transcription (in euk)

Question 1

what is the first step of transcription?

Answer 1

initiation

Question 2

what are the enzyme is used for RNA synthesis?

Answer 2

RNA polymerase II

Question 3

how to begin transcription?

Answer 3

RNA polymerase must bind to a region of DNA near the beginning of a gene known as the promoter

Question 4

where is the promoter sequence?

Answer 4

25-30 base pairs upstream from the transcription start point (is the nuclueotide where the RNA synthesis actually begins)

Question 5

what does the promoter consist of?

Answer 5

TATA BOX

Question 6

RNA polymerase II cannot recognise and bind to a promoter on its own.

Answer 6

• it requires proteins such as transcription factors to search for and attach to promoter DNA regions (known as TATA box) along the DNA molecules

Question 7

what does the transcription factors do?

Answer 7

they recognise the TATA box in the promoter and bind to DNA –> facilitates the binding of RNA polymerase

Question 8

what is it called after initiation is done?

Answer 8

• complete assembly of transcription assembly and RNA polymerase II bound to the promoter is called the transcription initiation complex

Question 9

what does the formation of transcription initiation complex do?

Answer 9

it causes the DNA double helix to unwind at the initiation site

Question 10

what is the template strand

Answer 10

one of the exposed strands of (-) DNA in the 3’ -> 5’ direction (is used as a template for transcription)

Question 11

what is the other strand called?

Answer 11

it is called non-template strand

Question 12

elongationnn

Answer 12

• a primer is not needed
• as the RNA polymerase moves along the DNA, it continues to unwind the double helix, exposing 10-20 DNA bases for base-pairing with ribonucleotides

Question 13

what happens when RNA polymerase moves along the template strand of DNA?

Answer 13

it adds the corresponding free ribonucleotides to the 3’ end of the growing RNA chain

Question 14

what direction is the RNA strand synthesised?

Answer 14

5’ to 3’ direction

Question 15

they areeeee

Answer 15

selected according to complementary base pairing rule (G C T A in the DNA would signal the addition of C G A U respectively to the RNA transcript)

Question 16

what bond are they joined together by?

Answer 16

they are joined together by phosphodiester bond

Question 17

differences btwn polymerases

Answer 17

DNA polymerase has proof reading while RNA polymerase does not have proof reading mechanism

Question 18

can it be transcribed simultaneously?

Answer 18

yes! a single gene in euk can be transcribed simultaneously by several molecules of RNA polymerase II

Question 19

what happens after transcription?

Answer 19

the newly formed RNA chain falls away from the DNA template and the DNA strand rewind into a double helix

Question 20

what happens at the termination of transcription?

Answer 20

it proceeds until the RNA polymerase II transcribes a sequence on the DNA called polyadenylation signal sequence (TTATTT) on the DNA –> (it codes for a polyadenylation signal AAUAAA in the pre-mRNA)

Question 21

how does termination occur?

Answer 21

• proteins associated with the growing RNA chain then cleave the growing RNA transcript 10-35 nucleotides downstream from the AAUAAA signal (termination site) –> this terminates transcription, releasing the primary RNA transcript –> RNA polymerase II dissociates from the DNA and the DNA rewinds to form a double helix

Question 22

what is the newly transcribed mRNA called?

Answer 22

pre-mRNA/primary mRNA transcript.immature mRNA (needs to be processed and undergo post-transcriptional modification)

Question 23

how to alter mRNA

Answer 23

• capping and polyadenylation
• splicing

Question 24

what is capping?

Answer 24

• additional of 7-methylguanosine (methylated GTP) (5’ cap) to the first base of the transcript
  this happens during elongation

Question 25

what is polyadenylation?

Answer 25

• addition of about 200 adenine to form a poly-A tail at the 3’ end
  it occurs immediately after cleavage behind the AAUAAA signal

Question 26

what is RNA splicing

Answer 26

• it undergoes processing where non-coding sequences (introns) are excised while the coding sequences (exons) are simultaneously spliced together to form the mature mRNA

Question 27

what does splicing reaction catalysed by?

Answer 27

• spliceosome –> it is assembled from proteins and small nuclear RNA molecules that recognise splice sites in the pre-mRNA sequence (it leaves the nucleus through the pores in the nuclear envelope)