Bio 50: Developmental Genetics - Lesson 3

Developmental Genetics: Transcription and RNA

• The central dogma of molecular biology describes the flow of genetic information: DNA → RNA → Protein.
• Gene expression involves transcribing gene sequences into functional products like proteins or functional RNAs (e.g., rRNA, tRNA).
• There are two main phases of gene expression: transcription and translation.

Ribonucleic Acids (RNA)

• RNA differs from DNA in several key ways:
  • Contains ribose sugar, while DNA contains deoxyribose.
  • Uracil (U) replaces thymine (T) in RNA.
  • Typically single-stranded, while DNA is double-stranded.
  • RNA is shorter, ranging from 75 to thousands of nucleotides.

Types of RNA

• Heterogeneous nuclear RNA (hnRNA): Direct product of DNA transcription; processed into mRNA.
• Messenger RNA (mRNA): Carries genetic instructions for protein synthesis to ribosomes.
• Small nuclear RNA (snRNA): Aids in converting hnRNA to mRNA.
• Ribosomal RNA (rRNA): Combines with proteins to form ribosomes, the sites of protein synthesis; does not carry genetic information.
• Transfer RNA (tRNA): Delivers amino acids to ribosomes for protein assembly.

Messenger RNA (mRNA) Synthesis

• Transcription occurs in the nucleus where DNA instructs the synthesis of hnRNA/mRNA.
• Genes consist of specific DNA sequences encoding for hnRNA/mRNA.

Steps in the Transcription Process

• Initiation:

  • The DNA double helix unwinds at a promoter region, allowing RNA polymerase to bind.
  • The transcription bubble forms, facilitated by RNA polymerase, which catalyzes the synthesis of a linear RNA chain from ribonucleotides.

• Elongation:

  • RNA polymerase aligns ribonucleotides with the DNA template strand, forming base pairs.
  • In RNA, U pairs with A instead of T.

• Termination:

  • Transcription concludes when RNA polymerase encounters a stop signal.
  • The completed hnRNA is released, and DNA reforms its double helix.

Eukaryotic RNA Processing

• RNA processing converts the primary transcript into its mature form through several steps:

• Covalent Modification of RNA Ends:

  • 5’ cap: A modified guanine nucleotide is added to the RNA's 5' end.
  • Polyadenylation: A chain of adenine nucleotides is added to the 3' end by poly-A polymerase.

• Removal of Introns:

  • Genes contain exons (coding regions) and introns (non-coding regions); both are transcribed.
  • RNA splicing removes introns and joins exons to form the final mRNA.
  • Splicing is facilitated by a complex of snRNA and proteins, forming spliceosomes.

Alternative Splicing

• A single gene can produce multiple mRNA variants through alternative splicing, allowing for diverse protein products by rearranging exons.