Lesson Explainer: DNA in Prokaryotes Biology

In this explainer, we will learn how to describe the structures that contain DNA within prokaryotic cells.

Antibiotic resistance in bacteria is a global threat. Numerous infections including pneumococcal pneumonia, tuberculosis, and gonorrhea are becoming harder to treat with antibiotics as the bacteria that cause these infections evolve ways to combat and resist antibiotics. This can lead to longer stays in the hospital, increased medical costs, and increased mortality (death rate). Antibiotics are overprescribed, easy to access, and misused by the public, leading to an increased number of antibiotic-resistant bacteria. According to the World Health Organization, without urgent action, antibiotic resistance may leave us vulnerable to death by common infections and minor injuries.

Antibiotic resistance has everything to do with bacterial DNA. Mutations in DNA can give bacteria resistance to antibiotics and can occur randomly.

Key Term: DNA (Deoxyribonucleic Acid)

DNA is the molecule that carries the genetic instructions for life. It is composed of two strands of nucleotides that coil around each other to form a double helix.

You will recall that bacteria, or prokaryotes, are cellular organisms that lack both a nucleus to contain their DNA and membrane-bound organelles. Eukaryotes, on the other hand, have cells with both a nucleus and membrane-bound organelles.

Key Term: Prokaryotic Cell

A prokaryotic cell is a cell that lacks a nucleus and membrane-bound organelles.

Key Term: Eukaryotic Cell

A eukaryotic cell is a cell that contains a membrane-bound nucleus and other membrane-bound organelles.

Although prokaryotes do not have a nucleus, they have a region where their chromosomal DNA is concentrated called the nucleoid. This is an irregular-shaped region within the cell that contains most of the genetic material of the prokaryote.

Definition: Nucleoid

The nucleoid is an irregular-shaped region within the cell that contains the chromosomal DNA, in addition to RNA and proteins, of the prokaryote.

Prokaryotic DNA exists within its chromosome and, in some cases, can also carry extrachromosomal DNA referred to as a plasmid. You can see this in Figure 1 below.

The chromosomal DNA contains the majority of the prokaryote’s genetic material. Most prokaryotes contain a single circular chromosomal DNA molecule that is highly compacted inside the cell. In fact, if you were to stretch out the chromosome in a bacterium, it would be about 1.4 millimetres long, compared to the bacterium itself that can vary from 0.2 to 2.0 micrometres long! Prokaryotic DNA is compacted by twisting or coiling it many times as can be seen in Figure 2. Unlike eukaryotes, most prokaryotic chromosomes are not associated with specialized histone proteins to assist in compaction. Because most prokaryotes only have a single chromosome, they are haploid.

Key Term: Chromosome

A chromosome is an organized structure of DNA and associated proteins that contains the genetic information of an organism in the form of genes.

Definition: Haploid

A haploid cell is a cell that only has a single set of chromosomes (n).

Example 1: Determining Where Prokaryotic DNA is Contained

Where in a prokaryotic cell is the chromosomal DNA contained?

  1. In the nucleolus
  2. In the nucleoid
  3. In the pili
  4. In the nucleus
  5. In the cell wall

Answer

Prokaryotic DNA exists within its chromosome and, in some cases, can also carry extrachromosomal DNA referred to as a plasmid. The chromosome is where most of the DNA is contained and is a single, circular molecule that is located in a region of the cell called the nucleoid.

Prokaryotes also contain plasmids that are usually small extrachromosomal pieces of circular DNA that can replicate independent of the prokaryotic chromosome. Plasmids often contain antibiotic resistance genes.

Let’s look at the possible answers to see which one best describes where prokaryotic DNA is located in the cell.

“The nucleolus” is incorrect. The nucleolus is a specialized region of the eukaryotic nucleus where ribosomes are assembled. Ribosomes are needed for translation, the process of producing proteins from mRNA.

“The nucleoid” seems to be correct. The nucleoid is a specialized region of a prokaryotic cell where the DNA is located. Let’s check the remaining answers to make sure.

“The pili” is incorrect. The pili are short, hairlike structures on the surface of prokaryotic cells that are involved in movement and sticking to surfaces.

