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Molecular Basis for
Relationship between Genotype and Phenotype
genotype
DNA
DNA sequence
transcription
RNA
translation
protein
function
phenotype
organism
amino acid
sequence
DNA Structure
1.Nucleotides - Building Blocks
2.Double Helix
3.Base Pairing - Chargaff’s Rule
Refer to Figure 7-5 from Introduction to Genetic Analysis, Griffiths et al., 2012.
DNA Double Helix
Two strands are held together
by hydrogen bonds between
complementary bases
(Chargaff’s rules).
Adenine pairs with Thymine.
Guanine pairs with Cytosine.
Two strands are antiparallel to
each other.
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
Refer to Figure 7-9 from Introduction to Genetic Analysis, Griffiths et al., 2012.
Refer to Figure 1-5 from Introduction to Genetic Analysis, Griffiths et al., 2012.
Refer to Figure 1-6 from Introduction to Genetic Analysis, Griffiths et al., 2012.
Muntjac (2n = 6)
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)
Nucleosome contains an
octomer of histones (2 each
of H2A, H2B, H3, and H4).
DNA wraps twice around an
octomer.
Refer to Figure 1-9 from Introduction to Genetic Analysis, Griffiths et al., 2012.
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.
Refer to Figure 1-10 from Introduction to Genetic Analysis, Griffiths et al., 2012.