Molecular Biology Transcription Overview

Transcription Overview

• Transcription is the process of copying a segment of DNA into RNA.
• It's the initial stage in gene expression, using a gene's information to create a functional product like a protein.
• The RNA synthesized during transcription is messenger RNA (mRNA).
• This mRNA carries the genetic code from DNA in the nucleus to ribosomes in the cytoplasm for protein synthesis.

Key Players in Transcription

• DNA: The template molecule holding genetic information.
• RNA polymerase: The enzyme facilitating RNA synthesis from DNA.
• Promoter region: A specific DNA sequence initiating transcription.
• Terminator sequence: A specific DNA sequence ending transcription.
• Transcription factors: Proteins interacting with DNA, influencing RNA polymerase's transcription initiation.
• mRNA: The RNA copy carrying the protein synthesis code.

Stages of Transcription

• Initiation: RNA polymerase binds to the promoter region of the DNA, often aided by transcription factors. The DNA double helix unwinds, exposing the template strand.
• Elongation: RNA polymerase traverses the DNA template strand (3' to 5') building a complementary RNA molecule (5' to 3'). RNA nucleotides complement DNA template strand nucleotides.
• Termination: RNA polymerase encounters a termination sequence. The RNA molecule and RNA polymerase detach from the DNA. Further RNA processing follows.

Types of RNA and Their Roles

• mRNA (messenger RNA): Carries genetic code from DNA to ribosomes for protein synthesis.
• tRNA (transfer RNA): Transfers specific amino acids to the ribosome during protein synthesis.
• rRNA (ribosomal RNA): Forms part of ribosome structure, the protein synthesis site.

Important Concepts

• Template strand: The DNA strand used as a template for RNA synthesis.
• Coding strand: The DNA strand with the same sequence as the RNA transcript (except uracil replaces thymine).
• Promoters: Sequences dictating transcription start points. Different promoters have varying strengths, affecting initiation rates.
• Transcription factors: Crucial proteins binding DNA to regulate transcription. They act as activators or repressors, mediating responses to signals such as hormones or environmental changes.
• RNA processing: Primarily in eukaryotes, involving capping, splicing, and polyadenylation. These processes enhance the stability and efficiency of mRNA for protein translation.

Differences between Transcription in Prokaryotes and Eukaryotes

• Location: Prokaryotic transcription occurs in the cytoplasm; eukaryotic in the nucleus.
• Complexity: Eukaryotic transcription is more complex due to introns requiring removal and specialized transcription factors.
• RNA processing: Eukaryotic mRNA undergoes processing (capping, splicing, and polyadenylation); prokaryotic mRNA does not.

Regulation of Transcription

• Transcription factors: Bind specific DNA sequences, regulating RNA polymerase binding and transcription rate.
• Enhancers and silencers: DNA sequences enhancing or suppressing transcription, sometimes acting distantly from the gene.
• Chromatin structure: DNA packaging into chromatin affects transcription factor and RNA polymerase access to DNA.
• Epigenetic modifications: Heritable changes to DNA or chromatin, influencing gene expression (including transcription), without altering the DNA sequence.

Transcription Errors and Their Implications

• Mutations: Transcription errors introduce mutations into the mRNA sequence, potentially leading to non-functional or malfunctioning proteins. Errors in germline cells can cause inherited diseases.
• Processing errors (splicing, capping) also have significant phenotypic consequences.