“The nucleus” is incorrect. The nucleus is a membrane-bound organelle inside eukaryotic cells that contains DNA. It is not found in prokaryotes and this is in fact a major difference between prokaryotes and eukaryotes.

Finally, “the cell wall” is also incorrect. Although prokaryotes do have a cell wall to give it shape and protect its interiors, the cell wall is not the location of prokaryotic chromosomal DNA.

Therefore, the correct answer is B: the nucleoid.

Additional genetic information that may not be essential for the prokaryote’s life can be stored in plasmids. Most plasmids are extrachromosomal circular molecules of DNA that are much smaller compared to the prokaryotic chromosome. You can see this in Figure 1. They can replicate independently of the prokaryotic chromosome and in some cases are able to integrate into the chromosomal DNA. Some prokaryotes can have as many as 20 different plasmids in a single cell. These can be transmitted to other prokaryotes to “share” genetic benefits such as genes that let them grow in special conditions or give them antibiotic resistance. You can see this in Figure 3. Plasmids can also be used to introduce foreign DNA into bacterial cells in genetic engineering. This can be done by inserting foreign genes into a plasmid and transferring the plasmid into bacterial cells.

Key Term: Plasmid

A plasmid is often a small, extrachromosomal, circular DNA molecule that contains additional genes not found in the bacterial chromosome, such as antibiotic resistance. They can be used to transport DNA into bacterial cells for DNA cloning.

Example 2: Identifying a Plasmid in a Diagram of a Prokaryote

The diagram provided shows a simplified structure of a bacterial cell. Which letter indicates a plasmid?

  1. Z
  2. Y
  3. W
  4. X

Answer

Prokaryotic DNA exists within its chromosome and, in some cases, can also carry extrachromosomal DNA referred to as a plasmid. The chromosome is where most of the DNA is contained and is a single, circular molecule that is located in a region of the cell called the nucleoid. Prokaryotes also contain plasmids that are usually small extrachromosomal pieces of circular DNA that can replicate independent of the prokaryotic chromosome. Plasmids often contain antibiotic resistance genes.

Let’s finish labeling this illustration of a bacterial cell to determine where the plasmid is.

“Z” indicates the plasma membrane. This is the double-layer lipid structure that separates the inside of the cell from the outside.

“Y” indicates a plasmid. This is usually a small, extrachromosomal piece of circular DNA that can replicate independently of the chromosome.

“W” indicates ribosomes. These are macromolecular machines that specialize in producing proteins for the cell.

“X” indicates the prokaryotic chromosome. This is where the majority of a prokaryote’s genetic information is stored.

Therefore, the correct answer is B, “Y”, which indicates a plasmid.

Antibiotic resistance genes are often found in plasmids and code for enzymes that interrupt how the antibiotic functions. For example, the antibiotic penicillin functions by inhibiting bacterial cell wall synthesis, and it causes the cell to die. In the 1940s, shortly after penicillin was introduced, the S. aureus species of bacteria became resistant to penicillin due to the plasmid-encoded penicillinase enzyme. This enzyme cleaves a critical component of penicillin, rendering the antibiotic ineffective. The development of new penicillin-based drugs has bypassed this, but only temporarily as new resistance genes evolve.

The differences between prokaryotic and eukaryotic DNA are listed in Table 1 below.

Example 3: Comparing the DNA of Prokaryotes and Eukaryotes

Which of the following statements correctly compares the DNA of eukaryotic cells with the DNA of prokaryotic cells?

  1. Eukaryotic DNA forms a single chromosomal loop, whereas prokaryotic DNA forms many small, circular plasmids.
  2. Eukaryotic chromosomes contain specialized proteins that help form chromatin, whereas most prokaryotic chromosomes do not.
  3. Both eukaryotic and prokaryotic DNA condense to form X-shaped chromosomes before the cell divides.

Answer

Prokaryotic DNA exists mostly as a single, circular chromosome that is contained in a region of the cell called a nucleoid. It is compacted by coiling and does not require the use of specialized proteins called histones.

In most eukaryotes, the DNA exists as multiple linear chromosomes that are compacted using specialized proteins called histones to form chromatin. Before cell division, eukaryotic chromosomes condense to form X-shaped structures. All of this occurs inside a nucleus.

