Functions of Caps Caps serve at least four functions: -Protect mRNAs from degradation Enhance translatability(可译性)of mRNAs Transport of mRNAs out of nucleus -Efficiency of splicing mRNAs 15-6
15-6 Functions of Caps Caps serve at least four functions: – Protect mRNAs from degradation – Enhance translatability (可译性) of mRNAs – Transport of mRNAs out of nucleus – Efficiency of splicing mRNAs
15.2 Polyadenylation(多聚腺苷酸化) The process of adding poly(A)to RNA is called polyadenylation A long chain of AMP residues is called poly (A) Heterogeneous nuclear mRNA is a precursor to mRNA Most eukaryotic mRNAs and their precursors have a chain of AMP residues about 250 nt long at their 3'-ends Poly(A)is added posttranscriptionally by poly(A) polymerase 15-7
15-7 15.2 Polyadenylation (多聚腺苷酸化) ✓ The process of adding poly(A) to RNA is called polyadenylation ✓ A long chain of AMP residues is called poly (A) ✓ Heterogeneous nuclear mRNA is a precursor to mRNA ✓ Most eukaryotic mRNAs and their precursors have a chain of AMP residues about 250 nt long at their 3’-ends ✓ Poly(A) is added posttranscriptionally by poly(A) polymerase
Functions of Poly(A) Poly(A)enhances both the lifetime and translatability of mRNA Relative importance of these two effects seems to vary from one system to another √In rabbit reticulocyte(网织红细胞)extracts, poly(A)seems to enhance translatability by helping to recruit mRNA to polysomes 15-8
15-8 Functions of Poly(A) ✓ Poly(A) enhances both the lifetime and translatability of mRNA ✓ Relative importance of these two effects seems to vary from one system to another ✓ In rabbit reticulocyte (网织红细胞) extracts, poly(A) seems to enhance translatability by helping to recruit mRNA to polysomes
Basic Mechanism of Polyadenylation Transcription of eukaryotic genes extends beyond the polyadenylation site √The transcript is: -Cleaved -Polyadenylated at 3'-end created by cleavage 5 (a) Cut Polyadenylate (c) Degrade 15-9
15-9 Basic Mechanism of Polyadenylation ✓ Transcription of eukaryotic genes extends beyond the polyadenylation site ✓ The transcript is: – Cleaved – Polyadenylated at 3’-end created by cleavage
Synthesis and Polyadenylation Copyright The McGraw-Hill Companies.Inc. Permission required for reproduction or display (a) Transcribe,polyadenylate,and splice. + + (b) Transcribe,terminate. Polyadenylate ■■ 2 Transcribe. Clip,polyadenylate. 15-10
15-10 Synthesis and Polyadenylation