ch 7 DNA structure and replication Flashcards
4 biological macromolecules
- polysaccharides
- nucleic acids
- lipids
- proteins
rough strain of S. pneumoniae
- lacks a polysaccharide coat
- avirulent (non-disease causing)
- immune system can detect and therefore destroy
smooth strain of s. pneumoniae
- has a polysaccharide coat
- virulent (disease causing)
- immune system can’t recognize cell because of the slime layer
Griffith experiment conclusion
a non-living substance is responsible for transforming avirulent R-strain into virulent S-strain
Avery, MacLeod, McCarty experiment
eliminated different compound of hear killed S strain
- only elimination of DNA caused elimination of transforming ability
Hershey and Chase expreiment
used radioactive labels to label T2 phage components
- hereditary compound must be injected into the host
35S - protein specific (proteins get labelled); liquid (supernatant) will be radioactive
32P - nucleic acid specific (nucleic acid gets labelled); cell pellet will be radioactive
3 pieces of info Watson and Crick discovered
- DNA is composed of 4 nucleotides
- Rules for nucleotide composition
- helical in structure
purines
adenosine
guanine
pyrimidines
cytosine
thymine
nucleoside
a molecule composed of a nitrogen base bound to a sugar molecule
nucleotide
a molecule composed of a nitrogen base, a sugar, and a phosphate group; the basic building block of nucleic acids
semiconservative replication
a model of DNA replication in which each strand of parental DNA serves as a template for new DNA synthesis resulting in both daughter molecules being composed of one parental and one newly synthesized strand
conservative replication
a model of DNA replication which predicts that half of the daughter DNA molecules should have both strands composed of newly polymerized nucleotides
- disproved
dispersive replication
a model of DNA replication which predicts the more or less random interspersion of parental and newly synthesized segments in daughter DNA molecules
Meselson and Stahl experiment
- labeled parental DNA by growing E. coli in 15N medium for many generations
- transferred to 14N medium
- extract DNA after the 1st and 2nd generations
- centrifuged the DNA in a CsCl gradient to separate DNA of different intensities
origin of replication
the start point of DNA replication
- recognition sequence with an associated AT rich region
DnaA
protein which binds to DnaA boxes and opens the helix
DnaB
- helicase
protein which binds to the ssDNA created by DnaA, continues to open the helix - directional slides 5’ to 3’
single-stranded binding proteins
SSBs
bind to open helix, keep strands apart
replication fork
the point at which the two strand of DNA are separated to allow for the replication of each strand
DNA polymerase III
adds nucleotides to the 3’ OH (complementary to the parental)
leading strand
the DNA strand that is being synthesized in the same direction as the replication fork is proceeding
lagging strand
the DNA strand that is being synthesized in the opposite direction as the replication fork is proceeding
okazaki fragments
a small segment of single stranded DNA, with a RNA primer at the 5’terminus, synthesized as part of the lagging strand during DNA replication
DNA polymerase I
degrades RNA, fills in with DNA
4 steps of synthesis on lagging strand
- primase synthesizes short RNA oligonucleotides (primer) copied from DNA
- DNA polymerase III elongates RNA primers with new DNA
- DNA polymerase I removes RNA at 5’ end of neighboring fragments and fills gap
- DNA ligase connects adjacent fragments
ligase
links DNA fragments by forming the phosphodiester bond
topoisomerases
relieves the strain created from unwinding the DNA helix
3 steps to remove strain from unwinding
- DNA gyrase cuts DNA strands
- DNA rotates to remove the coils
- DNA gyrase rejoins the DNA strands
unwinding components:
helicase - strand separation
topoisomerase - strain relief
SSBs - keeps helix open
catalytic components:
2 associated polymerase III’s (dimer)
Beta clamp for processivity
primase for lagging strand synthesis
okazaki fragment components:
DNA polymerase I
DNA ligase
replisome
responsible for DNA replication
overall DNA replication error rate
10^-10
exonuclease subunit of polymerase III
recognizes mispairings (proofreading), removes the incorrect nucleotide and replaces it
DNA polymerase III is capable of adding
1000nt/sec
eukaryotic differences from prokaryotes
- genomes are much larger
- replication is restricted to S phase
- chromosomes can only be replicated once
- chromosomes are comprised of chromatin (DNA and histones)
- chromosomes are generally linear
origin recognition complex (ORC)
binds to the origin (box element)
Cdc6 and Cdt1
- binds to ORC at the origin
- recruits the helicase
- regulates replication
helicase
separates DNA helix
inhibited by Cdc6 and Cdt1
regulation of replication
M: synthesis of Cdc6 and Cdt1
G1: pre-replication complex forms (ORC, Cdc6, Cdt1, and helicase (inactive))
right before S: Cdc6 and Cdt1 degraded and complex becomes active
telomere
the tip/end of a linear chromosome
telomere problem
for linear eukaryotic chromosomes, removal of the last primer of the lagging strand leaves a gap
- chromosomes would get shorter with each replication
- unable to add nucleotides due to lacking a 3’ OH
telomere solution
telomerase (RNA protein hybrid enzyme) adds repeated DNA sequence to 3’ end using a RNA template
- provides a buffer zone for shortening
steps in lengthening the 3’ overhang
- telomerase RNA acts as a template
- reverse transcriptase activity (DNA from RNA template)
- repetitive sequence allows repositioning that creates buffer that can be lost
werner syndrome and dyskeratosis congenita
deficiencies in telomerase
senescence
germ line cells have high activity of telomerase while somatic cell have lower activity
- deterioration with age
