Ch12TranscriptionTranslationV8+Rev_5c2ef498355e822edf9671bdc202474a

Introduction to Gene Expression

• Central Dogma: DNA -> RNA -> Protein

  • Genes need to be copied into RNA and translated into proteins.

  • Proteins perform various functions in the cell.

Eukaryotic Gene Structure

Key Components

• A: Promoter: Controls when and where a gene is transcribed into RNA.

  • Located upstream of the coding region.

• B-F: Coding Region: Part of the gene that is transcribed into RNA.

  • Contains start and stop sites for RNA synthesis.

• G: Termination Signal: Sequence that instructs RNA Polymerase to stop RNA synthesis.

Exons and Introns

• Exons (B, D, F): Regions that code for amino acids in proteins.

• Introns (C, E): Noncoding regions of DNA that are spliced out during RNA processing.

Transcription Process

Overview

• Occurs in the nucleus for eukaryotes.

• RNA Polymerase synthesizes RNA from a DNA template.

Steps in Transcription

1. Initiation:

   • Sigma factor binds to DNA sequences in the promoter.

   • RNA Polymerase binds to the sigma factor.

   • DNA unwinds at the transcription start site.

2. Elongation:

   • RNA Polymerase synthesizes RNA by adding nucleotides.

   • RNA is synthesized in the 5' to 3' direction, complementary to the DNA template strand.

3. Termination:

   • RNA Polymerase reaches a termination signal.

   • RNA synthesis ceases, releasing the newly formed RNA.

RNA Synthesis in Eukaryotes and Prokaryotes

• Eukaryotes: Transcription produces pre-mRNA that undergoes splicing.

• Prokaryotes: Transcription occurs directly in the cytoplasm, produces mRNA without splicing.

RNA Types

1. mRNA (Messenger RNA): Codes for amino acids in proteins.

2. tRNA (Transfer RNA): Brings amino acids to ribosomes during protein synthesis.

3. rRNA (Ribosomal RNA): Structural component of ribosomes.

RNA Splicing

Purpose

• To remove introns from pre-mRNA and ligate exons together, resulting in a mature mRNA ready for translation.

Mechanism

1. Formation of Spliceosome: snRNPs and other proteins assemble at intron splice sites.

2. Lariat Formation: Intron is cut at splice sites, forming a loop and connecting to a branch point.

3. Ligation: Exons are spliced together to form the mature mRNA.

Result

• Mature mRNA exists with a 5' cap, coding regions, and a poly-A tail at the 3' end.

Translation: From RNA to Protein

Process

• Translation occurs in the cytoplasm at ribosomes.

• mRNA codons are matched with tRNA anticodons to build polypeptides.

Steps in Translation

1. Initiation:

   • Ribosome binds to the mRNA start codon (AUG).

   • The first tRNA carrying the initiator amino acid (Methionine) binds.

2. Elongation:

   • Polypeptide chain is extended via peptide bonds between amino acids.

   • Ribosome moves along the mRNA, recruiting tRNAs corresponding to each codon.

3. Termination:

   • The ribosome reaches a stop codon on the mRNA.

   • A release factor binds, causing the disassembly of the translational complex and releasing the nascent polypeptide.

Molecular Components in Translation

• Ribosome: Composed of rRNA and protein; site of protein synthesis.

• Codons: Triplet sequences on mRNA that encode amino acids.

• Anticodon: Complementary sequence on tRNA that matches with mRNA codons.

Summary of Key Terms

• Codon: 3-nucleotide sequence on mRNA coding for an amino acid.

• Anticodon: 3-nucleotide sequence on tRNA matching a codon on mRNA.

• Release Factor: Protein that binds to stop codons, facilitating the release of the polypeptide from tRNA.