DNA Replication

• Biology
• Tertiary Education

Lesson Objectives

By the end of this session, the students will be able to

1. Recall the structure and function of DNA.

2. Analyze and infer about semi-conservative nature of DNA replication.

3. Recognize logical fallacies in reasoning about replication.

4. Rewrite the key requirements of DNA replication.

Let us review some basics

1.  Expand DNA.

2. State the location of DNA in a prokaryotic and an eukaryotic cell.

3. What is DNA made of?

4. What is the directionality of the strands present in DNA?

5. What is the main function of DNA?

Warm - up Activity

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Code - b196764

DNA Replication

The word replica means copy.

Hence DNA replication means making copies of DNA.

It is required because whenever a cell divides, the daughter cells must contain the same genetic information as the parent cell.

Where and when does it occur?

It occurs inside the nucleus in an eukaryotic cell and in the  cytoplasm in a prokaryotic cell.

It occurs during the S phase of Interphase of cell cycle.

Models of Replication

Meselson and Stahl proved the semiconservative nature of DNA replication in 1958

1. E. coli bacteria was used.

2. It was first grown on media containing heavy N or N-15.

3. Then it was grown on media containing light N or N-14 and samples of DNA was taken after every generation (20 min)

4.The samples are then mixed with CsCl and  Density gradient centrifugation process was carried out.

DNA replication is semi-conservative in nature

Let us re-cap

DNA replication is semi-conservative because

1. One strand is parental and the other new.
2. The daughter strands are dispersive
3. The daughter strands contain only new DNA
4. The daughter strands contain only parental DNA

If Meselson and Stahl's experiment was continued for three generations, what would be the ratio of hybrid and light DNA after three generations?

1. 1:1
2. 1:2
3. 1:3
4. 1:4

The method developed by Meselson and Stahl to separate heavy DNA containing N15 from light DNA containing N14 is

1. Iso pycnic centrifugation
2. Density gradient Centrifugation
3. Buoyant density Centrifugation
4. Ultracentrifugation

Assertion - In Meselson and Stahl's experiment, density gradient centrifugation technique is used to separate the heavy DNA from light DNA.
Reason - In 15NH4Cl, 15 N is a radioisotope of nitrogen

1. Both assertion and reason are true and reason is the correct explanation of assertion
2. Both assertion and reason are true but reason is not the correct explanation of the assertion
3. Assertion is true but reason is false
4. Both assertion and reason are wrong

Requirements of DNA replication

1. DNA template - This is the parent strand which acts as a base for the replication process.

2. DNA primer - It is a short strand of nucleic acid that provides the starting point of DNA synthesis

3. Enzymes - They are the catalysts which bring about the chemical reaction.

The various enzymes and their functions

DNA synthesis will always occur from 5' to 3' only. One of the new strand is synthesized continuously and is called leading strand. The other strand is synthesized in short fragments and is called as the lagging strand.

The DNA replication Enzymes and their location

Helicase

PRIMASE

DNA POL III

DNA LIGASE

ELONGATES THE NEW STRAND

SYNTHESIZES THE RNA PRIMER

UNWINDS THE DNA

JOINS THE NICK

Assertion - DNA replication needs a primer.
Reason - DNA polymerase can elongate the chain but cannot initiate the synthesis

1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true but R is not the correct explanation of A
3. A is true but R is false
4. Both A and R are false

What is your level of understanding of today's topic?

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