THE MOLECULAR BASIS OF INHERITANCE (DNA)

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THE MOLECULAR BASIS OF INHERITANCE
(DNA)
After T.H. Morgan’s fruit flies, the search was on for the actual
construction/makeup of chromosomes & genes.
Two key experiments:
1. Frederick Griffith (1928) w/ Streptococcus pneumoniae
 two strains, into mice…..
 Results supported the idea that some chemical component can cause
heritable traits to change--- “transformation”
2. Alfred Hershey & Martha Chase (1952)
 bacteriophages (T2) & Eschericia coli
 Results supported the idea that DNA was injected into bacterial cells
(and not the protein coat) thus, DNA is the controller/determiner of genetic
traits & heritable information
3. Erwin Chargaff
 analyzed bases from various organisms
 Results: found % of A = % of T and that % of C = % of G
4. James Watson, Francis Crick (and Rosalind Franklin too)
 used modeling, data from others’ investigations, and X-ray crystallography
to determine the structure of DNA
 found it to be a double helix, (like a twisted ladder) with sides of molecule
composed of alternating sugars (deoxyribose) and phosphates while the
rungs were composed of nitrogenous bases (bonded together via hydrogen
bonds)
Watson & Crick used other information in their deductions:
 adenine & guanine are “purines”, cytosine & thymine are “pyramidines”
 purine with pyramidines only, and number of hydrogen bonds specifies
(and matches with Chargaff’s Rule)
Ultimately, they noticed that …”It has not escaped out notice that the
specific pairing we have postulated immediately suggests a possible copying
mechanism for the genetic material!” Huh??
Each strand in DNA molecule can serve as a template for formation of the
other (thanks to base pairing rules).
The question remained: Is DNA replication a conservative process, a
dispersive process, or a semiconservative process?
used the findings of Meselson & Stahl experiment to explain
It was decided that DNA replication is a “semi-conservative” process.
Replication of DNA begins at “origins of replication”
proteins recognize sequences of bases and attach to the DNA separating
the two strands and opening up a replication bubble
multiple replication bubbles form
DNA polymerases catalyze elongation of new DNA strands
adds individual nucleotides to match complementary nucleotides
(about 500 nucleotides per second)
NOTE: each nucleotide is actually a “nucleoside triphosphate”
as each monomer joins the growing end of a DNA strand, it loses
two phosphates (exergonic reaction that drives polymerization
reaction
Interestingly, DNA replication (elongation) is an antiparallel process
the two original strands run in opposite directions so each has opposite 5’
and 3’ ends
 replication follows this same pattern
 nucleoside triphosphates can only be added to the 3’ end of a new
replicating strand…….WHY??
 a new DNA strand always elongates in the 5’3’ direction
DNA polymerase III can add bases along one strand in a continuous manner,
(this is called our “leading strand”)
 but must work away from the replication fork for the other strand (called
the “lagging strand”)
 thus the leading strand elongates continuously, but the lagging strand is
synthesized in a series of segments (“Okazaki fragments”…..about 100-200
nucleotides long)
Eventually, another enzyme, “DNA ligase” joins the sugar-phosphate
backbones of the Okazaki fragments, making a single new strand
It is important to note that DNA polymerases cannot initiate synthesis of a
polynucleotide
the initial nucleotide chain is a short one called a “primer” (5-10 bases long
 made of either DNA or RNA
an enzyme “primase” joins RNA nucleotides together at the location where
new DNA strand synthesis will begin
only one primer needed on leading strand, but each Okazaki fragment must
be primed separately
DNA polymerase I replaces RNA nucleotides of primers with DNA
versions, adding them one by one
NOTE: many other proteins are involved in DNA replication (what are these
weirdos responsible for???)
”helicase”
”topoisomerase”
”single-strand binding protein”
Beware that this process works so quickly that mistakes can (and frequently
are) made…(about 1 in 100,000 pairs)
In “mismatch repair”, cells use special enzymes to fix incorrectly paired
nucleotides
 series of enzymes, (including “nuclease”) cuts damaged section out and
enzymes fill in correct base pairs
process known as “nucleotide excision repair”
example: xeroderma pigmentosum (Michael Jackson)…..hmmmm, what is this?
(NOTE: ignore any and all Michael Jackson jokes at this point !!!)
Lastly, recall that we can only add bases to 3’ ends, thus we have no real way
to complete the 5’ ends….
as a result, repeated rounds of replication makes DNA molecules shorter &
shorter & shorter.
eukaryotes have sequences on ends of chromosomes called “telomeres”
(repeated sequences…i.e. TTAGGG)….this sequence repeat varies from about
100 to 1,000
 is this somehow connected to the aging process????
 “telomerase” catalyzes the lengthening of these telomeres in eukaryotic
germ cells (YEAH!!! This saves our chromosomes in germ cells from becoming
shorter and shorter and shorter!!)
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