Genetic Diversity in Prokaryotes

Prokaryotes are a lot less complex than eukaryotes when it comes to gene expression; however, being less complex is not always a bad thing. Prokaryotes are the oldest living organisms as they have been around long before eukaryotes. Prokaryotes are also a lot more diverse in their gene expressions, which allows them to live in crazy places. These organisms can be found in places such as the deepest parts of the oceans, within the soil around you, and even inside your own intestines! These organisms can survive anywhere due to their vastly diverse genomes. In this article, we will explore the genetic diversity of prokaryotes in more detail and discover what these tiny creatures are capable of.

Genetic diversity in eukaryotes and prokaryotes

As the world evolved, prokaryotes and eukaryotes needed to evolve in order to survive in new living conditions. This is where genetic diversity comes into play. Without genetic diversity, organisms would not be able to adapt to the ever-changing world and their populations would be wiped out. Prokaryotes and eukaryotes both have different methods of inducing genetic variation within their respective populations.

Genetic mutations function to affect the diversity of future generations both within eukaryotes and in prokaryotes. Since prokaryotes reproduce through binary fission, a mutation can be introduced into the population fairly quickly. The combination of mutations and binary fission speeds up the spread of genetic variation throughout the prokaryote population.¹

Genetic recombination is another way in which genetic diversity is achieved in prokaryotes.¹ During genetic recombination, the genetic material of two organisms is combined. This can occur in several ways. DNA can be transferred from one prokaryote to another through the process of conjugation.¹ During this process, the donor organism passes its DNA to the recipient through a tube called the pilus.¹

Another process that allows prokaryotes to maintain genetic diversity is transformation. During this process, prokaryotes pick up DNA in their environment from dead organisms or secretions from living organisms.

Mechanisms of bacterial transformation

Prokaryotic cells that have the ability to take up DNA are referred to as competent cells. Not all prokaryotes are capable of taking DNA from their environment. See Figure 2 for the steps of transformation.

A restriction enzyme isolates the gene to be transferred, and it is taken into the competent prokaryotic cell. Within the prokaryotic cell, the enzyme DNA ligase attaches the gene to the plasmid of the prokaryote which establishes the gene as part of the prokaryote DNA.

While prokaryotes achieve genetic diversity by physically acquiring new DNA from outside sources and taking them into their cell, eukaryotic cells achieve genetic diversity via sexual reproduction. Organisms that reproduce via sexual reproduction create new organisms that contain half DNA from the mother and half from the father. The process of creating a new organism through sexual reproduction is mediated through meiosis. During meiosis, one cell produces four daughter cells. This process affects genetic diversity in two ways. During the first stage of meiosis, chromosomes can exchange sections during crossing over. ¹

Crossing over is a cellular process that occurs when chromosomes with the same genes are lined up during prophase I. During prophase, two chromosomes join together to form a tetrad. The adjacent non-sister chromatids join together at points known as chiasmata.² These chiasmata are where crossing over occurs. At the chiasmata, the two non-sister chromatids exchange segments of their DNA. The process of crossing over results in two chromatids that are recombinants and two that are original. Crossing over allows for the production of new combinations of genes and is the basis for genetic diversity among eukaryotes (see Figure 3).

Genetic variation in prokaryotes

Another way that genetic variation is achieved in prokaryotes is through transduction. During transduction, genes are transferred from the donor to the recipient through a bacteriophage. There are two types of transduction: generalized transduction and specialized transduction.

During generalized transduction, the bacteriophage first injects the donor cells and starts the lytic cycle.¹ A virus then uses the host cell machinery to create its components. This results in the host cell DNA being integrated into the viral genome. When the virus infects another prokaryote, the donor DNA is transferred to the recipient.

In specialized transduction, the virus enters the prokaryote and integrates its genome into the host cell's DNA where it remains dormant and is passed from generation to generation as the prokaryotic reproduces.¹ When the cell is exposed to a stimulus that activates the viral genes, the lytic cycle begins, and the viral genome becomes a part of the host cell's genome.

3 types of genetic variation

Within eukaryotes and prokaryotes, there are three main types of genetic variation: mutation, recombination, and the immigration of genes. Mutations result when changes in base pairs result in the production of a different codon or a change in the reading frame of the DNA strand.¹ Mutations can both be helpful and can be harmful in the case of certain diseases. Mutations are discussed in great detail in the harmful mutations article as well as the point mutations article.

We have already discussed in some detail the process of genetic recombination. Within meiosis, chromosomes undergo random assortment during metaphase 1 in the process called independent assortment. During metaphase, the chromosomes are lined up in a random order which determines which chromosomes will be present in which daughter cells. As you can see in Figure 4 there are four different combination possibilities given the arrangement of the chromosomes. These arrangements allow for genetic diversity within the organisms as each daughter cell will not have an identical chromosomal arrangement.

The last major process involved in establishing genetic diversity is the immigration of genes. This process is also known as gene migration which is the introduction of genetic material from one population of a species to another population. This results in the changing of the composition of the gene pool of the receiving population. You can see this concept in Figure 5.

This phenomenon can be explained using a Punnett square. Here, in Figure 5, you can see that the offspring of the migrated bird can create a population with either a dominant or recessive homozygous population as a result of selection pressure. Selection pressure refers to factors that contribute to the selection of which genetic variations will provide the individual organism or population with an increased chance of surviving. Selective pressures give organisms with certain phenotypes an advantage when it comes to survival and reproduction.

References:

Byju's. Bacterial Genetics. 2022

Wendy Riggs, Independent Assortment, 2015