Download DNA - Santa Susana High School

DNA
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Timeline to the discovery of DNA:
1928 – Fredrick Griffith discovers non-virulent bacteria (Streptococcus
pneumoniae) become virulent when in contact with dead pathogenic bacteria
– calls the process transformation
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1944 – Oswald Avery (&colleagues) announced that DNA, not proteins, transferred
the genetic material that causes transformation
1947 – Erwin Chargaff demonstrated that although different organisms had
differing amounts of DNA, the ration of the 4 nucleotides remained constant.
Human DNA showed the following:
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A - 30.3%
T - 30.3%
G - 19.5%
C - 19.9%
The similarity ofA/T&C/G became known as Chargaff's rule.
1952 – Alfred Hershey & Martha Chase formed experiments showing that viral
DNA from the T2 bacteriophage (bacteria-eater) transforms E. coli and changes its
function
Early 1950s -many scientists, including Linus Pauling, Maurice Wilkins, and
Rosalind Franklin had discovered the arrangement of bonds of a single strand of
DNA
In 1953, James Watson and Francis Crick
discovered the structure of DNA .The discovery
would not have been possible, though, without the
work of Rosalind Franklin. Franklin's image of DNA
using X-ray crystallography allowed Watson to
calculate out the size and structure of the double
helix of DNA.
• From this picture Watson & Crick assembled
their model of double helix model of DNA with
the following properties:
– both strands are anti-parrallel
• 5' to 3'
• moving in opposite directions
– the sugar backbone (deoxyribose) resides on the
outside
– nucleotides are on the inside and paired in the
following function
• purines (A & G) are attached to pyrimidines (C & T)
– A is attached to T with two hydrogen bonds
– C is attached to G with three hydrogen bonds
• explained the basis for Chargaff's rule
Double Helix
DNA Replication
• DNA Replication is a semiconservative process where the new DNA
is copied onto a parental (conserved) strand. It takes place with
surprising efficiency and speed copying ~10 billion base pairs in a
few hours with little or no errors.
• Origin of replication: site of initiation of replication
– bacteria have a single site while Eukaryotes have multiple sites
– proteins recognize site and open up a replication bubble
– as replication begins a replication forks form as replication proceeds in
both directions
• nucleoside triphosphates are added 1 at a time by DNA polymerase (~50/sec)
in the 5' to 3' direction (copied 3' to 5')
• energy powering exergonic process comes from cleaving two of the 3 Pi from
the molecule
– replication forks eventually fuse completing the newly formed strands
• Antiparallel elongation
– since nucleotides can only be added to
the 3' end of the newly forming strand,
different mechanisms must be in place
for the antiparallel strand
• leading strand - 3' to 5'
– an RNA primer (5-10 nucleoside long fragment)
is needed for attachment of DNA pol III
» RNA attached with the enzyme primase
– DNA polymerase III attaches to the primer and
adds nucleosides one at a time in the 5' to 3'
direction
– replication continues until completion or
meeting another replication fork
• lagging strand - 5' to 3'
– DNA pol III attaches at the replication fork and
copies back to the growing strand in the 5' to 3'
direction is small 100 to 200 nucleotide
segments called Okazaki fragments
– replication continues until DNA pol III reaches a
primer then falls off
– DNA pol I replaces the RNA primer with DNA
– Okazaki fragments are joined (ligated) by DNA
ligase as DNA pol I detaches
Antiparallel
elongation
Other proteins involved
– helicase - unwinds the double helix for replication at the
replication fork
– topoisomerase - relieves supercoiling caused by helicase
– single-strand binding protein - stabilizes the DNA strand
that has been unwound until it is replicated
Telomeres
• Small sections of DNA at the 3' end of the DNA cannot be replicated
as the RNA primer occupies the space. As a result daughter
chromosomes are shorter that the parent chromosomes.
– telomeres are regions of DNA located at the ends of chromosomes
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contain 100 - 1000 repeating units (TTAGGG)
protect internal gene sequences from erosion
get shorter with each replication
associated with the aging process
– telomerase is an enzyme active in germ cells and restores the length
to the chromosomes
• is inactive in somatic cells
– may protect somatic cells from cancer
DNA Repair mechanisms
• Although the polymerases check the nucleotides for accuracy as they are
added, 1 in 100,000 base pairs ends in error.
• Mismatch repair - nucleotide excision repair
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incorrect nucleotide is excised with nuclease (protein that cuts DNA)
DNA polymerase fills in the sequence
ligase connects the strands
xeroderma pigmentosum is a disease caused by a mutation in the excision
repair mechanism which causes the person to be susceptible to skin damage
by the sun