Download DNA: The Genetic Material

DNA: The Genetic Material
• Frederick Griffith – 1928
– studied Streptococcus pneumoniae, a pathogenic
bacterium causing pneumonia
• 2 strains of Streptococcus
– S strain is virulent
– R strain is nonvirulent
• Griffith infected mice with these strains hoping to
understand the difference between the strains
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Live S strain cells killed the mice
Live R strain cells did not kill the mice
Heat-killed S strain cells did not kill the mice
Heat-killed S strain + live R strain cells killed the mice
• Transformation
– Information specifying virulence passed from the
dead S strain cells into the live R strain cells
• Our modern interpretation is that genetic material was
actually transferred between the cells
DNA: The Genetic Material
• Hershey & Chase –1952
– Investigated bacteriophages
• Viruses that infect bacteria
– Bacteriophage was composed of only DNA and
protein
– Wanted to determine which of these molecules is
the genetic material that is injected into the bacteria
• Bacteriophage DNA was labeled with radioactive
phosphorus (32P)
• Bacteriophage protein was labeled with radioactive
sulfur (35S)
• Radioactive molecules were tracked
• Only the bacteriophage DNA (as indicated by the 32P)
entered the bacteria and was used to produce more
bacteriophage
• Conclusion: DNA is the genetic material
DNA Structure
• DNA is a nucleic acid
• Composed of nucleotides
– 5-carbon sugar called deoxyribose
– Phosphate group (PO4)
• Attached to 5′ carbon of sugar
– Nitrogenous base
• Adenine, thymine, cytosine, guanine
– Free hydroxyl group (—OH)
• Attached at the 3′ carbon of sugar
• Phosphodiester bond
– bond between adjacent nucleotides
– formed between the phosphate group of one
nucleotide and the 3′ —OH of the next nucleotide
• The chain of nucleotides has a 5′-to-3′ orientation
• Phosphodiester
bond
– bond between
adjacent
nucleotides
– formed between
the phosphate
group of one
nucleotide and
the 3′ —OH of
the next
nucleotide
• The chain of
nucleotides has a
5′-to-3′ orientation
Chargaff’s Rules
• Erwin Chargaff determined that
– Amount of adenine = amount of thymine
– Amount of cytosine = amount of guanine
– Always an equal proportion of purines (A and G)
and pyrimidines (C and T)
Rosalind Franklin
• Performed X-ray diffraction studies to identify the 3-D
structure
– discovered that DNA is helical
– using Maurice Wilkins’ DNA fibers, discovered that
the molecule has a diameter of 2 nm and makes a
complete turn of the helix every 3.4 nm
James Watson and Francis Crick – 1953
• Deduced the structure of DNA using evidence from
Chargaff, Franklin, and others
• Did not perform a single experiment themselves
related to DNA
• Proposed a double helix structure
Double Helix
• 2 strands are
polymers of
nucleotides
• Phosphodiester
backbone –
repeating sugar and
phosphate units
joined by
phosphodiester
bonds
• Wrap around 1 axis
• Antiparallel
• Complementarity
of bases
• A forms 2
hydrogen bonds
with T
• G forms 3
hydrogen bonds
with C
• Gives consistent
diameter
DNA Replication
• Requires 3 things
– Something to copy
• Parental DNA molecule
– Something to do the copying
• Enzymes
– Building blocks to make copy
• Nucleotide triphosphates
• DNA replication includes
– Initiation – replication begins
– Elongation – new strands of DNA are
synthesized by DNA polymerase
– Termination – replication is terminated
DNA Replication is Semiconservative
• DNA polymerase
– Matches existing DNA bases with complementary
nucleotides and links them
– All have several common features
• Add new bases to 3′ end of existing strands
• Synthesize in 5′-to-3′ direction
• Require a primer of RNA
Semidiscontinous
• DNA polymerase can synthesize only in 1 direction
• Leading strand synthesized continuously from an
initial primer
• Lagging strand synthesized discontinuously with
multiple priming events
– Okazaki fragments
• Partial opening of helix forms replication fork
• DNA primase – RNA polymerase that makes RNA
primer
– RNA will be removed and replaced with DNA
DNA Repair
•
Errors due to replication
– DNA polymerases have proofreading ability
• Mutagens – any agent that increases the number of
mutations above background level
– Radiation and chemicals
• Falls into 2 general categories
1. Specific repair
– Targets a single kind of lesion in DNA and repairs
only that damage
2. Nonspecific
– Use a single mechanism to repair multiple kinds of
lesions in DNA
Photorepair
• Specific repair mechanism
• For one particular form of damage caused by UV light
• Thymine dimers
– Covalent link of adjacent thymine bases in DNA
• Photolyase
– Absorbs light in visible range
– Uses this energy to cleave thymine dimer
Excision Repair
• Nonspecific repair
• Damaged region is removed and replaced by DNA
synthesis
• 3 steps
1. Recognition of damage
2. Removal of the damaged region
3. Resynthesis using the information on the
undamaged strand as a template