Topic 2.
NUCLEIC ACIDS
Topic Outline AUDIO
Nucleotides
DNA versus RNA
Nitrogenous Bases
DNA Structure
Types of RNA
Watson & Crick
DNA Organisation
DNA RNA
The structure of DNA allows efficient storage of genetic information
Nucleotides
DNA and RNA are composed of monomers called nucleotides, which contain:
• A pentose (5C) sugar, a phosphate group and a nitrogenous base (A, C, G, T/U)
Phosphate Group
CH2 O
5
4 1 Nitrogen Base
Pentose Sugar 3 2
Structure of a Nucleotide:
The nucleic acids DNA and RNA are polymers of nucleotides
DNA versus RNA
DNA and RNA can be differentiated by certain key structural characteristics:
DNA RNA
Pentose Sugar Deoxyribose Ribose
Nitrogenous Bases Has Thymine (T) Has Uracil (U)
Number of Strands Double stranded Single Stranded
(double helix)
Master copy of Temporary copy of
Basic Function
genetic instructions a specific instruction
DNA differs from RNA in the number of strands present, the base composition and the type of pentose
Nucleic Acid Comparison
Phosphate Group Phosphate Group
CH2 O CH2 O
5 5
4 1 4 1
R D
3 2 Nitrogen Base 3 2 Nitrogen Base
(A, C, G, U) (A, C, G, T)
OH H
Ribose Sugar Deoxyribose Sugar
RNA Nucleotide DNA Nucleotide
Drawing simple diagrams of the structure of single nucleotides of DNA and RNA
Polynucleotide Formation
Nucleotides are linked to form a single polynucleotide
strand via condensation reactions (water is produced)
The 5’-phosphate group of one nucleotide attaches to
the sugar of another nucleotide (at the 3’-hydroxyl group)
• Nucleotide strands will run in a 5’ → 3’ direction
The bond that is formed between the two nucleotides
is a covalent phosphodiester bond (shown as red line)
Nitrogenous Bases
DNA and RNA nucleotides are each composed of one of four nitrogenous bases
• Adenine and Guanine are double-ringed purines
• Cytosine and Thymine / Uracil are single-ringed pyrimidines
N N
H H
O H2N H NH3 H3C O H O
H N N N N H N H N H H N H
N N N H N N
H2N H O O O
Guanine Adenine Cytosine Thymine Uracil
(DNA only) (RNA only)
DNA Structure: Complementary Base Pairing
DNA molecules form a double-stranded structure (ladder) C G
• Sugar-phosphate backbones are connected by bases
A T
The nitrogenous bases from each of the two strands form G C
a complementary pairing connected by hydrogen bonds
T A
• Cytosine (C) pairs with Guanine (G) via three H-bonds
• Adenine (A) pairs with Thymine (T) via two H-bonds G C
Mismatched pairing would destabilise the DNA molecule DNA ‘Ladder’
DNA strands are linked by hydrogen bonding between complementary base pairs
DNA Structure: Double Helix
In order for two sets of nitrogenous bases to pair,
C G
the two strands must run in antiparallel directions
A T
• I.e. Pentose sugars ‘point’ in opposite directions C G
G C
The double-stranded molecule then twists in order
A T
to adopt the most stable energy configuration C G
T A
DNA forms a stable double helix arrangement
T A
• Roughly 10 – 15 base pairs per rotation
DNA is a double helix molecule made of two antiparallel strands of nucleotides
RNA Structure and Types
RNA is usually single-stranded but can form loops via complementary base pairing
• Three main types are messenger RNA, transfer RNA and ribosomal RNA
Messenger RNA Ribosomal RNA Transfer RNA
mRNA is a transcript copy rRNA is the catalytic tRNA transports amino
of a gene (codes for protein) component of ribosomes acids to the ribosome
DNA Elucidation: Watson & Crick
James Watson and Francis Crick first elucidated the structural
organisation of a DNA molecule (i.e. a double helix) in 1953
Their efforts were guided by the contributions of other scientists:
Watson
• Pauling – Molecular distances and bond angles
• Levene – Nucleotide composition (sugar, phosphate, base)
• Chargaff – Base composition (equal purine / pyrimidine number)
• Franklin – Helical structure (data shared without permission)
Crick
Crick and Watson’s elucidation of the structure of DNA using model making
DNA Elucidation: Model Making
Watson and Crick built models using trial and error
to quickly assess the viability of potential structures
They assembled a DNA model that demonstrated:
• Anti-parallel DNA strands
• Outer sugar-phosphate backbone
• Inner base pairing (complementary)
• Overall shape of a double helix
Crick and Watson’s elucidation of the structure of DNA using model making
DNA Organisation
In prokaryotes, DNA is naked (not bound to protein)
• Also, the DNA adopts a circular configuration
In eukaryotes, DNA is linear and is bound to histone
Prokaryotic DNA
proteins to form a compact structure (chromatin)
This long strand of chromatin may further condense
into a chromosome during the process of cell division
• Each chromosome contains a single DNA molecule
Eukaryotic DNA
Topic Review
Can you do the following?
• Identify the components of a nucleotide
• Compare DNA and RNA
• Describe the formation of a polynucleotide
• Explain the structure of a DNA double helix
• Outline complementary base pairing
• Discuss the contribution of Watson and Crick
• Describe how DNA is organised within cells
