Transcription

Overview

•  Process
  • initiation
    • RNAp binds the template DNA strand at the promoter site with the help of transcription factors
  • elongation
    • RNAp synthesizes the mRNA strand in 5′ → 3′ direction
      • reads template strand in 3′ → 5′ direction
      • eukaryotic RNAps transcribe introns as well as exons
    • RNAp inhibited by alpha-amanitin (ingestion of the amanita phalloides mushroom) 
  • termination
    • prokaryotes
      • two mechanisms
        • rho-dependent
          • rho protein binds the mRNA
          • causes RNAp to fall off
        • rho-independent
          • mRNA has a GC-rich segment that naturally forms a hairpin
          • causes RNAp to fall off
    • eukaryotes
      • transcription continues past the gene and eventually falls off
• Mono/polycistronic
  • monocistronic = mRNA codes for 1 protein
    • eukaryotes only have monocistronic mRNA
  • polycistronic = mRNA codes for >1 protein 
• e.g. bacterial lac operon  

Introduction

Transcription is the process by which the genetic information encoded in DNA is used to synthesize RNA molecules. It is a key step in the central dogma of molecular biology, which describes the flow of genetic information from DNA to RNA to protein.

During transcription, a region of DNA is transcribed into a complementary RNA molecule by an enzyme called RNA polymerase. The RNA molecule is synthesized in the 5′ to 3′ direction and is complementary to the template strand of the DNA molecule.

Types

1. mRNA: Messenger RNA (mRNA) is the process by which the genetic information encoded in DNA is used to synthesize mRNA molecules. This type of transcription occurs in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells. mRNA molecules carry the genetic code from the DNA in the nucleus to the ribosomes in the cytoplasm, where they are translated into proteins.
2. rRNA: Ribosomal RNA (rRNA) is the process by which rRNA molecules are synthesized. These molecules are a key component of ribosomes, which are responsible for protein synthesis. rRNA transcription occurs in the nucleolus of eukaryotic cells and in the cytoplasm of prokaryotic cells.
3. tRNA: Transfer RNA (tRNA) transcription is the process by which tRNA molecules are synthesized. These molecules play a crucial role in protein synthesis by carrying amino acids to the ribosome, where they are incorporated into a growing polypeptide chain. tRNA transcription occurs in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells.

Function

The function is to convert the genetic information stored in DNA into RNA molecules that can be used to direct the synthesis of proteins or other functional RNA molecules. This process is essential for gene expression and cellular function.

1. Protein synthesis: It is the first step in the process of protein synthesis. The RNA molecules produced during transcription, such as mRNA, carry the genetic information from the DNA to the ribosomes, where it is used as a template for the synthesis of proteins.
2. RNA processing: The RNA molecules produced during transcription are often modified and processed before they are functional. For example, introns (non-coding regions) may be removed from pre-mRNA molecules by splicing, and the 5′ and 3′ ends of the RNA molecule may be modified. These modifications are critical for the stability, transport, and function of the RNA molecule.
3. Gene regulation: It plays a key role in the regulation of gene expression. By controlling the rate of transcription, cells can regulate the production of specific proteins or other functional RNA molecules. This regulation is critical for maintaining cellular homeostasis and responding to changes in the environment.
4. RNA-based regulation: In addition to encoding proteins, some RNA molecules, such as miRNA and siRNA, function as regulators of gene expression. These RNA molecules can bind to specific mRNAs and either inhibit or enhance their translation into protein.

Complications

1. Errors in transcription: During transcription, errors can occur, leading to mutations in the RNA molecule. These mutations can affect protein synthesis, and in some cases, they can lead to diseases such as cancer.
2. RNA processing defects: Processing defects can occur during RNA splicing, capping, and tailing, leading to unstable or non-functional RNA molecules. This can affect protein synthesis and lead to diseases such as spinal muscular atrophy.
3. Transcriptional interference: Transcriptional interference occurs when two genes are located in close proximity to each other, and the transcription of one gene interferes with the transcription of the other. This can lead to the downregulation or silencing of one or both genes.
4. Epigenetic modifications: Epigenetic modifications, such as DNA methylation and histone modifications, can affect the accessibility of DNA to transcription factors and RNA polymerase, leading to altered gene expression patterns.

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