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DNA Structure and Replication
Lifespan Gene
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Chromosomes
• Contain all
information for
an organism
• Karyotype =
arrangement of
chromosomes
Eukaryotic Chromosomes
• Chromatin =
– DNA
– Histone proteins
– condenses to
form strands
called
chromosomes,
– Strands occur just
prior and during
cell division
DNA
• Double
stranded,
coiled
molecule
Discovery of Hereditary Material
• Friedrich Miescher (1869)
– Isolates “nuclein” from nucleus
– was not called DNA until the 1920’s
– No notion it contained hereditary material
• Gregor Mendel (late 1800s)
– Pioneering work in genetics
– Traits are inherited discrete factors
Discovery of Hereditary Material
• Walter Sutton (early 1900s)
– Suggested chromosomes held hereditary factors
– No direct evidence for support
• Thomas Hunt Morgan (1910)
– Provided experimental evidence to support Sutton
– Identified X (sex) hormone
– worked with chromosomes of fruit flies and eye
color
Discovery of Hereditary Material
• Phoebus Levene
(1920s)
– Two types of nucleic
acid in cells,
composed of repeating
nucleotides
– Both types with nearly
identical structures
– DNA & RNA are the 2
types
Nucleotide Structure
• Three different
components
– Phosphate group
– Nitrogen containing
base (A,T,G,C,U)
– Five-carbon sugar
(ribose or deoxyribose)
Nitrogen Containing Bases
Discovery of Hereditary Material
• Oswald Avery (early 1940s)
– Preliminary evidence for DNA as hereditary
material
– Transformed bacteria by transferring DNA
Discovery of Hereditary Material
• Edwin Chargaff (late
1940s)
– Proportion of bases
varies in the DNA of
different type
organisms
– Portions of bases
roughly equal, (A=T
& C =G)
– Purines = pyrimidines
(A + G = C + T)
Discovery of Hereditary Material
• George Beadle & Edward Tatum (1950s)
– One-gene-one-enzyme theory
– Enzyme production under control of genes
• Alfred Hershey & Martha Chase (1952)
– Building of Avery’s work, confirmed DNA as
hereditary material
Discovery of Hereditary Material
• Rosalind Franklin & Maurice Wilkins (1953)
– Developed double helix model for DNA
– Evidence from X-ray diffraction
Discovery of Hereditary Material
• James Watson &
Francis Crick (1953)
– Structure of DNA
– Synthesized previous
research
– Nobel Prize in 1962,
along with Wilkins,
Franklin died before
nominations were
made
Structure of DNA
Discovery of Genetic Code
DNA is Self-Replicating
• Before Cells divide
– Enzymes break bonds between bases
– Complementary strands separate
– Complementary bases are added to strands
– Copy of DNA results
Replication Mechanisms
• Replication begins at various points
• Proceeds in both directions
Replication Mechanisms
• DNA molecule separates at its bases
• Forms split, or replication fork
• Each strand acts as a template
Replication Mechanisms
• Replication proceeds from 5‘ to 3‘ end
Genes
• One-gene-one-enzyme theory
– Production of a given enzyme is under control
of a specific gene
– Production of a given peptide is under control
of a single gene (revised statement)
Gene
• The unit of heredity
• Is a sequence of nucleotides
• Codes for amino acid sequences of
polypeptides or for RNA
DNA Sequences
• Genome = all genetic information
• Promoters = “start here” part of sequence
• Introns = extra sequences between
polypeptide-specifying portions, are not
expressed, interrupt most eukaryotic genes
• Exons = portions of a gene that are
expressed
Repeated Sequences
• Repetitive DNA
• 20-50% of
eukaryotic DNA
• May play
structural roles in
chromosomes
Repeated Sequences
• Telomeres
– At ends of chromosomes
– Shorten as cell replicates
– Cells do not function
when telomeres become
too short
Telomeres in yellow
Mutations
• Change can occur in DNA
• Point mutation or gene mutation
– Change in genetic message
• Chromosome mutation
Sources of Mutation
• Ionizing radiation
• Ultraviolet radiation
• Some chemicals
• Transposable
elements
Transposable Elements
• “jumping genes”
• Some DNA sequences move from one
position to another
• Make up more than 40% of human genome
• First reported by Barbara McClintock
Transposable Elements
• Significance
– Contribute to rate of mutation
– May cause visible changes
• Example: mottling in corn
End Chapter 9