As mentioned, all of the DNA necessary for the organism is contained within a single cell but not all genes are expressed. If the human DNA were arranged in a single piece and stretched out, it would be about 2 meters long per cell. This results in creative DNA packaging within the cell. The long pieces are separated into chromosomes and are wound, looped, coiled, and folded so that they fit within the nucleus. Histone proteins facilitate this folding and coiling so that DNA can be condensed into what’s called chromatin. Chromatin is DNA plus histone proteins. DNA can also be supercoiled, which is the case in prokaryotes, which do not have histone proteins. Supercoiling uses other proteins to tightly compact the DNA in what is usually a single circular chromosome. Eukaryotes have linear chromosomes in most cases, with different organisms having different numbers of chromosomes. The appearance of DNA when not actively in mitosis or meiosis is that of a bunch of string in a pile.
CHROMOSOMES AND GENES Chromosomes themselves are the totality of the DNA message, which involves thousands of different genes. A gene is considered any sequence of DNA (or RNA in some organisms) that codes for a protein that is functional in the cell. In most organisms, the DNA is first copied into RNA, which acts as an intermediary template molecule coding for a protein. The totality of the genes in an organism’s chromosomes is called a genotype, which interacts with both developmental and environmental factors in order to determine what the phenotype of the organism will be. We’ve talked about genes as though they have the power to do major things to the phenotype of the organism when in fact it is usually a combination of many different genes that impact the phenotype. These genes interact with the environment in order to determine the actual phenotype seen. The different variables in a specific gene cause the different alleles seen in a given population. So, when speaking of having a specific “gene” it basically means that the term refers to a specific allele of the gene and not the gene itself.
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