Gene Expression Flashcards

Gene Expression Overview

• The process by which the information in a gene is used to synthesize a functional gene product, often proteins.

Key Processes

• Transcription: The process of synthesizing RNA from a DNA template.
• Translation: The process of synthesizing proteins from mRNA.

Steps of Transcription

1. Initiation:
   • RNA Polymerase binds to the promoter region of the gene (commonly contains a TATA box).
   • Transcription factors assist in the assembly of the transcription initiation complex.
2. Elongation:
   • RNA Polymerase moves along the DNA template strand, unwinding the DNA and synthesizing RNA in a 5’ to 3’ direction, adding complementary RNA nucleotides.
   • The nascent RNA strand begins to peel off, allowing the DNA to re-form.
3. Termination:
   • RNA Polymerase reaches a termination sequence causing it to detach from DNA, releasing the primary mRNA transcript.

Post-Transcriptional Modifications

• 5' Capping:
  • Addition of a 7-methylguanylate cap to the 5' end of the mRNA for stability and recognition by ribosomes.
• Poly-A Tail:
  • Addition of approximately 200 adenine nucleotides to the 3' end to enhance stability and export from the nucleus.
• Splicing:
  • Removal of introns (non-coding regions) and joining exons (coding regions) to form mature mRNA.
  • Splicing is facilitated by a spliceosome.

Gene Structure

• Exons: Coding sequences for proteins.
• Introns: Non-coding sequences removed during splicing.
• Promoter: Sequence where transcription begins.
• Terminator: Sequence signaling the end of transcription.
• The coding sequence is not continuous; it alternates between introns and exons.

Translation Overview

• The process by which mRNA is decoded to produce a polypeptide chain (protein).
• Takes place in ribosomes, utilizing mRNA, rRNA, and tRNA.

Steps of Translation

1. Initiation:
   • mRNA binds to the small ribosomal subunit; the start codon (AUG) is recognized by the initiator tRNA.
   • The large subunit of the ribosome joins to complete the initiation complex.
2. Elongation:
   • tRNAs bring amino acids to the ribosome by matching their anticodons with codons on mRNA, forming peptide bonds between adjacent amino acids.
   • The ribosome moves along the mRNA, continuing the elongation process.
3. Termination:
   • Termination occurs when a stop codon (UAA, UAG, UGA) is reached.
   • Release factors bind to the stop codon, prompting the ribosome to disassemble and release the newly synthesized polypeptide.

Amino Acids

• Building blocks of proteins; there are 20 different amino acids.
• Each has a central carbon atom, an amino group (NH2), carboxyl group (COOH), and a unique R group.
• Amino acids are linked by peptide bonds to form polypeptides.

Protein Structure Levels

1. Primary Structure: Sequence of amino acids.
2. Secondary Structure: Local folding into alpha-helices and beta-sheets through hydrogen bonding.
3. Tertiary Structure: Overall 3D shape of the polypeptide chain resulting from interactions among R groups.
4. Quaternary Structure: Complex formed from two or more polypeptide subunits.

Regulation of Gene Expression

• Only a small percentage of genes are expressed at any time; gene regulation controls when and how genes are expressed.
• Regulation can occur at various levels: transcriptional, post-transcriptional, translational, and post-translational.

Mutations and Disease

• Changes in DNA can lead to mutations which may be silent, missense, or nonsense mutations.
• Mutations can also lead to frameshift mutations due to insertions or deletions.
• Prion diseases result from abnormal protein folding and can cause severe neurological conditions.