Molecular Basis for

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Molecular Basis for Relationship between Genotype
and Phenotype
DNA
genotype
DNA sequence
transcription
RNA
translation
amino acid sequence
protein
function
organism
phenotype
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DNA Structure

• Nucleotides - Building Blocks
• Double Helix
• Base Pairing - Chargaffs Rule

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DNA Double Helix
Two strands are held together by hydrogen bonds
between complementary bases (Chargaffs
rules). Adenine pairs with Thymine. Guanine
pairs with Cytosine. Two strands are
antiparallel to each other.
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DNA Double Helix
Nucleotides are linked 5-3 by phosphodiester
bonds. A - T pair (2 hydrogen bonds) G - C pair
(3 hydrogen bonds) Note antiparallel
nature 5 - 3 direction for one strand 3 - 5
direction for the other
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Muntjac (2n 6)
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Three-Dimensional Structure of Chromosomes
Human cell contains 1 meter of DNA per chromosome
set. Each metaphase chromosome appears as one
continuous fiber of 30 nm width.
Each chromosome contains one molecule of DNA.
How is DNA packaged? (Electron micrograph of
honeybee chromosomes)
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Nucleosome contains an octomer of histones (2
each of H2A, H2B, H3, and H4). DNA wraps
twice around an octomer.
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Model of a supercoiled chromosome during cell
division. Note the densely packed loops.
Relative scale of DNA and nucleosomes.
Scaffold is non-histone protein component that
stabilizes the chromosome structure.