Detailed Notes on Transcription, Translation, and Mutation in DNA

Transcription

• Definition: Process by which the DNA sequence of a gene is transcribed to produce a complementary RNA strand (mRNA).

• Steps of Transcription:

  1. Strand Separation: DNA strands are split open to expose the coding sequence.

  2. RNA Polymerase Enzyme:

  • Role: Binds to the promoter sequence on the DNA strand.

  • Function: Assembles complementary RNA nucleotides to form mRNA.

  • Completion: Continues until it reaches a terminator sequence, upon which it detaches from the DNA.

  1. Processing of mRNA:

  • Addition of Cap and Tail: A 5’ cap and a poly-A tail are added to the mRNA for protection and recognition.

  • Splicing:

    • Introns (non-coding regions) are removed.

    • Exons (coding regions) are joined together to produce the final, mature mRNA.

• Codons:

  • Definition: Sequences of three RNA bases.

  • Function: Each codon specifies a particular amino acid to be incorporated into a protein.

Translation

• Definition: The process of converting the sequence of the mRNA into a sequence of amino acids to form a protein.

• Steps of Translation:

  1. mRNA Attachment: mRNA attaches to a ribosome, which serves as the site for translation.

  2. tRNA Function:

  • Reading mRNA: tRNA reads the mRNA code from 5’ to 3' and aligns the appropriate amino acids.

  • Anticodons: Each tRNA molecule has an anticodon that pairs with the corresponding mRNA codon.

  • Each tRNA carries a specific amino acid determined by its anticodon.

  1. Amino Acid Binding Sequence:

  • The tRNA with the matching anticodon binds to the mRNA codon.

  • The tRNA for the second codon binds as well, and the amino acids on the two tRNAs link together.

  • The first tRNA leaves, leaving its amino acid behind.

  • This process is repeated until all codons are translated.

  1. Termination: mRNA contains specific codons that signal the start and stop of translation.

  • Start Codon: Indicates where translation begins.

  • Stop Codon: Indicates where translation ends.

Mutation

• Definition: A change in the sequence of bases in DNA, which can lead to changes in the structure and function of proteins.

• Types of Mutations:

  1. Base Substitution (Nucleotide Substitution):

  • Can alter the amino acid sequence of the resulting protein.

  • Example: Sickle-cell anemia, caused by a single base substitution.

  1. Redundancy in Genetic Code:

  • The genetic code is said to be redundant, meaning multiple codons can specify the same amino acid, allowing for some variations without effect.

  • 3rd Base Wobble: Refers to the flexibility of the third base in a codon, which can vary without changing the amino acid.

  1. Neutral Mutation:

  • A mutation that neither benefits nor harms the organism.

  1. Insertion or Deletion:

  • Involves adding or removing bases from the DNA sequence, potentially leading to frameshift mutations.

  • Frameshift Mutation: Alters the reading frame of the gene, affecting all subsequent codons.

• Mutagens: Agents that increase mutation rates, such as x-rays, radiation, UV light, and certain chemical substances.