Topic 2 - Nucleic acids

Question 1

Function of DNA and RNA

Answer 1

• DNA holds genetic information
• RNA transfers genetic information from DNA to ribosomes. Ribosomes are formed from RNA and proteins

Question 2

Structure of DNA nucleotide

Answer 2

• Contains pentose sugar, nitrogenous base and a phosphate group
• deoxyribose
• adenine, thymine, guanine or cytosine

Question 3

Structure of RNA nucleotide

Answer 3

• Contains pentose sugar, nitrogenous base and a phosphate group
• ribose
• adenine, uracil, guanine or cytosine

Question 4

Structure of DNA molecule

Answer 4

• polynucleotide chain
• DNA nucleotides joined together by condensation reactions forming phosphodiester bonds
• 2 strands joined in antiparallel, held together by hydrogen bonds between specific complementary base pairs, twisting into a double helix

Question 5

Structure of RNA molecule

Answer 5

• Polynucleotide chain
• RNA molecules joined together by condensation reactions forming phosphodiester bonds
• Single RNA polynucleotide strand

Question 6

Difference between DNA and RNA molecules

Answer 6

• DNA molecules have the pentose sugar deoxyribose whereas RNA nucleotides have the pentose sugar ribose
• DNA nucleotides can have the base thymine, whereas RNA nucleotides have uracil instead
• DNA molecules are double stranded (double helix) whereas RNA molecules are single stranded
• DNA is longer whereas RNA is shorter

Question 7

Structure of DNA related to it’s functions:

Answer 7

• Double stranded - both strands can act as templates for semi-conservative replication
• Weak hydrogen bonds between bases - can be unzipped for replication
• Complementary base pairing - accurate replication
• Many hydrogen bonds between bases - stable/strong molecule
• Double helix with sugar phosphate backbone - protects bases / H bonds
• Long molecule - store lots of genetic infomation (that codes for polypeptides)
• Double helix (coiled) - compact

Question 8

Process of DNA replication

Answer 8

1. DNA helicase breaks hydrogen bonds between bases, unwinded double helix
2. = two strands which both act as templates
3. free floating DNA nucleotides attracted to exposed bases via specific complementary base pairing, hydrogen bonds form (a-t, g-c)
4. DNA polymerase joins adjacent nucleotides on new strand by condensation, forming phosphodiester bonds (= sugar phosphate backbone)
5. Replication is semi-conservative - each new strand forms contains one original/template strand and one new strand
6. Ensure genetic continuity between generations of cells

Question 9

DNA polymerase moves in opposite directions along DNA strands

Answer 9

• DNA has antiparallel strands
• DNA polymerase is an enzyme with a specific shaped active site which can only bind to substrate with a complementary shape
• Can only bind to and add nucleotides to the phosphate (3’) end of the developing trend (so works in a 5’ to 3’ direction)

Question 10

Evidence of semi conservative replication (Meselson and Stahl)

Answer 10

• Bacteria grown in a nutrient solution containing heavy nitrogen (15N) for several generations
• nitrogen incorporate into bacterial DNA bases
• Bacteria then transferred to a nutrient solution containing light nitrogen (14N) and called to grow and divide twice.
• During this process, DNA from different samples of bacteria was extracted, suspended in a solution in separate tubes was spun in a centrifuge.

Sample 1. DNA from bacteria grown for several generations in nutrient solution containing 15N
–> DNA molecules contain 2 ‘heavy’ strands

Sample 2. DNA from bacteria grown originally in a nutrient solution containing 15N, then transferred for one division to a solution containing 14N
–> DNA molecules contain 1 original ‘heavy’ and 1 new ‘light’ strand

Sample 3. DNA from bacteria grown originally in a nutrient solution 15N, then transferred for two divisions to a solution containing 14N
–> 50% DNA molecules contain 1 ‘heavy’ strand and 1 new ‘light’ strand, 50% contain both ‘light’ strands.

