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Deoxyribonucleic acid (DNA) is the genetic material in prokaryotic (and eukaryotic) cells.

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Deoxyribonucleic acid (DNA) is the genetic material in prokaryotic (and eukaryotic) cells.

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One of the first key experiments that helped scientists towards the discovery of the genetic material was carried out by Frederick Griffith in 1928. The following figure shows an outline of Griffith’s experiment. In this experiment, Griffith used two strains of the bacteria Streptococcus pneumoniae: a nonvirulent strain that forms growth (colonies) with rough edges (R strain) and a virulent strain that forms smooth colonies (S strain). He then infected separate groups of mice with the R strain, S strain, heat-killed S strain and a mixture of live R strain with heat-killed S strain.

From this experiment it was concluded that a chemical substance ("transforming principle") from the heat-killed smooth strain must have transferred to the R strain bacteria, which transformed the R strain cells to become S strain (virulent).

Video link: https://youtu.be/OF533svW9FQ

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Griffith’s experiment was followed on by other scientific work that eventually led to the discovery that deoxyribonucleic acid (DNA) is the genetic material for cellular entities. In 1944, scientists Avery, MacLeod and McCarty modified Griffith’s experiment to determine which component (polysaccharides, lipids, proteins, RNA or DNA) in the heat-killed S strain extract was essential for making the R strain virulent.

Since it was only when DNA was destroyed that the R strain did not acquire virulence from the heat-killed S strain, DNA must be the component that transformed the R strain into a virulent strain. This provides evidence that supports DNA as the genetic material. (Many scientists however did not accept this still, until other subsequent scientific works supported it.)

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Scientists Hershey and Chase confirmed which biomolecule is the genetic material. They used viruses that infect bacteria (bacteriophages) marked with either radioactive phosphorus (³² P) or radioactive sulfur (³⁵ S). Bacteriophages inject their genetic material into the host bacterium to control its cellular machinery. Since DNA contains phosphorus in its backbone while proteins (making up the virus capsid) contains sulfur-containing amino acids, the location of radioactivity indicated whether it was DNA or proteins that was injected by the bacteriophages into the bacterial cells.

In the setup with radioactive sulfur, viruses in the supernatant were radioactive while bacteria in the pellet (at the bottom of the tube) were not. In the setup with radioactive phosphorus, bacteria in the pellet were radioactive, viruses in the supernatant were not. Therefore DNA, not proteins, entered the bacteria cells. Their results support that DNA is the genetic material. Video link:  https://www.youtube.com/watch?v=_kmb3wAgTS0

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Part B. Location and function of the genetic material in prokaryotes

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Part B. Location and function of the genetic material in prokaryotes

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In the nucleoid region within the cytoplasm (not in a membrane-bound nucleus) --- one bacterial chromosome (which is usually circular)

Also in the cytoplasm --- Plasmids (smaller, circular extrachromosomal DNA that exists in multiple copies)

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1. To control and regulate cellular function as it carries the information needed to produce proteins

2. To be able to pass on the information to the cell’s descendants (inheritance).

Chromosomal DNA contains essential genes for survival.

Plasmids carry additional genes that usually give the cell an advantage, i.e. antibiotic resistance, ability to survive harsh environments, ability to kill neighbouring cells, ability to make sex pilus, etc.

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Part C. How does DNA fit into a prokaryotic cell?

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Part C. How does DNA fit into a prokaryotic cell?

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If stretched out in a full circle, prokaryotic chromosomes will not fit inside the tiny cell. The cell needs to package the circular chromosome hundreds or thousands of times. For example, in Escherichia coli, it needs to be compacted about 1000 times.

This happens through the help of DNA-binding proteins that form a central scaffold, through which looped domains of DNA bind to. Within the looped domains, the DNA is supercoiled.

Video link: https://www.viddler.com/embed/d4cac474/

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Prokaryotic cells divide by through binary fission.
In the parent cell, the bacterial chromosome is copied (replicated*). Then, the cell elongates and forms a septum. The cell is split into two new cells upon the completion of the new cell walls. Each daughter cell receives an identical copy of the chromosome.

Video link: https://youtu.be/XlCA-cdvSvU

*Replication of prokaryotic chromosomes begin at a region called the "origin of replication". From here, the parent DNA strands are separated and new strands are formed using the parent DNA as the template (semi-conservative mode of replication). Replication occurs in two directions, until the entire circular chromosome is completed copied. Video link: https://www.viddler.com/embed/2e0e4017/

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Horizontal gene transfer is the transfer of genetic material between cells of the same generation/ non descendant cells.

In contrast, vertical gene transfer is the transfer of genetic material from parent to descendant/ offspring.

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The types of horizontal gene transfer include conjugation, transformation, and transduction.

In conjugation, the donor cell is alive and capable to form a sex pilus which it uses as a bridge to transfer its genetic material (plasmid DNA, transposons, or portions of the bacterial chromosome) to a recipient cell that it comes to direct contact with.

In transformation, the donor cell is a dead cell whose DNA is spread in the environment. The naked DNA is then uptaken by cells in the same enviroment, if they are competent cells (has proteins that allow the entry of the naked DNA and its protection the cytoplasm).

In transduction, a virus (bacteriophage) transfers genetic material from one bacterium to another. The donor bacterium's chromosome becomes fragmented due to the virus infection, and some of these fragments are packaged by the virus. The cell infected by this virus becomes a recipient of the donor cells' chromosome fragment.

The transferred DNA may either:
1. be degraded by the cells enzymes
2. recombine with the recipient cell’s chromosome and affect the cell's function
3. multiply in the cytoplasm and affect the cell's function, if they were plasmids

This in effect increases genetic variation among prokaryotes.