DNA Function, Structure And Replication

Question 1

Features of DNA in prokaryotes?

Answer 1

• no nuclear membrane
• DNA arranged in a single chromosome (may be circular)
• attachment point on DNA to cell wall

Question 2

Features of DNA in eukaryotes?

Answer 2

• DNA in the nucleus, bound to proteins (histones) to form a chromatin complex
• during mitosis chromatin condenses (coils around protein scaffold) into chromosomes
• each chromosome made of 2 identical strands joined by a centromere
• 5% of DNA found in mitochondria

Question 3

What are the functions of DNA?

Answer 3

• template and regulator for transcription and protein synthesis (can interact with proteins such as transcription factors)
• structural basis of heredity and genetic diseases

Question 4

How many structural genes in the human genome (genes which code for proteins)?

Answer 4

20,000 - 25,000

Question 5

What is the replication fork?

Answer 5

Structure created by helicase when it breaks H-bonds between the strands.

Question 6

What is meant by semi-conservative replication?

Answer 6

The daughter DNA strands are composed of a new and an old DNA strand.

Question 7

What is the function of DNA polymerase?

Answer 7

• synthesises DNA by adding nucleotides to the new DNA strand at the 3’ end
• proof reads DNA for replication mistakes and corrects them
• deoxyribonucleotides triphosphates are the substrates for DNA polymerase

Question 8

What is necessary for DNA polymerase to start DNA synthesis?

Answer 8

A primer is needed (short strand of RNA which provides a starting point for DNA synthesis)

Question 9

Where does the energy for DNA synthesis come from?

Answer 9

Breaking the covalent bond between phosphate groups of deoxyribonucleotides triphosphates releases energy.

Question 10

What is the function of DNA helicase?

Answer 10

Opens up DNA parent strand by breaking H bonds between base pairs.

Question 11

What is the function of single stranded binding proteins?

Answer 11

SSB proteins keep the parent strand open.

Question 12

What is the function of topoisomerase?

Answer 12

To unwind the DNA helix, ahead of DNA helicase.

Question 13

What is a lagging strand?

Answer 13

• the DNA strand replicated in the 3’ to 5’ direction
• synthesised in fragments (Okazaki fragments)
• fragments joined by ligase

Question 14

What are Okazaki fragments?

Answer 14

The fragments formed by the synthesis of the lagging DNA strand, joined by ligase.

Question 15

What are 3 sources of DNA damage?

Answer 15

• chemical
• radiation
• spontaneous insertion of incorrect bases during replication

Question 16

What is a thymine dimer?

Answer 16

UV light causes covalent bonds to form between consecutive thymine bases in the DNA helix. Distorts and destabilises DNA.

Question 17

How does p53 regulate the cell cycle?

Answer 17

• detects DNA damage and initiates repair mechanisms
• pauses cell cycle until repair is made
• halts cell cycle and initiates apoptosis if DNA is not repaired.

Question 18

Why do we develop drugs that interfere with DNA replication?

Answer 18

To treat cancer by interfering with replication of malignant cell DNA.

Question 19

What are some examples of therapeutic drugs targeting DNA replication?

Answer 19

• nucleotide synthesis inhibitors
• DNA polymerase inhibitors
• DNA template damaging agents
• DNA topoisomerase inhibitors