In a recent post, I began to provide some basic background on the subject of genes and inheritance by discussing the molecular structure of nucleic acids (DNA and RNA). I intend for this post to continue that discussion by taking a closer look at the role of deoxyribonucleic acid (DNA). If you remember from last time, I mentioned that the primary role of DNA is to serve as a repository for the genetic blueprints (the hereditary information) of almost all organisms. I say almost all, because some types of viruses store their genetic information in the form of ribonucleic acid (RNA), but in general, DNA is the preferred unit of storage. Regardless of whether an organism uses DNA or RNA, the sum total of an organism’s hereditary information is referred to as that organism’s “genome”. In human beings, our genome is packed into 46 chromosomes (pictured above), with copies of these chromosomes residing in the nucleus of most of the cells that make up our body (naturally, there are some exceptions). So what is a chromosome?
Chromosomes are how we store our DNA. As you may recall, DNA is usually represented as a long, double-standed helical structure when depicted in images. In most multicellular organisms, the strands of DNA are actually very long, and so for protection and organization, the strands are wrapped around proteins called histones and arranged into very compact structures known as chromosomes. In eukaryotic organisms (that is, organisms with cells that contain a nucleus: animals, plants, fungi, as well as many types of unicellular organisms), these chromosomes are housed in the nucleus of the cell. Humans, as I mentioned above, have 46 chromosomes, but it is important to note that there are actually two copies of each chromosome, and furthermore, there is typically a distinction made between the sex-determining chromosomes (denoted as X and Y), and the rest (known as autosomes). This means that you have 22 different chromosomes (two copies of each), as well as two sex chromosomes (two copies of X in women, or one X and one Y in men). Some of the more common chromosomal abnormalities are described here: http://anthro.palomar.edu/abnormal/abnormal_5.htm
The appearance of a chromosome varies, depending upon the phase of the cell’s reproductive cycle. During some phases, you will observe that chromosomes are made up of two copies that are joined together, as in the image above. The two copies are referred to as “chromatids”, and are joined at a point known as the “centromere”. On either side of the centromere are the “arms” of the chromasome, and the word “telomere” refers to the ends of those arms. The result is the X-shaped structure that is often used to depict chromosomes in images and popular media. As I mentioned before, all of this applies to eukaryotic organisms. Prokaryotes are organisms that lack a nucleus and are typically unicellular, like bacteria. Such small organisms might possess only a single chromosome, which can actually be circular in shape, rather than linear, as in the eukaryotic examples above. There are also much smaller DNA structures known as plasmids, which function like a small chromosome in bacteria, and can even be shared between bacteria. This is plays an important role in the spread of antibiotic resistance by allowing resistance genes to be passed between bacteria, which brings us to the idea of genes.
Genes are commonly defined as the basic unit of heredity, and in simplest terms a gene represents a particular stretch of DNA encoding something that might be used by the organism. The reality is actually a bit more complicated than that, but for the moment this definition will suffice. It is important to note that this unit is heritable, which is to say, that it can be passed on from parent to offspring. Copies of chromosomes, which consist of DNA and thus encode genes, are passed on through reproduction.
This seems to be a good stopping point for the moment. Future discussion topics along this line will likely involve how DNA is replicated, the role of RNA, and gene expression.
Thanks for reading!