Transcription (biology)

Your body has a plan.

This plan is in your DNA. It tells your cells what to do.

Cells need to read the plan. A tiny worker reads the DNA. It makes a new copy. This copy is called RNA.

This new copy carries a message. The message helps make things like food for your cells. It tells the cell how to work.

Sometimes, the worker makes a mistake. It is not perfect at copying. But it still helps your body grow.

This is how your body follows its plan.

It is a very busy job!

Your body has a set of instructions called DNA.

To use these instructions, cells must make a copy. This way of making a copy is called transcription.

A tiny worker called RNA polymerase reads the DNA. It makes a new strand called RNA. Some RNA carries messages to help make proteins. We call these messenger RNA, or mRNA.

Transcription has a few main steps. It starts with initiation. This is when the worker finds the right spot to start. Then comes elongation. This is when the worker builds the new RNA strand. Finally, the process ends with termination.

Other parts help control this work. Some parts are called enhancers. They can make a gene work much more. They do this by looping the DNA. This brings them close to the starting spot. Other parts can turn genes off. This can happen through methylation. This is when a small group called a methyl group attaches to the DNA.

This can stop the worker from reading the instructions.

Your body uses DNA as a master set of instructions. To use these instructions, cells must make a copy called transcription.

This process turns a segment of DNA into a molecule called RNA. Some of these molecules are messenger RNA, or mRNA, which help build proteins. Other segments become non-coding RNAs. Both DNA and RNA are nucleic acids made of nucleotide sequences. Transcription is a vital way for living things to express their genes.

Transcription works through a few specific steps. It starts with initiation, when RNA polymerase finds a starting spot. Next is promoter escape, where the enzyme moves away from the start. Then comes elongation, the phase where the new strand is built. Finally, the process reaches termination and stops.

During this, the RNA polymerase reads one DNA strand. It builds a new RNA strand that matches the DNA. This new strand uses a different building block called uracil instead of thymine.

Many parts of the cell help control this work. Enhancers are special regions that can increase transcription by up to 100-fold. They do this by looping the DNA to reach the promoter.

This loop can span very long distances. A protein called Mediator helps send signals from enhancers to the RNA polymerase. There are about 1,600 different transcription factors in a human cell. These proteins bind to DNA to guide the whole process.

Cells also use a method called methylation to control genes. This involves adding a methyl group to a part of DNA called cytosine.

This often happens at sites called CpG islands. When these islands are methylated, they can silence a gene. About 60% of gene promoters have these CpG islands. Proteins like MeCP2 bind to these spots to stop transcription. This helps the cell decide which instructions to follow.

Transcription is different from DNA replication in many ways. For example, it does not need an RNA primer to start. It also does not create Okazaki fragments like DNA replication does. Transcription has some ways to check for mistakes. However, these are not as strong as the controls used for DNA. This means transcription has a lower copying fidelity.

Even with these differences, it is a key part of life.

Transcription is a fundamental biological process used to duplicate a DNA segment into an RNA molecule. This process is essential for gene expression, which allows cells to use their genetic instructions. Some transcribed segments become messenger RNA (mRNA), which encodes proteins. Other segments become non-coding RNAs (ncRNAs). Both DNA and RNA are nucleic acids made of nucleotide sequences.

In virology, transcription refers to making mRNA from a viral RNA molecule. Many RNA viruses use negative-sense RNA as a template to create positive-sense viral mRNA. This step is necessary for synthesizing the proteins required for viral replication. This specific process is catalyzed by a viral RNA-dependent RNA polymerase.

The mechanism of transcription follows a specific sequence of steps. It begins with initiation, where RNA polymerase and general transcription factors bind to a DNA promoter. This forms an RNA polymerase-promoter closed complex. In this state, the promoter DNA remains fully double-stranded. The enzyme then unwinds about 14 base pairs of DNA. The process moves through promoter escape, elongation, and finally termination.

During elongation, RNA polymerase reads the antisense strand of DNA from the 3' end to the 5' end. It builds the complementary RNA strand in the 5' to 3' direction. This directionality exists because RNA polymerase can only add nucleotides to the 3' end of the growing chain. This method avoids the need for RNA primers or Okazaki fragments used in DNA replication.

Transcription involves several distinct parts and regulatory sequences. A DNA transcription unit often contains a coding sequence and regulatory sequences. The regulatory sequence located upstream from the coding sequence is the 5' untranslated region (5'UTR). The sequence downstream is the 3' untranslated region (3'UTR). The DNA strand that is not used as a template is called the coding strand. Its sequence matches the new RNA transcript, except that RNA uses uracil (U) instead of thymine (T). Because transcription has fewer proofreading mechanisms than DNA replication, it has lower copying fidelity.

In mammals, transcription is highly regulated by various cis-regulatory elements. These include core promoters and promoter-proximal elements located near transcription start sites. Other elements, like enhancers, silencers, and insulators, are often located far from the start site. Enhancers play a leading role in initiating transcription. They can increase transcription levels by up to 100-fold.

Enhancers often control cell-type-specific programs by looping through long distances of DNA. This looping brings the enhancer into physical proximity with a target gene's promoter. These loops are often stabilized by connector proteins like CTCF or YY1.

Complexes of proteins help coordinate these distant interactions. A large structure called the Mediator complex consists of about 26 proteins. The Mediator communicates signals from transcription factors bound to enhancers directly to RNA polymerase II. There are approximately 1,600 different transcription factors in a human cell. These proteins bind to specific motifs on enhancers to govern transcription levels. Additionally, active enhancers produce two enhancer RNAs (eRNAs) as RNA polymerases act on both DNA strands.

Another layer of control is DNA methylation, which affects about 60% of promoters. This involves adding a methyl group to cytosine, creating 5-methylcytosine (5-mC). This occurs mostly at CpG sites, where a cytosine is followed by a guanine.

There are about 28 million CpG dinucleotides in the human genome. In most mammals, 70% to 80% of CpG cytosines are methylated. However, active promoters often contain unmethylated groups called CpG islands. If these islands are methylated, they can silence gene transcription through methyl-binding domain (MBD) proteins.

Specific proteins like EGR1 demonstrate how cells respond to signals. EGR1 is a transcription factor that can regulate the methylation of CpG islands. In neurons, the binding of EGR1 can recruit TET1 enzymes to methylated promoters. These TET enzymes catalyze the demethylation of 5-methylcytosine. This process allows neurons to initiate the transcription of hundreds of genes following activation. This complex system of loops, factors, and chemical marks ensures that genes are expressed at exactly the right time and place.