RNA synthesis

Question 1

what are the different types of RNA polymerase enzymes

Answer 1

RNA polymerase I ——-> Most ribosomal RNA (rRNA)

RNA polymerase II ——-> Protein-coding, microRNA (miRNA), non-coding RNA

RNA polymerase III ——-> Transfer RNA (tRNA), 5S rRNA, other small RNAs

Question 2

Gene

Answer 2

• DNA segment containing instructions for making a particular product
• unit of heredity; contains instructions for an organisms phenotypes

Question 3

Transcription: The process

Answer 3

-TF2D has a TBP (tata binding protein) which binds to TATA on DNA strand
-TF2A and TF2B bind adjacent to the TF2D and stabilise the complex
-TF2E and TF2H to bind onto the DNA strand, which allowsRNA polymerase 2 (with TF2F and other TF on it) to assemble at the promoter
-this is called a TRANSCRIPTION INITIATION COMPLEX (TIC)
- TF2H pulls the strands apart and phosphorylates RNA polymerase 2.
- Phosphorylated RNA Pol 2 is released from the complex,
moves across DNA to recognise a start codon and begins transcription.

Question 4

capping

Answer 4

• 5’ end is modified
• Guanine nucleotide is inserted and methylated at carbon position 7 and forms a (5’-5’) triphosphate bridge
• Catalysed by a capping enzyme complex
• Happens as the mRNA is being transcribed

Question 5

polyadenylation

Answer 5

• the 3’ end is polyadenylated
• Cleavage signal is a specific DNA (RNA?) sequence which is recognised and “cleaved” by a specific endonucleases
• And this is a signal for another enzyme, poly (A) polymerase, which adds a sequence of A nucleotides to the end of the RNA molecule to make a poly A tail

Question 6

why cap and polyadenylate ?

Answer 6

-Stability (make sure it doesn’t get degraded)
-Integrity prior to translation - Cap protects the 5’ end and the polyadenylated tail protects the 3’ end
-Transport to cytoplasm - only transported when intact
-There are further controls which make sure that the mRNA is intact and stable
Over time the poly A starts degrading

Question 7

splicing

Answer 7

• binds at the end of the exons
• cleavage at the 5’ (of intron) splice site by a splisosome
• formation of lariat-like intermediate
• cleavage at the 3’ splice site
• ligation (sticking together) of exons

Question 8

splisosomes

Answer 8

• a type or ribonuclear complex (made up RNA and protein)

Question 9

alternative splicing

Answer 9

• the ligation of exons can differ and so the sequence of the mRNA also changes
• the proteins may have similar functions as there are common exons, but different function as the sequence of exons differs

Question 10

“why is alternative splicing important?”

Answer 10

so that more than one protein can be made from a single gene

Question 11

“why do we need introns?”

Answer 11

so that alternative splicing can take place

Question 12

export from the nucleus

Answer 12

-relevant proteins bind to the mRNA strand e.g.
~Cap-binding complex - binds whilst in nucleus and is swapped for intiation factors (for translation) outside nucleus
~polyA binding protein is the same inside and out the nucleus.
~exon-junction complex is required for recognition.
-recognition of these enzyme binding complexes allows the mRNA strand allows mRNA to leave via the nuclear pore

Question 13

“why is this step necessary for export out of the nucleus?”

Answer 13

this is a control step and will therefore ensure that the mRNA strand is stable enough to travel through the cytoplasm to be translated