translated into proteins, while other types need manipulation in order to be replicated into new viral particles.
CELLULAR GENOMES The entirety of the DNA message in the cell is called its genome. The genome is made from genes that are together stored in one or more chromosomes. Organisms vary from one to another in the size and arrangement of their genome. The DNA will code for all cellular activities with many segments that each encode for just one protein. This segment is called a gene. The genotype is all the genes in the genome. Not all genes are expressed at the same time within the cell and the genes are expressed differently, depending on the cell, in multicellular organisms. The gene will determine the phenotype, which is what is actually observed with a specific genotype. Constitutive genes are always expressed so, for this reason, these are known as housekeeping genes. A facultative gene, on the other hand, is only turned on when it is needed. The genotype is always constant but the phenotype is variable, depending on the environment. An example is Streptococcus mutans, which will make a slime layer only when it is exposed to sucrose or table sugar. Temperature also affects the phenotype of a specific organism. The genome is associated into chromosomes, which are considered discrete DNA segments. Prokaryotes generally have a single circular chromosome, while eukaryotes have multiple linear chromosomes. Each chromosome can have thousands of genes. Eukaryotic cells are often diploid, which means they have two copies of each chromosome. The chromosomes are tightly packed so they fit within the cell. This is due to supercoiling of the DNA, which shrinks it size. There are topoisomerases, which are enzymes that maintain the supercoiling of the genome. Histone proteins are DNAbinding proteins that help to organize and coil the DNA. Chromatin is what DNA plus the histone proteins are called.
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