2. Nucleic Acids

Question 1

Give 2 examples of nucleic acids

Answer 1

Deoxyribonucleic acid (DNA)
Ribonucleic acid (RNA)

Question 2

Give the 3 components that make up a nucleotide

Answer 2

• A pentose sugar (deoxyribose/ribose sugar)
• A phosphate group
• A nitrogen containing base

Question 3

What are the 4 nitrogen containing bases found in DNA?

Answer 3

Adenine
Thymine
Guanine
Cytosine

Question 4

What are the 4 nitrogen containing bases found in RNA?

Answer 4

Adenine
Uracil
Guanine
Cytosine

Question 5

How are mononucleotides formed?

Answer 5

The pentose sugar, phosphate group and organic base are joined by a condensation reaction, forming a mononucleotide.

Question 6

How are dinucleotides formed?

Answer 6

A result of a condensation reaction between the deoxyribose sugar of one mononucleotide and the phosphate group of another. The 2 nucleotides form a phosphodiester bond.

Question 7

What links the two strands of DNA together?

Answer 7

Hydrogen bonds link the two complementary nucleotide base pairs together. Adenine pairs with thymine, cytosine with guanine.

Question 8

What is the name of the coil of DNA that forms its backbone?

Answer 8

Double helix

Question 9

Why is DNA a stable molecule?

Answer 9

• The phosphodiester backbone protects the more chemically reactive organic bases inside the double helix.
• Hydrogen bonds link the organic base pairs forming bridges between the phosphodiester uprights. As there are 3 hydrogen bonds between cytosine and guanine, the higher proportion of C-G pairings, the more stable the DNA molecule

Question 10

What is the function of DNA?

Answer 10

• Hereditary material that is responsible for passing genetic material from cell to cell and down generations.
• A total of approx. 3.2 billion base pairs in the DNA of a typical mammalian cell. Therefore almost an infinite variety of base sequences, providing genetic diversity in living organsims.

Question 11

How is DNA adapted to carry out its functions?

Answer 11

• It’s a very stable structure which normally passes from generation to generation without change.
• Its two separate strands are joined only by hydrogen bonds, allowing them to separate during DNA replication and protein synthesis.
• Its an extremely large molecule and therefore carries a large amount of genetic information.
• By having the base pairs within the helical cylinder of the deoxyribose-phosphate backbone, the genetic information is to some extent protected from being corrupted by outside chemical and physical forces.
• Base pairing leads to DNA being able to replicate and transfer information as mRNA.

Question 12

What does the function of DNA rely on?

Answer 12

The sequence of DNA base pairs.

Question 13

Suggest why base pairing of cytosine with adenine and guanine with thymine doesn’t occur?

Answer 13

The base pairs are linked by hydrogen bonds. The molecular structures could be such that hydrogen bonds don’t form between adenine and cytosine and between guanine and thymine.

Question 14

If the base sequence on one strand of DNA is TGGAGACT, determine the sequence of the other strand.

Answer 14

ACCTCTGA

Question 15

If 19.9% of base pairs on human DNA is guanine, calculate the percentage of thymine

Answer 15

19.9% guanine so 19.9% cytosine
Total C-G is 39.8%
So total A-T is 60.2%
so thymine must be 30.1%

Question 16

Give the 2 main stages of cell division

Answer 16

• Nuclear division

- Cytokinesis

Question 17

What are the 4 requirements needed for semi-conservative replication to occur?

Answer 17

• The 4 types of nucleotide, with their bases present.
• Both strands of DNA act as a template for the attachment of these nucleotides.
• The enzyme DNA polymerase.
• A source of chemical energy.

Question 18

Describe the process of semi-conservative replication.

Answer 18

The enzyme DNA helicase breaks the hydrogen bonds linking the base pairs of DNA.
As a result, the double helix separates itself into 2 strands and unwinds.
Each exposed polynucleotide strand then acts as a template to which complimentary free nucleotides bind by specific base pairing.
Nucleotides are joined together in a condensation reaction by the enzyme DNA polymerase to form the missing polynucleotide strand on each of the 2 original polynucleotide strands of DNA.

Question 19

Explain why the process of DNA replication is described as ‘semi conservative’

Answer 19

Because half of the original DNA is built into the new DNA strand.

