Gene Function 1 - Replication and Transcription

Question 1

What is replication?

Answer 1

Replication is a process to replicate both strands of the DNA prior to cell division.

Question 2

What are the raw materials needed for replication?

Answer 2

1) DNA to be copied - it will act as the template (pattern) for the newly synthesised strands
2) dNTPS (deoxyribonucleotides - dATP, dCTP, dGTP, dTTP)
3) DNA polymerase - the enzyme that will carry out the reaction
4) Primer - short piece of RNA required by DNA polymerase to start the reaction

Question 3

What does replication start with?

Answer 3

A double stranded parental DNA molecule with each base paired to its complementary partner.

Question 4

What is the first step of replication?

Answer 4

Separation of the two DNA strands

Question 5

What is the role of the parental strand in replication?

Answer 5

Each parental strand serves as the template for the order of complimentary nucleotides in the new daughter strand.

Question 6

Why is replication described as semi-conservative?

Answer 6

Because the parental strands have not been lost but are no longer connected.

Question 7

Where does DNA replication begin?

Answer 7

Specific sites called origins of replication.

Question 8

What are origins of replication?

Answer 8

Sequences of DNA that are recognised by the replication machinery.

Question 9

How are replication bubbles formed?

Answer 9

When proteins attach to origins of replication they separate the DNA strands, forming replication bubbles. These replication bubbles grow in both directions and eventually join together.

Question 10

How does DNA polymerase work?

Answer 10

It moves along the template DNA strand and catalyses the formation of the new daughter strand.

Question 11

Why are many origins of replication needed?

Answer 11

So that DNA can be replicated in a relatively short amount of time.

Question 12

What are the requirements of DNA polymerase?

Answer 12

1) Must have a 3’OH group to add on to
2) Consequently,
* will only elongate DNA in the 5’ –> 3’ direction
* cannot initiate DNA synthesis unless there is a primer (short piece of RNA) that contains a 3’OH for addition of new nucleotides

Question 13

Which enzymes are involved in the initiation of replication?

Answer 13

Helicases and single-stranded binding proteins help with double helix unwinding and providing single-stranded DNA templates.

Question 14

Which enzymes are involved in synthesis of leading strand?

Answer 14

Primase –> priming
DNA polymerase –> elongation and replacement of RNA primer by DNA

Question 15

Which enzymes are involved in synthesis of lagging strand?

Answer 15

Primase –> priming for Okazaki fragment
DNA polymerase –> elongation of fragment and replacement of RNA primer by DNA
Ligase –> joining of fragments

Question 16

How can nucleotide analogues be used to treat disease?

Answer 16

AZT is an analogue of thymidine:
* blocks DNA replication
* used to treat AIDS (HIV) infection

Question 17

What is transcription?

Answer 17

The process to synthesise a short piece of RNA using a specific single strand of DNA prior to cell division.

Question 18

What are the raw materials needed for transcription?

Answer 18

1) Single strand of DNA to be copied - it will act as the template
2) NTPs (ribonucleotides - ATP, CTP, GTP, UTP)
3) RNA polymerase - the enzyme that will carry out the reaction
4) Transcription factors - proteins that bind to the DNA to indicate where the gene sequence commences

Question 19

What is the nitrogenous base of RNA?

Answer 19

Uracil

Question 20

What is the pentameric sugar in RNA?

Answer 20

Ribose (as opposed to deoxyribose in DNA)

Question 21

What are the three steps of transcription?

Answer 21

Initiation, elongation and termination

Question 22

What is the first step of initiation?

Answer 22

Enzyme called RNA polymerase attaches to a region of the DNA called the promoter.
Promoter is A/T rich (strands can be split).
Other proteins bind to promoter region and help RNA polymerase recognise this sequence –> these transcription factors determine what genes to transcribe.

Question 23

What is the second step of initiation?

Answer 23

RNA polymerase pries the DNA strands apart and one strand of DNA, the template strand, serves as a template for the sequence of complimentary RNA nucleotides.

Question 23

What is the final step of initiation?

Answer 23

RNA polymerase links the nucleotides together.

Question 24

What does elongation involve?

Answer 24

RNA polymerase moves along the DNA, adding nucleotides to the end of the growing RNA molecule.
As the enzyme moves along, the RNA strand peels away, allowing the two DNA strands to come together again.

Question 25

What does termination involve?

Answer 25

Once the RNA polymerase moves past a sequence called the terminator, the enzyme releases the completed RNA. The terminator causes the association of the RNA polymerase to be relaxed.

Question 26

How does this process work in prokaryotic cells?

Answer 26

Prokaryotic cells do not have a nucleus. As soon as the mRNA is made, the ribosome binds to it and protein synthesis begins.

Question 27

How does this process work in eukaryotic cells?

Answer 27

Eukaryotic cells do have a nucleus. mRNA is formed as pre-mRNA in the nucleus. It is then extensively modified before it is exported to the cytoplasm where the ribosomes are (processing).

Question 28

What is mRNA protected by?

Answer 28

The addition of a 5’ cap (a modified guanine) and a poly(A) tail.

Question 29

What are introns and exons?

Answer 29

Introns = intervening sequences
Exons = coding or expressed sequences

Question 30

When are introns spliced out?

Answer 30

Before the mRNA leaves the nucleus.