Gene expression refers to the process through which genetic instructions are used for purposes of synthesizing gene products. The purpose of genes is information storage as such, each gene contains information that is needed to make a protein or in other cases, RNA non-coding. Genes are expressed in order to produce functional protein molecules and RNA in the cells.
Gene expression can occur in two stages which include transcription and protein synthesis:
In this case, the gene is copied so it can produce RNA molecule (primary transcript) with basically similar sequence as the gene. In most cases, human genes can be divided into introns and exons. Only exons carry information needed for protein synthesis. The most primary transcripts are consequently processed through splicing in order to remove intron sequence and as such, generate mature messenger RNA (mRNA) or transcript that is comprised of exons only.
This is the second stage and it is also known as translation for the simple reason it does not have any direct correspondence between the sequence of amino acids in the protein and nucleotide sequence in DNA (RNA). As a matter of fact, there are three nucleotides which are needed in order to specify one amino acid. The amino acid chain must also fold up in order to generate final tertiary structure of protein.
All genes in human genome are not expressed in the same manner. There are those that expressed in all cells at all times and they are known as housekeeping genes. They are essential for the purposes of carrying out basic cellular functions.
There are other genes that are expressed in specific cell types or at particular development stages. For instance, genes that encode protein muscle like myosin and actin are expressed in muscle cells only not in brain cells. Still, there are other genes that are inhibited or activated by signals circulation in the body like hormones.
Differential gene expression is achieved through regulation translation and transcription. All genes are also surrounded by DNA sequences that control expression. Proteins known as transcription factors bind the sequences and can switch genes off and on. As such, gene expression is controlled by activity and availability of different kinds of transcription factors.
Note transcription factors are in themselves, proteins as such, they must be produced by genes as well and the genes should be regulated through other transcription factors. In such a manner, all proteins and genes can be linked into regulatory hierarchy starting with transcription factors in the egg at the start of development. There are some diseases that are caused by malfunction or absence of transcription factors as such, they cause gene expression interruption.
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