10 - From DNA to RNA

What happens when RNA pol II is phosphorylated at the C-terminal domain (CTD)

The CTD lengthens and more proteins associated with with it. They then “hop off” to the new RNA molecule to begin processing with the 5’ cap, RNA splicing, and 3’ tail

What happens after 20 nucleotides are transcribed

The cap is put in place to modify the pre-mRNA transcript; first modification of pre-mRNA

What is the mRNA 5’ cap

A guanine nucleotide modified by methylation and other things

How is the mRNA 5’ cap put in place

By 3 enzymes that were bound to RNA pol tail

What does the 5’ cap act as

Marker to cell that this is an mRNA and essential for translation

What does the 5’ cap bind

Protein complex as a recognition factor (cap-binding complex CBC)

Exons

Coding sequences

Introns

Intervening sequences

Are both exons and introns transcribed into RNA

Yes

How are introns removed

By splicing

How are exons joined

With help of small nuclear RNAs (snRNAs) that recognize the splice sequences

SnRNA + 7 protein subunits

SnRNP (ribonucleotide protein)

What do snRNPs form

Core of spliceosome

Look at splicing method

Kk

What does splicing mechanism involve

6 snRNAs and their associated proteins

What happens after a two-step enzymatic reaction

Intron is removed (as a “lariat”) and two neighboring exons are joined together

What plays a critical role in the enzymatic reaction

Branch point A residue

What marks success in splicing

An exon junction protein complex binds at the site of the former intron

Are all eukaryotic exons generally the same length

Yes, 150 nucleotides

How are exons marked and spliceosomes called

Special serine and arginine-rich proteins assemble on exons (SR proteins)

What are the three things that gene splicing depends on

Affinity of the splicing machinery for the three “signals” on the mRNA (splice junctions and branch point)

Assembly of the spliceosome

Exon definition

When is splicing flexible

If alternative proteins need to be made

If there are mutations present in the gene

What do cap and tail message

Tell our nuclear pore that it is mature mRNA ready to leave the cell

What does poly A polymerase do to add the 3’ tail

Add 200 As to 3’ end of mRNA transcript

What is the length of the tail determined by

Poly A binding proteins

What happens when tail is done

RNA pol lets go of template

steps of RNA transcription maturation into an mRNA

5’ end cap as it emerges from RNA polymerase

Spliceosome assembles to delineate intron/exon boundaries “done by CTD tail, exon definition)

When RNA pol reaches specific consensus sequences, special proteins are called forth hat are ventral translated to the 3’ end processing sequence

Can the same gene make multiple different mRNAs

Yes, each can have their own function

How do you get to the cytoplasm

Through selective transport

What signals that an mRNA is ready for export

The dissociation of proteins

What do some proteins on the mRNA signify

Incomplete splicing or other processing; these mRNA’s would not be exported

What happens to mRNA that isn’t exported

Degraded in exosomes

What packages mRNA and are most abundant

HnRNPs

What does mRNA require to get through the nuclear membrane

Nuclear transport receptor (similar to a key that opens the pore complex lock)

What is the most abundant RNA

rRNA

What transcribes rRNA

RNA polymerase I

What prevents rRNA from leaving the nucleus

The enzyme has no tail, so cannot add cap or polyA tail to transcripts

What happens in the nucleolus

Site of rRNA processing and incorporation of rRNAs into ribosome subunits; “ribosome factory”

Is the nucleolus membrane-bound

No

Where does telomerase assembly happen

Nucleolus

Where are tRNAs processed

Nucleolus

Subnuclear structures besides the nucleolus

Canal bodies

GEMS

Interchromatin granule clusters (speckles)

What are the subnuclear structures thought to be involved in

Processing and storage of RNA-processing components; form a staging area for assembly of part of the spliceosome

What causes Spinal Muscular Atrophy

SMN mutations