Replication

Question 1

DNA

RNA

Answer 1

Deoxyribonucleic acid (H at C2)
Ribonucleic acid (OH at C2)

Question 2

Monomers of nucleic acids

Answer 2

Nucleotides, linked by phosphodiester bonds

Question 3

Nucleotides

Answer 3

1. 5-carbon sugar → pentose → ribose/deoxyribose
2. Nitrogenous base → purine/pyrimidine
3. Phosphate group → attached to C5 → -ve charge

Question 4

Nucleoside

Answer 4

1. 5-carbon sugar

2. Nitrogenous base

Question 5

Purine

Answer 5

• 2 rings

- Adenine (A) and guanine (G)

Question 6

Pyrimidine

Answer 6

• 1 ring

- Thymine (T)/uracil (U) and cytosine

Question 7

Structure of DNA

Answer 7

Pentose sugar + nitrogenous base
→ nucleoside + phosphoric acid
→ nucleotides → (Deoxy)nucleoside[no. of phosphates]
→ nucleic acid (2nm, -ve) + Histones (+ve), electrostatic interactions

→ nucleosomes (euchromatin, 10nm fibre) + linker
→ chromatin fibre/solenoid (heterochromatin, 30nm) + protein scaffold
→ looped domains (300nm)
→ chromatid (700nm)
→ replicated chromosome (1400nm/1.4μm)

Question 8

Nucleic acid

Answer 8

• Deoxyribonucleotide is basic unit
• Width btwn 2 sugar phosphate backbones is constant (2.0nm) → combined width of purine and pyrimidine
• 1 complete turn → 10 base pairs → 3.4 nm
• Double helix → antiparallel → right-handed
• Held by weak H bonds btwn nitrogenous bases of opposite strands
• A=T, G≡C → complementary base pairing

Question 9

Structure of RNA

Answer 9

• Ribonucleotide is basic unit
• Uracil instead of thymine
• Single stranded (except in some viruses)

Question 10

DNA Replication

Answer 10

• Semi-conservative
• Many enzymes and proteins
• Rapid
• Accurate
• S-phase of interphase
• Requires free deoxyribonucleoside triphosphate (dNTP)

Question 11

Semi-conservative

Answer 11

• Both parental strands of DNA separate through breakage of H bonds and each act as a template for the synthesis of a new strand through complementary base pairing
• Each DNA molecule formed is a hybrid → 1 original strand, 1 newly synthesised strand

Question 12

DNA Replication steps (4)

Answer 12

1. Before replication/overview
2. Unzipping of parental strands
3. Addition of primer
4. Synthesis of daughter strands

Question 13

1. Before replication/overview (4)

Answer 13

1. DNA replication occurs at the S phase of interphase
2. It is semi-conservative where the original strands of double helix separates and act as templates for synthesis of two new strands
3. This gives rise to two new DNA molecules, each consisting of one original and one newly synthesised strand
4. Before replication, free activated deoxyribonucleoside triphosphates (dNTP) are manufactured in the cytoplasm and transported into the nucleoplasm via nuclear pores

Question 14

2. Unzipping of parental strands (4)

Answer 14

1. Replication of DNA begins at origin of replication,
2. where helicase will bind and unzip and separate the DNA molecule by breaking hydrogen bonds between complementary base pairs
3. Single-strand binding proteins keep the strands apart so that they can serve as templates for the synthesis of new strands
4. Topoisomerase relieves overwinding strain ahead of replication forks by breaking, swivelling and rejoining DNA strands

Question 15

3. Addition of primer (3)

Answer 15

1. Enzyme primase catalyses the synthesis of a short RNA primer
2. which provides a free 3’ OH end for DNA polymerase to recognise and start DNA synthesis of the complementary daughter strand
3. as it can only add deoxyribonucleotides to a pre-existing free 3’ OH end of a nucleotide

Question 16

4. Synthesis of daughter strands (Basic mechanism) (5)

Answer 16

1. dNTPs are added to the newly synthesised strand via complementary base pairing with the template strand
2. Whereby A forms 2 H bonds with T and G forms 3 H bonds with C
3. DNA polymerase catalyses the formation of phosphodiester bonds between adjacent daughter DNA
   nucleotides of the newly synthesised strand
4. Removal of the pyrophosphate (PPi) from the dNTP and the subsequent hydrolysis of PPi provides the energy to drive the polymerisation reaction
5. As DNA polymerase moves along the template, it proof reads the previous region for proper base pairing. Any incorrect deoxyribonucleotide is removed and replaced by the correct one

Question 17

4. Synthesis of daughter strands (Leading vs lagging strand) (5)

Answer 17

1. The leading strand is synthesised continuously in the 5’ to 3’ direction
2. The lagging strand is synthesised discontinuously, giving rise to Okazaki fragments. Each fragment is initiated by an RNA primer before the addition of DNA nucleotides
3. A different DNA polymerase then removes the RNA primer and replaces it with DNA
4. DNA ligase seals the nicks by catalysing the formation phosphodiester bonds between Okazaki fragments
5. At the end of replication, the complementary parental and newly synthesised DNA strands form a double helix

Question 18

Role of DNA (4)

Answer 18

• Main role → store information and pass it on from one generation to the next
• It is a suitable store of information as:

1. Can be replicated accurately
2. Stable molecule
3. Backup of the code
4. Coded information readily utilised/accessed

Question 19

1. Can be replicated accurately → cbp

Answer 19

• Daughter cells have identical copies of DNA as the parent cell
• Weak H bonding between the 2 strands allow them to separate and act as a template for new strand synthesis
• A forms 2 H bonds with T and C forms 3 H bonds with G through complementary base pairing

Question 20

2. Stable molecule → bonds

Answer 20

• Can be passed on to next generation without loss of the coded information
• Collectively, numerous H bonds hold the two strands of DNA together
• Adjacent nucleotides in each strand joined by strong covalent phosphodiester bonds

Question 21

3. Backup of the code → repair template

Answer 21

• DNA is double stranded

- 1 strand to serve as a template for the repair of the other if a mutation occurs spontaneously on either one

Question 22

4. Coded information readily utilised/accessed → transcription, cbp

Answer 22

• Weak H bonding allows the template strand to separate from the non-template strand allowing transcription to take place
• Complementary base pairing allows faithful transfer of info from DNA to RNA in transcription, which will subsequently be translated to protein

Question 23

Telomeres

Answer 23

• Nucleotide sequences found at both ends of eukaryotic chromosomes
• Non-coding regions of DNA made up of tandem repeat sequences
• Single-stranded region → 3’ overhang
• Ensure genes are not lost/eroded with each round of DNA replication due to end replication problem
• Prevent loss of vital genetic info with each replication cycle

Question 24

End replication problem

Answer 24

• DNA polymerase is unable to replace RNA primers with DNA
• RNA primer removed without replacement → 3’ overhang
• Each round of DNA replication results in shortening of daughter molecules at the telomeres
• Critical length → apoptosis
• Telomerase extends telomeres