DNA History, Structure, and Replication Flashcards
1
Q
Fred Griffith
A
- Discovered transformation
- Showed that some “active genetic substance” could be transferred from dead bacteria capable of causing disease to live harmless bacteria, making these live bacteria dangerous
- Attempting to develop vaccine for type of bacteria – streptococcus
- Virulence – disease-causing organism (pathogenic)
-
S strain
- Forms smooth colonies
- Polysacc. coat protects it from attack by immune system
- Virulent / pathogenic
-
R strain
- Rough colonies
- No protective coating
- Avirulent / nonpathogenic
- Something from S strain had been able to transform the R strain into a mouse killer
2
Q
Avery, McLeod + McCarthy
A
-
Proteases (protein-destroying enzymes)
- Added to heat-killed S strain (then mixed with live R strain)
- Mice still died, factor was not protein
-
DNAse (DNA-destroying enzyme)
- Added to heat-killed S strain (then mixed with live R strain)
- Mice didn’t die, factor was likely DNA
3
Q
Hershey and Chase
A
- Proved DNA was the genetic material
- Used the T2 bacteriophage (virus that infects and kills bacterial cells)
- Infects living cells and multiplies inside, explode the cell, infect more cells
- Batch of radioactive capsid proteins (35S)
- Batch of radioactive DNA (32P)
- Allowed each to infect bacteria (E. coli), then remove the viruses on the outside of the bacterial cells by agitating the cells in blender
- Spin tubes in centrifuge – dense bacteria sinks to bottom (forms pellet)
- E. coli bacteria in the pellet contained no 35S
- The offspring of the virus contained lots of 32P
- Proteins: CHONS
- DNA: CHNOP
4
Q
Chargaff
A
- Noticed that in every analysis of DNA that he performed:
- amount of adenine = amount of thymine
- amount of cytosine = amount of guanine
- Pairs present in equal ratios
5
Q
Watson-Crick-Wilkins-Franklin (and Pauling)
A
- Wilkins and Franklin – crystallographer, developed X-ray diffraction images of DNA – 2 twisted strands
- Franklin’s data – DNA was an anti-parallel double-stranded molecule
- 2 stranded double helix
- Backbone of molecule composed of the phosphates and deoxyribose sugars of the nucleotides (covalently bonded)
- Rungs composed of nitrogenous base pairs (H bonding)
- Watson and Crick published journal Nature
- Watson, Crick, Wilkins – Nobel Prize
- Watson – published The Double Helix
6
Q
What are the components of a nucleotide?
A
- Nitrogenous base (A, C, T, G)
- Phosphate group
- Sugar
7
Q
Timeline of discovery
A
- Griffith discovers that bacteria can change from one form to another (transformation)
- Avery followed up Griffith’s earlier discovery and concluded that the transforming factor is DNA
- Franklin and Wilkins provide evidence that DNA is in the form of a double helix
- Chargaff’s rule
- Hershey and Chase conduct experiments which further prove that DNA was the hereditary material, sufficient to code for growth of a new organism
- Watson and Crick publish 3D structure and composition of DNA
8
Q
What types of bonds hold these molecules together?
A
- Hydrogen bonds hold the nitrogenous bases together
-
Covalent bonds hold the nucleotides together
- Between the sugar of one nucleotide and the phosphate of the next – sugar-phosphate backbone
9
Q
Base pairing rules in DNA?
A
- There should be a near 1:1 ratio of C to G, and A to T (or U in RNA) in any living organism (Chargaff’s rule)
- Adenine can bond to Thymine or Uracil (A — T / U)
- Guanine can only bond to Cytosine (G — C)
10
Q
Differences between DNA and RNA?
A
- Both are polymers of nucleotides (sugar + nitrogenous base + phosphate)
-
RNA
- sugar is ribose
- A, G, C, U
- uracil (U) is substituted for thymine (T)
-
DNA
- sugar is deoxyribose
- A, G, C, T
- missing an oxygen atom in a hydroxyl group in the sugar,
11
Q
What are 5’ and 3’ ends? (where do these terms come from?)
A
- Refer to the specific carbon molecules in the deoxyribose backbone of DNA/RNA
- 5’ is the carbon closest to the phosphate group
12
Q
Why is DNA referred to as antiparallel?
A
- The two strands of DNA that make up the double helix are parallel but are oriented in opposite directions
13
Q
What does it mean to call the DNA strands complementary?
A
- The two strands that create the double helix are related by the rules of base pairing, pairs are opposites
14
Q
Replication takes place where in a Eukaryotic cell? Prokaryotic cell?
A
- The nucleus in eukaryotic
- Cytoplasm in prokaryotic
15
Q
Why is DNA replication called semi-conservative?
A
- Half of the strand used when attaching to the new base pairs
16
Q
What is a replication bubble?
A
- Where DNA starts to replicate along the strand
- There are many of these along a piece of DNA
17
Q
Origin of replication?
A
- Where replication bubbles start
18
Q
Replication fork
A
- Where new DNA is polymerized by DNA Polymerase
19
Q
What are Okazaki fragments? Why are these created?
A
- DNA polymerase can only add on to the 3 prime end
- So when synthesizing the lagging strand it has to read from inside as the strand opens up (copies backwards to the direction it’s opening), which means that it has to copy in fragments as the lagging strand opens up bit by bit
- And since it can only add onto 3’ without ligase, it doesn’t connect making fragments
20
Q
What are RNA primers? What enzyme creates these?
A
- RNA primers are short strands of RNA or DNA that serve as starting points for DNA synthesis on the lagging strand
- Primase makes these
21
Q
Helicase
A
- Unwinds the helix during DNA replication
22
Q
Primase
A
- Sets down RNA Primers for DNA Polymerase
23
Q
DNA polymerase
A
- Adds new DNA nucleotides to the parent strand
24
Q
Ligase
A
- Covalently bonds Okazaki fragments
25
Q
How does replication proceed from a replication fork?
A
- Proceeds until it reaches another replication bubble, DNA replication is complete
26
Q
Differences between leading and lagging strands
A
- The leading strand gets polymerized continuously 5—>3, while the lagging strand gets polymerized 3–>5 overall in the 5–>3 direction in Okazaki fragments
