Download Lecture 3

Genetics
Instructor: Dr. Jihad Abdallah
Lecture 3
Structure of DNA
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• In 1953, James Watson and Francis Crick proposed
that DNA is a double-stranded molecule twisted into
a helix (the double helix structure)
• Each spiraling strand, comprised of a sugarphosphate backbone and attached bases, is connected
to a complementary strand by non-covalent hydrogen
bonding between paired bases.
• These two strands run in opposite directions to each
other and are therefore anti-parallel.
• The bases are adenine (A), thymine (T), cytosine (C)
and guanine (G).
• A and G are purines while T and C are pyrimidines
• A and T are connected by two hydrogen bonds. G
and C are connected by three hydrogen bonds.
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• In the DNA molecule, the sum of Purines (A and G)
is equal to the sum of the Pyrimidines (T and C)
• The distance between two base pairs is 3.4 A°
(Angistrom).
• Each complete turn of the helix is 34 A° (10 base
pairs).
• In any segment of the molecule, alternating larger
(major grooves) and smaller “minor grooves” are
apparent along the axis.
• The double helix measures 20 A° (2 nm) in diameter.
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DNA molecule
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Building units of DNA are called nucleotides
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(Pyrimidines )
(Purines)
The double helix of the DNA is shown along with details of how the bases, sugars
and phosphates connect to form the structure of the molecule.
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3.4 A°
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Nucleotides
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Nucleotides are the building blocks of DNA. Each
nucleotide consists of 3 components:
- Nitrogen base
- Pentose sugar
- Phosphate group
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Nitrogen bases
• There are two kinds of nitrogen bases:
1. Purines: double ringed (include Adinine and
Guanine)
2. Pyrimidines: single-ringed (include Thymine,
Cytosine, and Uracil). Thymine is specific to DNA
and Uracil is specific to RNA.
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Pyrimidines
Purines
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Pentose sugars
Found in RNA
Found in DNA
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Polynucleotides
• Are formed by the joining of mononucleotides.
• The linkage between two mononucleotides consists of
a phosphate group linked to two sugars forming a
phosphodiester bond
• The phosphate groups link the 3’ carbon of one
deoxyribose molecule to the 5’ carbon of the next (3’
– 5’ orientation).
• Joining of two mononucleotides forms a dinucleotide,
joining of three nucleotides forms a trinucleotide and
so on.
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Phosphate group forming
3` - 5` phosphodiester bond
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