Week 19- Duplication and transcription

Question 1

Why and when is DNA replication necessary?

Answer 1

It is necessary in: 1) Duplication of the genome during S phase (prior to mitosis or meiosis) 2)To repair damage to the genome.

Question 2

What is the mechanism of DNA replication?

Answer 2

DNA replication occurs through semiconservative mechanism.

Question 3

Describe semiconservative mehcanism of replication:

Answer 3

Each parent strand serves as a template for a new strand and the new strand is made of an old strand and a new strand.

Question 4

What are the two steps involved in DNA replication?

Answer 4

1) DNA helix is unwound to separate the two template strands. 2)New nucleotides are added and form complementary base paring with the template DNAs.

Question 5

What are the four things that DNA replication needs in order to occur?

Answer 5

1)A template 2)Nucleotides 3)Enzymes that catalyze the addition of nucleotides 4)A primer to initiate the polymerization reaction.

Question 6

Where does the enrgy used to drive the condensation reaction comes from?

Answer 6

It comes from the hydrolisis of dNTPs which contains three phospates group and two are removed during the formation of phosphodiester linkages.

Question 7

What’s the first step in the polymerization of DNA during replication?

Answer 7

The replication begins with the binding of the pre-replication complex to the origin of replication.

Question 8

What happens after the pre-replication complex binds to the origin of replication?

Answer 8

DNA is unwound by helicase and replication proceeds in both direction forming two replication forks.

Question 9

What are replication forks?

Answer 9

The sites ehere DNA unwinds to exposes the bases so they can act as templates.

Question 10

What does helicase do?

Answer 10

Helicase uses energy from ATP to unwind the two strands of DNA held together by hydrogen bonds.

Question 11

What do single strand binding proteins (SSBP) do?

Answer 11

They bind to the single strands in order to avoid reassociation.

Question 12

What does DNA polymerase III do?

Answer 12

DNA polymerase III is an enzyme that catalyses the addition of nucleotides by covalently bonding them and it requires a primer.

Question 13

What’s a primer?

Answer 13

A short strand of RNA that is complementary to DNA template and it’s synthetized by primase.

Question 14

What’s primase?

Answer 14

Primase is an RNA polymerase that requires a DNA template and generates a primer for DNA polymerase III.

Question 15

What’s DNA polymerase I?

Answer 15

An enzyme that hydrolise the RNA primer and fill the gap adding DNA.

Question 16

What are the two types of strands formed after replication forks?

Answer 16

-Leading strand -Lagging strand

Question 17

Describe leading strand:

Answer 17

Leading strand is orientated so that it can continuously grows at its 3’ end as the fork open up. I t requires just one primer.

Question 18

Describe the lagging strand:

Answer 18

Lagging strand is oriented so that as the fork opens up its exposed 3’ end gets further and further away from the replication fork.

Question 19

Describe the synthesis of the lagging strand:

Answer 19

It requires the synthesis of small, discontinous strands of DNA called Ozazaki fragments and each of those requires its own primer.

Question 20

What’s DNA ligase?

Answer 20

Is an enzyme that attache the Ozazaki fragments together.

Question 21

Why is DNA polymerase III processive?

Answer 21

Because they catalyse the fromation of many phosphodiester linkages at the same time.

Question 22

How is processivity of DNA polymerase III mantained

Answer 22

It’s maintained with the help of a sliding DNA clamp.

Question 23

What does the sliding DNA clamp do?

Answer 23

It keeps the enzyme and the DNA associated so that they remain attached longer.

Question 24

What are the three different mechanism to correct DNA errors?

Answer 24

-Proofreading -Mismatch Repair -Escision repair.

Question 25

Describe proofreading

Answer 25

It corrects errors in replication ad the DNA polymerase III makes them by exciding the wrong nucletides and replacing them with correct nucleotides.

Question 26

Describe mismatch repair:

Answer 26

It occurs after proofreading and scans DNA immediately after it has been replicated correcting any base pairing mismatching.

Question 27

Describe Excision repair:

Answer 27

It removes abnormal bases that have been formed and replace them with functional bases.

Question 28

Describe telomere and its function:

Answer 28

Telomere is a repetitive sequence at the end of the chromosome that prevents DNA repair system to recognize them as broken.

Question 29

What happens during transcription?

Answer 29

The information in the DNA sequence (a gene) is copied into a complementary RNA sequence

Question 30

State the three components required for transcription to occurs:

Answer 30

1) A DNA template for complementary base pairing. 2)The four ribonucelotides trisphosphate. 3)RNA polymerase.

Question 31

Describe RNA polymerase:

Answer 31

An enzyme that catalyze the synthesis of RNA nucleotides from a DNA template.

Question 32

Is all the sequence of DNA transcribed during transcription?

Answer 32

Only gene are transcribed during transcription.

Question 33

What are the three steps involved in transcription?

Answer 33

1) Initiation 2) Elongation 3) Termination

Question 34

Describe initiation:

Answer 34

RNA polymerase binds to a special sequence of DNA called the promoter and locally separates the two DNA strands.

Question 35

What’s a promoter and what are its functions?

Answer 35

A sequence of DNA with three functions: 1)Orient the RNA polymerase in a particular direction 2)Tells where to start to transcript 3)Tells which DNA has to be transcribed.

Question 36

What’s the initiation site?

Answer 36

The region where transcription begins located on the promoter.

Question 37

What’s promoter strength?

Answer 37

The binding affinity of the σ factors and it depends on the sequence of the promoter.

Question 38

What do σ factors do?

Answer 38

They allow for the expression of particular sets of genes.

Question 39

Describe elongation:

Answer 39

RNA poylmerase unwind DNA and read its template. It creates an RNA transcript adding new nucleotides to the 3’ end.

Question 40

Describe termination:

Answer 40

A particular palindromic base sequence specify the termination of transcriptionby forming a harpin loop in the RNA transcript the cause the RNA polymerase to stall.