DNA
Replication of the genetic code
DNA = recipe book
• Instructions for ALL proteins are
encoded by DNA
• DNA resides in the nucleus
• To pass on instructions for life, need to
replicate DNA prior to reproduction
How do we know DNA is the
genetic code of life?
• Some late 19th century observations
of dividing cells gave us some clues
Observations
• Late 1800’s; Walther
Flemming sees “threads”
moving & changing during
cell division
• Threads appear paired
prior to cell division
• Paired threads separate
just prior to division
• Named the “thread
separating” process
Mitosis
Observations on thread #
• Thread # differs between species
– Roundworms = 4; Peas = 14; Humans = 46
• Thread # identical between individuals
within a species
– All roundworms = 4
• Same between cells within an individual
• Threads were named chromosomes which
consist of both DNA & protein
Is it DNA or proteins that
are important?
• Chromosomes consist of both, so how
did scientists identify which one
holds instructions for reproduction of
cells?
DNA’s “discovery”
• 1952: Hershey & Chase
find that bacteriophage
virus infects and
“reprograms” bacteria to
make more virus
– Consists only of external
protein coat and internal
DNA
– Inserts its DNA into
bacteria, protein coat
remains outside
• A perfect model!
Label DNA & protein separately
• Radioactive Sulfur incorporates into proteins
only. Why?
• Heavy bacterial cells settle, while lighter phage
particles remain in solution. (where’s the radioactivity?)
Label DNA & protein separately
• Radioactive Phosphorous incorporated into
DNA. Why?
• Heavy bacterial cells settle, while lighter phage
particles remain in solution. (where’s the radioactivity?)
DNA encodes instructions
for replication!
DNA’s structure
What’s it look like? Does its
structure suggest how
replication is accomplished?
Monomers of DNA = Nucleotides
• Repeated phosphate, sugar, base motif of ALL
nucleotides
• Phosphate-sugar backbone
• Base = only difference between nucleotides
Nitrogenous Bases
• Purines: G, A; 2 nitrogenous rings
• Pyrimidines: C, T; 1 nitrogenous ring
Who discovered the structure?
• J. Watson & F. Crick deduced doublestranded, helical structure from Rosalind
Franklin’s X-ray crystallographic image of
a DNA molecule.
Chemical structure
Conclusions
• Molecule is of uniform width
• Amounts of A & T are
identical; same for C & G
• H-bonds hold bases of
neighboring strands together
– suggests precise
complimentarity between
nucleotides
– Adenine always pairs with
Thymine; Cytosine always pairs
with Guanine
Extensions
• Sequence possibilities
are limitless (variation
in sequence could
account for the
diversity of life.)
• Those “threads”
(chromosomes) we saw
separating with dividing
cells must be DNA
molecules
Structure also suggests
mechanism of replication
• Pull strands apart; now each strand serves as
template for a new strand
• Semiconservative model: ½ parent molecule is
conserved in each daughter molecule
Replication
• Begins @ multiple
replication centers
• Helicase unwinds
and separates DNA
strands (bubble)
• DNA polymerase
adds bases
opposite the
template (parent
strand)
Structure determines
direction
• Strands are anti-parallel
• Each has a 5’ and a 3’ end
– Refers to Carbon atom in
sugar ring (i.d. purposes)
• DNA polymerase can only
add nucleotides to the 3’
end of a strand
Consequences of polarity
• One strand is
continuously replicated
• The other is replicated
in fragments (Okazaki
fragments)
• DNA ligase joins these
fragments to complete
the new molecule
• Other polymerases
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