Transcription

Central Dogma

Flow of genetic information within biological systems

Transcription

the synthesis of RNA using information in DNA

Gene

DNA segment that codes for a single molecule (RNA or polypeptide)

messenger RNA (mRNA)

Ribosomal RNA (rRNA)

Transfer RNA (tRNA)

other small RNAs

RNA’s involved in Transcription

promoters

are DNA sequences that signal the start of transcription of a gene

prokaryotic promoters

relatively simple and conserved in structure

typically located upstream of the transcription start site

prokaryotic promoters

consists of conserved sequences of promoter at -10 (TATAAT) and -35 (TTGACA) positions

recognized by RNA polymerase and associated sigma factor

eukaryotic promoters

complex and diverse in structure

typically located upstream of the transcription start site

eukaryotic promoters

contains core promoter elements, such as the TATA box, and regulatory elements like enhancers

recognized by RNA polymerase II and general transcription factors

Initiation

What step of transcription: Polymerase recognizes and binds to the promoter

DNA dependent RNA polymerase

Enzyme uses DNA as a template to synthesize the complementary RNA strand during transcription

The enzyme that synthesizes RNA during transcription is a

all types of RNA

In Prokaryotes, single RNA polymerase synthesizes ….

initiation (step one of transcription)

RNA polymerase binds to the promoter and starts to unwind the DNA strand

RNA polymerase I- most rRNAs (18S and 28S)

RNA polymerase II- mRNA

RNA polymerase III- 5S rRNA, tRNAs

In eukaryotes, 2 types of RNA polymerase synthesize different types of RNA:

Elongation (step two of transcription)

RNA polymerase moves along the DNA template strand from 3’ to 5’ and produces the RNA transcript by adding nucleotides complementary to the DNA template to the 3’ end of the growing RNA

nucleotides are added to 3’ OH

RNA is synthesized as a single strand

Template Strand

DNA strand used as a template to synthesize RNA molecules

Elongation (step two transcription)

RNA strand synthesized is complementary to the template strand

essentially identical to the non-template (coding) strand

elongation (step two of transcription)

DNA-RNA hybrid is temporary

DNA double helix reforms as soon as RNA exits

encounters termination signal

Termination (Step 3 of transcription)

when RNA polymerase reaches the termination site, the RNA transcript and polymerase and released from the template

termination (step three of transcription)

release of the newly synthesized RNA molecule and the disengagement of RNA polymerase from the DNA template

termination signals

release of the RNA transcript

Rho protein involvement (prokaryotes)

post-termination events

Major points for the termination step

the p helicase binds to a rut site

RNA polymerase continues synthesizing the RNA strand in the same 5’ to 3’ direction- the p helicase migrates along the mRNA to the elongating RNA polymerase, catches up to the RNA polymerase

Rho reaches the RNA polymerase, helicase unwinds DNA-RNA complex- the p helicase separates the mRNA from the DNA template

rho dependent termination releases mRNA

p-dependent termination in prokaryotes steps

Rho

acts as helicase when bound to RNA

terminator sequences

acts as stop signs for RNA polymerase

Does not need Rho protein

palindromic sequence

region of nucleotide order is the same on both strands (reading from both directions)

RNA synthesis encounters a terminator sequence

An RNA hairpin is formed at a palindromic sequence, reducing the length of the RNA-DNA hybrid

mRNA is released

p- independent termination in prokaryotes steps

RNA hairpin

secondary structure formed when a single-stranded RNA molecule folds back on itself, creating a double helical region and a loop

Transcription termination in Eukaryotes

transcription is terminated by a terminator protein that recognizes a termination sequence

transcript needs to be modified

Transcription Termination in eukaryotes

RNA polymerase II

mRNA……

prokaryotic rho-independent termination

Transcription Termination in eukaryotes

RNA polymerase III

5S rRNA and tRNAs

termination is similar to what?

Transcription (Termination) in Eukaryotes

the mRNA is initially synthesized as a pre-mRNA transcript that needs to be modified in several ways

A modified nucleotide is added to the 5’ end to 5’ cap

RNA polymerase II transcribes the polyadenylation signal sequence

RNA transcript is released 10-35 nucleotides past this polyadenylation signal

Poly A polymerase adds As to the 3’ end of the transcript (poly A tail)

Poly A tail is template independent

Transcription (Termination) in Eukaryotes

Eukaryotic pre-mRNA has coding regions interspersed with noncoding regions

RNA processing in Eukaryotes

noncoding regions

called intervening sequences or introns

coding regions

called exons because they are eventually expressed

RNA splicing

removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence

small nuclear ribonucleoproteins (snRNPs) bind to the pre-mRNA near the 5’ donor and branch point consensus sequences

binding of snRNPs recruits many other proteins- gathering of snRNP forms spliceosome

a cut is made between the upstream exon and the intron

after the first cut at the 5’ end, the intron forms a closed loop

the free 3’ OH group at the end of the cut upstream exon reacts with the 5’ phosphate of the downstream exon

RNA processing in Eukaryotes steps

spliceosome

complex of proteins and RNA that carries out RNA splicing

spliceosome

catalyzes splicing reactions

splicesome

consists of small nuclear ribonucleoproteins (snRNPs) and other associated proteins

Transcription in Eukaryotic cell

mature mRNA is transported to the cytoplasm

transcription in Eukaryotic cell

nuclear transport factors- help guiding mRNA on its trip

crosses nuclear pore complex