Amino acids and DNA

Question 1

How many naturally occurring amino acids are there

Answer 1

20

Question 2

What type of amino acid are all of the 20 naturally occurring amino acids and what does this mean

Answer 2

2-amino acids which means that the amine group is on the carbon next to the -COOH group.

Question 3

Describe the properties of amino acids as acids and bases

Answer 3

• Amino acids have an acidic and a basic functional group.
• The carboxylic acid has a tendency to lose a proton (act as an acid)
• The amine group has a tendency to accept a proton (act as a base).

Question 4

What is a zwitterion

Answer 4

• zwitterions are how amino acids exist in solid form
• The carboxyl group loses a hydrogen and has a negative charge
• The amine group accepts a proton so has a positive charge.
• This means overall, the charge of a zwitterion is neutral.
• Because amino acids exist as zwitterions they are ionic compounds.

Question 5

What does their zwitterions causing amino acids to be ionic cause its properties to be

Answer 5

• They have high melting points.
• They dissolve well in water but poorly in non-polar solvents
• A typical amino acid is a white solid at room temperature and behaves much like an ionic salt.

Question 6

What happens to the structure of an amino acid in strongly acidic conditions

Answer 6

The lone pair on the -NH2 group accepts a proton to form a positive ion

Question 7

What happens to the structure of an amino acid in strongly alkaline conditions

Answer 7

The -OH group loses a proton (is deprotonated) to form a negative ion.

Question 8

What linkage joins amino acids

Answer 8

An amide linkage

Question 9

How do you break a protein into its constituent amino acids

Answer 9

• Boil it with 6 moldm^-3 HCl
• It breaks down into a mixture of its constituent amino acids.
• All peptide linkages are hydrolysed by the acid.

Question 10

List and explain the three different ways that amino acids in a protein chain can bond together

Answer 10

1) Hydrogen bonding: This occurs between the C=O groups and -NH2 groups
2) Ionic attractions: These occur between groups on the side chains of amino acids such as -COO- and -NH3+
3) Sulfur-sulfur bonds (disulphide bridges): Under suitable oxidising conditions, two amino acids with sulphur containing R groups may react to form a disulphide bridge.

Question 11

Describe what the primary structure of a protein is

Answer 11

• The primary structure of a protein is the sequence of amino acids along the protein chain
• This structure is held together by covalent bonds which means it is stable.
• It requires harsh conditions such as boiling with 6moldm^-3 HCl to break the amino acids apart.

Question 12

Describe what the secondary structure of a protein is

Answer 12

• The amino acid chain folds to form an a-helice or b-pleated sheet.
• This secondary structure is held together by hydrogen bonds.
• Hydrogen bonds are much weaker than covalent bonds and so this level of structure is easily disrupted by gentle heating or changes in PH.

Question 13

Describe what the tertiary structure of a protein is

Answer 13

• The a-helix or b-pleated sheet can be folded into a 3-dimensional shape
• This tertiary structure is held together by a mixture of hydrogen bonds, ionic interactions and disulphide bridges.

Question 14

Explain why wool is stretchy

Answer 14

• Wool is a protein fibre with a helix that is held together by hydrogen bonds
• When wool is gently stretched, the hydrogen bonds stretch and the wool extends.
• Releasing this tension allows the hydrogen bonds to return to their normal length and the fibre returns to its original shape.
• However, washing at high temperatures can permanently break the hydrogen bonds and a garment may permanently lose its shape.

Question 15

What are enzymes

Answer 15

• Proteins that are biological catalysts
• They can speed up reactions by factors of up to 10^10 and can act upon 500,000 molecules per second.
• They are globular proteins that have a cleft called the active site to which the substrate bonds.

Question 16

What is a key feature of enzymes

Answer 16

They are stereospecific: only one enzyme enantiomer will catalyse a reaction/ an enzyme will only act on one substrate enantiomer because of the specificity of the active site.

Question 17

What are the different ways in which drugs block the action of enzymes

Answer 17

• The enzyme can be denatured by changing the temperature and pH. The problem with this is that the conditions in the human body cannot be changed enough for this to happen.
• Enzyme inhibition: A molecule with a similiar shape to the substrate bind to the enzymes active site,therefore blocking it’s action.

Question 18

How does penicillin work

Answer 18

It inhibits the enzymes that control the building of cell walls in bacteria.

Question 19

What is a nucleotide made up of

Answer 19

A phosphate, a sugar and a base

Question 20

How do nucleotides link together

Answer 20

The -OH group of a phosphate on one nucleotide reacts with the -OH group on a sugar molecule of another nucleotide to eliminate a molecule of water.

Question 21

Describe the shape of DNA and how this is held together

Answer 21

• DNA exists as a double helix held together by hydrogen bonding.
• Thymine and adenine form two hydrogen bonds
• Cytosine and guanine form three hydrogen bonds.

Question 22

What is cisplatin

Answer 22

An anti-cancer drug

Question 23

What is the shape of cisplatin

Answer 23

Square planar

Question 24

Explain how cisplatin works

Answer 24

• Cisplatin works by bonding to strands of DNA, distorting their shape and preventing replication of cells.
• It bonds to nitrogen atoms on two adjacent guanine bases on a strand of DNA.
• This works because the nitrogen atoms of the guanine molecules have lone pairs of electrons that form dative covalent bonds with the platinum.
• The chloride ions in cisplatin are displaced by water.
• The water ligands are then displaced by nitrogen on guanine because the nitrogen is a better ligand.
• This is an example of a ligand substitution reaction.

Question 25

What type of cells does cisplatin act on

Answer 25

• Cisplatin bonds to DNA in healthy cells as well as in cancerous ones.
• Cancer cells replicate faster than healthy cells and so the effect of the drug is greater on cancer cells than normal cells.
• However healthy cells that replicate quickly such as hair follicles are also significantly affected.