DNA Replication

What is semi-conservative DNA replication?

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

What does it mean that DNA strands are antiparallel?

One strand runs 5'→3' and the complementary strand runs 3'→5'

What is the leading strand?

The strand synthesized continuously in the 5'→3' direction toward the replication fork.

What is the lagging strand?

The strand synthesized discontinuously in short fragments (Okazaki fragments) away from the replication fork.

Where does DNA replication begin?

At origins of replication.

What is a replication fork?

The Y-shaped region where DNA is unwound and new DNA is being synthesized.

How do replication forks move?

In opposite directions away from the origin of replication

What enzyme makes RNA primers?

Primase

Why are primers needed?

DNA polymerase can only extend from an existing 3' OH and requires a double-stranded starting point

What are RNA primers made of?

Short 10–20 nucleotide sequences complementary to the DNA template

In which direction is new DNA synthesized?

Always 5'→3'

What determines the sequence of the new DNA strand?

Complementary base pairing to the template DNA strand.

What enzyme adds nucleotides to the growing DNA strand?

DNA polymerase

Where does DNA polymerase add new nucleotides?

To the 3' OH end of the growing strand.

What provides the energy for DNA polymerase to add nucleotides?

The nucleoside triphosphate itself; two phosphates are released during bonding.

Why is the lagging strand synthesized differently?

Because DNA is antiparallel and polymerase can only build 5'→3', requiring discontinuous synthesis.

What are Okazaki fragments?

Short segments of DNA synthesized on the lagging strand

What enzyme proofreads newly made DNA?

DNA polymerase (self-correcting proofreading)

What is the purpose of proofreading?

To reduce the error rate during DNA replication.

Why can’t the very end of the lagging strand be fully replicated?

Removing the final primer leaves no 3' OH for DNA polymerase to extend.

What happens to chromosomes without telomerase?

They shorten with each round of replication.

What enzyme extends chromosome ends (telomeres)?

Telomerase

Where is telomerase most active?

Germ cells, stem cells, and some cancer cells

What is the function of telomerase?

It adds repeated telomere sequences to prevent chromosome shortening

DNA replication is described as “semi-conservative” because:

A. Each daughter DNA molecule is completely new
B. Each daughter molecule has one old and one new strand
C. Both strands are replaced each replication
D. Only half the genome is copied

B. Each daughter molecule has one old and one new strand

What does it mean that DNA strands are antiparallel?

A. Both run 5′→3′
B. One strand runs 5′→3′ and the other runs 3′→5′
C. They wind in opposite helical directions
D. They contain opposite bases

B. One strand runs 5′→3′ and the other runs 3′→5′

The leading strand is synthesized:

A. Discontinuously away from the replication fork
B. In short fragments
C. Continuously toward the replication fork
D. Only after the lagging strand

C. Continuously toward the replication fork

The lagging strand is synthesized:

A. Continuously
B. In the 3′→5′ direction
C. Using Okazaki fragments
D. Without primers

C. Using Okazaki fragments

DNA replication begins at:

A. Ribosomes
B. Origins of replication
C. Telomeres
D. The nuclear envelope

B. Origins of replication

Replication forks move:

A. In the same direction
B. Randomly
C. Toward each other from both ends
D. In opposite directions away from the origin

D. In opposite directions away from the origin

Primase synthesizes:

A. DNA primers
B. RNA primers
C. Telomeres
D. Spindle fibers

B. RNA primers

What does DNA polymerase require to begin DNA synthesis?

A. A free 5′ phosphate
B. A free 3′ OH on a primer
C. Telomerase activity
D. A fully condensed chromosome

B. A free 3′ OH on a primer

RNA primers used in replication are typically:

A. 2–4 nucleotides
B. 10–20 nucleotides
C. 50–100 nucleotides
D. The same length as Okazaki fragments

B. 10–20 nucleotides

New DNA is always synthesized in which direction?

A. 3′→5′
B. 5′→3′
C. Both directions simultaneously
D. Randomly

B. 5′→3′

The correct nucleotide sequence of the new DNA strand is determined by:

A. Telomerase
B. Helicase
C. Complementary base pairing
D. Ligase

C. Complementary base pairing

DNA polymerase adds nucleotides to the:

A. 5′ end of the new strand
B. 3′ OH end
C. Middle of the strand
D. RNA primer’s phosphate group

B. 3′ OH end

The energy for adding new nucleotides comes from:

A. ATP
B. Nucleoside triphosphates
C. DNA ligase
D. Primase

B. Nucleoside triphosphates

Which fragments are found on the lagging strand?

A. Telomere fragments
B. Okazaki fragments
C. rRNA fragments
D. Histone fragments

B. Okazaki fragments

DNA polymerase proofreading is important because it:

A. Speeds up replication
B. Reduces replication errors
C. Replaces RNA primers
D. Forms phosphodiester bonds

B. Reduces replication errors

Why can’t the lagging strand replicate the very end of DNA?

A. Helicase stops working at the ends
B. The last RNA primer cannot be replaced
C. Ligase cannot seal the fragments
D. The leading strand blocks it

B. The last RNA primer cannot be replaced

What happens to chromosomes after many rounds of replication without telomerase?

A. They elongate
B. They shorten
C. They duplicate twice per cycle
D. They stop replication entirely

B. They shorten

Which enzyme prevents chromosome shortening?

A. Ligase
B. Telomerase
C. Helicase
D. Primase

B. Telomerase

Where is telomerase MOST active?

A. Mature neurons
B. Stem cells and germ cells
C. All somatic cells
D. Red blood cells

B. Stem cells and germ cells

The main function of telomerase is to:

A. Add DNA to the ends of chromosomes nts

A. Add DNA to the ends of chromosomes