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DNA’s Discovery and Structure
Honors Objectives
SOL.BIO.6f
Today’s Objectives:

TSW investigate and understand the common
mechanisms of protein synthesis, including the
structure of DNA and its discovery.
Our Genetic Code
 The
genetic code is a sequence of
nucleotides in the __________ of
eukaryotic cells.
 There are two polymers of nucleic
acids, _____ and ______.
Important Scientists that
determined DNA’s Importance
Frederick Griffith – bacterial
transformation
 Oswald Avery – DNA = key to
transformation
 Alfred Hershey & Martha Chase
– Bacteriophage transformation
experiment
 Erwin Chargaff – base-pairing rules

Important Scientists in the “Race
for the Double Helix”

Maurice Wilkins & Rosalind Franklin
–
–
–

X-ray diffraction
DNA must be a some type of spiral (helix)
DNA’s diameter is constant and stable
James Watson & Francis Crick
–
–
Created the first accurate model of DNA
Won the Nobel Prize
DNA’s Structure
 DNA is
a double helix
 DNA contains four nucleotide bases:
adenine (A)
– cytosine (C)
– guanine (G)
– thymine (T)
–
Chargaff’s Rule

The bases form the “rungs” on the DNA ladder
by complementary pairing
A == T
C == G
T == A
G == C
A == T
T == A
Sugarphosphate
backbone
DNA Replication

Replication = Duplication
–
–
Happens in the S phase of Interphase
Must replicate before mitosis or meiosis I
Part of the double helix is unwound
 Replication in small pieces (Okazaki
fragments)
 Enzyme stitches pieces together later

Semi-Conservative Replication


One-half of
each new
molecule of
DNA is old
(template
strand)
One-half of new
molecule of
DNA is new
(complementary
strand)
What is a Gene?
A length of DNA on a chromosome
 May be in several parts

–
–

Exon – the expressed parts of the DNA
sequence
Intron – the intervening, “junk DNA”, not
expressed
Parts are cut, put together and then used
to make RNA and proteins
Odd Types of Genes
 Multigene
families
 Transposons (Barbara McClintock)
 Protective Genes
 Viruses
may give us new DNA that
can be kept over time to cause new
evolutionary changes!