DNA genetic material and replication Flashcards
1
Q
Acetabularia experiment showing DNA is the genetic material
A
- Experiments that show DNA is the genetic material
left- photograph of plant- most complex single cell- has base with grabbers on bottom - Nucleus at bottom- top has crown thing specialized for photosynthesis
- genetic information at nucleus
experiment: 2 types, one darker with wide spread cap, one green with narrow cap
–> cut stalks and switch–> –> in every case, new cap grows back consistent with base
2
Q
Griffith-Avery experiment (1)
A
- Griffith had bacteria of 2 types, identical in every way, but one have a slimy layer around the outside (S), one did not(R)
- Pneumoniae (S) causing bacteria
- If you inject mice with R, immune system can fight off infection and survive
- If you inject mice with S, cannot fight off infection, die
- Take heated S type, mice fight off infection
- If take heated S type and normal R type, mice die–> can isolate smooth strain of bacteria
–> proposed that there was some form of the transforming factor that must be present in these heat ones S that is transferred into R
“transforming factor”
3
Q
Avery, MacLeod & McCarty, ~1940 (2)
A
- Investigator names Avery wanted to know nature of transforming factor (after Griffith-Avery)
- Did same experiments but took mixed (heat killed S) and treated with some isolated enzymes for breaking down protein and enzymes for breaking down RNA and DNA
–> get 3 different tubes, one with no protein, one with no RNA, one with no DNA - Found that no S cells appeared when there was no DNA (but did appear when there was no protein or RNA)
–> can’t transform unless DNA is present
4
Q
Hershey-Chase experiment- 1950s
A
- Grew up a batch of bacteriophages in the presence of radioactive sulfur
- Amino acids that get incorporated into protein with sulfur in them (methionine and cysteine), create radioactive capsules, but DNA is normal
- Experiment- took radioactive phage and injects genetic material–> mix around so phage heads fall off and genetic material is inside cell
- Then do parallel experiment- grow another batch of phages, DNA labeled with phosphorous 32 (no phosphorous in amino acid), but onto bacteria and mix so phage heads fall off, DNA is injected
- -> now have one cell with radioactive sulfur, one cell with radioactive phosphorous
- convinced that DNA carries genetic information
5
Q
Bacteriophage
A
- nucleic acid (DNA) at the top, rest of stalk/head is protein, no RNA, no cytoplasm - biological entity
6
Q
DNA double helix structure
A
- Way to store information
2. Way to ensure accurate copying
7
Q
Direction of synthesis
A
- elongation is at the 3’ end, so we say, “Synthesis of a nucleic acid strand is always in the 5’ to 3’ (5’–>3’)
- original strand serves as template
8
Q
DNA synthesis phase
A
- Double stranded DNA, origins open up little bubbles
- enzyme fuses strands when bubbles run into each other
- -> end up with 2 strands, but held together at centromere
- Eukaryotes have multiple origins of replication –may have evolved because we have much more DNA than prokaryotes (which have 1 origin)
9
Q
semi-conservative DNA replication
A
- 2 parental strands are copied
- New daughter DNA “conserves” one of the two original or parental strands
- Daughter strand sequence is identical to parental sequence
- each new strand has an parental template strand and a new strand
10
Q
Meselson and Stahl experiment 1957
A
- testing if DNA replication actual semi-conservative
- What actually happens–> each parental strand acts as a template and end up with a hybrid
1. Bacteria were grown in a medium containing a heavy isotope of nitrogen.
2. Bacteria were then allowed to grow in a medium containing a light isotope of nitrogen.
3. At various times, the DNA from bacterial cells was extracted.
4. The DNA was suspended in a cesium chloride solution.
–> shows that heavy strands separate and you get 1 light, 1 heavy strand, a mixture –> semi-conservative
11
Q
Replication origin
A
- A specific DNA sequence which is recognized by proteins involved in initiating DNA replication
12
Q
Replicon
A
- The length of DNA replicated from a single origin
13
Q
timing of replication
A
- Replications happen at same speed, but have staggered initiation time
14
Q
replication fork
A
- In real life both new strands are being synthesized at the same time
- Strands are antiparallel, 5’ with 3’ replicating
15
Q
favorable that phosphate will add onto strand
A
- Take triphosphate and break 2 phosphate off of one phosphate– high energy bond between first and second phosphate
- Energy for making this linkage is contained in that bond
- When you break bond, change in delta G is negative–> attach to strand
- Other 2 phosphate = pyrophosphate, are less stable, and tend to fall off into two inorganic phosphates
- Very favorable that the phosphate will add onto strand- still need an enzyme to do it and recognize 3’ end, bind to phosphate precursor, and look at template and know it needs C to bind with G
