Nucleic acids

Question 1

Nucleotide structure

Answer 1

A nucleotide has 3 components: a pentose (5-carbon) sugar, a phosphate group, and an organic nitrogenous base

Question 2

Polynucleotides

Answer 2

• Formed by condensation reactions between nucleotides, which form phosphodiester bonds
• They are broken by hydrolysis
• e.g. deoxyribose (DNA) and ribonucleic acid (RNA)

Question 3

DNA

Answer 3

• The sugar in DNA nucleotides is deoxyribose

- Each DNA nucleotide has one of four bases: adenine (A), thymine (T), cytosine (C), or guanine (G)

Question 4

Complementary base pairing

Answer 4

• DNA exists as a double helix (i.e. two polynucleotides, running in opposite directions, bounded together)
• The two DNA polynucleotide strands are held together by hydrogen bonds between their bases
• C pairs with G, and A pairs with T

Question 5

Which nitrogenous bases are purine?

Answer 5

Adenine and guanine

Question 6

Which nitrogenous bases are pyramidine?

Answer 6

Thymine and cytosine

Question 7

How many hydrogen bonds are there between A and T?

Answer 7

2

Question 8

How many hydrogen bonds are there between C and G?

Answer 8

3

Question 9

RNA

Answer 9

• RNA exists in 3 forms: messenger, transfer and ribosomal - all of which play a role in polypeptide synthesis
• The pentose sugar in RNA is ribose
• Uracil (U) is present instead of thymine (T), meaning it bonds with A
• It has one phosphate group (in a nucleotide of RNA)
• RNA is single-stranded and does not form a double helix

Question 10

How to extract DNA from plant tissue (practical)

Answer 10

1. Grind tissue (to break down cell walls
2. Mix with detergent (to break down membranes)
3. Add salt (to break hydrogen bonds between DNA polynucleotides)
4. Add protease enzyme (to break down proteins surrounding DNA)
5. Add alcohol (to precipitate DNA out of solution)
6. Remove DNA (which can be seen as white strands below the alcohol layer

Question 11

DNA replication (8 steps)

Answer 11

1. Histone proteins are removed
2. The DNA double helix is unwound (by an enzyme called helicase)
3. Hydrogen bonds between strands are broken (unzipping; catalysed by helicase)
4. Both DNA strands act as template strands
5. Free (monomer) nucleotides are activated (phosphate groups are added to them)
6. Free nucleotides form hydrogen bonds with bases on the template strands
7. C bonds with G, A bonds with T (complementary base pairing)
8. Phosphodiester bonds join nucleotides in the new strands (catalysed by DNA polymerase)

Question 12

What properties does the genetic code have?

Answer 12

• It uses a triplet code (i.e. a sequence of 3 nucleotides, known as a codon, codes for the production of one amino acid)
• The code is degenerate (i.e. in most cases, several codons code for the same amino acid)

Question 13

Start codon (indicates beginning of a gene sequence)

Answer 13

Methionine

Question 14

Stop codons (pinpoint the end of a gene)

Answer 14

TAG, TAA, TGA

Question 15

Transcription (5 steps)

Answer 15

1. Helicase unwinds and unzips DNA (but usually only along the base sequence of one gene)
2. RNA nucleotides bind to complementary DNA bases on the template (antisense) strand
3. RNA polymerase joins RNA nucleotides together with phosphodiester bonds
4. A stop codon causes RNA polymerase to detach
5. The mRNA molecule detaches from the DNA and leaves the nucleus

Question 16

Translation (8 steps)

Answer 16

1. mRNA attaches to a ribosome (in a groove between the two subunits)
2. The first mRNA codon binds to a tRNA molecule with a complementary anticodon
3. The tRNA is bound to an amino acid specific to its anticodon
4. A second tRNA (carrying another amino acid) binds to the adjacent mRNA codon
5. A peptide bond forms between the two amino acids
6. The ribosome continues along the mRNA molecule, from codon to codon
7. Amino acids are bonded together in a polypeptide chain until a stop codon is reached
8. The polypeptide is released

Question 17

ATP structure

Answer 17

It contains an adenine base, a ribose sugar and 3 phosphate groups

Question 18

How are ATP and ADP formed?

Answer 18

• ATP is formed in respiration through aa condensation reaction
• Hydrolysis of ATP produces ADP by removing one of the phosphates
• The hydrolysis reaction releases energy that can be used in metabolic processes

Question 19

How is ATP suited to energy transfer?

Answer 19

1. Its soluble
2. It releases energy in small quantities (which prevents energy wastage)
3. It has an unstable phosphate bond (which is easily broken)