Gene & Proteins Notes

Chapter 15: Gene & Proteins

Genetic Code

• The genetic code is composed of sequences of nucleotides that dictate protein synthesis.
• Each amino acid is specified by codons which are triplets of nucleotides:
  • e.g., Glycine (G), Alanine (A), Valine (V), etc.
• Codons: A sequence of three nucleotides that codes for an amino acid.
• Example of codons mapping to amino acids:
  • AAA: Lysine (K)
  • UUU: Phenylalanine (F)

Central Dogma of Molecular Biology

• Flow of Genetic Information: DNA → RNA → Protein
  • Replication: Copying of DNA.
  • Transcription: Converting DNA to mRNA.
  • Translation: Synthesizing protein from mRNA.

Definition of a Gene

• A gene is a specific sequence of nucleotides in DNA that encodes for a protein.
• Genes typically code for traits (e.g., blood type).
• Example:
  • The smallest human gene, SRY, encodes for 204 amino acids.

Beadle & Tatum’s Experiment

• Proposed the one gene-one enzyme hypothesis:
  • Each gene encodes a single enzyme, suggesting a direct link between genes and proteins.
• Conducted experiments using bread mold (a fungus) to demonstrate the influence of genetic mutations on metabolic pathways.
  • Used X-rays to induce mutations which led to different growth capabilities in minimal media.

Transcription vs. Translation: Eukaryotes vs. Prokaryotes

• Prokaryotic Transcription/Translation:
  • Occurs simultaneously in the cytoplasm, does not require RNA modification.
• Eukaryotic Transcription/Translation:
  • Transcription occurs in the nucleus and undergoes mRNA processing before translation in the cytoplasm.

Key Steps in Transcription

1. Initiation: RNA polymerase binds to a promoter sequence.
2. Elongation: RNA synthesis proceeds.
3. Termination: Release of RNA transcript upon reaching the termination sequence.

Eukaryotic Pre-mRNA Processing

• Key steps include:
  • Addition of a 5' cap (modified guanine) and 3' poly-A tail (sequence of adenines).
  • Removal of introns and splicing of exons through spliceosomes to form mature mRNA.

Translation: Overview

• Translation occurs at ribosomes, involving mRNA and tRNA:
  • mRNA provides the template for polypeptide assembly.
  • Each tRNA carries a specific amino acid and has an anticodon that pairs with mRNA codons.
• Stages of Translation:
  1. Initiation: mRNA, tRNA, and ribosomal subunits form a complex.
  2. Elongation: Amino acids are linked to form a polypeptide chain.
  3. Termination: The process stops when a stop codon is reached on the mRNA.

Mutations & Mutagens

• Mutations: Changes in genetic material, classified as:
  • Point Mutations: Changes in a single nucleotide base pair.
  • Types: Silent, Missense, Nonsense.
  • Frameshift Mutations: Insertions or deletions that alter the reading frame.
  • Chromosomal Mutations: Large scale alterations in chromosome structure.
• Mutagens: Agents that cause mutations (e.g., UV radiation, chemical substances).

Conclusion

• Understanding gene structure, function, and the processes of transcription and translation is crucial for grasping molecular biology and genetics. Insights into these mechanisms underpin advances in biotechnology, medicine, and evolutionary biology.
• The principles outlined in the central dogma provide a framework for understanding how genes direct cellular function and the development of traits.