Chapter 2: Nucleic Acids
Structure of DNA and RNA
Nucleotide structure
Nucleotides are made up of a pentose sugar, a phosphate
group and a nitrogen-containing organic base: Cytosine,
Thymine, Uracil, Adenine and Guanine.
Two mononucleotides are joined as a result of a condensation
reaction between the deoxyribose sugar of one
mononucleotide and the phosphate group of another.
The bond formed between them is called a phosphodiester
bond. The new structure is called a dinucleotide. The continued
linking of mononucleotides in this way forms a long chain
known as a polynucleotide.
RNA
Ribosomes are formed from RNA and proteins. RNA is a
polymer made up of nucleotides.
It is a single, relatively short, polynucleotide chain in which the
pentose sugar is always ribose, and the organic bases are
adenine, guanine, cytosine and uracil.
DNA
DNA structure
In DNA, the pentose sugar is deoxyribose, and the organic
bases are adenine, thymine, guanine and cytosine. DNA is
made up of two strands of nucleotides (polynucleotides).
Each of the two strands is extremely long, and they are
joined together by hydrogen bonds formed between
certain bases.
Base pairing
• Adenine always pairs with thymine.
• Guanine always pairs with cytosine.
The quantities of adenine and thymine in DNA are always
the same, and so are the quantities of guanine and
cytosine. However, the ratio of adenine and thymine to
guanine and cytosine varies from species to species. 1
, The stability of DNA
The phosphodiester backbone protects the more chemically
reactive organic bases inside the double helix.
Hydrogen bonds link the organic base pairs forming bridges
(rungs) between the phosphodiester uprights. As there are three
hydrogen bonds between cytosine and guanine, the higher the
proportion of C-G pairings, the more stable the DNA molecule.
Structure of DNA related to its functions
• Weak hydrogen bonds between bases can be separated
during DNA replication and protein synthesis.
• Double helix with sugar-phosphate backbone protects
the genetic information from being corrupted by outside
chemical and physical forces.
• It is an extremely large molecule, stores lots of genetic
information.
• Double helix (coiled) helps the structure to be compact.
DNA Replication
1. The enzyme DNA helicase breaks the
hydrogen bonds linking the base pairs of
DNA. As a result, the double helix separates
into its two strands.
2. Each exposed polynucleotide strand then
acts as a template to which complementary
free nucleotides bind by specific base
pairing.
3. Nucleotides are joined together in a
condensation reaction by the enzyme DNA
polymerase to from the 'missing'
polynucleotide strands of DNA.
4. Replication is semi-conservative – each new
strand formed contains one original /
template strand and one new strand.
Evidence - Meselson and Stahl
Watson and Crick discovered the structure of DNA in 1953 helped by
Rosalind Franklin's research on x-ray diffraction.
Hypothesis 1: Semi-conservative replication
• Each replicated DNA molecule contains one of the original
DNA strand and one newly synthesised DNA strand from
free floating nucleotides. 2
Structure of DNA and RNA
Nucleotide structure
Nucleotides are made up of a pentose sugar, a phosphate
group and a nitrogen-containing organic base: Cytosine,
Thymine, Uracil, Adenine and Guanine.
Two mononucleotides are joined as a result of a condensation
reaction between the deoxyribose sugar of one
mononucleotide and the phosphate group of another.
The bond formed between them is called a phosphodiester
bond. The new structure is called a dinucleotide. The continued
linking of mononucleotides in this way forms a long chain
known as a polynucleotide.
RNA
Ribosomes are formed from RNA and proteins. RNA is a
polymer made up of nucleotides.
It is a single, relatively short, polynucleotide chain in which the
pentose sugar is always ribose, and the organic bases are
adenine, guanine, cytosine and uracil.
DNA
DNA structure
In DNA, the pentose sugar is deoxyribose, and the organic
bases are adenine, thymine, guanine and cytosine. DNA is
made up of two strands of nucleotides (polynucleotides).
Each of the two strands is extremely long, and they are
joined together by hydrogen bonds formed between
certain bases.
Base pairing
• Adenine always pairs with thymine.
• Guanine always pairs with cytosine.
The quantities of adenine and thymine in DNA are always
the same, and so are the quantities of guanine and
cytosine. However, the ratio of adenine and thymine to
guanine and cytosine varies from species to species. 1
, The stability of DNA
The phosphodiester backbone protects the more chemically
reactive organic bases inside the double helix.
Hydrogen bonds link the organic base pairs forming bridges
(rungs) between the phosphodiester uprights. As there are three
hydrogen bonds between cytosine and guanine, the higher the
proportion of C-G pairings, the more stable the DNA molecule.
Structure of DNA related to its functions
• Weak hydrogen bonds between bases can be separated
during DNA replication and protein synthesis.
• Double helix with sugar-phosphate backbone protects
the genetic information from being corrupted by outside
chemical and physical forces.
• It is an extremely large molecule, stores lots of genetic
information.
• Double helix (coiled) helps the structure to be compact.
DNA Replication
1. The enzyme DNA helicase breaks the
hydrogen bonds linking the base pairs of
DNA. As a result, the double helix separates
into its two strands.
2. Each exposed polynucleotide strand then
acts as a template to which complementary
free nucleotides bind by specific base
pairing.
3. Nucleotides are joined together in a
condensation reaction by the enzyme DNA
polymerase to from the 'missing'
polynucleotide strands of DNA.
4. Replication is semi-conservative – each new
strand formed contains one original /
template strand and one new strand.
Evidence - Meselson and Stahl
Watson and Crick discovered the structure of DNA in 1953 helped by
Rosalind Franklin's research on x-ray diffraction.
Hypothesis 1: Semi-conservative replication
• Each replicated DNA molecule contains one of the original
DNA strand and one newly synthesised DNA strand from
free floating nucleotides. 2
