14.3: Eukaryotic cells modify RNA after transcription

Question 1

RNA processing

Answer 1

both ends of the primary transcript are altered and certain segments are cut out before it leaves the nucleus, only in eukaryotes

Question 2

5’ cap

Answer 2

is a modified form of a guanine nucleotide added onto the 5’ end

Question 3

poly-A tail

Answer 3

at the 3’ end, and enzyme adds 50-250 more adenine nucleotides

Question 4

What are the functions of the mRNA mods ? 4

Answer 4

1. they facilitate the export of the mature mRNA from the nucleus
2. They help protect the mRNA from degradation by hydrolytic enzymes
3. They help ribosomes attach to the 5’ end of the mRNA once it reaches the cytoplasm
   - In addition to the tail and cap, UTRs exist at both ends before the cap and tail are added, they are not translated by helping ribosomes bind

Question 5

RNA splicing

Answer 5

large portions of the RNA molecule are removed and the remaining portions are reconnected

Question 6

introns

Answer 6

noncoding segments, cut out

Question 7

exons

Answer 7

coding segments

Question 8

alternative RNA splicing

Answer 8

genes give rise to polypeptides depending on which segments are treated as exons during RNA processing

Question 9

splicesome

Answer 9

removes introns, made of proteins and small RNAs, binds to nucleotides sequences along the intron, and releases and degrades it, consist of a variety of proteins and several small nuclear ribonucleoproteins (snRNPs) that recognize the splice sites

These proteins work with small Nuclear RNA (snRNA) that help in the catalytic excision of the introns.

Question 10

ribozymes

Answer 10

RNA molecules that function as enzymes

the intron RNA functions as a ribozyme and catalyzes its own removal

Question 11

3 properties of RNA enable some RNA molecules to function as enzymes

Answer 11

1. RNA is single-stranded, a region of an RNA molecule may base pair, in an antiparallel arrangement, with a complementary region elsewhere in the same molecule; this gives the molecule a partial 3 dimensional structure, A specific structure is essential to the catalytic function of ribozymes, just as it is for enzymatic proteins.
2. like certain amino acids in an enzymatic protein, some of the bases in RNA contain functional groups that can participate in catalysis
3. The ability of RNA to hydrogen bond with other nucleic acid molecules adds specificity to its catalytic activity