ch 16 Flashcards
(38 cards)
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
DNA replication step by step
the parent moleculle has two comp. strands
first step is separation of two DNA strands (breaking hydrogen bonding between base pairs)
now, each parental strand serves as a template that determines the order of nucloetides along a new, complementary strand
the nucleotides are connected to form sugar-phosphate back-bones of the new strands
each daughter cell consissts of one parental strand and one new strand
what does watson and crick’s semioconservative model of replication predict
when a double helix replicates, each daughter molecule will have one old strand and one newly made strand
conservative model off replication
two parental strands reassociate after acting as templates for new strands, thus restoring the parental double helix
dispersive model of replication
each strand of both daughter molecules contains a mixture of old and newly synthesized DNA
replication bubble
replication begins at special sites called origins or replication, where the two DNA strands are separated, opening up a replication “bubble” –o–
replication fork
at the end of each replication bubble is a replication fork
a y-shaped res=gion where new dna strands are elongating
DNA polymerase
catalyzes the elongation of new DNA at a replication fork
adds nucleotides only to the free 3’ end of a graowing strand (5’ –> 3’)
only adds next ucleotid to the OH- group at the end of the growing strand and releases 2 phosphates
leading strand
new strand
3’ template strand
DNA polymerase can synthesize a complementary strand continuously, moving towrd the replication fork
lagging strand
nucleotide added in 5’ to 3’ in small sections
must be made in overall 3’to 5’ direction, dna polymerase works in direction away from replication fork
series of okazaki fragments joined together by DNA ligase and added in direction toward replication fork
