DNA Structure

Question 1

What is Chargaff’s Rule?

Answer 1

• Chargaff’s rules state that DNA from any cell of any organisms should have a 1:1 ratio of pyrimidine and purine bases and, more specifically, that the amount of guanine should be equal to cytosine and the amount of adenine should be equal to thymine. This pattern is found in both strands of the DNA

Question 2

What are the purine bases?

Answer 2

• Thymidine

- Guanosine

Question 3

What are the Pyrimidine bases?

Answer 3

• Adenosine

- Cytidine

Question 4

What structural features can be seen from Franklin and Wilkins DNA Photo?

Answer 4

• The base is perpendicular to the axis

- The Helical Structure of the DNA

Question 5

What structural features of DNA were discovered by Watson and Crick

Answer 5

• The base is perpendicular to the axis
• The Helical Structure of the DNA
• Double stranded DNA: allows base pairing (A → T, G → C)
• Phosphate backbone on the outside
• Meets Chargaff’s rule

Question 6

What type of chemical bonds are important in DNA structure?

Answer 6

• Covalent Bonds
• Hydrogen Bonds
• Hydrophobic “Bonds”

Question 7

What are Hydrogen Bonds?

Answer 7

• A weak bond between an electronegative atom and a hydrogen atom (electropositive), that is, covalently linked to a second electronegative atom.

Question 8

What are Covalent Bonds?

Answer 8

• Strong Bonds formed by the sharing of electrons between atoms

Question 9

What are Hydrophobic “Bonds”?

Answer 9

• The association of non-polar groups with each other while present in aqueous solutions because of their insolubility in water.

Question 10

Where in the DNA are Covalent Bonds found?

Answer 10

• In Bases and Sugars
• In phosphodiester linkages
  https: //docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing

Question 11

What is the Hydrophobic Core?

Answer 11

The stacked base pairs in the DNA are bonded together and these bonds create a Hydrophobic core.
https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing

Question 12

How do Hydrophobic “Bonds” work?

Answer 12

• Water molecules are very polar
• Compounds that are similarly polar are very soluble in water (“Hydrophilic”)
• Compounds that are nonpolar (no charged groups) are very insoluble in water (“Hydrophobic)

Question 13

Draw and label diagrams of the Hydrogen Bonds and Hydrophobic “Bonds”

Answer 13

https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing

Question 14

What is the overall polarity of DNA?

Answer 14

• DNA does not have an overall polarity because the two strands of the double helix have opposite polarity

Question 15

Draw and label an example of how the two strands in DNA are of opposite polarity.

Answer 15

https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing

Question 16

Draw and label the Space-filling diagram of DNA.

Answer 16

https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing

Question 17

In the Space-filling diagram of DNA, what features are demonstrated?

Answer 17

Space-filling diagram of DNA shows:

• Stacked nature
• Two grooves; the major and minor groove.
• The difference between important for recognition by enzymes involved in gene regulation and histone binding
• Right-handed double helix (B-DNA)

Question 18

What are the alternate forms of DNA?

Answer 18

• Right handed (A)
• Right handed (B)
• Left handed (Z)

Question 19

What are the features of Right handed (A) DNA?

Answer 19

• It has 11 base pairs per turn and a Helix diameter of 2.3nm

Question 20

What are the features of Right handed (B) DNA?

Answer 20

• It has 10 base pairs per turn and a Helix diameter of 1.9nm

Question 21

What are the features of Left handed (Z) DNA?

Answer 21

• It has 12 base pairs per turn and a Helix diameter of 1.8nm

Question 22

Draw a diagram of the Alternate forms of DNA

Answer 22

https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing

Question 23

What is DNA Supercoiling?

Answer 23

Supercoiling occurs when the molecule relieves the helical stress by twisting around itself.
- This supercoiling causes a DNA molecule to collapse into a tightly coiled structure - Overtwisting leads to
postive supercoiling, while undertwisting leads to negative supercoiling

Question 24

When does Supercoiling occur?

Answer 24

Introduced into a DNA molecule when one or both strands are cleaved and when the complementary strands at one end are rotated or twisted around each other with the other end held fixed in space—and thus not allowed to spin

Question 25

What causes super coiling?

Answer 25

Supercoils are introduced into and removed from DNA molecules by enzymes that play essential roles in DNA replication

Question 26

Analyse the image of DNA supercoiling.

Answer 26

https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing

Question 27

How does Negative Supercoiling occur?

Answer 27

Cleaving one strand of a covalently closed, circular double helix of DNA, and rotating one end of the cleaved strand a complete turn (360°) around the complementary strand while holding the other end fixed, will introduce one supercoil into the molecule

Question 28

Why is Negative Supercoiling important?

Answer 28

Negative supercoiling is involved in replication, recombination, gene expression, and the regulation of gene expression

Question 29

Draw a diagram of the process of DNA negative supercoiling.

Answer 29

https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing