DNA and RNA

Question 1

What 3 classic experiments led to Watson and Crock suggesting the double helical model for DNA structure in 1953

Answer 1

1. Griffith 1928: Discovered the Transforming Principle
2. Avery, MacLeod + McCarty 1944: Established transforming principle was DNA
3. Hershey + Chase 1952: Demonstrated that a biological organism (bacteriophage T2) had DNA as its genetic material

Question 2

Describe the basis for Griffith’s 1928 experiment

Answer 2

• Uses Streptococcus pneumonia
• Descovered two different strains defined by serotypes
• Serotype ll: no capsule - ‘rough’ colony (R) - non-pathogenic. When injected into mice they could fight it off
• Serotype lll: Capsule - ‘smooth’ colony (S) - pathogenic in mice causing them to die

Question 3

Describe Griffith’s (1928) experiment

Answer 3

• If the Smooth type lll bacteria were heat-killed the mouse would survive
• However if the Smooth type lll bacteria were heat-killed and then Rough type ll bacteria were added, the mice would die
• Transformation occurs because DNA is released from heat killed smooth type bacteria and taken up by living rough type bacteria and incorporated into their genome changing their properties

Question 4

Describe Dawson’s (1931) experiment

Answer 4

• In vitro = in a test tube rather than using live mice
• Heat killed smooth type lll cells will transform living rough type ll cells in vitro
• Living smooth type lll cells recovered without injection into mice

Question 5

What did Avery, MacLeod and McCarty (1944) discover

Answer 5

That the transforming principle was DNA

Question 6

Describe the experiment done by Avery, MacLeod and McCarty (1944)

Answer 6

• Used 75L of heat killed smooth type lll cells to give a soluble extract containing the transforming principle
• Successively removed different components (including lipids, protein and polysaccharide)
• The only thing remaining was DNA
• This confirmed in a final experiment where they treated with an enzyme DNAse which breaks down DNA and finally the activity by the transforming principle was lost

Question 7

What did Avery, MacLeod and McCarty’s experiment (1944) fail to show

Answer 7

It didn’t fully demonstrate that biological organisms use nucleic acids as their genetic material

Question 8

Describe the life cycle of a Bacteriophage

Answer 8

Question 9

Describe Hershey-Chase’s experiment (1952)

Answer 9

• The experiment demonstrated that the genetic material of Phage T2 is DNA
• They had two batches of phage labelled with different radioisotopes: 35S which was a coat protein and 32P which attached to DNA
• The 35S lable remained in the supernant as phage ghost parts, while the 32P became intergrated into the bacterial cells and ended up in a pellet

Question 10

What other experiment by Wilkins, Franklin and Gosling also played a key part of the Watson and Crock proposing the structure of DNA

Answer 10

They used x-ray diffraction to study DNA fibres
The image created showed spots forming a cross in the centre denote a helical structure
Heavy bands at the left and right arise from the recurring bases

Question 11

What new piece of information was found about DNA resulting in Chargaff’s Laws (1949-1959)

Answer 11

• Total number of pyrimidines (T, C) = total purines (A, G)
• Hence deducing that A bonds with T and C bonds with G

Question 12

Which is the major and which in the minor grove on the diagram

Answer 12

Question 13

DNA strands run

Answer 13

antiparallel

Question 14

The two DNA strands are held together by hydrogen bonds
How many bonds form between C+G and A+T

Answer 14

3 bonds between G+C pairs
2 bonds between A+T pairs

Question 15

Because there are only 2 bonds between A+T bases, these two bases are found at higher quantities at

Answer 15

Places where the DNA will need to unwind to all the start of transcription

Question 16

In a basic unit of a nucleic acid (nucleotide), what determines if it is a DNA or RNA nucleotide

Answer 16

Carbon 2 position determines whether DNA or RNA
hydroxyl group forms the sugar ribose (RNA)
hydrogen atom forms the sugar deoxyribose (DNA)

Question 17

Which carbon is the phosphate attached to of the pentose sugar

Answer 17

Carbon 5

Question 18

What affect does the phosphate group have on the DNA

Answer 18

It cause it to have a negative charge

Question 18

Which carbon does the Purine/Pyrimidine base attach to

Answer 18

At carbon 1

Question 19

In RNA, adenine bind to which base?

Answer 19

Uracil

Question 20

What is the main difference between purine and pyrimidines

Answer 20

Purine - double nitrogenous ring structure
Pydrimidines - single ring structure

Question 21

Through which group on the nitrogenous bases do they bind to the pentose sugar

Answer 21

N-H groups
For purine, the group on the smallest carbon ring
For pyrimidine, the group next to the carboxyl and methyl group

Question 22

Using which function groups on Adenine and Thymine, will they form hydrogen bonds

Answer 22

Question 23

Using which functional groups on Guanine and Cytosine, will they form hydrogen bonds

Answer 23

Question 24

What is the difference between a nucleoside and a nucleotide

Answer 24

Nucleoside: sugar + base
Nucleotide: sugar + base + phosphate

Question 25

What does dAMP, dGMP, dCMP and dTMP stand form

Answer 25

deoxyadenosine monophosphate
deoxyguanosine monophosphate
deoxycytidine monophosphate
deoxythymidine monophosphate

Question 26

Bases are said to be stacked in a what arrangement in DNA

Answer 26

Bases are stacked in a planar arrangement - horizontal running up through the centre of the DNA

Question 27

What is the significance of the major and minor grove

Answer 27

There is a lot more space in the major grove for proteins to come and interact with the bases within DNA

Question 28

DNA is a right-hand helix, meaning?

Answer 28

It spins to the right

Question 29

How is the phosphodiester bond formed in DNA

Answer 29

is the ester bond formed between the hydroxyl group on the 3’ carbon and the carboxyl group on the phosphate AND the ester bond between the hydroxyl group on the 5’ carbon and another carboxyl group on the phosphate

Question 30

There is other forms of DNA, how do they differ from regular DNA

Answer 30

A - form similar to normal but squashed down form of DNA (found in double stranded RNA)
Z - form is a left handed helix that zig zags rather than coils elegantly (found in vitro in repeated DNA)

Question 31

For normal DNA, how many base pairs are there per turn?
In Å, what is the distance between base pairs
And in Å, what is the diamter of the helix

Answer 31

Base pairs per turn: 10
Distance between BP: 3.4Å
Diameter of helix: 19Å

Question 32

What property of DNA in vivo (in cells) but also in labs, is important

Answer 32

The double helix can denature when heat is applied which breaks the hydrogen bonds between the strands

Question 33

What is the main two differences between DNA and RNA on a molecular level

Answer 33

• RNA contain uracil instead of thymine
• has ribose sugar, instead of deoxyribose

Question 34

What are the 3 most important types of RNA within cells

Answer 34

mRNA, rRNA, tRNA

Question 35

What is mRNA

Answer 35

messenger RNA - copied from DNA and used to make protein (intermediate between DNA and protein)

Question 36

What is rRNA

Answer 36

Ribosomal RNA - functional RNA component of the ribosome (translational machinery in cells)

Question 37

What is tRNA

Answer 37

transfer RNA - involved in process of translation - tRNA brings amino acids to ribosomes during translation

Question 38

What is the basic structure of t-RNA

Answer 38

• RNA is single stranded but folds up on itself to form complicated 3D structure. Base pairing between different parts of the molecule to form stem and loop structures
• Forms these stem and loop structures