RNA polymerase synthesizes RNA during the process of transcription in cells.
Key takeaways:
- RNA polymerase synthesizes RNA during transcription.
- The polymer synthesized during transcription is RNA.
- Key stages of transcription are initiation, elongation, and termination.
- RNA polymerase unwinds DNA and assembles an RNA strand.
- Eukaryotic pre-mRNA undergoes modifications like capping and splicing.
What You Will Learn
Transcription Overview
Transcription is a fundamental biological process, where DNA is used as a template to create a complementary RNA strand. This happens in the nucleus of the cell. The sequence of nucleotides in DNA is transcribed into RNA, which then carries the genetic information necessary for protein synthesis.
The polymer synthesized during this process is RNA (ribonucleic acid), which comes in various forms including mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA). The main points to understand about transcription are:
- Initiation: RNA polymerase binds to DNA at a specific region called the promoter. This signals the start of the gene to be transcribed.
- Elongation: The RNA polymerase travels along the DNA, unwinding the double helix and assembling the RNA strand by matching RNA nucleotides with their complementary DNA partners.
- Termination: Once the RNA polymerase reaches a stop signal in the DNA, transcription ends, and the newly formed RNA strand is released.
The resulting RNA strand is a single-stranded molecule that will undergo further processing before it fulfills its role in the cell.
RNA Polymerase
Acting as a crucial enzyme in the transcription process, RNA polymerase binds to DNA at specific regions known as promoters. Its role is to read the DNA template strand and assemble a corresponding RNA strand.
Here’s a breakdown of its function:
- Binding Affinity: RNA polymerase recognizes promoter sequences, signaling the start point for transcription.
- Unwinding DNA: The enzyme unwinds the DNA helix to expose the genes to be transcribed.
- RNA Strand Assembly: With use of one DNA strand as a template, RNA polymerase matches RNA nucleotides with their complementary DNA partners, forming an RNA strand.
- Moving Along DNA: It moves step-by-step along the DNA, elongating the RNA strand until it reaches a termination signal, indicating the end of the gene.
- Detaching: Once the full RNA sequence is synthesized, RNA polymerase releases the new RNA molecule.
- Repeating the Process: The enzyme can now begin a new transcription cycle.
RNA polymerase is essential for the synthesis of RNA, which includes messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA), each serving distinct roles in the cell’s protein synthesis machinery.
Stages of Transcription
Transcription unfolds in three main stages: initiation, elongation, and termination.
During initiation, RNA polymerase binds to a specific region on the DNA called the promoter. This signals the start of the gene that needs to be transcribed. Certain proteins called transcription factors are typically required to ensure that the RNA polymerase attaches to the right place.
Once bound, the RNA polymerase unwinds a small portion of the DNA double helix to expose the template strand. The polymerase then begins synthesizing a strand of RNA by adding complementary RNA nucleotides to the DNA template.
Elongation is the stage where the RNA strand gets longer. As RNA polymerase moves along the DNA, it continues unwinding the DNA and adding more nucleotides to extend the RNA strand, elongating the chain in the 5’ to 3’ direction.
Termination is the final stage where the synthesis of RNA concludes. The polymerase keeps extending the RNA strand until it encounters a specific sequence on the DNA called a terminator. Upon reaching this sequence, the RNA polymerase releases both the DNA template and the newly created RNA strand, which results in the cessation of the transcription.
The freshly synthesized RNA, often called primary transcript or pre-mRNA in eukaryotes, may undergo further processing before it is ready to be used by the cell.
RNA Capping and Splicing: Key RNA Modifications in Eukaryotes
After the initial stages of transcription, eukaryotic pre-mRNA undergoes essential modifications that are crucial for its stability and function before it can be translated into a protein. RNA capping is the first modification, where a special ‘cap’ is added to the beginning of the RNA strand. This cap is a modified guanine nucleotide and it serves multiple purposes: it protects the RNA from degradation, helps in the export of the mRNA from the nucleus, and is recognized by the protein synthesis machinery.
Once capping is done, splicing takes place. During splicing, non-coding regions called introns are removed from the pre-mRNA. This process is facilitated by a molecular complex known as the spliceosome. The remaining coding segments, or exons, are then joined together to form the mature mRNA. This mature mRNA now contains a continuous coding region that translates into a functional protein.
Both of these modifications are critical for creating a stable and functional mRNA that can then travel out of the nucleus into the cytoplasm, where protein synthesis occurs.
Eukaryotic MRNA Maturation and Export From the Nucleus
After initial synthesis, eukaryotic mRNA undergoes several processing steps before it is ready to be translated into a protein. Here are the key points of mRNA maturation:
- 5′ capping: Soon after transcription begins, a unique cap is added to the 5′ end of the nascent RNA. This cap is a modified guanine nucleotide and it protects the RNA from degradation, while also assisting in ribosome binding during translation.
- Polyadenylation: At the 3′ end, an enzyme adds a long chain of adenine nucleotides, known as a poly-A tail. This modification stabilizes the mRNA and facilitates its export from the nucleus to the cytoplasm.
- Splicing: Introns, or non-coding regions, are removed from the mRNA transcript through a process called splicing. The remaining exons, which are coding sequences, are then joined together to form a continuous sequence that will dictate protein synthesis.
- Exit to the cytoplasm: Mature mRNA is then exported out of the nucleus through the nuclear pore complexes. The export is tightly regulated and involves transport proteins that recognize signals on the mature mRNA.
- Quality control: Before exporting, mRNAs are subject to a quality control mechanism. Only properly processed and fully mature mRNAs are allowed to pass through the nuclear pores.
Once in the cytoplasm, the mature mRNA is available to ribosomes for translation, ultimately resulting in the production of a new protein.
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