Chromosomes and Genes in Prokaryotes and Eukaryotes

Through the study of population genetics, the ideas leading to modern synthesis concepts in evolution were created. Before population genetics, ideas like orthogenesis and Lamarckism were used to explain why there was such a great deal of genetic diversity in the world. These theories have been refuted through the study of population genetics. Now, mathematical models are used to explain things like the genetic diversity seen in small isolated populations, such as that which exists on the Galapagos Islands.

Another related theory coming out of the study of population genetics is called neutral theory. The idea behind this is that most mutations that can happen in a population are considered deleterious for those organisms who have them so they do not persist in the population. The rest are considered to be neutral, with only a few being advantageous to the organism. Neutral mutations may or may not get passed on but this is largely through chance of through the processes of genetic drift.

Other things that are studied in population genetics are things we will talk further about as you listen to this course. These include things like natural selection and sexual selection, which determine whether or not a specific trait survives in a population, mutation, which is how genetic variation in a population occurs in the first place, genetic drift, which is the change in the frequency of an allele caused by random events rather than environmental pressures, and gene flow, which involves the actual transfer of alleles between populations caused by immigration of some of the population’s individuals.

CHROMOSOMES AND GENES IN PROKARYOTES AND EUKARYOTES

We will talk more about the genetics of prokaryotes and eukaryotes later on but, in this section, we will talk about the chromosomes seen in these types of organisms. Prokaryotes are organisms that do not have membrane-bound organelles inside each cell. This means they do not have separate, bound nuclei in the cell to house genetic material. Bacteria and Archaea organisms are both types of prokaryotes. Eukaryotes have membrane-bound organelles in the cell so they do have nuclei where genetic material is stored. Fungi, plants, and animals of all types are eukaryotic organisms.

The genome of prokaryotes is much simpler than in eukaryotes. Remember that the genome of an organism is the sum total of genetic material in each cell. Prokaryotes usually have a single chromosome plus one or more small, circular plasmids, which contain just one or a few genes. Figure 1 shows you what a prokaryotic chromosome would look like:

Figure 1.

Chromosome studies certainly could be done on any prokaryotic organism but most have been performed through studying the chromosome of the E. coli bacterium. The chromosome is made from double-stranded DNA that clustered together in an area called the nucleoid, which is not bound by a membrane.

Despite their small size, prokaryotes have rather large genomes. The chromosome is able to fit into such a small space by being rolled into coils in a process called supercoiling, which twists the fibers so they form a tight ball. Positive coiling happens

in the same direction as the double helix, while negative coiling is in the opposite direction of the coiling. Most of the time, bacterial DNA is negatively coiled.

Prokaryotes reproduce through asexual means most of the time and the DNA is considered haploid, which means there is only one copy of any gene per cell. Plasmids, as mentioned, are also circular but can be linear. Most plasmid DNA isn’t necessary for the bacterial organism to survive but give it an added advantage, such as antibiotic resistance. Plasmids replicate themselves separately from the replication of the chromosome.

Chromosomes in prokaryotes need to be so efficient in order to be able to fit in the cell. Because of this, there isn’t very much of the bacterial chromosome that isn’t directly involved in coding for proteins. While eukaryotes have 98 percent of their DNA that doesn’t get coded into known proteins, prokaryotes have much more coding DNA with only 12 percent of the DNA not coding for proteins.

Eukaryotic chromosomes are much more complex that is true for prokaryotes. There is a great deal more DNA involved overall but the majority of it does not seem to code for any proteins yet discovered. Chromosomes in eukaryotes are linear and paired so that there are generally two copies of each gene, except for those stored on the sex chromosomes. The number of chromosomes in each species varies with the species and, interestingly, it is not true that more complex organisms have more chromosomes. Plant, in fact, often have many more chromosomes than is true for complex animals.

Eukaryotes have the same problem as prokaryotes in that the DNA must fit in a small space. This is accomplished by wrapping pieces of the DNA around histone proteins. The combination of DNA plus its histone proteins is called chromatin. Figure 2 shows what chromatin looks like. Each bead on the string represents an area that is wrapped around a histone in a structure called a nucleosome.

Chromosomes in eukaryotes look differently depending on what part of the cell cycle you look at. During mitosis or the cell division stage, for example, the chromosomes are much more visible under the microscope. In between those times, however, the chromatin is thin and the chromosomes cannot be individually seen.

When the chromatin is visualized in eukaryotes, you can see three main aspects to it. There is a central centromere near the middle that is particularly important in cell division. It is this part that must separate during cell division. There are two types of arms. The short arm is called the p-arm and the long arm is called the q-arm. Figure 2 shows what the different types of chromosomes might look like, although you do not have to know the different types:

Figure 2.

The ends of each arm is a section called the telomere. Telomeres do not code for any specific gene but serve to protect the end of the DNA arm of the chromosome.

Chromosomes do not have any color themselves but can be dyed with Giemsa dye. When you do this, you will see striped chromosomes. The stripes do not necessarily mean anything except for the fact that certain base pairs that make up the long DNA

chain will take up more dye than others. Scientists do use these stripes to look for areas where the chromosome is missing a piece or has an extra piece associated with it. In other situations, fluorescent dyes are used to light up the chromosome using certain microscopes that detect fluorescence. Figure 3 shows the human karyotype using Giemsa dye staining:

Figure 3.

The karyotype of a cell is a picture of what each of their paired chromosomes look like. A picture can be taken and the chromosomes sorted out and ordered, resulting in what you see in figure 4.