9.3 Transcription

Genes are like recipes for proteins, and transcription is the first step in following those recipes. It's how cells copy DNA instructions into RNA messages that can be read by protein-making machines.

Transcription involves three main steps: initiation, elongation, and termination. Along the way, the RNA message gets some special modifications to help it do its job better. Understanding this process is key to grasping how genetic information flows in cells.

Transcription and Gene Expression

Flow of genetic information

• Central dogma of molecular biology describes the flow of genetic information from DNA to RNA to protein
  • DNA serves as the genetic blueprint and is transcribed into RNA (messenger RNA or mRNA)
  • mRNA carries the genetic information from the nucleus to the cytoplasm where it is translated into protein by ribosomes
• Transcription is the process of synthesizing RNA from a DNA template
  • Occurs in the nucleus of eukaryotic cells (plants, animals)
  • Produces mRNA which serves as the intermediate between DNA and protein
• Translation is the process of synthesizing proteins using the genetic code carried by mRNA
  • Occurs in the cytoplasm of eukaryotic cells
  • Ribosomes read the genetic code on mRNA and assemble amino acids into proteins (enzymes, structural proteins)

Steps of transcription process

1. Initiation
   • RNA polymerase enzyme binds to the promoter region of the DNA
     • Promoter region contains specific DNA sequences that signal the start of transcription (TATA box)
     • Transcription factors (proteins) help recruit RNA polymerase to the promoter
   • Sigma factor helps RNA polymerase recognize and bind to the promoter region
2. Elongation
   • RNA polymerase unwinds the DNA double helix, exposing the template strand
   • RNA polymerase reads the template strand of DNA in the 3' to 5' direction
   • RNA nucleotides (A, U, G, C) are added to the growing RNA strand in the 5' to 3' direction
     • RNA strand is complementary to the DNA template strand (A pairs with U, T pairs with A, C pairs with G)
   • A transcription bubble forms as the DNA temporarily unwinds, allowing RNA synthesis
3. Termination
   • RNA polymerase reaches a termination sequence on the DNA (poly-A signal)
   • Newly synthesized RNA strand (pre-mRNA) is released
   • RNA polymerase dissociates from the DNA
   • In some cases, rho factor assists in terminating transcription

DNA strands in transcription

• Template strand (antisense strand) is the DNA strand that is read by RNA polymerase
• Coding strand (sense strand) is complementary to the template strand and has the same sequence as the RNA transcript (except for T instead of U)

Eukaryotic mRNA modifications

• Post-transcriptional modifications occur after transcription but before translation
  • Necessary for the proper function and stability of mRNA
• 5' Capping adds a 7-methylguanosine cap to the 5' end of the pre-mRNA
  • Protects mRNA from degradation by exonucleases (enzymes that break down RNA)
  • Facilitates mRNA export from the nucleus to the cytoplasm through nuclear pore complexes
  • Assists in the initiation of translation by helping ribosomes recognize the start codon (AUG)
• Polyadenylation adds a poly(A) tail (multiple adenine nucleotides) to the 3' end of the pre-mRNA
  • Protects mRNA from degradation by exonucleases
  • Enhances mRNA stability and translation efficiency by facilitating mRNA export and ribosome binding
• Splicing removes non-coding sequences (introns) and joins coding sequences (exons) to form mature mRNA
  • Carried out by the spliceosome, a complex of proteins and small nuclear RNAs (snRNAs)
  • Alternative splicing allows for the production of multiple protein isoforms from a single gene (CD44, tropomyosin)