Question 11

The structure of ATP (adenosine triphosphate)

Answer 11

• Ribose
• Molecule of adenine
• 3 phosphate groups
• Nucleotide derivative

Question 12

ATP hydrolysis

Answer 12

• ATP –> ADP + Pi
• Catalysed by the enzyme ATP hydrolase
• Can be coupled to energy requiring reactions within cells, to provide energy for active transport, protein synthesis, etc.
• The inorganic phosphate released can be used to phosphorylate other compounds e.g. glucose, often making them more reactive (i.e. lowers the activation energy)

Question 13

ATP condensation

Answer 13

• ADP + Pi –> ATP
• Catalysed by the enzyme ATP synthase
• Happens during respiration or photosynthesis
• Also called phosphorylation of ADP

Question 14

The properties of ATP that make it a suitable immediate source of energy

Answer 14

IMPORTANT - ATP cannot be stored
- ATP releases energy in small, manageable amounts (no energy wasted)
- Only one bond is hydrolysed (single reaction) to release energy (which is why energy release is immediate)

Question 15

How hydrogen bonding occurs between water molecules

Answer 15

• Water is a polar molecule (oxygen molecule has a partial negative charge; hydrogen atoms have a partial positive charge)
• Slightly negatively charged oxygen atoms attract slightly positive charged hydrogen atoms of other water molecules
• Hydrogen bonds form between water molecules

Question 16

Properties of water that make it important

Answer 16

• High specific heat capacity
• High latent heat of evaporation
• Cohesive
• Solvent
• Metabolite

Question 17

High specific heat capacity explanation and importance

Answer 17

Explanation:
- Polar so many H bonds form between water molecules
- These allow water to absorb a relatively large amount of heat energy before its temperature changes

Importance:
- Good habitat for aquatic organisms
- Organisms most made of water so helps to maintain a constant internal body temperature - important as temperature affects enzyme activity

Question 18

High latent heat of evaporation explanation and importance

Answer 18

Explanation:
- Polar so many H bonds form between water molecules
- These can absorb a lot of energy before breaking, when water evaporates

Importance:
- Evaporation of small amount of water (e.g. sweat) is an efficient cooling mechanism
- Helping organisms maintain constant body temperature

Question 19

Cohesive explanation and importance

Answer 19

Explanation:
- Polar so many H bonds form between water molecules
- So water molecules tend to stick together

Importance:
- Column of water doesn’t break when pulled up a narrow tube e.g. xylem during transpiration
- Produces surface tension at an air water surface so invertebrates can walk on water e.g. pond skaters

Question 20

Solvent explanation and importance

Answer 20

Explanation:
- Polar (has a slightly positive and negative ends)
- Can separate (dissolve) ionic compounds

Importance:
Can dissolve other substances e.g. inorganic ions, enzymes, urea, etc. so water…
- Acts as a medium for metabolic reactions (which can happen in water)
- Acts as a transport medium e.g. in xylem to transport nitrates which are needed to make amino acids

Question 21

Metabolite explanation and importance

Answer 21

Explanation:
- Water is reactive

Importance:
Condensation releases H2O and forms a chemical bond; hydrolysis requires H2O to break a bond; e.g. amino acids joined by condensation reactions for form polypeptides

Question 22

Inorganic ions

Answer 22

• Occur is solution in the cytoplasms and body fluids of organisms
• Some in high concentrations and others in very low concentrations
• Each type of ion has a specific role, depending on it properties

Question 23

Phosphate: chemical symbol and role

Answer 23

Chemical symbol:
PO4-3

Role:
Attached to other molecules as a phosphate group for example:
- In DNA nucleotides, enabling nucleotides to join together forming phosphodiester bonds
- In ATP - bonds between these store / release energy

Question 24

Hydrogen: chemical symbol and role

Answer 24

Chemical symbol:
H+

Role:
Maintain pH levels in the body
- Too much H+ = acidic (low pH)
- Too little H+ = alkaline (high pH)
- Affects rate of enzyme controlled reactions as can cause enzymes to denature

Question 25

Iron chemical symbol and role

Answer 25

Chemical symbol: Fe2+ Role: - Component of (haem group of) haemoglobin which is contained in red blood cells - Transports oxygen around the body - oxygen temporarily binds to it, so it becomes Fe3+

Question 26

Sodium chemical symbol and role

Answer 26

Chemical symbol: Na+ Role: - Co transport of glucose and amino acids across cell membranes - Involved in generating nerve impulses and muscle contraction