RNA Synthesis - Transcription

How do genes specify proteins?

Genes specify proteins via transcription and translation

Gene

Segment of DNA coding for one RNA molecule or one protein

Eukaryotic and Prokaryotic genes

Organised differently

Prokaryotic genes

Contain 5' UTR, 3' UTR, and a continuous coding sequence without introns

Eukaryotic genes

Contain introns (noncoding) and exons (coding)

RNA

Intermediate between DNA and the protein it encodes

Codon

Sequence of 3 consecutive mRNA bases read during translation

Gene expression

Process by which DNA directs protein synthesis

Transcription

Synthesis of RNA from DNA

Translation

Synthesis of a polypeptide from mRNA

Prokaryotic transcription and translation

Occur simultaneously without RNA processing

Coupling in prokaryotes

Translation begins before transcription ends

Eukaryotic transcription

Occurs in nucleus and includes RNA processing before translation

RNA processing

Modifies eukaryotic primary RNA into mature mRNA

Exons

Coding regions retained in mature mRNA after splicing

Introns

Noncoding regions removed during RNA splicing

Splicing

Removal of introns and joining of exons

Prokaryotic transcription

Generation of RNA from DNA by RNA polymerase

RNA polymerase

Enzyme that synthesizes RNA from DNA

RNA strand

Complementary to DNA template strand

RNA polymerase primer requirement

RNA polymerase does not require a primer to initiate transcription, unlike DNA polymerase.

RNA base pairing

Uses uracil (U) instead of thymine (T)

RNA synthesis direction

5' to 3'

Phosphodiester bond formation

Catalyzed by RNA polymerase between ribonucleotides

Promoter

DNA region where RNA polymerase binds to initiate transcription

RNA product

Single-stranded RNA complementary to DNA template strand

Initiation

RNA polymerase binds promoter, unwinds DNA, and begins RNA synthesis

RNA synthesis initiation

Starts without primer, in 5' to 3' direction

Elongation (transcription)

RNA polymerase adds nucleotides, moves along DNA

DNA-RNA helix length

8 base pairs

RNA-DNA separation

Enzyme structure forces hybrid to separate so RNA can exit

DNA unwinding/rewinding

RNA polymerase unwinds ahead and rewinds behind

Unwound DNA during elongation

17 base pairs remain unwound

Termination (transcription)

Stop signals (termination sequences) cause transcription to end

RNA polymerase at terminator

Releases RNA, detaches from DNA, and DNA reanneals

Multiple RNA polymerases

Can transcribe a single gene simultaneously

Many RNA transcripts

Can be made from one gene

RNA transcription errors

Less critical

All RNA molecules

Eventually degraded and replaced

Termination sequence

Palindromic GC-rich region followed by a run of adenines on template strand

Hairpin structure

Forms from GC-rich palindrome in RNA, causing RNA polymerase to pause

Hairpin loop + poly-U

Causes unstable binding and halts RNA polymerase

RNA-DNA dissociation

Occurs after hairpin and weak poly-A/U interaction

Final steps of termination

RNA is released