Transcription is a fundamental process in molecular biology that involves copying genetic information from a specific segment of one DNA strand into an RNA molecule.
Let's delve into the details of transcription, including its principles, boundaries, and why both DNA strands are not copied:
Principle of Complementarity:
Complementarity governs the transcription process, just as it does in DNA replication. However, in transcription, adenine (A) in DNA forms base pairs with uracil (U) in RNA instead of thymine (T).
Selective RNA Synthesis:
1. Unlike DNA replication, which duplicates the entire genome, transcription only involves copying a specific segment of DNA, and only one of the DNA strands is transcribed. This segment corresponds to a particular gene.
2. Defining Transcription Boundaries: To demarcate the region and strand of DNA that will be transcribed, certain factors and signals, such as promoter regions, are involved. These signals help initiate and terminate transcription at the appropriate locations along the DNA strand.
Why Both Strands Aren't Copied:
1. Avoiding Ambiguity: If both DNA strands were transcribed, they would code for RNA molecules with different sequences. Consequently, the RNA products would code for proteins with different amino acid sequences, creating confusion and ambiguity in the genetic code.
2. Preventing Double-Stranded RNA Formation: When two RNA molecules with complementary sequences are produced simultaneously, they can form a double-stranded RNA. Double-stranded RNA molecules are not suitable for translation into proteins, rendering the transcription process futile.
Singular RNA Products:
Transcription ensures that only one segment of the DNA strand is transcribed into a specific RNA molecule. This RNA molecule, known as messenger RNA (mRNA), serves as a template for protein synthesis during translation.
In the context of DNA, a transcription unit is defined by three distinct regions, each serving a crucial role in the transcription process:
- The promoter is a region of DNA located towards the 5'-end (upstream) of the structural gene within the transcription unit.
- Its primary function is to provide a binding site for RNA polymerase, an enzyme responsible for initiating transcription.
- The presence of a promoter in a transcription unit also plays a key role in defining the template and coding strands of DNA.
- The structural gene is the central part of the transcription unit and contains the genetic information that will be transcribed into RNA.
- Within the structural gene, one of the two DNA strands acts as a template for RNA synthesis, while the other strand plays a different role.
- The terminator is positioned towards the 3'-end (downstream) of the coding strand within the transcription unit.
- Its main function is to define the endpoint of the transcription process, indicating where RNA synthesis should cease.
Template and Coding Strands:
- Conventionally, in the structural gene of a transcription unit, the strand with a 3'→5' polarity acts as the template strand.
- The template strand serves as the blueprint for RNA synthesis during transcription.
- The other strand, which has a 5'→3' polarity and a sequence identical to RNA (except for thymine in place of uracil), is displaced during transcription.
- This strand, despite not coding for any product, is referred to as the coding strand.
- It's important to note that all reference points for defining a transcription unit are made with respect to the coding strand.
Representation of Template and Coding Strands:
As an illustration, consider the following hypothetical DNA sequence from a transcription unit:
3' -ATGCATGCATGCATGCATGCATGC-5' (Template Strand)
5' -TACGTACGTACGTACGTACGTACG-3' (Coding Strand)
In this example, the template strand guides the synthesis of RNA during transcription, while the coding strand serves as the reference point for defining the transcription unit.
Types of RNA
In bacteria, the transcription process is essential for generating three major types of RNA molecules: mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA). Each of these RNA types plays a crucial role in protein synthesis within the cell:
1. mRNA (Messenger RNA): mRNA serves as the template for protein synthesis. It carries the genetic code from the DNA to the ribosomes, where proteins are synthesized.
2. tRNA (Transfer RNA): tRNA molecules bring amino acids to the ribosome during protein synthesis. Each tRNA molecule is specific to a particular amino acid and contains an anticodon that can base-pair with the codon on the mRNA, ensuring accurate protein assembly.
3. rRNA (Ribosomal RNA): rRNA is a structural component of ribosomes, which are the cellular machinery responsible for protein synthesis. rRNA also plays a catalytic role during translation, aiding in peptide bond formation.
Process of transcription
- In bacteria, a single DNA-dependent RNA polymerase catalyzes the transcription of all three types of RNA.
- Transcription initiates at a specific DNA region called the promoter, where RNA polymerase binds and starts RNA synthesis (Initiation).
- During transcription, RNA polymerase uses nucleoside triphosphates as substrates and polymerizes RNA in a template-dependent manner following the rule of complementarity.
- RNA polymerase also facilitates the separation of DNA strands to expose the template and continues elongation, synthesizing a complementary RNA strand.
- As RNA polymerase reaches the terminator region of DNA, the newly formed RNA transcript falls off, along with the RNA polymerase, leading to the termination of transcription.
- Factors Involved in Transcription in Bacteria:
- An intriguing aspect of bacterial transcription is how RNA polymerase performs initiation, elongation, and termination.
- RNA polymerase associates transiently with initiation factor σ (sigma) to initiate transcription and with termination factor ρ (rho) to terminate transcription.
- The association with these factors alters the specificity of RNA polymerase, allowing it to initiate or terminate transcription as needed.
- In bacteria, transcription and translation can be coupled because mRNA does not require extensive processing and because both processes occur in the same cellular compartment (cytoplasm).
- As a result, translation of mRNA can often begin before transcription is fully completed, allowing for efficient protein synthesis.
- Transcription in eukaryotes introduces two significant complexities that distinguish it from the process in bacteria. These complexities contribute to the regulation and processing of RNA before it becomes functional:
(i) Three RNA Polymerases:
Eukaryotes have three distinct RNA polymerases located in the nucleus, each with specific roles:
RNA Polymerase I is responsible for transcribing genes that code for ribosomal RNA (rRNA) components, including 28S, 18S, and 5.8S rRNA.
RNA Polymerase III transcribes genes related to small RNA molecules, such as transfer RNA (tRNA), 5S rRNA, and small nuclear RNAs (snRNAs).
RNA Polymerase II plays a central role in transcribing precursor heterogeneous nuclear RNA (hnRNA), which serves as the template for mRNA synthesis.
(ii) Splicing and Post-Transcriptional Processing:
Eukaryotic primary transcripts, known as hnRNA, possess both exons (coding regions) and introns (non-coding regions). These primary transcripts are non-functional.
To produce functional mRNA, a process called splicing is employed, which removes introns and joins exons in a specific order.
Additional processing steps include capping (adding a methyl guanosine triphosphate to the 5'-end) and tailing (adding adenylate residues, around 200-300, to the 3'-end). These modifications transform hnRNA into fully processed mRNA.
The mature mRNA is then transported out of the nucleus for translation.
- Significance of Eukaryotic Transcription Complexities:
Division of Labor: The presence of multiple RNA polymerases in eukaryotes allows for specialized roles in transcribing different RNA types, ensuring efficient and regulated gene expression.
Intron-Exon Structure: Introns within eukaryotic genes reflect an ancient feature of genome organization. Splicing of introns represents a dominance of RNA-based processes in early life forms.
RNA Processing: Post-transcriptional modifications like capping and tailing enhance RNA stability and functionality, contributing to the control and precision of gene expression.
- Understanding these complexities of eukaryotic transcription sheds light on the intricate mechanisms that govern gene expression, RNA processing, and the regulation of genetic information in complex organisms.