Concept 17.3: Eukaryotic cells modify RNA after transcription

alteration to the 5’ end vs 3’ end

5’ end: 5’ cap (modified form of guanine nucleotide) added after first 20-40 nucleotides transcribed

3’ end: poly-A tail of 50-250 adenine nucleotides

purpose of AAUAAA

pre-mRNA is cut and released soon after the polyadenylation signal is transcribed

purpose of 5’ cap and poly-A tail

1. facilitate the export of mature mRNA from nucleus

2. protect mRNA from degradation

3. help ribosomes attach to 5’ end of mRNA

untranslated regions

UTRs are parts of mRNA that aren’t translated into proteins but instead do things like ribosome binding

RNA splicing

large portions of RNA primary transcript molecules are removed and remaining portions are connected

introns vs exons

intron: noncoding segment that lies between coding regions (INtervening sequence)

exon: eventually EXpressed by being translated into amino acid sequences or EXiting the nucleus

spliceosome

this large complex of proteins and small RNAs accomplishes the removal of introns. when the introns are released and rapidly degraded, the spliceosome joins the two exons together

ribozymes

RNA molecules that function as enzymes (catalytic role)

what can an intron function as

a ribozyme, by catalyzing its own removal in the splicing process

properties that allow some RNA molecules to function as enzymes

1. RNA is single-stranded so a region of an RNA molecule might base-pair with a complementary region somewhere else in the same molecule (giving it a 3d structure)

2. like certain amino acids, some bases in RNA contain functional groups that can participate in catalysis

3. ability of RNA to hydrogen-bond with other nucleic acid molecules adds specificity to its catalytic activity

alternative RNA splicing

eukaryotic gene regulation where different polypeptides can be made depending on which RNA segments are treated as exons vs introns

domains

structural and functional regions of protein architecture (region with active site vs region allowing enzyme to bind to cell membrane)

exon shuffling

when the presence of introns facilitates evolution of new proteins by increasing the probability of crossing over between exons due to more terrain for crossovers