DNA Replication Overview

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Questions and Answers

What is the typical size of a bacterial chromosome?

4,000,000 bp (correct)

8,000,000 bp

2,000,000 bp

6,000,000 bp

How many base pairs must a bacterium replicate per minute to keep up with its division rate?

200,000 bp (correct)

100,000 bp

250,000 bp

150,000 bp

What process was shown by the Meselson-Stahl experiment?

Semiconservative replication (correct)

Lagging strand replication

Dispersive replication

Conservative replication

What was used in the medium to label the DNA in the Meselson-Stahl experiment?

15NH4Cl Signup and view all the answers

What method was used to distinguish between 15N-DNA and 14N-DNA?

Density gradient centrifugation Signup and view all the answers

At what rate must the DNA molecule rotate during replication in a bacterium?

20,000 revolutions per minute Signup and view all the answers

Which isotope of nitrogen was used in the experimentation to mark DNA?

15N Signup and view all the answers

How often can a bacterium divide under the conditions described?

Every 20 minutes Signup and view all the answers

What is the primary purpose of DNA replication?

To ensure equal amounts of DNA are passed during cell division Signup and view all the answers

What type of replication produces two identical DNA strands, each containing one original strand?

Semiconservative replication Signup and view all the answers

Which enzymes are critical for resolving the unwinding of the DNA double helix during replication?

Topoisomerases Signup and view all the answers

Why was semiconservative replication initially thought to be impossible?

Because DNA is plectonemic and unwinding was deemed challenging Signup and view all the answers

What misconception about DNA replication was common in the 1950s?

Semiconservative replication was thought to be impossible Signup and view all the answers

In which types of DNA structure is unwinding particularly problematic during replication?

Circular DNA and plectonemic DNA Signup and view all the answers

What was the widely accepted theory of DNA replication before semiconservative replication was established?

Dispersive replication was the main belief Signup and view all the answers

What is one consequence of defects in DNA replication?

Development of various diseases, including cancer Signup and view all the answers

What is the primary function of DNA topoisomerases during DNA replication?

To remove or prevent supercoiling of DNA Signup and view all the answers

Which segment of DNA is completely cut by a Type II DNA topoisomerase?

G Segment Signup and view all the answers

How does a Type I DNA topoisomerase function during DNA replication?

By separating strands without unwinding the helix Signup and view all the answers

What is the role of the T Segment in the action of Type II DNA topoisomerase?

It is the DNA strand that gets passed through Signup and view all the answers

What type of supercoiling occurs when DNA strands are separated?

Positive supercoiling Signup and view all the answers

Which one of the following describes the main action of Type II DNA topoisomerases?

They pass one DNA strand through another Signup and view all the answers

Which of the following best describes the mechanism of action for DNA topoisomerases?

They cut and rejoin DNA strands to relieve tension Signup and view all the answers

What is required to initiate DNA synthesis during DNA-dependent DNA synthesis?

A primer Signup and view all the answers

What direction does DNA synthesis occur in during DNA-dependent DNA synthesis?

5´ to 3´ Signup and view all the answers

Which type of exonuclease activity allows a DNA polymerase to correct errors it has just made?

3´ to 5´ exonuclease Signup and view all the answers

What is the name of the enzyme that carries out DNA-dependent DNA synthesis?

DNA-dependent DNA polymerase Signup and view all the answers

What happens if a DNA synthesis occurs without a primer?

No DNA synthesis occurs Signup and view all the answers

What role do DNA polymerases play in DNA replication?

They synthesize DNA strands from nucleotides. Signup and view all the answers

What distinguishes leading strand synthesis from lagging strand synthesis at the replication fork?

Leading strands are synthesized continuously, while lagging strands are synthesized in fragments. Signup and view all the answers

What is the function of helicases during DNA replication?

To break the hydrogen bonds between base pairs. Signup and view all the answers

Why is semiconservative replication considered an important mechanism?

It ensures that each daughter DNA molecule contains one original and one new strand. Signup and view all the answers

What do mutations in DNA polymerases potentially lead to?

Colorectal cancer. Signup and view all the answers

What is the primary role of single-strand binding proteins (SSBs) during DNA replication?

To prevent DNA strands from re-attaching Signup and view all the answers

How does the initiation of DNA synthesis differ between bacteria and eukaryotes?

Eukaryotes extend the RNA primer with DNA pol alpha before DNA pol delta, bacteria do not Signup and view all the answers

What is the main difference in the synthesis of the leading strand compared to the lagging strand?

The leading strand is synthesized continuously, while the lagging strand is made in segments Signup and view all the answers

Which of the following statements about the RNA primer during DNA replication is correct?

It serves as a starting point for DNA synthesis Signup and view all the answers

What direction does DNA synthesis occur during the replication process?

5' to 3' direction Signup and view all the answers

What are the short segments of DNA synthesized on the lagging strand called?

Okazaki fragments Signup and view all the answers

What must happen to Okazaki fragments after their formation?

They must be joined together and RNA primers removed Signup and view all the answers

Why were small DNA fragments only observed at shorter timepoints in the discovery of Okazaki fragments?

They were labeled with radioactive nucleotides Signup and view all the answers

Which enzyme is known for joining two Okazaki fragments in eukaryotes?

FEN1 Signup and view all the answers

What is the primary characteristic of the lagging strand during DNA replication?

It is synthesized in a non-linear fashion Signup and view all the answers

What is the primary reason for the incomplete DNA synthesis during replication?

