DNA

Question 1

Draw the structure of a phosphodiester bond

Answer 1

Question 2

What is the base paring for DNA?

Answer 2

adenine = thymine

guanine = cytosine

Question 3

Why is DNA a stable molecule?

Answer 3

• 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
• because there are three hydrogen bonds between cysteine and guanine, the higher proportion of C—G pairings, the more stable the DNA molecule

Question 4

How is DNA adapted to carry out its function?

Answer 4

• very stable structure. Rarely mutates
• two separate strands are joined only with hydrogen bonds, which allows them to separate during DNA replication and protein synthesis
• extremely large molecule and therefore carries an immense amount of genetic information
• by having 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 of DNA being able to replicate and to transfer information and mRNA

Question 5

Name the two purine bases. How are they different from the pyrimidine bases? (name all the pyrimidine bases)

Answer 5

• The bases adenine and guanine are purines
  – they have a double ring structure
• The bases cytosine, thymine and uracil are pyrimidines
  – they have a single ring structure

Question 6

Which bases form two hydrogen bond?

Answer 6

adenine and thymine

Question 7

What are the two requirements for semi-conservative replication?

Answer 7

• the four types of nucleotides (each with a different base) must be present
• a source of chemical energy to drive the process

Question 8

Describe the process of semi-conservative replication?

Answer 8

• The enzyme DNA helicase breaks the hydrogen bonds between the two polynucloetide DNA strands causing the double helix to unzip, forming two single strands.
• Each original single strand then acts as a template for a new strand. Free-floating DNA nucleotides join to the exposed bases on each original template strand via complementary base pairing.
• The nucleotides of the new strands are joined together by the enzyme DNA polymerase, which catalyses a condensation reaction between the two adjacent nucleotides (joined together by phosphodiester bonds)
• This forms the sugar-phosphate backbone. Hydrogen bonds form between the bases on the original and new strand and the strands twist to reform the double helix.

Question 9

What does semi-conservative replication of DNA ensure?

Answer 9

genetic continuity between generations of cells

Question 10

Describe the anti-parallel nature of DNA

Answer 10

As the strands run in opposite directions (they are antiparallel), one is known as the 5’ to 3’ strand and the other is known as the 3’ to 5’ strand

Question 11

Draw a dna polynucleotide

Answer 11

Question 12

Describe the experiment that prove semi-conservation

Answer 12

Replicating Bacterial DNA in 2 types of Nitrogen Isotopes, 15N and 14N

Nitrogen found in nitrogenous bases of DNA

Bacterial DNA made from 15N will have a Heavy Density

Bacterial DNA made from 14N will have a Light Density

Experiment =

Sample of 14N and 15N in separate test tubes are centrifuged : 14 at top and 15 at bottom

Bacterial DNA made of 15N is extracted and is replicated in an environment of 14N – produces DNA molecules with half 15/half 14 (semi-conservative replication, original strand = 15N & new strand = 14N), therefore, DNA molecule has medium density

Question 13

What are the components of ATP?

Answer 13

• Adenine
• ribose
• three phosphates group

Question 14

Why is ATP easily broken?

Answer 14

the bonds between the phosphate groups are unstable = low activation energy = easily broken

Question 15

The synthesis of ATP from ADP involves the addition of a phosphate molecule to ADP in three ways. What are these three ways?

Answer 15

• 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 16

How many bases make up an amino acid?

Answer 16

3

Question 17

What is the chemical equation of ATP hydrolysis

Answer 17

Question 18

What are the roles of ATP? (6)

Answer 18

• immediate energy source
• provides energy needed to build up macromolecules from their basic units (metabolism)
• provides energy for muscle contraction, by providing the energy for gilaments of muscle to slide past one another and therefore shorten the overall length of the muscle fibre (movement)
• provides the energy to change the shape of carrier proteins in plasma membranes, allowing molecules to move against the concentration gradient (active transport)
• needed to form the lysosomes necessary for the secretion of cell products
• activation of molecules = inorganic phosphate released during hydrolysis of ATP, can be used to phosphorylate other compounds in order to make them more reaction, lowering the activation energy.

Question 19

Why is ATP a better immediate energy source than glucose?

Answer 19

• each ATP molecule releases less energy, so energy for reactions is released in smaller, more manageable quantities.
• hydrolysis of ATP to ADP is a single reaction tha releases immediate energy vs the breakdown of glucose is a long series of reactions

Question 20

How is the hydrolysis of ATP used in cells

Answer 20

• release of energy = good for cells which require energy
• donates its phosphate to other substances, it can make these substances more reactive