Gene expression

Your body uses tiny plans to work.

These plans are called genes. First, the cell makes a copy of a plan. This copy helps the cell make food or parts.

Next, the cell uses that copy to build a protein. Proteins are like tiny tools that do many jobs. Some plans work all the time. Other plans only work when they are needed.

This helps your body change when it needs to. It is how your body stays healthy.

Your body uses genes to get things done.

This is called gene expression. It is the way a cell uses a gene to make a product. This product can be a protein or a special RNA molecule.

First, the cell must copy the gene. This step is called transcription. A tool called RNA polymerase makes the copy. It reads the DNA and builds a new strand of RNA.

In some cells, this RNA must change before it is ready. This is called RNA processing. The cell cuts out some parts and joins others together. This is called splicing.

Next, the cell uses the RNA to make a protein. This step is called translation.

A tiny machine called a ribosome reads the RNA. It links small pieces called amino acids into a long chain. This chain folds into a protein.

Cells do not use all genes at once. They can turn genes on or off. Some genes work all the time to keep the cell running. These are called housekeeping genes. Other genes only work when the cell needs them.

This helps a cell adapt to its world.

Every living thing uses genes to carry out important jobs. This whole way of working is called gene expression. It is how a cell uses the information in a gene to make a product. This product might be a protein or a functional RNA molecule.

These products allow cells to do many different biological functions. Without gene expression, the instructions in our DNA could not be used.

The first major step is called transcription. An enzyme called RNA polymerase reads a DNA template to build a new RNA strand.

In bacteria, one type of RNA polymerase does this work. In eukaryotes, there are three different types of RNA polymerase. RNA polymerase I makes ribosomal RNA, while RNA polymerase II makes protein-coding genes. RNA polymerase III makes other types like transfer RNA. This step ends when the enzyme hits a sequence called a terminator.

In eukaryotes, the new RNA often needs extra work called processing. The first part is called 5' capping, which adds a special cap to protect the RNA. Another step is adding a poly(A) tail to the end of the strand.

A very important part of this is RNA splicing. A large complex called a spliceosome removes pieces called introns. It then joins the useful pieces, called exons, back together. This can even create different proteins from just one single gene.

The next step is called translation. This is when the cell uses the RNA to build a protein.

A machine called a ribosome reads the RNA in groups of three called codons. Each codon tells the cell which amino acid to add next. Transfer RNA molecules bring the correct amino acids to the ribosome. These amino acids are chained together to form a protein. In mammals, one mRNA can make about 2,800 protein molecules.

Cells are very smart about when they use their genes. This is called regulation. Some genes are constitutive, meaning they work all the time. Others are facultative, meaning they only work when they are needed.

There are also housekeeping genes that maintain basic cell functions. Examples of these include proteins like actin or ubiquitin. Regulation helps cells adapt to changes in their environment. It even helps create different patterns, like the colors on a tortoiseshell cat.

Gene expression is the fundamental process used by cells to turn genetic information into functional products. These products are typically proteins or functional RNA molecules. This process allows a cell to carry out a vast range of biological functions. Without gene expression, the instructions stored in DNA could not be used to build or operate a living thing. Cells must carefully control this process to adapt to environmental changes or internal needs.

The first major stage is transcription, which creates an RNA copy from a DNA template. This task is performed by an enzyme called RNA polymerase (RNAP). The enzyme moves along the DNA, adding one ribonucleotide at a time to a growing RNA strand. It follows the complementarity law of nucleotide bases. In this process, the RNA is complementary to the DNA template, though uracils (U) replace thymines (T). In bacteria, a single type of RNA polymerase performs this task. It requires a sigma factor protein to bind to a specific DNA sequence called a Pribnow box.

In eukaryotes, transcription is more complex and occurs within the nucleus. There are three distinct types of RNA polymerase used for different tasks. RNA polymerase I transcribes ribosomal RNA (rRNA) genes. RNA polymerase II transcribes all protein-coding genes and some non-coding RNAs. RNA polymerase III transcribes transfer RNA (tRNA) genes and 5S rRNA. Each of these polymerases requires a promoter sequence and transcription factors to begin. The process concludes when the polymerase reaches a sequence called a terminator.

Eukaryotic cells often require mRNA processing to turn a primary transcript, or pre-RNA, into mature RNA. One step is 5' capping, which adds 7-methylguanosine (m7G) to the start of the strand. This cap protects the RNA from being broken down by exonucleases. Another step is 3' cleavage and polyadenylation. If a specific signal sequence is present, a series of about 200 adenines is added to the end. This creates a poly(A) tail that protects the molecule and helps it move to the cytoplasm.

A critical part of eukaryotic processing is RNA splicing. Most eukaryotic pre-mRNAs contain alternating segments called exons and introns. A large complex called a spliceosome removes the introns and joins the exons together. Sometimes, the cell uses alternative splicing to create different transcripts from a single gene. This process increases the complexity of the species' proteome. In prokaryotes, transcription and translation happen almost at the same time. In eukaryotes, the nuclear membrane separates these steps, allowing more time for processing.

The final stage is translation, where the RNA information is used to build a protein. The messenger RNA (mRNA) carries the code to a ribosome. The ribosome reads the mRNA in groups of three nucleotides called codons. Each codon corresponds to a specific binding site on a transfer RNA (tRNA). These tRNA molecules carry specific amino acids to the ribosome. The ribosome then chains these amino acids together into a protein.

In mammals, a single mRNA molecule can produce an average of 2,800 protein molecules.

Cells regulate gene expression to control the amount and timing of product appearance. Some genes are constitutive, meaning they are transcribed continually. Others are facultative, meaning they are only transcribed when needed. Housekeeping genes, such as actin or ubiquitin, are required for basic cellular functions and are expressed in all cell types. Inducible genes respond to environmental changes or the cell cycle. This regulation is essential for cellular differentiation and organism adaptability.

Gene regulation can produce striking physical results. For example, the patchy colors of a tortoiseshell cat result from different levels of pigmentation gene expression in the skin. Regulation also helps control vital biological signals. In mammals, it controls insulin expression for blood glucose regulation. It also manages X chromosome inactivation in females to prevent an overdose of genes. These systems ensure that every cell performs its specific role within the larger organism.