Chapter 6: DNA replication

Question 1

Main functions of DNA and RNA?

Answer 1

DNA: Store heritable information
RNA: Synthesis of proteins

Question 2

How is DNA shaped?

Answer 2

Double-helix with 5’ and 3’ ends that run antiparallel to each other

Question 3

When is DNA replication happening in the cell cycle?

Answer 3

S phase of interphase for both mitosis and meiosis

Question 4

What does Helicase do?

Answer 4

Unwinds DNA strands (breaks hydrogen bonds)

Question 5

What does DNA polymerase do?

Answer 5

• Synthesizes DNA by catalyzing the formation of phosphodiester bonds between nucleotides
• Uses parental strand as a template (adds complementary nucleotides)
• Reads the template strand from 3’ to 5’
• Synthesizes DNA in the 5’ to 3’ direction (links 5’ phosphate group of new nucleotide to 3’-OH
  group of growing strand)

Question 6

What does Primase do?

Answer 6

• An RNA polymerase that synthesizes an RNA primer on the template strand
• Required because DNA polymerase can only add a nucleotide to a free 3’-OH group

Question 7

What does ligase do?

Answer 7

• Glues together nicks in DNA sugar-phosphate backbone (forms phosphodiester bond)
• Needed after RNA primers are replaced by DNA and for Okazaki fragments

Question 8

What is the role of single-stranded binding proteins?

Answer 8

Stabilizes single stranded DNA

Question 9

Where does DNA polymerase get its energy from?

Answer 9

breaking of bonds between phosphates of nucleoside triphosphate (E coupling???)

Question 10

Where does replication start?

Answer 10

origins of replication

Question 11

How many helicases are there per origin of replication?

Answer 11

2

Question 12

What binds to the single-stranded DNA to stabilize it?

Answer 12

Single stranded binding proteins

Question 13

How does elongation happen?

Answer 13

• Primase makes a complementary RNA primer
• DNA polymerase synthesizes a new strand of
  DNA using the parental strand as a template

Question 14

What characterizes the leading strand?

Answer 14

• Continuous DNA synthesis
• DNA polymerase is synthesizing DNA in the
  same direction as DNA unwinding
• One RNA primer

Question 15

What characterizes the lagging strand?

Answer 15

• Discontinuous DNA synthesis (Okazaki
  fragments)
• DNA polymerase must synthesize DNA in the
  opposite direction of DNA unwinding
• Many RNA primers

Question 16

What are the steps for a leading strand?

Answer 16

1. Helicase unwinds DNA
2. Primase makes RNA primer
3. DNA pol extends from primer
4. DNA pol replaces RNA primer with DNA
5. DNA ligase repairs nick in backbone
   between replaced primer and rest of
   new strand

Question 17

What are the steps for a lagging strand?

Answer 17

1. Helicase unwinds DNA
2. Primase makes RNA primer 1
3. DNA pol extends from primer 1
4. Primase makes RNA primer 2
5. DNA pol extends from primer 2
   ….
6. DNA pol replaces RNA primers with DNA
7. DNA ligase repairs nicks in backbone between
   replaced primers and rest of strand and between
   Okazaki fragments

Question 18

What is the problem with the end of replication?

Answer 18

• Removal of RNA primer from beginning
  of lagging strand leaves a gap and a free
  5’-phosphate end

Question 19

Why can’t the gap at the end of replication be filled by DNA polyermerase?

Answer 19

Gap cannot be filled by DNA pol
because can only add DNA to 3’-OH end

Question 20

What are the consequences of the end of replication problem?

Answer 20

• Each replication results in shorter DNA!
• Can result in the deletion of genes
• Limiting factor in life span of cell

Question 21

What is eukaryotic DNA flanked by?

Answer 21

Telomeres

Question 22

What are telomeres?

Answer 22

Telomeres are repeats of
noncoding nucleotide sequences

Question 23

What are telomeres synthesized by?

Answer 23

Telomerase (RNA and protein)

Question 24

Where is telomerase active?

Answer 24

Germ and Stem cells (not somatic cells!)