A nuclear gene is a gene located in the cell nucleus of a eukaryote. The term is used to distinguish nuclear genes from the genes in the mitochondrion, and in case of plants, also the chloroplast, which host their own genetic system and can produce proteins from scratch. The term "gene" most often refers to nuclear genes.
The distinct genomes of eukaryotes are thought to have arisen through symbiotic relationships. The mitochondrion represents a eubacterium that integrated into its archaeal host cell's physiology to such an extent that it became an integral cell component, or organelle. The process repeated in plants, with a cyanobacterial symbiont in a eukaryote host becoming the chloroplast. The nuclear genes represent the genome of the original host cell, while both of these organelles still retain a small genome, although many of the genes of the organelles have moved to the nucleus during the course of evolution.
The majority of the proteins of a cell are the product of messenger RNA transcribed from nuclear genes, including most of the proteins of the organelles, which are produced in the cytoplasm like all nuclear gene products and then transported to the organelle. In addition, there are nuclear genes that encode non-translated regulatory RNAs. Mutations can occur when translating the messenger RNA into amino acid. The gene’s frame is read three bases at a time called codon, there is only one starting codon and three stopping codons, they are key codons for the determining of the reading frame of the genetic information. Each codon corresponds to an amino acid used in the synthesis of proteins. The mutation occurs when there is a deletion or insertion of base into the reading frame of the gene. Mutation can result in different amino acid which codes different protein. Genes in the nucleus are arranged in a linear fashion upon chromosomes, which serve as the scaffold for replication and the regulation of gene expression. As such, they are usually under strict copy-number control, and replicate a single time per cell cycle.
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