Genetic Code Study Notes

Genetic Code

Criteria for Genetic Material

  • Replicate: The ability to make copies of itself.
  • Express information: The capability to convey genetic information for functioning and development.
  • Store information: The ability to hold genetic information - related to the concept of the genetic code.
  • Increase variation via mutation: The potential for genetic change through mutations, leading to diversity.

Goals of Study

  1. Understand the basic steps of how DNA provides information for protein synthesis (known as the central dogma of biology).
  2. Familiarize oneself with the general features of the genetic code.
  3. Differentiate between the reading frame and the open reading frame, which is the code for the frame.
  4. Identify open reading frames encoded in the genome.

Flow of Genetic Information from DNA to Protein

  • Gene: A segment of DNA that contains the instructions for synthesizing a protein.
  • DNA Sequence Example:
    • DNA Template Strand:
      • Sequence: TACCACAACTCG
    • Transcription results in:
      • mRNA Sequence: AUGGUGUUGAGC
  • The triplet code in mRNA is read:
    • Protein Polypeptide Sequence: Met Val Leu Ser (corresponding amino acids).

Mechanism of Protein Synthesis

  1. Transcription occurs in the nucleus where the genetic information encoded in DNA is transcribed into mRNA.
  2. Translation occurs on ribosomes, where mRNA is translated into a polypeptide chain consisting of amino acids.
Messenger RNA (mRNA)
  • Acts as an intermediate in transferring genetic information from DNA to proteins.
  • Ribonucleic acid (RNA) is primarily located in the nucleus and utilized by ribosomes for protein synthesis.
Nucleic Acids Coding for Proteins
  • A codon consists of three nucleic acids coding for one amino acid.
  • The relationship established by Sydney Brenner & Francis Crick indicates:
    • 4 nucleotides (A, U, C, G) correspond to 20 amino acids.

Genetic Code Characteristics

  • Triplet Code: The genetic code is read in triplets of bases, known as codons.
  • Reading Frame:
    • Definition: The organization of nucleotide sequences into non-overlapping triplet codons.
  • Out of Frame Examples:
    • If nucleotides are added or deleted (e.g., insertion of C or CAG), the reading frame shifts, altering the resulting protein.
Codons and their Amino Acids
  • Codon Table Overview:
    • UAA (Stop), UGA (Stop), AUG (Start, Methionine), etc.

Wobble Hypothesis

  • Definition: The initial two ribonucleotides of triplet codes carry more significance than the third nucleotide.
  • The third position of a codon is less constrained and may pair with varied bases outside normal rules.
  • Base Pairing Examples:
    • A pairs with U, G.
    • C pairs with A, G.
    • Codons discrepancies are tolerated at the third position.

Features of the Genetic Code

  • Written in a linear form composed of ribonucleotide bases forming mRNA.
  • A Codon is defined as a sequence of three nucleotides (triplet).
  • Unambiguous: Each codon corresponds to one specific amino acid.
  • Degenerate: Multiple codons can code for the same amino acid (e.g., Leucine).
  • Contains signals for start and stop: triplets communicated at the beginning and end of translation.
  • Commaless: Once translation initiates, it is continuous until a termination signal is encountered.
  • Non-overlapping: Codons do not overlap; each is distinct in its meaning.
  • Colinear: The sequence of codons aligns directly with the sequence of amino acids in the protein.
  • Universality: The majority of organisms utilize the same codon table for protein synthesis.

Open Reading Frames (ORFs)

  • Definition: Open reading frames refer to nucleotide sequences that can produce RNA with a defined start and end, encoding for proteins.
  • Both strands of DNA can code for protein products, leading to multiple potential reading frames.
  • Identifying ORFs: To find ORFs, start and stop codons within the gene sequence must be identified.
  • Each DNA double helix contains six possible reading frames:
    • For example, a sequence like 5'-ATGCCTTATCGAAATTCGG-3' indicates different frames:
    • Frame 1: 5'-ATG CCT TAT CGA AAT TCGG-3'
    • Other frames derived accordingly.

DNA Mutations

  • Upcoming module: Will discuss types of mutations at various biological levels:

    • Molecular Level: Changes to the sequence of nucleotides.
    • Chromosomal Level: Alterations involving chromosomes, such as deletions, duplications, inversions, or translocations.
    • DNA-Level: Impact within the structure of DNA itself.
    • Phenotypic Level: Observable expression of mutations.

  • Task: Come prepared with definitions of the different types of mutations for class discussion.