5. Chapter 14.1 - 14.4: pre-mRNA processing in eurkaryotes

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transcription: nucleus translation: cytoplasm = more time to modify pre-mRNA

name posttranscriptional modifications to eukaryotic pre-mRNA

Addition of 5’ cap, 3 cleavage and addition of poly(A) tail, RNA splicing/ editing (rare)

explain the addition of the 5’ cap:

addition of an extra guanine nucleotide to the 5 end of the mRNA and the addition of methyl groups (CH ) to the base in that guanine, forming 7 methylguanosine; Methyl groups are also added to the 2-OH group of the sugar of the first and sometimes second nucleotides adjacent to 7 methylguanosine

When does the 5’ cap actually pop on the pre-mRNA?

addition of the 5’ cap takes place rapidly after the initiation of transcription

When does the cap complete it’s function?

Initiation of translation; why is this important?

Why is the 5’ cap important?

Cap-binding proteins recognize the cap and attach to it; a ribosome then binds to these proteins and moves downstream along the mRNA until the start codon is reached and translation begins. The presence of a 5 cap also increases the stability of mRNA and influences the removal of introns.

What is the Poly(A) tail?

Adenine nucleotides added AFTER transcription - called polyadenylation 

What is the main point fo the 5’ cap and the poly-A tail?

stabilize RNA and protect it against degradation; thus increasing the time during which the mRNA remains intact and available for translation

What is one-way eukaryotic cells regulate the expression of genes?

controlling degradation of mRNA

What is RNA splicing and where does it take place?

The removal of introns in pre-mRNA; in the nucleus

What does splicing require?

3 sequences in the intron

One end of the intron is referred to as the

5’ splice site

The other end of the intron is the

3’ splice site

What does each end of the intron contain?

short consensus sequences

5’ splice site consensus sequence

GU A/G AGU; changing a single nucleotide at either of these sites prevents splicing

3’ splice site consensus sequence

CAGG; changing a single nucleotide at either of these sites prevents splicing

The third sequence important for splicing is the

branch point

What is the branch point?

an adenine nucleotide that lies 18 to 40 nucleotides upstream of the 3’ splice site

Since the A nucleotide at the branch point isn’t strong, what can happen?

Deletion or mutation of the adenine nucleotide at the branch point prevents splicing.

What is the splicing code?

—the 5 splice site, the 3 splice site, and the branch point