How DNA Works

Proteins and DNA

Why is DNA replication important?

It allows DNA to be copied accurately so genetic information can be passed from one cell or generation to another.

Define DNA replication.

The process by which a DNA molecule makes an identical copy of itself.

When does DNA replication occur?

Before a cell divides.

What are the two main models that were proposed for DNA replication?

Conservative replication and semiconservative replication.

Define conservative replication.

A model in which the original DNA double helix remains intact and a completely new double helix is produced.

What would the two DNA molecules contain after conservative replication?

One molecule would contain both original strands and the other would contain two newly synthesised strands.

Define semiconservative replication.

A model of DNA replication in which each new DNA molecule contains one original parental strand and one newly synthesised strand.

What does "semi" mean in semiconservative replication?

Half – half of each new DNA molecule is conserved from the original DNA molecule.

Which model of DNA replication is accepted?

Semiconservative replication.

Why did Watson and Crick's DNA model suggest semiconservative replication?

The two DNA strands are held together by hydrogen bonds, allowing them to unzip so each strand can act as a template for a new strand.

Who provided experimental evidence for semiconservative DNA replication?

Matthew Meselson and Franklin Stahl.

Which organism did Meselson and Stahl use?

Escherichia coli (E. coli) bacteria.

Why was E. coli useful in the Meselson and Stahl experiment?

It reproduces rapidly, allowing several generations of DNA replication to be studied.

What two nitrogen isotopes were used by Meselson and Stahl?

¹⁵N and ¹⁴N.

What is an isotope?

Atoms of the same element with the same number of protons but different numbers of neutrons.

Which nitrogen isotope is heavier?

¹⁵N.

Which nitrogen isotope is lighter?

¹⁴N.

How did Meselson and Stahl initially produce bacteria containing heavy DNA?

They grew E. coli for several generations in a medium containing ¹⁵N as the only nitrogen source.

Why did growing E. coli in ¹⁵N produce heavy DNA?

The bacteria incorporated ¹⁵N into nitrogen-containing molecules, including DNA.

What did Meselson and Stahl do after producing bacteria containing ¹⁵N DNA?

They transferred the bacteria to a medium containing ¹⁴N and allowed them to replicate.

What did Meselson and Stahl measure after DNA replication?

The density of the DNA.

What would conservative replication predict after one replication in ¹⁴N?

Two DNA densities: heavy ¹⁵N DNA from the original molecule and light ¹⁴N DNA from the newly produced molecule.

What would semiconservative replication predict after one replication in ¹⁴N?

All DNA molecules would have an intermediate density because each would contain one ¹⁵N strand and one ¹⁴N strand.

What did Meselson and Stahl observe after one replication cycle?

All the DNA had an intermediate density.

Why did the results after one replication cycle reject conservative replication?

Conservative replication predicts separate heavy and light DNA, but only intermediate-density hybrid DNA was observed.

What is hybrid DNA in the Meselson and Stahl experiment?

DNA containing one heavy ¹⁵N strand and one light ¹⁴N strand.

What did Meselson and Stahl observe after a second replication in ¹⁴N?

Approximately half the DNA was hybrid and half was light DNA.

Why does the second generation support semiconservative replication?

Each hybrid DNA molecule produces one hybrid molecule and one completely light molecule during replication.

What proportion of DNA is hybrid after the first generation in ¹⁴N?

100%.

What proportion of DNA is light after the first generation in ¹⁴N?

0%.

What proportion of DNA is hybrid after the second generation in ¹⁴N?

Approximately 50%.

What proportion of DNA is light after the second generation in ¹⁴N?

Approximately 50%.

What did the Meselson and Stahl experiment demonstrate?

DNA replicates semiconservatively.

What are the three enzymes involved in DNA replication described in this section?

DNA helicase, DNA polymerase and DNA ligase.

What is the role of DNA helicase in DNA replication?

It breaks the hydrogen bonds between complementary bases, causing the two DNA strands to unzip and separate.

What bonds does DNA helicase break?

Hydrogen bonds between complementary base pairs.

What happens after DNA helicase separates the DNA strands?

Each original strand acts as a template for the synthesis of a new complementary strand.

What is a template strand?

An original DNA strand that determines the sequence of nucleotides in a newly synthesised complementary strand.

What happens to the bases when the DNA strands separate?

The bases become exposed and attract complementary free DNA nucleotides.

How do free DNA nucleotides align with the template strand?

By complementary base pairing.

Which DNA bases pair together during replication?

Adenine pairs with thymine and cytosine pairs with guanine.

What happens between complementary bases during DNA replication?

Hydrogen bonds form between them.

What is the role of DNA polymerase in DNA replication?

It lines up and catalyses the linking of new complementary nucleotides along the DNA template strand.

What is the role of DNA ligase in DNA replication?

It catalyses the formation of phosphodiester bonds between nucleotides.

What type of bonds join adjacent nucleotides within a DNA strand?

Phosphodiester bonds.

What happens after the new nucleotides have been joined together?

The two DNA molecules coil into their double-helix structures.

What is produced at the end of DNA replication?

Two identical DNA molecules.

Why are the two new DNA molecules identical?

Complementary base pairing ensures that the sequence of each new strand is determined by its original template strand.

What does each DNA molecule contain after semiconservative replication?

One original parental strand and one newly synthesised strand.

Describe DNA replication in order.

DNA helicase breaks hydrogen bonds and unzips the DNA → each original strand acts as a template → free DNA nucleotides pair with exposed complementary bases → DNA polymerase lines up and links the nucleotides → DNA ligase catalyses phosphodiester bond formation → two identical DNA molecules form.

Why is complementary base pairing important during DNA replication?

It ensures that the new DNA strands have the correct base sequence and therefore preserves the genetic information.

If the template DNA sequence is TACGGA, what sequence will be synthesised?

ATGCCT.

Why is DNA replication described as semiconservative rather than conservative?

Each daughter DNA molecule conserves one strand of the original DNA rather than keeping the entire original double helix together.