what is gene expression?

Answer 1

You must be knowing that gene is a stretch of DNA with promoter etc and introns and exons.Let us not get into the changing definition of gene. Instead focus on the answer; what gene expression is. In shortest term gene expression means TRANSCRIPTION, that is, reading of one strand of the gene so as to get a copy of RNA. In case of messenger RNA, it will be translated into protein. In case of non-translatable RNAs,( transfer RNA and ribosomal RNA), they will go to cytoplasm to perform the function they are supposed to. Transfer RNA will carry the amino acid and ribosomal RNA will constitute the ribosome.Gene expression is also called gene activation.Opposed to this, is gene silencing or gene inactivation.
RNA polymerase is the enzyme which transcribes RNA from DNA. There are three types of RNA polymerase, each for each class of RNA.This enzyme sits on the promoter and begins the job of polymerization going downstream. Transcription requires the units of RNA and several other factors, some of them are proteins.

Answer 2

Gene expression, or simply expression, is the process by which a gene's DNA sequence is converted into the structures and functions of a cell. Non-protein coding genes (e.g. rRNA genes, tRNA genes) are not translated into protein.
Gene expression is a multi-step process that begins with transcription of DNA, which genes are made of, into messenger RNA. It is then followed by post transcriptional modification and translation into a gene product, followed by folding, post-translational modification and targeting. The amount of protein that a cell expresses depends on the tissue, the developmental stage of the organism and the metabolic or physiologic state of the cell.