Answer:
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:
tropical, temperate, and polar
Answer: A) Two atoms share electrons so they can fill their outer shells
Explanation: A covalent bond is a type of bond formed by sharing equal number of electrons between two non-metal atoms so that both of them can attain a stable octet or duplet structure of noble gases.
In covalent bonding, there is no loss or gain of electrons between atoms. For example, the formation of chlorine molecule, chlorine atom has seven electrons in its outermost shell, remaining one electron to attain an octet structure of noble gases. Two chlorine atoms share one pair of electron, each chlorine atom donating one electron to form a pair so that each atom in the chlorine molecule attains an octet structure..
