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
Dispersive replication-
parent strands and new strands
would become
randomly mixed during replication
Telomeres-
caps on each end of chromosomes that contain short noncoding DNA sequences that repeat many times. Shortening of telomeres contribute to cell aging and apoptosis-
programmed cell death.
Purpose of telomeres
After a DNA strand finishes replication, there is a small
segment on the lagging strand that
does not get replicated
Therefore, a cell can therefore divide many times before it starts
to lose important genetic info
Telomerase
special DNA replication enzyme that can lengthen
telomeric DNA. This is usually present in cells that divide an unlimited number of times, like
protists, and also cancer cells
Matthew Meselson and Franklin Stahl
Used density gradient centrifugation to prove that DNA
replicated in a semi conservative fashion. Labeled the bases of DNA with 15N, a heavy isotope of
nitrogen
Put it in an area containing just 14N and allowed it to
replicate
After one generation, new DNA had an intermediate
density, between 14N and 15N, because it had
half of each type
After another round of cell division, two types of DNA
appeared in density gradient- one with intermediate
density, one just 14N density proved
replicated in a semi conservative fashion
Semiconservative replication explains
how mutations, or genetic changes, could arise in genes and then be transmitted to succeeding generations
First step of replication
DNA helicase unwinds and unzips the two strands of DNA at various points within the DNA molecule called origins of replication
What happens in replication after helicase?
The two strands separate from one another, forming a
replication fork
What happens in replication after replication fork is formed?
Single-strand binding proteins (SSBs) bind to the individual strands and stabilize them, preventing them from
reforming the double helix
What happens in replication after individual strands stabilize
Topoisomerases produce breaks in the DNA molecule and then rejoin the strands, preventing
excessive coiling in the rest of the strand
How is DNA replication initiated if DNA polymerase only adds
nucleotides to an existing nucleotide?
An RNA primer (short
piece of RNA) is synthesized first at the region where
replication begins, by an enzyme called DNA primase. DNA polymerase then replaces the primase and adds
nucleotides to the 3’ end. Enzymes later
break down the primer and fill the space in w DNA
DNA assembly direction
DNA is read from 3’ to 5’, synthesized from 5’ to 3’
Leading strand –
strand where one DNA polymerase is
constantly adding nucleotides to one strand from the 5’ end to
the 3’ end of new (3-5), moving smoothly and continuously towards the replication fork
Lagging strand-
strand where another polymerase molecule
adds nucleotides to the 3’ end of the other new strand. Since this moves away from the replication fork, only short pieces, called Okazaki fragments, can be synthesized at a time.
Okazaki fragment initiation
Each Okazaki fragment is initiated by its own primer.
The fragments are joined together by DNA ligase- forms
a phosphodiester linkage between nucleotides
DNA proofreading mechanisms-
DNA polymerase can proofread new bases pairings
can correct mismatches and reverse additions of base pairs
When they cannot, replication stops
Repair enzymes can repair some changes-can sniff out damaged sites or mismatches
Prokaryotic replication
Bacteria, which has DNA in a circle, have one origin of replication, so
the replication forks eventually meet each other
Eukaryotic replication
Eukaryotic chromosomes are linear, so many origins of replication exist at one time until newly synthesized strand is formed
What happens in replication after topoisomerases
DNA polymerases catalyze the formation of two brand-new strands of DNA from free nucleotides
