Describe the key features of the watson-crick model for dna structure.

Watson and Crick proposed the double helical model of DNA.

Explanation:

- This model says that DNA exist as double helix. It has two polynucelotide strands running in opposite direction i.e the strands are anti-parallel.

- Each polynucleotide strand is made up of several nucleotide liknked together by phophodiester bonds.

- The two strands are not coiled upon each other but the double strand is coiled upon itself around a common axis like the spiral stair case. Here base pairs forms the stairs and backbone of two strands form railing. Back bone contains sugar and phosphate.

- The base pairing is specific, adenine always pairs with thymine and guanine always pairs with cytosine. Thus if we see any base pair, it consists of one purine and one pyrimidine.

- The two strands of DNA thus show complementary base pairing.

- The strands are held together because of hydrogen bonding between the bases in opposite strand.

- One full helical turn involves ten base pairs and the base pairs are stacked  3.4 A apart.

- This form is called as B-form of DNA.

answer: ice crystals in air

the water from the water body or another source of water gets evaporated as water vapor as an effect of heat on water. these water vapor rises in the upper layers of the earth atmosphere under high atmospheric pressure. the atmospheric pressure of these water molecules starts to decrease due to the effect of cold air. the invisible water vapor condenses into visible water droplets and ice crystals.these ice crystals and water droplets being less denser than the surrounding air floats on air as clouds.