Transcription (biology)

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Transcription (biology) is a fundamental molecular biology and genetics process where the DNA sequence of a gene is copied (transcribed) into RNA (ribonucleic acid). This process is essential for the synthesis of proteins, a central dogma in molecular biology which dictates the flow of genetic information from DNA to RNA to protein. Transcription occurs in the cell nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells.

Overview

During transcription, one of the two DNA strands, known as the template strand, serves as a template for the synthesis of a complementary RNA strand. This RNA strand is called messenger RNA (mRNA), which later undergoes translation to form proteins. Transcription is carried out by the enzyme RNA polymerase, which binds to a specific region of the DNA called the promoter and unwinds the DNA strands. As RNA polymerase moves along the DNA, it synthesizes a single strand of RNA by adding RNA nucleotides that are complementary to the DNA template strand.

Stages of Transcription

Transcription is divided into three main stages: initiation, elongation, and termination.

Initiation

Initiation is the first step of transcription where RNA polymerase binds to the promoter region of the gene. In eukaryotes, this process is assisted by various transcription factors that help RNA polymerase recognize and bind to the promoter.

Elongation

During elongation, RNA polymerase unwinds the DNA and synthesizes the mRNA strand by adding RNA nucleotides that are complementary to the DNA template. The RNA strand grows in a 5' to 3' direction, and the DNA that has been transcribed rewinds back into its double helix structure.

Termination

Termination occurs when RNA polymerase reaches a specific sequence in the DNA template called a terminator. In eukaryotes, the process is more complex and involves cleavage of the newly synthesized mRNA and addition of a poly(A) tail.

Types of RNA

Transcription results in the synthesis of different types of RNA, each serving a unique function in the cell. These include:

• Messenger RNA (mRNA): Carries the genetic information from DNA to the ribosome, where it is translated into protein.
• Ribosomal RNA (rRNA): Forms the core of the ribosome's structure and catalyzes protein synthesis.
• Transfer RNA (tRNA): Brings amino acids to the ribosome to be added to the growing polypeptide chain.
• MicroRNA (miRNA) and small interfering RNA (siRNA): Involved in RNA silencing and post-transcriptional regulation of gene expression.

Regulation of Transcription

The rate of transcription is tightly regulated by a variety of mechanisms. These include the binding of transcription factors to enhancer or silencer regions of DNA, modifications to the DNA structure, such as methylation, and alterations to the histone proteins around which DNA is wound.

Significance

Transcription is a critical process in the expression of genes and the synthesis of proteins, which perform most of the functions in a cell. Errors in transcription can lead to mutations, which may cause diseases such as cancer. Understanding transcription is also crucial for biotechnology and medicine, as it allows for the development of gene therapy, drugs, and genetically modified organisms.