Chromosomes and Genes

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.

The idea of genes was first suggested by Gregor Mendel, whose genetic theories have already been discussed. He did not understand genes themselves but proved the existence of distinct inheritable units in his study of pea plants. He was also the first to describe independent assortment of genes and to recognize that there were dominant and recessive genes. He also understood the concepts of heterozygosity and homozygosity. The actual term “gene” wasn’t developed until 1905, when Wilhelm Johannsen first introduced it.

Gene sequencing was first accomplished in 1972 and became more efficient in 1977 by Frederick Sanger. This was further automated, which lead to the Human Genome Project, which successfully sequenced the entire human genome by 2003.

As mentioned, DNA runs from the five-prime end to the three-prime end on one strand and from the three-prime end to the five-prime end on the other strand. When a gene is transcribed from DNA to RNA (and when DNA is replicated, these are done from the five-prime end to the three-prime end. The exposed hydroxyl end at the three-prime end is called a nucleophile because it attaches a nucleotide to this end.

When a gene is transcribed, it takes the message and creates an RNA molecule. This is similar to DNA but has uracil instead of thymine and is a less-stable molecule because it does not form a double helix; it is a single-stranded molecule. The reading sequence involves codons that are three base pairs in length. The base sequence ATT, for example will code for something different than the base sequence TAA. As a gene is being transcribed, mutational errors can occur such that the reading sequence is off by a base pair so that the entirety of the rest of the sequence is completely misread. There can also be single base mutations, which means that a single amino acid is wrong; however, this may or may not affect the structure of the protein.

The total complement of genes is referred to as the organism’s genome. Thousands of genes are located on a single chromosome. The locus is the region on the chromosome where the gene is located. The portion of the gene that is tightly locked into histone proteins is usually not expressed. Those that are unraveled are accessible for the reading of the gene. As mentioned, prokaryotes have genes that are arranged one after the other