ribose pinto sugar, 2. a phosphate group, 3. a nitrogen bade. The sugar
can be two possible sugars: 1. Ribose gives RNA with the molecular
formula, C5H10O5, 2. Deoxyribose gives DNA with the molecular formula
C5H10O4. The phosphate’s molecular formula is H3PO4.
Building a nucleotide: The nucleotide is usually represented as follows.
Topic 2: The Chemistry of Life
Sub-topic: 2.4 DNA Structure
Page: 2
Since the reactions involves are condensation reactions, the equation
becomes phosphate + sugar + organic base = nucleotide + two waters.
2.4.2 1 State the names of the four bases of DNA.
The organic bases can be one of the five different bases: Adenine,
Guanine, Cytosine, Thymine, and Uracil. The bases found in DNA are:
C, T, A, and G. The bases found in RNA are: U, A, G, and C.
2.4.3 2 Outline how the DNA nucleotides are linked together by covalent
bonds into a single strand.
The nucleotide can be linked together by a covalent bond between the
phosphate of one nucleotide and the sugar of another. This way the sugar
and phosphate from the “backbone” of the nucleic acid with the nitrogen
base sticking out. Covalent bonds are formed between the phosphate and
(deoxy)ribose and a nitrogen base attached to every sugar. Three
nucleotides represent a codon, which typically codes for an amino acid.
Draw one below:
Topic 2: The Chemistry of Life
Sub-topic: 2.4 DNA Structure
Page: 3
2.4.4 3 Explain how a DNA double helix is formed using complementary base
pairing and hydrogen bonds.
Knowing that the structure of DNA is a double helix (twisted ladder) we
need to fit two DNA molecules together. Since organic bases form
hydrogen bonds with other organic bases, the DNA strand can be
combined. In DNA, adenine pairs with Thymine and vice versa, guanine
pairs with cytosine and vice versa. In RNA, adenine pairs with Thymine,
and Guanine pairs with Uracil and vice versa. The sugar phosphate run
antiparallel forming the sides of the ladder as the organic base pairs as
rungs in between.
2.4.5 1 Draw and explain a simple diagram of the molecular structure of
DNA. Be sure to indicate bonds.
Topic 2: The Chemistry of Life
Sub-topic: 2.4 DNA Structure
Page: 4
The diagram above is DNA as it would look if it was uncoiled. The DNA
molecule coils in suck a way that approximately 10 nucleotides complete
one turn of the helix. Adenine, thymine and uracil form two hydrogen
bonds, cytosine and guanine can form three hydrogen bonds. So, again,
adenine can form a base pair with thymine (or uracil in RNA) and cytosine
forms a base pair with guanine. All nitrogen bases have a complicated
molecular structure involving ring compounds. Adenine and guanine are
purines, big 2-ring structures. Cytosine, thymine and uracil are
pyrimidines, smaller 1-ring structures. In a DNA molecule the backbone
is made of alternating phosphates and sugar groups. The “rungs” of the
twisted ladder are the nitrogen bases. The available space for these rungs
is equal to three of the rings. This means to fill the space, an organic bade
pair must have a total of three rings. To allow them to have hydrogen
bonding, they must be capable of forming the same number of bonds.