Transcription

Overview of Expression

  • The Central Dogma

    • States that DNA makes RNA makes Protein

    • Gene expression involves turning a gene’s DNA information into a polypeptide

    • Transcription: the process of making mRNA using DNA

    • Translation: the process of making a polypeptide using mRNA

Eukaryotic vs. Prokaryotic Expression

  • Eukaryotic Cells

    • RNA is synthesized in the nucleus

    • Polypeptides are synthesized outside the nucleus

    • RNA processing is required before translation

  • Prokaryotic Cells

    • Transcription and translation happen almost simultaneously

The Genetic Code

  • Genetic information is stored in triplets of nucleotides

    • RNA serves as a single-stranded copy of DNA

    • The sugar in RNA is ribose instead of deoxyribose

    • RNA contains uracil (U) instead of thymine (T)

    • mRNA has 5’ and 3’ ends

    • Codon: a group of three nucleotides read from the 5’ to the 3’ end

    • The sequence of codons determines the amino acids during translation

    • Reading frame consists of three codons

Codon Structure

  • Given mRNA example:

    • 5'-AUG-UCA-UCC-GGC-UGA represents a sequence

    • AUG is the start codon which signals the start of translation

    • UGA is a stop codon which terminates translation

  • Codon Chart Insights

    • There is one start codon and multiple stop codons

    • The code is generally redundant but not ambiguous

    • Wobble phenomenon: the first two nucleotides of the codon are the most critical for determining the amino acid

Messaging RNA (mRNA) Synthesis

Transcription Process

  • Involves synthesizing RNA from a DNA template

    • Promoter: a DNA sequence that binds to RNA polymerase

    • RNA Polymerase

    • Binds to the promoter, separates DNA strands, and synthesizes RNA (elongation process)

    • In eukaryotes, often requires transcription factors

    • Upon termination, RNA polymerase detaches, releasing the RNA transcript

RNA Polymerase Mechanics

  • Utilizes base pairing to facilitate RNA synthesis

    • RNA elongates as RNA polymerase progresses down the DNA strand

    • Following passage of RNA polymerase, DNA automatically winds back up

RNA Processing in Eukaryotes

  • Before RNA exits the nucleus, it undergoes several modifications for assistance and protection:

    • Addition of a 5’ cap

    • Addition of a Poly-A tail

  • Splicing process:

    • Introns are removed, while exons are spliced together

  • Coding segments are defined as regions between start and stop codons

    • Some noncoding, untranslated regions (UTR) remain

  • Alternative splicing allows for multiple proteins to be produced from a single gene by varying the way exons are combined