2. Nucleic Acids

Question 1

What are the 2 most important types of information-carrying molecules?

Answer 1

Deoxyribonucleic acid (DNA)
Ribonucleic acid (RNA)

Question 2

What makes up a nucleotide?

Draw!

Answer 2

A pentose sugar
A phosphate group
A nitrogenous organic base (adenine, thymine, uracil, cytosine and guanine)

Question 3

How do nucleotides join together?

Answer 3

Through condensation reactions between the pentose sugar in one nucleotide and the phosphate group in another to form a polynucleotide chain
The bonds are called phosphodiester bonds and they help form the sugar-phosphate backbone

Question 4

What is the pentose sugar and bases in DNA?

Answer 4

The pentose sugar is deoxyribose and the bases will either be adenine, thymine, cytosine or guanine

Question 5

What is the pentose sugar and what are the bases in RNA?

Answer 5

The pentose sugar is ribose and the bases will be adenine, uracil, cytosine and guanine

Question 6

Explain the structure of DNA

Answer 6

DNA is made up of 2 extremely long polynucleotide chains, joined by hydrogen bonds formed between certain bases (complementary base pairing)
These 2 chains coil around each other in a double helix

Question 7

Explain the structure of RNA

Answer 7

RNA is a single, relatively short polynucleotide chain

Question 8

Describe the process of semi-conservative replication

Answer 8

• DNA helicase unwinds the molecule and breaks the hydrogen bonds between complementary base pairs
• these two separated strands will then act as a template for DNA replication
• free activated DNA nucleotides then complementary base pair to the newly exposed bases on the template strands
• DNA polymerase then moves along and catalyses the formation of phosphodiester bonds between adjacent nucleotides, forming the sugar-phosphate backbone and the newly synthesised polynucleotide chain
• the result is two identical DNA molecules, genetically identical to both each other and the original DNA

Question 9

Why is it called semi-conservative replication?

Answer 9

In each daughter DNA molecule, one strand is the original DNA molecule and the other strand is newly synthesised

Question 10

Which 3 facts did Meselson and Stahl bade their work on?

Answer 10

• All bases of DNA contain nitrogen
• nitrogen has 2 isotopes, N14 and N15 (N15 is heavier as it contains an extra neutron)
• bacteria will incorporate nitrogen from their growing medium into any new DNA they make (into the bases)

Question 11

Describe work by Meselson and Stahl

Answer 11

• they began by growing bacteria on a medium of N15 for many generations, so that all of the DNA was made up of this isotope
• DNA was then extracted and analysed
• bacteria was then transferred to a growth medium of N14 for one generation and the DNA was extracted and analysed
• data was also analysed after 2 and 3 generations

Question 12

Why is DNA said to be antiparallel

Answer 12

Because when nucleotides are arranged into the double helix, one strand runs in a 5’ to 3’ direction and the other runs in a 3’ to 5’ direction

Question 13

Why is DNA a stable molecule?

Answer 13

The sugar-phosphate backbone protects bases inside of the double helix (which carries the genetic information) from external physical and chemical forces
- the base pairs and joined by hydrogen bonds which form bridges and these are strong when multiple act together

Question 14

How many hydrogen bonds form between the base pairs?

Answer 14

3 between C and G,

2 between A and T

Question 15

How is DNA adapted to its functions?

4

Answer 15

• very stable structure which normally passes from generation to generation without change (only rarely mutates)
• strands are joined only by hydrogen bonds which allows easy separation during replication
• extremely large and so can carry a large amount of genetic information
• genetic information is protected to some extent from external forces

Question 16

What makes up a molecule of ATP

Draw!

Answer 16

Adenine, ribose and a chain of 3 phosphate groups

Question 17

Describe the hydrolysis of ATP

Answer 17

ATP hydrolase (ATPase) hydrolyses ATP into ADP + Pi (inorganic phosphate)

This is because the bonds between the 3 phosphate groups are easily broken and when they do break they release a considerable amount of energy

Question 18

How is ATP resynthesised?

Answer 18

ATP synthase catalyses the resynthesis of ATP from condensation of ADP + Pi

This reaction requires energy

Question 19

Which ways can ATP be synthesised by the addition of a phosphate molecule

Answer 19

• in chlorophyll containing plant cells during photosynthesis (photophosphorylation)
• in plant and animal cells during respiration (oxidative phosphorylation)
• in plant and animal cells when phosphate groups are transferred from donor molecules to ADP (substrate-level phosphorylation)

Question 20

Why is ATP a better immediate energy source than glucose?

Answer 20

• hydrolysis of ATP is a single reaction that releases immediate energy rather than a long series of reactions
• each ATP molecule releases less energy than each glucose therefore is released in smaller and more manageable quantities

Question 21

Explain the structure of water

Answer 21

A water molecule is made up of 2 hydrogen atoms and one of oxygen
Although the molecule has no overall charge, the oxygen atom has a slight negative charge and the hydrogen atom has a slight positive charge
This makes water molecules polar

Question 22

What does it mean by water being extensively hydrogen bonded?

Answer 22

Hydrogen bonds form between the slight negative oxygen and the slight positive hydrogen molecules
These bonds act together to form important forces

Question 23

How many hydrogen bonds can each water molecule form?

Answer 23

4

Question 24

What is cohesion and why is it important?

Answer 24

The tendency of molecules to stick together is cohesion and it is caused by the hydrogen bonding
This allows water to move up and give support to xylem vessels through a combination of cohesion and adhesion (sticking to the xylem walls)
It also allows water droplets to form

Question 25

What is surface tension?

Answer 25

When water molecules meet air, they tend to be pulled back into the body of water forming a skin-like layer
This supports small organisms such as pond skaters

Question 26

What is high specific heat capacity and what is the cause?

Answer 26

This means a large amount of energy is needed to raise the temperature of water
It is due to the extensive hydrogen bonding

Question 27

What does it mean when water has a large latent heat of vaporisation?

Answer 27

It means that a large amount of energy is needed for a small amount of water to evaporate
Which is why loss of water allows the body to cool

Question 28

Why is the large latent heat of vaporisation and specific heat capacity important?

Answer 28

It makes water thermostable, providing an stable environment for many forms of life and acting as a buffer against sudden changes in temperature

Question 29

How is water a metabolite?

Answer 29

It is involved in many metabolic reactions - used to break down complex molecules in hydrolysis and is produced during condensation reactions
Chemical reactions take place in an aqueous medium and water is also a major raw material in photosynthesis

Question 30

Why are hydrogen ions(H+) important?

Answer 30

Important in determining the pH of solutions and therefore important in the functioning of enzymes

Question 31

Why are ion ions (Fe2+) important?

Answer 31

Iron ions are found in haemoglobin where they play a role in the transport of oxygen

Question 32

Why are phosphate ions (PO43-) important?

Answer 32

Phosphate ions form a structural role in DNA molecules and an energy storage role in ATP

Question 33

Why are sodium ions (Na2+) important?

Answer 33

Important in the transport of glucose and amino acids across plasma membranes