ch 16 Flashcards
How did the discovery of genetic role of DNA begin
Griffith worked with two strains of a strep bacterium
pathogenic S strain (smooth w capsule ) and harmless R strain (rough due to lack of capsule)
when he mixed heat-killed remains of the pathogenic strain with living cells of R strain, some living cells became pathogenic
called this transformation
transformation
a change in genotype and phenotype due to assimilation of foreign DNA
How did scientists confirm that the transforming substance was DNA
they purified various molecules from heat-killed pathogenic bacteria and tried to transform live nonpathogenic bacteria with each type
pnly DNA worked
How was DNA’s function confirmed by Hershey and Chase
What was the experiment
they used bacteriophages which use bacterial cells for reproduction. they only consist of a protein coat and a nucleic acid core
they labeled viruses with either radioactive phosphorus (only found in dna) or radioactive sulfur (found only in proteins
they then allowed viruses to infect bacteria cells
results of hershey and chase’s experiment
the labeled P virus into the bacteria cell made the bacteria radioactive, while its absence made the viral coats inactive
the labeled S virus into the bacteria cell made the bacteria inactive, while it kept the viral coats radioactive
meant that presence of DNA in p virus made bacteria active
Erwin Chargaff
1.reported that DNA composition of bases (A,T,C,G) varies from one species to the next
^ this evidence of diversity made DNA a more credible candidate for genetic material
2. Chargaff also showed that %of A always = % of T and % of G always = %of C
3. purines always with pyrimidines
DNA structure
polymer of nucleotides with nitrogenous base, a sugar, and a phosphate group
two antiparralel sugar-phosphate backbones, with the nitrogenous bases paired in the molecule’s interior
how was a picture of the DNA molecule produced
wilkins and franklin used X-ray crystallography
a beam of x-rays strike a crystal and cause the beam of light to spread into many specific directions
what did watson deduce from franklin’s images of dna
that dna was helical
and deduced the width of the helix and the spacing of nitrogenous bases
width suggested that DNA molecule was made up of two strands forming a double helix
two purines
two rings
adenine and guanine
two pyrimidines plus one
1 ring
thymine and cytosine
uracil
dna characteristics
sequence of bases in a nucleotide strand is different from species to species
length of a strand of DNA (number of base pairs) is different from species to species
more complex organisms generally have greater #’s of base pairs
how does DNA’s structure contribute to its function
antiparralel arrangement of two strands ensures that the bases are oriented properly, so they can interact/bond
each nucleotide has a phosphate group at the 5’ position of the sugar
how are nucleotides joined
by linking 5’’ phsphate to 3’ position
DNA is always made in the 5’ to 3’ direction
you only add to 3’ end
DNA replication how it can occur
since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication
in dna replication, the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules