2.3.3 DNA: Structure

Question 1

DNA Structure

Answer 1

The nucleic acid DNA is a polynucleotide
- it is made up of many nucleotides bonded together in a long chain
DNA molecules are made up 2 polynucleotide strands laying side by side, running in opposite directions
- the strands are said to be antiparallel
Each DNA polynucleotide stand is made up of alternating deoxyribose sugars and phosphate groups bonded together to form the sugar-phosphate backbones
These bonds are covalent bonds known as phosphodiester bonds
- the phosphodiester bonds link the 5-carbon of one deoxyribose sugar molecule to the phosphate group from the same nucleotide, which is itself linked by another phosphodiester bond to the 3-carbon of the deoxyribose sugar molecule of the next nucleotide in the strand
- each DNA polypeptide strand is said to have a 3’ end and a 5’ end (these numbers relate to which carbon on the pentose sugar could be bonded with another nucleotide)
- as the strands run in opposite directions (they are antiparallel), one if known as the 5’ to 3’ strands and the other is known as the 3’ to 5’ strand
The nitrogenous bases of each nucleotide project out from the backbone towards the interior of the double-stabbed DNA molecule

Question 2

Hydrogen Bonding

Answer 2

The 2 antiparallel DNA polynucleotide strands that make up the DNA molecule are held together by hydrogen bonds between the nitrogenous bases
These hydrogen bonds always occur between the same pairs of base:
- the purine adenine (A) always pairs with the pyrimidine thymine (T) - two hydrogen bonds are formed between these bases
- the purine guanine (G) always pairs with the pyrimidine cytosine (C) - three hydrogen bonds from between these bases
This is process is known as complementary base paring and the pairs are known as complementary base pairs

Question 3

Double Helix

Answer 3

DNA is not two-dimensional
DNA is described as a double helix
- this refers to the three-dimensional shape formed by the twisting of the DNA molecule