Lecture 12

Question 1

Purines

Answer 1

Adenine and Guanine

Question 2

Pyrimidines

Answer 2

Thymine, cytosine and uracil

Question 3

What makes DNA a better candidate for the genetic material than protein?

Answer 3

Proteins have a variation in function and structure in comparison to DNA that maintains the same structure. DNA is relatively more stable than protein.

Question 4

RNA VS DNA

Answer 4

RNA is more flexible than DNA. It tends to form loops within its structure and is involved in:

• transcription + translation
• reverse translation
• regulation of gene expression
• component of ribosomes
• genetic material (viral)

Question 5

What are nucleotides?

Answer 5

Nucleotides are the monomers of nucleic acids. Each nucleotide contains a phosphate group, a pentose sugar and a nitrogenous base. (Ex: ATP is a nucleic acid, AMP is a nucleotide)

Note: FAD and NAD are also nucleotides. Free nucleotides usually exist in a triphosphate form and only loses the two extra phosphates groups after it gets added to DNA

Question 6

What is different about the sugar groups in DNA and RNA?

Answer 6

RNA contains a ribose sugar while DNA contains a deoxyribose sugar (1 less oxygen)

Question 7

How is the width of the helix maintained?

Answer 7

A nucleotide with a purine always binds with a nucleotide with a Pyrimidine.

Question 8

How are the 5’ and 3’ ends allocated?

Answer 8

Starting with the sugar, the carbon on it that binds with the base is labelled 1’. The labelling moves clockwise until it reaches the methyl group on the end of the sugar which is labelled 5’. The phosphate group attaches to the 5’ end and 3’ end of the sugar

Question 9

Which base pair forms weaker bonds with each other?

Answer 9

All base pairs attach via hydrogen bonds. However, the pairing between G-C contains 3 hydrogen bonds in comparison to A-T which only contains 2 hydrogen bonds

Question 10

Drawing a double strand of DNA

Answer 10

5’ is always on the top left corner. Bottom strand is complementary

Question 11

How is the DNA organised?

Answer 11

The double stranded DNA is wrapped around proteins called histones. This forms a nucleosome. The nucleosomes coil around each other to form chromatin. This eventually leads to the formation of condensed chromosomes.

Question 12

Major and minor grove

Answer 12

Although the DNA is in a shape of a helix, there is a slight tilt in its shape. This causes it to form major and minor groves in the molecule of DNA. The major grove occurs where the backbones are furthest apart whereas the minor grove is where they are the closest

Question 13

Naming of strands

Answer 13

The original double strands of DNA is called the parental DNA double helix. Each of these strand is called the template strand as it is the template for DNA replication. When replicated, the two double helices formed are called the daughter DNA double helices.

Question 14

What are the different forms of DNA?

Answer 14

There are 3 different forms of DNA; A-form, B-form and Z-form. Both the A and B form are right handed helices where as the z form is a left handed helix.

Question 15

What cellular processes involve DNA?

Answer 15

DNA is involved in DNA synthesis, transcription

Question 16

What cellular processes involve RNA?

Answer 16

RNA is involved in transcription, translation, DNA synthesis (RNA primers), reverse translation

Question 17

What are the major differences between DNA and RNA?

Answer 17

RNA has uracil instead of thymine as a base. It also has a ribose sugar instead of a deoxyribose sugar. RNA is more flexible than DNA

Question 18

A form DNA

Answer 18

• Right handed helix
• 11 base pairs in every turn
• Turn every 2.3nm

Question 19

B form DNA

Answer 19

• “Normal” right handed helix
• Turns every 3.4 nm
• 10 base pairs in every turn

Question 20

Z form DNA

Answer 20

• Left handed helix

- One turn spans 4.6 nm, comprising 12 base pairs

Question 21

What is the type of chemical bond that exists between the phosphate and the pentose of the neighbouring nucleotide?

Answer 21

Covalent bonds

Question 22

What does it mean that DNA is anti parallel?

Answer 22

The two strand of DNA run in opposite directions to each other. The 5’ end of one DNA strand is directly next to the 3’ end of the opposite strand.

Question 23

What is different about the tertiary structures of DNA and RNA?

Answer 23

While DNA is naturally found as a double stranded structure, RNA can tolerate a mix of single stranded and double stranded regions. RNA is able to form hairpin loops, bulges, internal loops and single strands that are protected by SSBP.