formation of mature mRNA from pre-mRNA

Question 1

where does post-transcriptional modification occur?

Answer 1

• in the nucleus

Question 2

what are the 3 steps involved in post-transcriptional modification?

Answer 2

1. addition of 5’ cap
2. polyadenylation at 3’ end
3. RNA splicing

Question 3

what are the 3 steps involved in post-transcriptional modification?

Answer 3

1. addition of 5’ cap
2. polyadenylation at 3’ end
3. RNA splicing

Question 4

where does the addition of the 5’ cap occur?

Answer 4

it occurs rapidly after the initiation of transcription

Question 5

what is done to the pre-mRNA during the addition of the 5’ cap?

Answer 5

a methylated guanine nucleotide is added to the 5’ end of the newly synthesized pre-mRNA

Question 6

what is done to the pre-mRNA when polyadenylation at the 3’ end occurs?

Answer 6

a poly(A) tail consisting of 50 to 250 nucleotides are added to the cleaved 3’ end of the pre-mRNA by an enzyme

Question 7

what is RNA splicing?

Answer 7

it is the process where introns in the pre-mRNA transcript are excised while the exons are spliced together to form a continuous coding sequence

Question 8

what is required before RNA splicing can occur?

Answer 8

the formation of a spliceosome involving small nuclear ribonucleoproteins (snRNPs)

Question 9

what are snRNPs made of?

Answer 9

• they are made up of small nuclear RNAs & proteins

Question 10

what is the role of snRNPs?

Answer 10

to recognize & bind to splice sites, which are short RNA nucleotides at both ends of each intron, to facilitate RNA splicing

Question 11

describe the mechanism of RNA splicing. [3]

Answer 11

1. snRNAs in the snRNPs recognize & bind to the splice sites at each end of the intron by complementary base pairing
2. additional snRNPs join the snRNAs to form a spliceosome
3. the spliceosome cuts the intron at specific points to release the intron & splice the exons together
   
   • during the excision of the intron, the 5’ end of the intron is cut & attached near the 3’ end of the intron, forming a lariat
   • afterwards, the 3’ end of the intron is cut
4. the exons are spliced, & the spliceosome dissociates, releasing the mature mRNA

Question 12

what is alternative splicing?

Answer 12

• alternative splicing is the removal of different exons together with introns, resulting in different mature mRNA formed from the same pre-mRNA

Question 13

why does alternative splicing occur?

Answer 13

• this is because at different stages of development, different cell types or organisms contain different snRNPs
• as the snRNAs in the different snRNPs recognize different splice sites of the introns, some exons may be treated as ‘introns’ & be removed

Question 14

what is the outcome of alternative splicing?

Answer 14

• alternative splicing allows different mature mRNA, each coding for a slightly different polypeptide, to be formed from the same pre-mRNA

Question 15

why is the total number of different proteins a eukaryote can synthesize more than the total number of genes in the DNA?

Answer 15

• this is because alternative splicing allows 1 gene to give rise to different proteins

Question 16

what happens to the introns after they have been excised from the pre-mRNA transcript?

Answer 16

• the introns are degraded to be used to form free ribonucleotides

Question 17

how does the mature mRNA leaves the nucleus?

Answer 17

• it leaves the nucleus via pores in the nuclear envelope

Question 18

what is the role of the 5’ cap & the 3’ poly(A) tail?

Answer 18

1. to facilitate the export of mature mRNA from the nucleus into cytoplasm via nuclear pores of the nuclear envelope
2. to protect mature mRNA from degradation by RNase in the cytoplasm
3. facilitate binding of translation initiation factors & small ribosomal subunit to 5’ end of the mature mRNA for initiation of translation

Question 19

what is the role of RNA splicing?

Answer 19

• to form a mature mRNA molecule with a continuous coding sequence

Question 20

can alternative splicing occur? if yes, what is the purpose of alternative splicing?

Answer 20

• yes, as it allows for different mature mRNAs to form from the same pre-mRNA, hence, different types of proteins may be synthesized from the same gene

Question 21

why does the length of the poly(A) tail matter?

Answer 21

• the length of the poly(A) tail determines the stability of the mRNA, and hence the duration of translation before the mRNA is degraded by enzymes

Question 22

what makes a mature mRNA strand more stable?

Answer 22

• a longer poly(A) tail!!
  → hence, more polypeptide chains would be translated

Question 23

what is the role of the coding sequence?

Answer 23

• it contains the codons which will be translated to amino acids
• it also includes the start codon, AUG, which codes for methionine

Question 24

what are the non-coding sequences in the mature mRNA?

Answer 24

• 5’ cap
• poly(A) tail
• 5’ untranslated region (5’ UTR)
• 3’ untranslated region (3’ UTR)

Question 25

what are the roles of 5’ cap & the poly(A) tail?

Answer 25

• transport of mature mRNA
• protective function
  → increase the half life of the mRNA, and hence increase duration of translation
• facilitate initiation of translation

Question 26

what are the roles of the 5’ untranslated region (5’ UTR) & 3’ untranslated region (3’ UTR)?

Answer 26

• facilitate the initiation of translation
• regulate the rate of translation