Dna and Proteins

Compare and contrast the biopolymer DNA (polynucleotide) and protein (polypeptide) in terms of their composition, structure and synthesis

Composition and structure of DNA (polynucleotide)

DNA, also known as deoxyribonucleic acid consists of two strands which coil around each other in opposite directions to form a double helix. The two strands are biopolymers which are known as polynucleotides composed of smaller units called nucleotides [3]. Both strands are composed of sugar-phosphate backbone. The sugar molecule is known as deoxyribose which contains 5 carbons within the molecule. The 3C of one sugar molecule is connected through a phosphate group to the 5C of the next sugar molecule. This linkage is also known as ‘‘3’-5’ phosphodiester linkage’’ [4]. All the DNA strands are read from 5’ to 3’ end where the ‘‘5’ end terminates in a phosphate group and the 3’ end terminates in a sugar molecule’’ [4]. [1][2][3][4][5]

Figure 1: An image showing the phosphodiester linkage. [4]

DNA also consists of 4 possible organic bases (nitrogenous bases): adenine, guanine, cytosine and thymine. Adenine and guanine belong to purines, meaning that they only contain 2 rings in their structure. Cytosine, thymine and uracil belong to pyrimidines, meaning they only have 1 ring in their structure. Purines are larger molecules than pyrimidines. [1][2][3][4][5]

The sugar group and the organic base join together to form nucleoside with glycosidic bonds. The nucleoside then joins together with the phosphate group to form a nucleotide. Hence the sugar group, the phosphate group and an organic base join together to form a single nucleotide as shown below.

Figure 2: A single nucleotide showing the joining of the sugar group, phosphate group and the organic base (CHEMGUIDE, CLARK, J.) [2]

The two DNA biopolymer strands coil in such a way so that thymine always pairs up with adenine and cytosine always pairs up with guanine hence why they coil in opposite...