Introduction to Molecular Biology: Transcription and Translation

Introduction to Molecular Biology

  • Understanding of how DNA and protein structure correlate with organism traits.

    • DNA dictates protein synthesis, affecting both structure and function.

    • Proteins serve as a bridge between genotype (genetic makeup) and phenotype (observable traits).

Gene Expression

  • Definition: Gene expression is the process of creating a protein from a gene.

    • Involves two main stages: Transcription and Translation.

    • Transcription: Synthesis of RNA from DNA.

    • Translation: Synthesis of proteins from RNA.

  • Importance of terminology: Certain biological terms have definitions in everyday language that overlap, ensuring understanding without redefinition.

The Central Dogma of Molecular Biology

  • Key Process Flow: DNA → RNA → Protein

    • DNA is transcribed to RNA, which is then translated to produce proteins.

    • RNA acts as the intermediary connecting DNA and protein.

Transcription Process

Definition of Transcription

  • Transcription refers to the copying of genetic information from DNA to RNA.

    • Analogous to language transcription where information is conveyed from one form (spoken) to another (written).

  • In biological terms, it specifically means transcribing DNA (nucleic acid) into mRNA (another nucleic acid).

Types of RNA

  • Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes for protein synthesis.

Detailed Steps of Transcription

  1. Initiation

    • The process begins when the DNA moves apart at the promoter region, which contains specific sequences indicating where transcription should start.

    • RNA polymerase binds to the promoter with help from transcription factors.

    • Example of a common promoter region: TATA box (sequence: TATAAA) that recruits transcription factors.

  2. Elongation

    • RNA polymerase adds RNA nucleotides complementary to the DNA template strand, synthesizing mRNA.

    • RNA polymerase synthesizes mRNA in the 5’ to 3’ direction at a rate of about 40 nucleotides per second.

    • Key point: RNA does not use thymine (T); it uses uracil (U) instead.

  3. Termination

    • Transcription terminates when RNA polymerase reaches a terminator sequence, resulting in the release of the mRNA transcript.

    • The newly formed RNA strand (pre-mRNA in eukaryotes) still requires processing.

mRNA Processing (Eukaryotes Only)

  • Pre-mRNA undergoes modifications before leaving the nucleus:

    • 5’ cap: A special structure added for stability and to facilitate ribosome binding.

    • Poly-A tail: A long sequence of adenine nucleotides added to the 3’ end for export and stability.

    • Splicing: Removal of introns (non-coding sequences) and joining of exons (coding sequences) by spliceosomes.

    • Exons represent expressed sequences, while introns are removed from the final mRNA transcript.

  • Result: A mature mRNA that is ready for translation.

Transfer of Information to Proteins: The Translation Process

Definition of Translation

  • Translation is the process by which ribosomes read mRNA sequences and translate them into amino acid sequences to form proteins.

    • Requires tRNA: Transfer RNA brings specific amino acids to the ribosome based on the mRNA sequence.

Ribosome Structure and Function

  • Ribosomes consist of rRNA and proteins and are located in the cytoplasm or attached to the rough endoplasmic reticulum (ER).

  • Structure of Ribosomes:

    • Contains two subunits (large and small).

    • Three functional sites on the ribosome during translation: A (aminoacyl), P (peptidyl), E (exit);

    • A site: Where new tRNA carrying amino acids enters.

    • P site: Holds the growing polypeptide chain.

    • E site: Where empty tRNAs exit the ribosome.