D

Genetics and Gene Expression Notes

Genetics Overview

  • Definition: Genetics is the study of molecules that control inheritance, primarily focusing on nucleic acids: DNA and RNA.

Nucleic Acids

  • Deoxyribonucleic Acid (DNA): Contains genetic instructions, structured as a double helix, located in the nucleus of the cell.
  • Ribonucleic Acid (RNA): Involved in protein synthesis, serves as a copy of the genetic information in DNA.

Gene Expression

  • Definition: Process by which information from a gene is used to synthesize proteins responsible for inherited traits.
  • Components:
    • Genes: Segments of DNA that provide instructions for making RNA and proteins.
    • Ribosomes: Molecular machines in the cytoplasm responsible for protein synthesis; many are attached to the rough endoplasmic reticulum.

Process of Gene Expression

  1. Transcription:
    • Location: Nucleus
    • Process: The process of copying a gene in DNA into RNA (transcript).
    • RNA Types:
      • Messenger RNA (mRNA): Carries instructions to ribosomes.
      • Ribosomal RNA (rRNA): Combines with proteins to form ribosomal subunits.
      • Transfer RNA (tRNA): Carries amino acids to ribosomes and matches them to the mRNA template.

Steps in Transcription

  • Unwinding DNA: The double helix unwinds at the gene location.

  • Template Strand: One strand of DNA serves as a template for complementary RNA synthesis.

  • Enzyme Involved: RNA polymerase catalyzes the formation of the RNA polymer by matching RNA nucleotides to the DNA base sequence (A pairs with U, C pairs with G).

  • RNA Processing:

    • Pre-mRNA is modified before it leaves the nucleus, where introns (non-coding regions) are removed and exons (coding regions) are spliced together to form mature mRNA.
    • Introns vs Exons: Introns are non-coding; exons are expressed and coding.

Translation

  • Definition: The process of translating mRNA into a polypeptide (protein).
  • Location: Cytoplasm, specifically at the ribosome.
  • Stages of Translation:
    • Initiation:
    • Formation of initiation complex involving the small ribosomal subunit, mRNA, and the initiator tRNA carrying the first amino acid.
    • The start codon on the mRNA is recognized by the tRNA’s anticodon.
    • Elongation:
    • New tRNAs bring amino acids corresponding to the exposed mRNA codons, adding to the growing polypeptide chain while the ribosome moves along the mRNA.
    • Peptide bonds form between amino acids.
    • Termination:
    • A stop codon is reached; a release factor binds, leading to the assembly disassembling, releasing the newly formed protein.

Genetic Code

  • Relationship: The correlation between DNA sequences, mRNA codons, and corresponding amino acids. Each codon consists of three bases that code for specific amino acids.
  • Redundancy: Multiple codons can encode the same amino acid due to the code's degeneracy (e.g., serine can be coded by multiple codons).
  • Start and Stop Codons: AUG is the start codon, while UAA, UAG, and UGA are stop codons.

Summary of Concepts

  • Transcription Process: Occurs in the nucleus, produces RNA, involves unwinding and copying DNA, using RNA polymerase, and processing mRNA.
  • Translation Process: Occurs in the cytoplasm at ribosomes, involves codons from mRNA, tRNA bringing amino acids, elongation of the chain, and termination upon reaching a stop codon.
  • Importance of mRNA Processing: Essential for creating functional proteins from genes that can have multiple expressions based on splicing and processing.