The majority of proteins in 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. 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. Nuclear cells such as platelets do not possess nuclear DNA and therefore must have alternative sources for the RNA that they need to generate proteins. With the nuclear genome's 3.3 billion DNA base pairs in humans, one good example of a nuclear gene is MDH1 or the malate dehydrogenase 1 gene. In various metabolic pathways, including the citric acid cycle, MDH1 is a protein-coding gene that encodes an enzyme that catalyzes the NAD/NADH-dependent, reversible oxidation of malate to oxaloacetate. This gene codes for the cytosolic isozyme, which is involved in the malate-aspartate shuttle, which allows malate to cross past the mitochondrial membrane and be converted to oxaloacetate to perform further cellular functions. This gene among many exhibits its huge purposeful role in the entirety of an organism’s physiologic function. Although non-nuclear genes may exist in its functional nature, the role of nuclear genes in response and in coordination with non-nuclear genes is fundamental.