Unit 1.2 Replication of DNA

Question 1

What is meant when we say that the replication of DNA is semi-conservative?

Answer 1

Each strand acts as a template for the synthesis of a new DNA molecule, by the addition of complementary base pairs, thereby creating a new DNA strand that is complementary in sequence to the parental DNA strand.

In this way each daughter cell ends up with one of the original strands from the parent molecule and one newly synthesized strand of DNA, hence DNA replication is said to be semi-conservative.

Question 2

What happens in the first stage of DNA replication?

Answer 2

DNA is unwound and the hydrogen bonds between the complementary bases are broken and the DNA is unzipped, Two replication forks form and open the double strand in different directions exposing the DNA bases.

Question 3

What is the second stage of DNA replication?

Answer 3

On the leading strand a primer binds to the 3’ end of the DNA and DNA polymerase adds free DNA nucleotides continuously to the parent strand in a 5’ to 3’ direction, this is done in accordance with the rules of complementary base pairing.

Question 4

What is meant by the term leading strand in terms of DNA replication?

Answer 4

In the leading strand a single primer will be added to the 3’ end of the parent strand of DNA, this singular primer will allow free nucleotides to be added continuously in a 5’ to 3’ direction.

Question 5

What is meant by the term lagging strand in terms of DNA replication?

Answer 5

In the lagging strand multiple primers are required, since primers must be added to the 3’ ends of the parent strand as the DNA replication fork expands. DNA polymerase will then add free nucleotides in the 5’ to 3’ direction.

Question 6

name the enzyme that joins the DNA fragments together in the lagging strand?

Answer 6

Ligase

Question 7

What role does DNA polymerase play in DNA replication?

Answer 7

DNA polymerase catalyses the formation of strong covalent bonds between DNA nucleotides and adds free nucleotides in a 5’ to 3’ direction to the parent strand.

Question 8

What is the role of a primer in DNA replication?

Answer 8

Primers are a short strand of nucleotides that bind to the 3’ end of the parent strand and enable DNA polymerase to begin to add free nucleotides in a 5’ to 3’ direction to the parent strand.

Question 9

What is an important aspect of DNA replication?

Answer 9

The daughter molecule must be genetically identical to the parent molecule to ensure that no genetic information has been lost.

Question 10

What happens in the final stage of DNA replication?

Answer 10

The primers (RNA) are replaced with DNA and the enzyme ligase then joins the fragments in the lagging strand together, and the new DNA molecules re-twist into two genetically identical DNA double helixes.

Question 11

DNA replication is an ________________ process?

Answer 11

Enzyme controlled

Question 12

What enzymes are needed in DNA replication?

Answer 12

(for purposes of the higher course) DNA polymerase, and DNA ligase.

Question 13

What is the genotype of an organism?

Answer 13

The genetic constitution of a particular organism

Question 14

What are the requirements for DNA replication to take place?

Answer 14

• DNA template (Each parent strand acts as a template strand for the newly forming strand)
• Supply of free nucleotides (DNA polymerase requires a supply of free nucleotides to act on)
• Enzymes (DNA replication is an enzyme controlled process and the enzymes DNA polymerase and DNA ligase are required)
• Primers (A short strand of RNA nucleotides that bind to the 3’ end of the template strand and allow DNA polymerase to begin to add free nucleotides to the forming strand)

Question 15

What can be said about the genotypes of two different organisms?

Answer 15

Each genome will be unique, unless the organisms are clones

Question 16

What is PCR used for?

Answer 16

Amplification of a DNA sample using complementary primers for a specific target sequence

Question 17

What makes PCR especially useful?

Answer 17

• It is highly specific
• It is easily automated
• It is capable of amplifying a minute quantity of the sample

Question 18

Give two examples of how we could use PCR?

Answer 18

• Crime scene investigation
• Identifying mutations in organisms

Question 19

What apparatus is used when carrying out a PCR?

Answer 19

A thermocycler

Question 20

Explain the process of PCR?

Answer 20

• Firstly The sample will be heated to temperatures between 92 and 98 degrees C, and this acts to separate the strands of DNA by breaking down the hydrogen bonds between bases.
• Next The sample should be cooled to between 50 and 60 degrees, this allows Primers to bind to the 3’ end of the template strands and enable DNA polymerase to add DNA nucleotides. The primers act as a starting point for DNA polymerase.
• Lastly the solution should be heated to between 70 and 80 degrees C, these are the optimum temperatures for the heat tolerant strain of DNA polymerase being used in this process to act at. This enables the heat tolerant DNA polymerase to add free nucleotides to the forming strand in a 5’ to 3’ direction producing a new molecule of DNA.
• The process is then repeated to amplify the quantity of DNA produced.

Question 21

What do we use to cut DNA into smaller fragments?

Answer 21

Restriction endonucleases in a process called a restriction digest

Question 22

What does gel eletro-phoresis do?

Answer 22

Separates DNA fragments based upon their size, the results of such a technique can be used to match DNA samples, the closer the bands are to matching the more closely related the samples will be.