The lagging strand is copied discontinuously. Signup and view all the answers

What role does telomerase play in DNA replication?

It elongates the telomeres to prevent DNA shortening. Signup and view all the answers

What are the small sections of DNA synthesized on the lagging strand called?

Okazaki fragments Signup and view all the answers

Which enzyme is responsible for creating the RNA primer in prokaryotes during DNA replication?

Primase Signup and view all the answers

What enzyme prevents the shortening of chromosomes in eukaryotic cells?

Telomerase Signup and view all the answers

What happens to the RNA primer after DNA pol III or DNA pol delta has extended the new strand?

It is replaced by DNA nucleotides. Signup and view all the answers

What must occur to complete the synthesis of the lagging strand after Okazaki fragments are formed?

Okazaki fragments must be joined together Signup and view all the answers

In which direction does DNA synthesis occur on both the leading and lagging strands?

5' to 3' Signup and view all the answers

What is the consequence of the inability to synthesize the final Okazaki fragment on a linear DNA molecule?

Gradual shortening of DNA molecules Signup and view all the answers

What is a significant consequence of the end replication problem?

Chromosomes become progressively shorter over generations. Signup and view all the answers

In eukaryotic cells, which enzyme is specifically involved in removing RNA primers during lagging strand synthesis?

FEN1 Signup and view all the answers

How does the synthesis of the leading strand differ from that of the lagging strand?

The leading strand synthesis is continuous and requires one primer. Signup and view all the answers

What additive sequence does telomerase add to the ends of chromosomes?

TTAGGG Signup and view all the answers

What is the role of RNA primers in the synthesis of Okazaki fragments?

They provide a starting point for DNA synthesis Signup and view all the answers

During the synthesis of the leading strand, DNA is created in which manner compared to the lagging strand?

Continuously Signup and view all the answers

What is characteristic of senescent cells in culture during replication?

They exhibit shortened ends of chromosomes. Signup and view all the answers

What role does telomerase play in Tetrahymena thermophila?

It maintains the length of telomeres. Signup and view all the answers

Which pattern of DNA replication occurs first during the S phase?

Euchromatin Signup and view all the answers

Why might telomerase not be present in all cells in the body?

Its presence may lead to abnormal cell behaviors. Signup and view all the answers

What is a result of DNA replication involving modified nucleotides?

It enhances the ability to visualize DNA replication sites. Signup and view all the answers

In normal somatic cells, why do chromosomes shorten over time?

Regular cell division processes. Signup and view all the answers

Which of the following best describes telomerase activity in cancer cells?

Telomerase is activated leading to longer chromosome ends. Signup and view all the answers

What phase of the cell cycle is primarily concerned with DNA replication?

S phase Signup and view all the answers

Study Notes

DNA Replication

DNA replication is essential for cell division and life.
DNA needs to double to distribute equal amounts of DNA when cells divide.
Defects in DNA replication can cause diseases like cancer.
DNA topoisomerases are crucial for efficient and accurate DNA replication. They solve the problem of DNA double helix unwinding during strand separation.

Types of DNA Replication

Semiconservative replication: Each new DNA molecule consists of one original strand and one new strand. This was experimentally proven.
Conservative replication: Both parental strands remain together and create two new strands that form a new double helix.
Dispersive replication: Both parental strands are dispersed into new DNA strands.

Semiconservative replication proof

Meselson-Stahl experiment (1959): Distinguished semiconservative replication from other models by using isotopes of nitrogen.
Bacteria were grown in a medium containing heavy nitrogen (15N).
The bacteria's DNA was then transferred to a medium containing normal nitrogen (14N).
DNA was extracted and analyzed by density gradient centrifugation.
The results indicated that after one cycle of replication, the DNA was intermediate in density. This supported the semiconservative model.

The role of topoisomerases in DNA replication

DNA topoisomerases prevent supercoiling during DNA replication.
Supercoils form when DNA strands are separated.

How topoisomerases work

Type I Topoisomerases: Create a single-strand break in the DNA, allows another segment of DNA to pass through, and reseals the break. Type I topoisomerases allow the DNA to relieve twisting stress by making a single cut.
Type II Topoisomerases: Create a double-strand break, pass a segment of DNA through the gap, and then reseal the breaks. Type II topoisomerases resolve the knots formed by removing twists, and the DNA continues to unwind for replication.

DNA Supercoiling

Supercoiling occurs when DNA strands are separated.
Topoisomerases remove or prevent supercoiling.

Additional notes

A typical bacterial chromosome is a circular DNA molecule (approximately 4 million base pairs).
Bacteria can divide every 20 minutes.
A circular DNA molecule would have to rotate very rapidly to replicate its genome every 20 minutes (approximately 20,000 revolutions per minute).
DNA synthesis proceeds in the 5' to 3' direction.
Important enzymes involved in replication: DNA Polymerase I, DNA Polymerase III, DNA polymerase α, DNA polymerase δ, primase, FEN1.
Reading Materials: Brown, Chapter 10 (pages 187-191, 192-193) and Alberts, Chapter 6 (pages 199-205, 212).
Online resources are available on Blackboard.

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Description

This quiz explores the fundamental concepts of DNA replication, including its importance for cell division and the different types of DNA replication processes. It also highlights the crucial role of DNA topoisomerases and the historical Meselson-Stahl experiment that proved semiconservative replication. Test your knowledge on how DNA is accurately replicated and its significance in preventing diseases like cancer.