Gene Expression-From Gene to Protein

Gene Expression: From Gene to Protein

1. Overview of Gene Expression

  • Gene expression is the process where DNA instructs the synthesis of proteins.

  • Each cell (except sperm and egg) contains identical DNA but expresses different genes based on cell function.

2. Information Flow in Gene Expression

  • Central Dogma of Molecular Biology: DNA → RNA → Protein

  • Transcription: Synthesis of RNA using DNA information, primarily leading to mRNA formation.

  • Translation: Synthesis of a polypeptide from the information in mRNA at the ribosome.

3. Differences in Transcription and Translation Locations

Prokaryotes

  • Both transcription and translation occur in the cytoplasm.

  • Translation can start before transcription ends.

Eukaryotes

  • Transcription occurs in the nucleus and translation in the cytoplasm (must complete transcription first).

4. Mechanism of Transcription Initiation

  • RNA Polymerase: Recognizes the transcription start point via the promoter.

  • Promoter: Sequence signaling the start point, extends upstream.

  • Transcription Factors: Proteins that assist RNA polymerase binding to the promoter

  • Terminator: Signals the end of transcription.

  • Start Point and Upstream: The start point is where transcription begins, while 'upstream' refers to the direction opposite to that of transcription.

5. Predicting RNA Sequence from DNA

  • Can derive the RNA sequence complementary to a given DNA strand.

6. Steps of Transcription

  • Initiation: RNA polymerase binds the promoter.

  • Elongation: RNA polymerase synthesizes RNA by adding nucleotides.

  • Termination: RNA polymerase reaches terminator; differs in bacteria and eukaryotes.

7. Understanding Strands in Transcription

  • Template Strand: Provides the template for ordering the sequence of nucleotides.

  • Coding Strand: Matches the RNA (except T→U).

8. Pre-mRNA and RNA Modifications

  • pre-mRNA: Primary transcript before processing.

  • RNA Modification in Eukaryotes:

    • 5' Cap: Modified nucleotide added at 5' end.

    • Poly-A Tail: 50-250 A’s added at the 3' end.

9. RNA Splicing

  • RNA Splicing: Cutting out introns and joining exons.

  • Introns: Non-coding segments.

  • Exons: Coding regions that are expressed.

10. Significance of Introns

  • Some introns may regulate gene expression.

  • Alternative RNA Splicing: Leads to the production of different polypeptides from one gene, enhancing protein diversity.

11. Codons and the Genetic Code

  • Codon: Three-nucleotide sequence on mRNA coding for one amino acid.

  • Amino Acids: The building blocks of proteins translated from codons.

  • Genetic Code: Redundant (multiple codons for one amino acid) and unambiguous (each codon only specifies one amino acid).

12. Reading Frame in Translation

  • Codons must be read in the correct reading frame to produce the intended polypeptide.

13. Structure and Function of tRNA

  • tRNA Structure: Cloverleaf shape that carries amino acids to the ribosome.

  • Each tRNA molecule has an anticodon that base-pairs with the mRNA codon.

14. Translation Process

Steps of Translation

  • Initiation: Start codon (AUG) recognized; small subunit of ribosome binds to mRNA and tRNA.

  • Elongation:

    • Codon Recognition: tRNA brings appropriate amino acid to the A site.

    • Peptide Bond Formation: Amino acids in P site and A site join.

    • Translocation: Ribosome moves to the next codon.

  • Termination: Stops at codon (UAA, UAG, UGA); release factors break down machinery.

15. Ribosomal Structure and Function

  • Ribosome consists of two subunits (large and small) made of proteins and rRNAs.

  • P Site: Holds tRNA with the growing polypeptide.

  • A Site: Holds the tRNA carrying the next amino acid.

  • E Site: Exit site for tRNA.

16. Cracking the Genetic Code

  • 64 codons in total, with 61 coding for amino acids and 3 as stop signals.

  • Genetic Code is universal across organisms.