Genetics Exam 2 Quizlet 2 - RNA Molecules & RNA-Processing

Addition of Poly(A)Tail - additional info

The 3' end of mRNAs is modified with the addition of a poly(A) tail 1) AAUAAA consensus sequence 2) 3' end cleavage 3) U rich sequence 4) Multiple adenine residues 5) Stability

multiple 3' cleavage sites

After 3' cleavage and polyadenylation, 1) cleavage can occur at different sites, 2) mRNA products of differing lengths are produced

Alternative splicing

After 3' cleavage and polyadenylation, 1) two introns can be removed to yield an mRNA molecule 2) two introns, with an exon conjoined in the middle, can be removed to yield a different mRNA

Spliceosome

One of the largest molecular complex structures -> catalyzes splicing 1) Consists of 5 molecules (SnRNAs) and almost 300 proteins 2) snRNA & proteins: Small nuclear ribonucleoprotein particles (snRNPs: pronounced 'snurps') 3) Spliceosome is composed of five snRNPs: U1, U2, U4, U5, U6

RNA editing

Alters nucleotide sequence of mRNA

Nuclear pre-mRNA introns

protein encoding genes in the nucleus of eukaryotes -> spliceosomal splicing mechanism

Addition of Poly(A) Tail - AAUAAA consensus sequence

recognition site in pre-mRNA

mRNAs three primary regions

1) A mature mRNA contains 5′ untranslated region (5′ UTR, or leader sequence) - Shine Dalgarno sequence 2) Protein coding region 3) 3′ untranslated region

Introns and RNA splicing

1) Average human gene contains 8-9 introns 2) All introns and exons are initially transcribed into RNA but after transcription, the introns are removed by splicing and exons are joined to yield mature RNA

The gene includes

1) DNA sequences that code for all exons and introns 2) Those sequences at the beginning and end of the RNA that are not translated into a protein, including the entire transcription unit - The promoter - The RNA coding sequence - The terminator

3' cleavage and addition of poly (A) tail purpose

1) Increases stability of mRNA 2) facilitates binding of ribosome to mRNA

5' Cap Addition process

1) One of the three phosphate groups at the 5' end of the mRNA is removed 2) A guanine nucleotide (with its phosphate group) is added 3) Methyl groups are added to position 7 of the base terminal guanine nucleotide 4) Methyl groups are added to the 2' position of the sugar in the second and third nucleotides 5) the base on the initial nucleotide may also be methylated

Addition of 5' Cap - additional details

1) Only on transcripts generated by RNA polymerase II 2) the cap consists of a guanine with a methyl group (CH3) at the position 7 of the base -> 7-methylguanosine (7mG) cap -> Increase mRNA stability 3) Functions in the initiation of translation

Addition of the Poly(A) tail - process overview

1) Pre-mRNA is cleaved, at a position 11 to 30 nucleotides down-stream of the consensus sequence in the 3' untranslated region 2) The addition of adenine nucleotides (polyadenylation) takes place at the 3' end of the pre-mRNA, generating the poly(A) tail. Conclusion: in pre-mRNA processing, a poly(A) tail is added through cleavage and polyadenylation

RNA Processing in Eukaryotes

1) RNA Polymerase II synthesis 2) RNA processing occurs in nucleus in eukaryotic 3) The trimmed mature mRNA transports to cytosol for translation

RNA splicing

1) Removes noncoding introns from pre-mRNA 2) Facilitates export of mRNA to cytoplasm 3) allows for multiple proteins to be produced through alternative splicing

Spliceosome Process

1) U1 attaches to the 5' splice site 2) U2 attaches to the branch point 3) A complex of U4, U5, and U6 joins the spliceosome 4) U1 and U4 are released 5) 5 Base pairing between sequences in the mRNA and the snRNAs hold the spliceosome together 6) the exons are joined together and the intron is released as a lariat

RNA Processing Steps - Overview

1) introns, exons, and a long 3' end are all transcribed into pre-mRNA 2) a 5' cap is added 3) Cleavage at the 3' end is approximately 10 nucleotides downstream of the consensus sequences 4) Polyadenylation at the cleavage produces of the poly(A) tail 5) Introns are removed 6) mature mRNA is produced

RNA splicing full process

1) the mRNA is cut at the 5' splice site 2) the 5' end of the intron attaches to the branch point 3) A cut is made at the 3' splice site 4) The intron is released as a lariat 5) the two exons are spliced together 6) The bond holding the lariat is broken, and the linear intron is degraded 7) The spliced mRNA is exported to the cytoplasm and translated Lariat is formed by a phosphodiester bond between C2 of ribose

RNA Editing Process

1) unediting mRNA pairs with guide RNA 2) The guide serves as a template for the addition, deletion, or alteration of bases 3) The mature mRNA is then released Conclusion: Guide RNA adds nucleotides to the mRNA that were not encoded by the DNA

Addition of Poly(A) Tail - 3' end cleavage

3' end is cleaved 11 to 30 nucleotides from AAUAAA -> cleavage involves several proteins

exon junction complex (EJC)

A group of protein completes exon-exon junction

Spliceosome branch point

Adenine, 18 to 40 nucleotides upstream of the 3' splice site

Exons

Coding segments of eukaryotic DNA.

tRNA introns

Enzymatic splicing mechanisms

Addition of Poly(A) Tail - Multiple adenine residues

Multiple adenine residues (50-250) -poly (A) are then added by the enzyme poly- adenylate polymerase (PAP)

RNA Editing

Pre-mRNA can be altered after transcription (not typical) 1) One base is substituted for another 2) Nucleotide is inserted/deleted 3) Guide mRNA (gRNA) serves as the template Editing may be involved in mutations

RNA Processing in Eukaryotes - RNA Polymerase II synthesis

RNA Pol II synthesizes a pre-mRNA that needs to be processed in order to become a functional mRNA 1) Addition of 5' cap 2) 3' cleavage and addition of poly(A)-tail 3) RNA splicing (removal of non coding introns) 4) RNA editing (in some cases)

Group 1 and 2 introns

Splice through self-splicing

Addition of Poly(A) Tail - stability

The poly(A) tail confers stability because it slows down degradation

Addition of Poly(A) Tail - U rich sequence

U rich sequence is typically downstream of the cleavage site

#of genes and amino acids

With colinearity, the number of nucleotides in a gene is proportional to the number of amino acids in the protein

Can a single gene yield multiple mRNA and protein products?

Yes, through alternative processing and multiple 3' cleavage sites

Introns

a non-coding, intervening sequence within a eukaryotic gene

addition of 5' cap purpose

facilitates binding of ribosome to 5' end of mRNA, increases mRNA

Calcitonin and CGRP

in thyroid cells, cleavage and polydenylation take place at the end of exon 4, while in brain cells it takes place in exon 6 This results in different molecules being produced Example of multiple 3' cleavage sites

Introns - full overview

interspersed non-coding sequences between exons ➢ 1) Common (not absolutely) in eukaryotic genomes ➢ 2) Observed in certain genes of prokaryotes 3) Need to be removed before mature mRNA is released 4) Sequences mark the 5' and 3' splice sites 5) Branch point A for 5' intron to form a lariat structure