Transcription

• Process Overview: Transcription occurs in the nucleus and involves the synthesis of mRNA from a DNA template.

• Steps Involved:

  1. Strands Separation: The DNA strands are split open to expose the genetic code.
  2. RNA Polymerase Function:
  • Attachment: The enzyme RNA polymerase attaches to a specific region known as the promoter sequence on the DNA strand.
  • Nucleotide Addition: It subsequently adds appropriate RNA nucleotides to form a complementary strand of mRNA.
  • Termination: The process continues until the RNA polymerase reaches a terminator sequence, at which point it detaches from the DNA.
  1. Processing of mRNA:
  • A 5' cap and a poly-A tail are added to the mRNA for stability and protection.
  • Splicing: Introns (non-coding regions) are removed from the pre-mRNA, while exons (coding regions) are retained and spliced back together to form mature mRNA.

• Codon Structure: The information in mRNA is encoded in codons, which are sequences of three bases; each codon specifies a particular amino acid.

Translation

• Process Overview: Translation occurs at the ribosome, where the mRNA is decoded to build proteins.

• Steps Involved:

  1. Attachment of mRNA: The mRNA strand attaches to a ribosome.
  2. tRNA Function:
  • Transfer RNA (tRNA) molecules “read” the mRNA code, working from the 5' to 3' direction, and align the corresponding amino acids to form the protein.
  • Each tRNA has:
    • An anticodon that pairs with the mRNA codon.
    • An attached amino acid, which depends on the anticodon.
  • Binding Process:
    • The tRNA with the anticodon that matches the first codon binds to the mRNA.
    • The tRNA that matches the second codon attaches to the mRNA.
    • The amino acids on both tRNAs bind together.
    • The first tRNA leaves, releasing its amino acid, while the process repeats for subsequent codons.
  1. Start and Stop Codons: mRNA contains specific codons that signal the beginning (start codon) and the end (stop codon) of the translation process.

Mutation

• Definition: A mutation is a change in the sequence of bases in DNA, which can affect protein synthesis.

• Types of Mutations:

  1. Base Substitution (Nucleotide Substitution): This may change the amino acid sequence of a protein.
  • Example: Sickle-cell anemia is a result of a base substitution mutation.
  • Genetic Code Redundancy: The genetic code is described as redundant because the third base of a codon can often wobble (change) without affecting the amino acid.
  1. Neutral Mutation: This mutation does not impact the organism positively or negatively.
  2. Insertion or Deletion: This involves adding or removing bases from the DNA sequence, potentially leading to significant changes.
  • Frameshift Mutation: Such mutations change all subsequent codons in the gene, which can radically alter protein function.

• Mutagens: External factors that increase the rate of mutations, such as x-rays, radiation, UV light, and certain chemicals.