Genes are transcribed into RNA by the process of transcription. Some RNAmolecules are functional in and of themselves. Examples include the RNA components of certain enzymes (e.g., telomerase); transfer RNA (tRNA), which delivers amino acids to the ribosome for protein synthesis; and ribosomal RNA (rRNA), a structural component of the ribosome. Other RNA molecules are intermediaries in protein synthesis. Such RNAs are processed in the nucleus into messenger RNA (mRNA) molecules. mRNAs are then exported to the
cytoplasm, where they bind ribosomes and direct synthesis of the encoded protein by the process of translation.
Genes are expressed when their ultimate products (RNAs or proteins) are produced. Sometimes, genes encoding proteins are considered expressed when they are simply transcribed, but it should be remembered that the proteins, not the transcripts, are the functional products.
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as rRNA genes or tRNA genes, the product is a functional RNA. The process of gene expression is used by all known life - eukaryotes (including multicellular organisms), prokaryotes (bacteria and archaea) and viruses - to generate the macromolecular machinery for life. In genetics gene expression is the most fundamental level at which genotype gives rise to the phenotype. The genetic code is interpreted" by gene expression, and the properties of the expression products give rise to the organism's phenotype. The gene itself is typically a long stretch of DNA and does not perform an active role. It is a blueprint for the production of RNA.
The production of RNA copies of the DNA is called transcription, and is performed by RNA polymerase, which adds one RNA nucleotide at a time to a growing RNA strand. This RNA is complementary to the DNA nucleotide being transcribed; i.e. a T on the DNA means an A is added to the RNA. However, in RNA the nitrogen-containing base Uracil is inserted instead of Thymine wherever there is an Adenine on the DNA strand. Therefore, the mRNA complement of a DNA strand reading "TAC" would be transcribed as "AUG".
Transcription of protein encoding genes creates a primary transcript of RNA at the place where the gene was located. This transcript can be altered before being translated, this is particularly common in eukaryotes. The most common RNA processing is splicing to remove introns. Introns are RNA segments which are not found in the mature RNA, although they can function as precursors, e.g. for snoRNAs, which are RNAs that direct modification of nucleotides in other RNAs. Introns are common in eukaryotic genes but rare in prokaryotes.
RNA processing, also known as post-transcriptional modification, can start during transcription, as is the case for splicing, where the spliceosome removes introns from newly formed RNA.
In most organisms non-coding genes (ncRNA) are transcribed as precursors which undergo further processing. In the case of ribosomal RNAs (rRNA), they are often transcribed as a pre-rRNA which contains one or more rRNAs, the pre-rRNA is cleaved and modified (2′-O-methylation and pseudouridine formation) at a specific sites by approximately 150 different small nucleolus-restricted RNA species, called small nucleolar RNAs(snoRNAs), which like snRNAs, snoRNAs associate with proteins, forming snoRNPs. In eukaryotes, in particular a snoRNP, called RNase MRP cleaves the 45S pre-rRNA into the 28S, 5.8S, and 18S rRNAs. The rRNA and RNA processing
factors are form large aggregates called the nucleolus. In the case of transfer RNA (tRNA), for example, the 5' sequence is removed
by RNase P, whereas the 3' end is removed by the tRNase Z enzyme.. In the case of micro RNA (miRNA), miRNAs are first transcribed as primary transcripts or pri-miRNA with a cap and poly-A tail and processed to short, 70-nucleotide stem-loop structures known as pre-miRNA in the cell nucleus.