Part-5.3 translation and trancription

Overview of Transcription and Translation

• Central dogma of biology explains the flow of genetic information: DNA replication → Transcription → Translation

• Goal: Produce proteins that perform various jobs in the body (e.g., insulin, pigments)

Transcription Process

• Definition: Transcription is the conversion of a specific gene from DNA into messenger RNA (mRNA).

• RNA Polymerase: The enzyme primarily responsible for transcription; synthesizes RNA from the DNA template.

• Strands of DNA:

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

  • Coding Strand: strand that has the same sequence as the mRNA (except thymine in DNA is replaced by uracil in RNA).

• Directionality: DNA is read in a 5' to 3' direction.

• Process:

  • DNA strands are denatured (separated).

  • RNA polymerase attaches to the template strand and synthesizes mRNA complementary to it.

  • Example: if the template strand is 5'-AUG-3', the RNA will be 5'-AUG-3' after transcription.

Translation Process

• Definition: Translation is the process of synthesizing proteins from the mRNA transcript.

• Ribosome Function:

  • Ribosomes read the mRNA sequence to produce a polypeptide chain of amino acids.

  • mRNA is fed into the ribosome, where codons are read in sets of three nucleotides.

• Transfer RNA (tRNA):

  • tRNA molecules have anticodons that are complementary to mRNA codons.

  • Each tRNA delivers a specific amino acid corresponding to the codon.

• Polypeptide Formation:

  • Amino acids link together via peptide bonds to form a growing polypeptide chain.

  • The process continues until a stop codon is reached, signaling the end of protein synthesis.

Codons and Amino Acids

• Codon: A sequence of three nucleotides that correspond to specific amino acids.

• Start Codon: AUG, which codes for methionine and signifies the start of translation.

• Stop Codons: UAA, UAG, UGA (do not code for any amino acids; terminate translation).

• Degeneracy of the Code:

  • 64 codons exist, but only 20 amino acids, meaning most amino acids have multiple codons coding for them.

  • Example: Proline is indicated by CCU, CCC, CCA, CCG.

Practice Exercise

• If given a DNA sequence, transcribe to mRNA by replacing T with U.

• Translate by using the codon table, starting at the AUG codon, to find corresponding amino acids.

• Stop upon reaching a stop codon, completing the amino acid sequence.