2.1.3 Nucleotides and nucleic acids (e)

DNA replication - Notes

2.1.3e: semi-conservative DNA replication

1. Explain why DNA replication is necessary.

2. Define the term “semi-conservative replication”. (F)

3. Draw and annotate a diagram to show the sequence of events in DNA replication.

4. Describe the process of DNA replication in a series of bullet points. (F)

5. State the roles of the enzymes DNA polymerase and DNA helicase in DNA replication.

6. Outline the experimental procedure used by Meselson and Stahl to prove that DNA replicated by semi-conservative replication. (S+C)

7. Draw and annotate a series of diagrams to show how the results of Meselson and Stahl’s experiment demonstrate that DNA is replicated by semi-conservative replication. (S+C)

8. Describe how, and explain why, DNA replication occurs by continuous replication of one strand and discontinuous replication of the other strand. (S+C)

9. Explain the importance of DNA replication conserving genetic information with accuracy.

10. Define the term “mutation”. (F)

The importance of DNA replication

The sequence of nucleotides in DNA is the genetic code. This is the information cells need to make proteins. Without this code cells wouldn’t be able to make any proteins and so wouldn’t be able to do any jobs a cell needs to do.

When cells divide each daughter cell needs a full copy of the cell’s DNA. The DNA from the original parent cell is copied in the S phase of interphase by a process called semi-conservative replication.

This replication is incredibly accurate. Very few mistakes are made, and cells have checking and correcting processes that minimise the mistakes that get through even further.

A change in the sequence of nucleotides in DNA is called a mutation. One way mutations can occur is by errors during DNA replication.

Mutations occur randomly and are usually harmful because they can change the sequence of amino acids in a protein produced so that it can’t do its job anymore (or as well). This is why the DNA replication process, and the associated checking and correcting processes have evolved such amazing accuracy.

The process of DNA replication

Where DNA is replicating (called the replication fork) there is a cluster of enzymes all playing different roles in the process.

1. Gyrase unwinds the DNA helix (not required at A-level but occasionally present as an additional mark point).

2. Helicase unzips the DNA. It breaks hydrogen bonds between complementary base pairs and the two strands in a DNA molecule separate.

3. Free DNA nucleotides bind to the exposed nitrogenous bases on both original template strands following the rules of complementary base pairing (A with T and C with G).

4. DNA polymerase forms phosphodiester bonds (in condensation reactions) between adjacent free DNA nucleotides to make a new DNA strand bound to each original strand.

5. Each new DNA molecule winds back into a double helix.

6. The two DNA molecules produced are identical to each other and to the original DNA molecule.

Each new DNA molecule contains one strand from the original DNA molecule and one new strand made from free DNA nucleotides. Because one strand is conserved from the original molecule and one strand is new, this process is called ‘semi-conservative replication’.

A bit more detail

The information on the previous page is sufficient for A-level but is a simplification. Sometimes other information is on mark schemes as additional things you can say (even that is not the whole story).

1. DNA polymerase can only build new DNA strands in the 5’ to 3’ direction.

2. This means that DNA polymerase can move along one template strand from its 3’ end towards the 5’ end in the same direction as the replication fork is progressing. It can continuously build the new strand in a 5’ to 3’ direction (because the strands in DNA are anti-parallel). The templates strand used here is called the leading strand.

3. The other template strand is called the lagging strand. This is because DNA polymerase cannot directly follow the replication fork and a length of unused template strand exists for a little while before the new strand forms.

4. On the lagging strand the DNA polymerase binds near the replication fork and then travels away from it. Because the two strands of the original DNA molecule are anti-parallel, this DNA polymerase is moving along the lagging strand in the 3’ to 5’ direction and can build the new strand in the 5’ to 3’ direction.

5. Eventually the DNA polymerase is going to get to a point where the lagging strand already has a complementary strand built on it (by a different DNA polymerase molecule earlier on in the process).

6. In this way the new strand attached to the lagging strand is built in sections (rather than continuously on the leading strand). These sections are called Okazaki fragments.

7. DNA ligase forms phosphodiester bonds (in condensation reactions) to join the Okazaki fragments together and complete the new strand.

DNA replication explained

What is DNA?

• DNA is like a recipe book for cells, telling them how to make proteins.

Why is DNA Replication Important?

• When cells divide, they need a full copy of DNA to function properly.

• Replication makes sure the new cells have the right instructions.

How Does DNA Replication Work?

• Enzymes like helicase unzip the DNA.

• New DNA pieces match up with the original DNA to make copies.

• DNA polymerase helps build new DNA strands in the right direction.

Why is Accuracy Important?

• Mistakes in replication can cause mutations, which can harm cells.

• Checking processes help catch and fix errors to keep cells healthy.

Fun Fact:

• DNA replication is like making a copy of a treasure map to find the hidden treasure accurately!

The Meselson and Stahl experiment

Matthew Meselson and Frank Stahl worked at the California Institute of Technology. In 1958 they grew bacteria in growth medium containing ammonium ions (NH₄⁺) as the source of nitrogen. The type of DNA made by the cells depended on the type of nitrogen present in the bacteria’s growth medium. They used two isotopes of nitrogen – ¹⁴N and ¹⁵N. ¹⁴N is the common, light form (isotope) of nitrogen. ¹⁵N is the heavier form. They then extracted DNA from the bacterial cells and centrifuged the resulting solution to isolate the DNA. The DNA made with ¹⁴N and the DNA made with ¹⁵N accumulated at different levels in the centrifuged solutions, according to the DNA’s density.

Bacteria containing DNA made with ¹⁵N were allowed to divide once in a solution containing only ¹⁴N. Any nucleotides used to make new DNA would contain ¹⁴N. After a single replication the DNA was extracted and centrifuged.

The resulting DNA solution was then analyzed to determine the distribution of 14N and 15N in the replicated DNA strands. The experiment aimed to observe the behavior of the original 15N-labeled DNA after one round of replication in a medium containing 14N. The centrifugation process helped separate the newly synthesized DNA strands based on their densities, allowing for the comparison of the distribution patterns of the two types of DNA. This experimental setup provided insights into the semi-conservative nature of DNA replication and how the incorporation of 14N nucleotides affected the overall DNA composition.

The Meselson and Stahl Experiment Explained

Matthew Meselson and Frank Stahl were scientists who wanted to understand how DNA copies itself. They used bacteria that grow in a special liquid with different types of nitrogen. They used a light nitrogen called 14N and a heavy nitrogen called 15N. By spinning the DNA in a machine called a centrifuge, they could separate the DNA made with 14N from the DNA made with 15N because they have different weights.

They let the bacteria with 15N DNA grow in 14N liquid. After one round of copying, they checked the DNA. They found that the new DNA had a mix of 14N and 15N. This showed that when DNA copies itself, it uses one old strand and one new strand, which is called semi-conservative replication.

By using this cool experiment, Meselson and Stahl proved how DNA makes copies of itself and how the new DNA is a mix of old and new parts. This helped scientists understand more about how DNA works and how it passes on information from one generation to the next.