Topic 6 - DNA Replication

Question 1

What are the three proposed models of DNA replication?

Answer 1

Conservative, Semi-conservative, Dispersive.

Question 2

Describe the conservative model.

Answer 2

The original double helix remains intact; an entirely new double helix is synthesized.

Question 3

Describe the semi-conservative model.

Answer 3

Each daughter DNA molecule contains one original strand and one newly synthesized strand.

Question 4

Describe the dispersive model.

Answer 4

Each strand contains interspersed segments of old and new DNA.

Question 5

What question did Meselson and Stahl answer?

Answer 5

How does DNA replicate?

Question 6

How did Meselson and Stahl label DNA?

Answer 6

Grew bacteria in ¹⁵N (heavy nitrogen), then transferred to ¹⁴N (light nitrogen).

Question 7

What happened after one round of replication?

Answer 7

A single intermediate (hybrid) band formed. (half labeled half unlabeled)

Question 8

What happened after two rounds of replication during Melhson-Stahl experiment.

Answer 8

One light band and one hybrid band appeared.

Question 9

Which model was supported?

Answer 9

Semi-conservative replication.

Question 10

How many origins of replication in prokaryotes?

Answer 10

One (circular chromosome).

Question 11

How many origins in eukaryotes?

Answer 11

Multiple origins per chromosome.

Question 12

Major structural difference affecting replication between Prokaryotes and Eukaryotes?

Answer 12

Prokaryotes have circular DNA; eukaryotes have linear chromosomes.

Question 13

Which polymerases are found in E. coli?

*Polymerases are the enzymes that help with adding nucleotides to (R)DNA

Answer 13

Polymerase I and III.

Question 14

Name the five eukaryotic DNA polymerases.

Answer 14

α, β, γ, δ, ε.

Question 15

Which polymerase replicates mitochondrial DNA?

Answer 15

Polymerase γ.

Question 16

Which polymerase is involved in DNA repair?

Answer 16

Polymerase β.

Question 17

What enzyme unwinds DNA?

Answer 17

Helicase.

Question 18

What prevents strands from re-annealing?

Answer 18

Single-strand binding proteins (SSBPs).

Question 19

What relieves supercoiling tension?

Answer 19

Topoisomerase (Gyrase).

Question 20

Why is a primer required?

Answer 20

DNA polymerase cannot initiate DNA synthesis from scratch

Question 21

Which complex forms the primer in eukaryotes?

Answer 21

Polymerase α–primase complex.

Question 22

What type of primer is formed after a primase complex attaches to ssDNA?

Answer 22

Short RNA (hybrid RNA-DNA) primer in 5’-3’ direction

Question 23

Which polymerase synthesizes the leading strand?

Answer 23

Polymerase ε (continuous).

Question 24

Which polymerase synthesizes the lagging strand?

Answer 24

Polymerase δ (discontinuous).

Question 25

What are short lagging strand fragments called?

Answer 25

Okazaki fragments (150–200 nucleotides).

Question 26

Why is the lagging strand discontinuous?

Answer 26

DNA Polymerase III only reads from 3' to 5' direction thus the lagging strand which is the 5' to 3' strand is always playing catch up with the uncoiling at the start 3'. This results in needing many primers to place down DNA which leaves empty spaces called Okazaki fragments.

Question 27

What removes RNA primers?

Answer 27

DNA polymerase I (replaces RNA with DNA).

Question 28

What enzyme seals the Okazaki fragments?

Answer 28

DNA ligase I.

Question 29

Which strand is synthesized continuously?

Answer 29

Leading strand.

Question 30

Which strand requires multiple primers?

Answer 30

Lagging strand.

Question 31

What problem occurs at chromosome ends?

Answer 31

The lagging strand cannot be fully replicated.

Question 32

What sequence is repeated in human telomeres?

Answer 32

TTAGGG.

Question 33

What enzyme extends telomeres?

Answer 33

Telomerase.

Question 34

What type of enzyme is telomerase?

Answer 34

Reverse transcriptase. (RNA --> cDNA)

Question 35

What does telomerase carry internally?

Answer 35

Its own RNA template.

Question 36

When is telomerase most active?

Answer 36

During development and in stem cells; reactivated in many cancers. (S-Phase)

Question 37

Why are telomeres important?

Answer 37

They prevent chromosome ends from being recognized as DNA breaks.

Question 38

What happens if telomeres continually shorten?

Answer 38

Cellular aging and eventual senescence (cell permanently stops dividing).

Question 39

Where are nucleotides added?

Answer 39

To the 3’ OH group.

Question 40

Where does replication begin?

Answer 40

Origins of replication (ori).

Question 41

How does DNA structure enable replication?

Answer 41

After getting 3' OH group, Complementary base pairing allows each strand to serve as a template.

Question 42

What is a primer

Answer 42

A short RNA strand that provides a free 3′ OH group for DNA synthesis that is added by primase. DNA polymerase cannot start synthesis from nothing.

Question 43

Why can't the ends of chromosomes not be completely copied?

Answer 43

Because of the lagging strand mechanism of DNA replication

Question 44

Why does the lagging strand create a replication problem at chromosome ends?

Answer 44

The lagging strand requires multiple RNA primers to initiate synthesis.

Question 45

What happens when the final RNA primer is removed?

Answer 45

The new daughter strand is shortened by ~100 nucleotides.

Question 46

What sequences are gradually lost after repeated replication?

Answer 46

Telomeric sequences.

Question 47

What is shelterin?

Answer 47

Shelterin is a specialized protein complex that binds specifically to telomeres and protects chromosome ends from being recognized as DNA damage

Question 48

When is telomerase active during the cell cycle?

Answer 48

S-Phase

Question 49

Why do telomeres shorten in normal somatic cells?

Answer 49

Because of the end-replication problem during lagging-strand synthesis.

Question 50

Approximately how much telomeric DNA is lost per cell division?

Answer 50

~50–100 nucleotides per replication cycle.

Question 51

What happens when telomeres become critically short?

Answer 51

Chromosome ends are recognized as DNA damage.

Question 52

What enzyme allows cancer cells to maintain telomere length?

Answer 52

Telomerase

Question 53

How are telomeres connected to aging?

Answer 53

Progressive telomere shortening limits the replicative capacity of somatic cells.

Question 54

What are two proposed mechanisms linking telomere shortening to aging?

Answer 54

1. Loss of telomeres damages nearby gene regions. 2. Short telomeres activate cell-cycle inhibition signals. (Stop cell division)

Question 55

DNA Synthesis comparisons between DNA Polymerase, RNA polymerase and Ribosome

Answer 55

DNA Polymerase: the template strand is read 3'-5' to synthesize new strand 5'-3'. RNA Polymerase: the gene sequence is read in the 3' to 5' in order to synthesize mRNA in the 5' to 3' direction. Ribosome: mRNA is read in a 5'-3' direction results in polypeptide

Question 56

Where do DNA Polymerase I & III come from?

Answer 56

E.Coli

Question 57

What does DNA Polymerase III do?

Answer 57

Places down new DNA on strand