas and/or Nitrococcus.
• The nitrite is further oxidised to nitrate with the help of the bacterium Nitrobacter and the step is known as nitrification.
• In the leaves, nitrates reduced to form ammonia that finally forms the amine group of amino acids, and the step is known as assimilation.
• Denitrification is the process in which nitrate in the soil is reduced to molecular nitrogen by Pseudomonas and Thiobacillus.

Fig. Nitrogen fixation