Question 20

If an inhibitor of DNA polymerase were introduced to a cell, explain the effect on DNA replication.

Answer 20

The linking together of new nucleotides could not take place. While the nucleotides would match up to their complementary nucleotides on the original DNA strand, they wouldn’t join together to form a new strand.

Question 21

What type of molecule is ATP?

Answer 21

A phosphorylated macromolecule

Question 22

What are the 3 parts of ATP?

Answer 22

• Adenine: a nitrogen-containing organic base.
• Ribose: a pentose sugar, acting as the backbone of the molecule.
• Phosphates: a chain of 3 phosphate groups.

Question 23

How does ATP store energy

Answer 23

The bonds between the phosphate groups are unstable and so have a low activation energy, which means they’re easily broken. When they break they release a considerable amount of energy.

Question 24

State the equation for the conversion of ATP into energy.

Answer 24

ATP + (H2O) -> ADP + Pi (inorganic phosphate) + Energy

Question 25

What type of reaction converts ATP to energy?

Answer 25

Hydrolysis- water is used to convert ATP to ADP.

Question 26

Name the enzyme that catalyses the reaction converting ATP into energy

Answer 26

ATP hydrolase

Question 27

What is the type of reaction which synthesises ATP from ADP.

Answer 27

Condensation- water is removed.

Question 28

Name three ways ATP is synthesised from ADP

Answer 28

- In chlorophyll-containing plant cells during photosynthesis. - In plant and animal cells during respiration. - In plant and animal cells where phosphate groups are transferred from donor molecules to ADP

Question 29

Why is ATP an unsuitable long term energy store?

Answer 29

Due to the instability of its phosphate bonds. Fats and carbohydrates are better stores.

Question 30

Why is ATP a better immediate energy source than glucose?

Answer 30

- Each ATP molecule releases less energy than each glucose molecule. The energy for reactions is released in smaller, more manageable quantities. - The hydrolysis of ATP to ADP is a single reaction that releases immediate energy. The breakdown of glucose is a long series of reactions and therefore energy re,ease takes longer.

Question 31

What processes require ATP in cells?

Answer 31

- Metabolic processes - Movement - Active transport - Secretion - Activation of molecules

Question 32

Explain why ATP is referred to as an 'immediate energy source'

Answer 32

ATP releases its energy very rapidly. This energy is released in a single step and transferred directly to the reaction requiring it. It's too unstable to be a long term store.

Question 33

Explain how ATP can make an enzyme-catalysed reaction take place more readily.

Answer 33

ATP provides a phosphate that can attach to another molecule, making it more reactive and so lowering its activation energy. As enzymes work by lowering activation energy they have less work to do and function more readily.

Question 34

Explain why water is described as a 'dipolar' molecule

Answer 34

The oxygen atom has a slight negative charge, while the hydrogen atoms have a slight positive one.

Question 35

Explain how water molecules form hydrogen bonds.

Answer 35

The positive pole of one water molecule will be attracted to the negative pole of another. The attractive force between these molecules is called a hydrogen bond.

Question 36

Why is the boiling point of water higher than expected?

Answer 36

Because water molecules stick together, it takes more energy to separate them.

Question 37

What are the advantages of water having a high specific heat capacity?

Answer 37

Water acts as a buffer against sudden temperature variations, making the aquatic environment temperature-stable. As organisms are mostly, water, it also buffers them against sudden temperature changes especially in terrestrial environments.

Question 38

Describe cohesion and give an example

Answer 38

Cohesion is the tendency of molecules to stick together. With its hydrogen bonding, water has large cohesive forces and these allow it to be pulled up a tube, such as xylem vessel in plants.

Question 39

Describe surface tension and give an example

Answer 39

Where water molecules meet the air they tend to be pulled back into the body of water rather than escaping from it. This surface tension allows water to be strong enough to support small organisms such as pond skaters.

Question 40

What is the importance of water in metabolism?

Answer 40

- Water is used to break down complex molecules by hydrolysis and is produced in condensation reactions. - Chemical reactions take place in an aqueous medium. - Water is a major raw material in photosynthesis.

Question 41

As a solvent, name substances that water can readily dissolve

Answer 41

- Gases- oxygen, carbon dioxide - Wastes- ammonia, urea - Inorganic ions, small hydrophilic molecules - Enzymes, whose reactions take place in a solution.