Transcription
Step 1 of Protein Synthesis
Molecular Components of Transcription

RNA polymerase is responsible for separating the DNA strands
and synthesizing the mRNA from a gene on the template strand.

The stretch of DNA that is transcribed into RNA is called a
transcription unit.

Assembly of nucleotides only occurs in the 5’3’ direction.
(“downstream”)

The “start” sequence of nucleotides is called the promoter.

The “stop” sequence of nucleotides is called the terminator.

Prokaryotes only have 1 type of RNA polymerase, whereas
eukaryotes have 3.
Synthesis of an RNA Transcript

Three stages: initiation, elongation, and termination

Initiation

Promoter sequence serves as a binding site for RNA polymerase,
determines where transcription starts, and indicates which strand of the
DNA is used as the template.

Transcription factors: a collection of proteins that mediate the binding of
RNA polymerase and the initiation of transcription.

Transcription Initiation Complex: transcription factors and RNA
polymerase

TATA box: a crucial promoter nucleotide sequence containing TATA that is
upstream from the transcriptional start point.

All of these features increase the cell’s control of the process.
Synthesis of an RNA Transcript

Elongation

As RNA polymerase adds nucleotides to the 3’ end of the growing
RNA molecule, the DNA helix reforms behind it and the RNA peels
away from the template.

Rate – about 60 nucleotides per second in eukaryotes!

Multiple RNA polymerases can be transcribing a single gene at once
(like trucks in a convoy), which helps the cell make the encoded
protein in large amounts.
Synthesis of an RNA Transcript

Termination


Prokaryotes

Transcription proceeds through a terminator sequence in the DNA

The transcribed terminator (RNA sequence) causes the polymerase to detach from the DNA
and release the transcript, which is ready for immediate use as mRNA
Eukaryotes

Pre-mRNA is cleaved from the growing RNA chain while RNA polymerase II continues to
transcribe the DNA.

A sequence called the polyadenylation signal sequence is transcribed which adds AAUAAA
to the pre-mRNA.

Transcription ends when the polymerase eventually falls off of the DNA

Pre-mRNA is then modified during RNA processing…
Eukaryotic cells modify RNA after
transcription

Alteration of mRNA ends

A modified guanine nucleotide is added to the 5’ end of the premRNA: 5’ cap.

At the 3’ end, between 50-250 adenines are added: poly-A tail.

Functions of these modifications:

Facilitate export from the nucleus

Protect mRNA from degradation by hydrolytic enzymes

Help ribosomes attach to the 5’ end of the mRNA
Eukaryotic cells modify RNA after
transcription

Split Genes and RNA Splicing

RNA splicing: a cut-and-paste job on the RNA molecule that is initially
synthesized

Introns (intervening sequences): noncoding segments of nucleic acid that
are cut out of the RNA

Exons (expressed sequences): coding segments of nucleic acid that are
spliced back together to form the mRNA

snRNPs (small nuclear ribonucleoproteins) and other associated proteins
form a spliceosome that cuts out the introns and pastes the exons back
together.
Eukaryotic cells modify RNA after
transcription

Split Genes and RNA Splicing


Ribozymes

RNA molecules that function as enzymes

Sometimes the intron RNA functions as a ribozyme and catalyzes its own excision!
The Functional Importance of Introns

Play regulatory roles in the cell (control gene activity)

Splicing is necessary for the passage of mRNA from the nucleus to the cytoplasm

A single gene can encode more than one kind of polypeptide, depending on which
segments are treated as exons: alternative RNA splicing

Several domains (discrete structural & functional regions of a polypeptide) can be
encoded on the same gene… exon shuffling for a variety of polypeptides