Nucleic acids (a,b)

Question 1

What are nucleic acids?

Answer 1

Large molecules that were discovered in cell nuclei, e.g. DNA and RNA. Polymers formed from nucleotides.

Question 2

What do they contain?

Answer 2

C, H, O, N, P

Question 3

What do the nucleotides consist of?

Answer 3

• a pentose monosaccharide
• a phosphate group - PO₄²⁻ - inorganic molecule - acidic and negatively charged
• a nitrogenous base - a complex organic molecule containing 1 or 2 carbon rings in its structure as well as nitrogen

Question 4

How do the nucleotides link together?

Answer 4

• by condensation reactions to form polynucleotide
• the phosphate group at the 5th carbon on the pentose sugar (5’) of one nucleotide forms a covalent bond with the hydroxyl group at the third carbon (3’) of the pentose sugar of an adjacent molecule
• phosphodiester bonds formed
• forms a long, strong sugar - phosphate ‘backbone’ with a base attached to each sugar

Question 5

What are pyramidines?

Answer 5

• smaller bases containing single carbon ring structures
• thymine, T and cytosine, C
• contains one C-N ring

Question 6

What are purines?

Answer 6

• larger bases, containing double carbon ring structures
• adenine, A and guanine, G
• contains 2 C-N rings

Question 7

How many bonds does each base form?

Answer 7

T and A form 2 hydrogen bonds

C and G form 3 hydrogen bonds

Question 8

Describe DNA - deoxyribonucleic acid

Answer 8

• deoxyribose sugar (1 fewer O atoms than ribose)
• double helix structure
• 2 strands of polynucleotides coiled into a helix
• strands held together by hydrogen bonds between the bases
• each strand has a phosphate group (5’) at one end and a hydroxyl group (3’) at the other end
• strands are anti-parallel - arranged so they run in opposite directions
• pairing between the bases allows DNA to be copied + transcribed

Question 9

Base pairing rules

Answer 9

• complementary base pairing: A+T and C+G
• specific due to number of hydrogen bonds
• purines pair with pyramidines
• arrangement maintains a constant distance between the DNA ‘backbones’ resulting in parallel polynucleotide chains
• thus equal amounts of A+T and C+G always

Question 10

What is RNA + its role?

Answer 10

• ribonucleic acid
• essential role in the transfer of genetic information from DNA to the protein that make up the enzymes and tissues of the body

Question 11

Describe the structure of RNA

Answer 11

• ribose sugar
• (U is a pyramidine) A+U and C+G, no thymine, T
• single polynucleotide chain

Question 12

What happens to RNA after protein synthesis?

Answer 12

• the RNA molecules are degraded in the cytoplasm

- phosphodiester bonds are hydrolysed + the RNA nucleotides are released and reused

Question 13

Practical investigations to obtain DNA

Answer 13

1. Grind sample in a mortar and pestle to break down cell walls
2. Mix with detergent to break down the cell membrane to release cell contents into solution
3. Add salt to break the hydrogen bonds between DNA and water molecules
4. Add protease to break down the proteins associated with the DNA in the nuclei
5. Add a layer of ethanol to cause the DNA to precipitate out of solution
6. Seen as white strands forming between the layer of sample and layer of alcohol. Spool it onto a glass rod.