Transcription, also known as RNA synthesis, is the process by which a segment of DNA is copied into RNA by the enzyme RNA polymerase.
This is the first step in the expression of genes, leading to the synthesis of proteins. Here's an overview of the transcription process:
Steps of Transcription or RNA Synthesis
1) Initiation
Promoter Region: Transcription begins at a specific region called the promoter, which is a sequence of DNA that signals the start of a gene.
RNA Polymerase Binding: RNA polymerase binds to the promoter region with the help of transcription factors.
DNA Unwinding: RNA polymerase unwinds a small section of the DNA double helix to expose the template strand.
2) Elongation
RNA Synthesis: RNA polymerase moves along the DNA template strand, adding complementary RNA nucleotides (A, U, C, G) to the growing RNA strand.
Direction: RNA synthesis occurs in the 5' to 3' direction, meaning new nucleotides are added to the 3' end of the RNA molecule.
Temporary DNA-RNA Hybrid: A temporary hybrid structure forms where the RNA strand is base-paired with the DNA template.
3) Termination
Termination Signals: Transcription continues until RNA polymerase encounters a termination sequence in the DNA.
Release of RNA: The newly synthesized RNA strand, also called the primary transcript or pre-mRNA (in eukaryotes), is released from the DNA template.
DNA Rewinds: The DNA double helix re-forms as RNA polymerase and the new RNA strand dissociate.
Post-Transcriptional Modifications (in Eukaryotes)
1) Capping
5' Cap Addition: A modified guanine nucleotide is added to the 5' end of the pre-mRNA, protecting it from degradation and aiding in ribosome binding during translation.
2) Polyadenylation
Poly-A Tail Addition: A sequence of adenine nucleotides is added to the 3' end of the pre-mRNA, further protecting it from degradation and assisting in export from the nucleus.
3) Splicing
Intron Removal: Non-coding sequences (introns) are removed from the pre-mRNA.
Exon Joining: The remaining coding sequences (exons) are joined together to form the mature mRNA.
Transcription Regulation
Transcription Factors: Proteins that bind to specific DNA sequences, regulating the binding of RNA polymerase to the promoter.
Enhancers and Silencers: DNA sequences that can increase or decrease the rate of transcription when bound by specific proteins.
Disorders Associated with Transcription
Genetic Disorders: Mutations in promoter regions, transcription factors, or RNA polymerase can lead to various genetic disorders by affecting gene expression.
Cancer: Dysregulation of transcription factors can lead to uncontrolled cell growth and cancer.
