Cellular Genomes

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.

Prokaryotes, with their circular DNA, are generally haploid. This means there is one copy of the genome in each cell. Supercoiling also happens to help the DNA shrink to a more acceptable size. Topoisomerases are also involved, one of which is DNA gyrase, which keeps the DNA from winding too much. There are no histone proteins in prokaryotes but there are similar proteins involved in the packaging of DNA. Some regions of DNA are not supercoiled and can be expressed directly.

There can be a great many regions of DNA called noncoding DNA, which does not encode for protein or stable RNA segments. These are located between genes. An intron does not code for a protein, while an exon does encode for a protein. There are start and stop sequences at the beginning and end of a gene that are not coding segments. There are regulator segments that are also not coded but help regulate the expression of the gene. Prokaryotes have fewer noncoding segments compared to eukaryotes.

Extrachromosomal DNA is located outside the main chromosome. These are a part of the genome. These extrachromosomal DNA segments can be found in chloroplasts, mitochondria, and plastids. The DNA of some latent viruses can also represent extrachromosomal DNA. This is seen in humans when human papillomaviruses infect the cell. Plasmids are found in prokaryotes mainly and can confer certain benefits to the host, such as antibiotic resistance.

Each organism has a different size of genome. Humans have 46 chromosomes with three billion base pairs. Plants have larger genomes that are generally polyploid, meaning they have multiple copies of the chromosome. The smallest genomes are found in viruses, although this is not universally true, with small, obligate intracellular organisms having the next smallest genome. Other bacteria will have larger genomes. As mentioned, plants have the largest genomes.