DNA structure + function Flashcards
Frederick Griffith (1928)-
Studied two strains of bacteria:
o S strain (smooth)
o R strain (rough)
S strain
virulent, capsuled, causes disease
R strain
avirulent- didn’t cause disease. rough bc lacked capsule
Griffith found that when injected mice with S strain-
mouse died
When injected mice with living R strain-
mouse lived
When injected mice with S strain that was heat-killed first-
mouse lived
when injected mice with a mixture of heat killed S and
living R-why?
mice died, and there were living S cells inside the mice–Heat killed S did not become spontaneously alive- rather-
although the heat killed the cells, the hereditary material
(including the part that coded for infection) wasn’t destroyed and was transferred from S stream to R stream. permanent transformation.
transformation
bacteria can absorb DNA and make it part of them
original thought on strains
Scientists originally thought that this hereditary material
was the proteins bc they are the most structurally complex
Oswald Avery (with MacLeod and McCarty)
further studied these
bacteria and found that adding a DNA digesting enzyme prevented the transformation This points to the fact that DNA is the hereditary material
Alfred Hershey and Martha Chase (1950s) -
studied the replication
of viruses called bacteriophages (viruses that enter bacteria). Knew that a bacteriophage had to enter the bacteria cell in order
to reproduce- but which part of the virus entered? nucleic acid or protein coat?
o Labeled a protein coat of a bacteriophage with a radioactive
substance- S35 – sulfur
o Labeled the DNA of bacteriophage with radioactive substance-P32–phosphorus
Allowed bacteriophage to attach to cell and infect them. Found S35 outside the bacteria cell and P32 inside the cell. Concluded that bacteriophages inject their DNA into the cells,
leaving protein on the outside- therefore the DNA is significant in reproduction and codes for traits
Bacteriophage-
composed of nucleic acid surrounded by a protein coat
Nucleotide-
building block of DNA. Consists of: Deoxyribose, phosphate group, 1/ 4 possible nitrogenous base
Deoxyribose
pentose (=5carbon) sugar
4 nitrogenous base
Adenine and Guanine- purines (2 ringed)
Thymine and Cytosine- pyrimidines (1 ringed)
phosphodiester bond
The phosphate and sugar form the backbone and are linked by
covalent bond called phosphodiester bond. 5’ end starts with
phosphate group
o 3’ end starts with
OH group of the sugar
Erwin Chargoff
in DNA, the number of adenines equals the number
of thymines, the number of guanines = the number of cytosines. Chargoff’s rules–Adenine bonds w Thymine,
Guanine w Cytosine
Rosalind Franklin-
Took pictures of DNA using X-ray diffraction- directed beam of x-rays at DNA, which would be scattered by the atoms in the DNA in specific patterns.
James Watson and Francis Crick, 1953-
got Nobel Prize for structure of DNA; Studied the pictures and determined it to be a helical shape with
repeating patterns. Built models and determined that it was a double helix shape.
o Sugar-phosphate backbone formed the
outside of the chains
Bases, in pairs, formed the
rungs of the chain
o 10 base pairs present in a full turn of the helix= 3.4 nm
o Two chains are antiparellel-
o Each rung had the same length- 2.0 nm= one purine (two rings) bound to one pyrimidine (one ring)
o Hydrogen bonds hold the bases together-
o 3’—- AGCTAC —–5’
5’—–TCGATG——3’
antiparallel
run in opposite directions,
meaning that one starts with 5’ end (phosphate group) while the
other starts with
the 3’ end (the OH group of the sugar)
how many H bonds between nitrogenous bases
two between adenine and thymine, and three between cytosine and guanine-
complementary to each other
DNA replication
- method by which DNA is copied
o Because nucleotides form complementary pairs, each strand can
separate from the other and act as a
template for synthesizing the opposite strand
semi-conservative replication-
correct. each new DNA
double helix is made from one original strand and one newly synthesized complimentary strand
Conservative replication-
wrong. parent strands remained together, new strands would form a double helix