Prokaryotes also contain plasmids. Plasmids are often extrachromosomal, small, circular pieces of DNA that often can contain antibiotic resistance genes. Plasmids can replicate independently of prokaryotic chromosomal DNA and can be transmitted to other prokaryotes.

Let’s look over these answers to see which correctly compares the DNA of eukaryotic cells with the DNA of prokaryotic cells.

In A, the answer “eukaryotic DNA forms a single chromosomal loop, whereas prokaryotic DNA forms many small, circular plasmids” is incorrect. The first part of the answer that states, “eukaryotic DNA forms a single chromosomal loop” is incorrect. This is because eukaryotic DNA does not form a single chromosomal loop, instead it is made up of multiple linear chromosomes. The second part of the answer, which states, “prokaryotic DNA forms many small, circular plasmids,” is incorrect as prokaryotes have two types of DNA, not just plasmids. They have a single, circular chromosome that contains the majority of their DNA and they may have multiple circular DNA molecules called plasmids.

In B, the answer “eukaryotic chromosomes contain specialized proteins that help form chromatin, whereas most prokaryotic chromosomes do not” seems like a good choice. The first part of the answer “eukaryotic chromosomes contain specialized proteins that help form chromatin” is correct. Eukaryotic chromosomes contain histones that help package DNA into chromatin. The second part of the answer, that most prokaryotic chromosomes do not “contain specialized proteins to help form chromatin,” is correct. Most prokaryotic chromosomes do not contain histones to form chromatin, but eukaryotes mostly have these features.

In C, the answer “both eukaryotic and prokaryotic DNA condense to form X-shaped chromosomes before the cell divides” is incorrect. The chromatin in eukaryotes condenses before cell division to form X-shaped chromosomes. This does not happen in prokaryotes. Most prokaryotes do not have specialized proteins called histones to package their DNA into chromatin like eukaryotes do, and prokaryotic DNA does not condense to form X-shaped chromosomes.

Therefore, the correct answer is B: eukaryotic chromosomes contain specialized proteins that help form chromatin, whereas most prokaryotic chromosomes do not.

Mitochondria and chloroplasts, two organelles that are found in eukaryotes, are believed to have evolved from prokaryotes! They both have circular DNA and divide in a similar way to prokaryotes.

Early eukaryotes may have ingested a prokaryote that either was capable of aerobic respiration (and went on to develop into a mitochondrion) or was capable of photosynthesis (and went on to develop into a chloroplast) as shown in Figure 4.

Symbiosis is a long-term biological “partnership” where two organisms may mutually benefit. In this case, the larger eukaryotic cell provided the prokaryote with nutrients, and the smaller prokaryotic cells provided the eukaryotic cell with ATP by cellular respiration. A similar process likely occurred with chloroplasts.

Over billions of years, these prokaryotes gradually lost most of their genes (which would not be needed since the eukaryote would provide for their survival) and would eventually become mitochondria or chloroplasts. This is known as the endosymbiotic theory, with “endo-” meaning within or inner.

Key Term: Endosymbiotic Theory

The endosymbiotic theory proposes that mitochondria and chloroplasts may have once been prokaryotes that were ingested by a eukaryote. This formed a symbiotic relationship, and eventually the prokaryote evolved into the organelle we know today.

Example 4: Understanding the Endosymbiotic Theory of the Origins of the Mitochondrion

Mitochondria, organelles found in eukaryotic cells (shown in the figure), have their own circular loops of DNA. According to the endosymbiotic theory, what does this suggest about the evolution of mitochondria?

  1. Mitochondria have had many mutations in their DNA, so they now resemble the structure of bacterial DNA.
  2. Mitochondria have evolved to be separate, parasitic organelles within the cells of a eukaryote.
  3. Mitochondria were previously separate, prokaryotic organisms before being incorporated in eukaryotes.

Answer

The prokaryotic chromosome is where most of the DNA is contained and is generally a single, circular molecule that is located in a region of the cell called the nucleoid. Prokaryotes can also contain plasmids, which are much smaller circular pieces of DNA that often have genes that give antibiotic resistance.

Mitochondria and chloroplasts also contain circular DNA and divide in a similar way to prokaryotes. Mitochondria and chloroplasts, two organelles that are found in eukaryotes, are believed to have evolved from prokaryotes! They both have circular DNA and divide in a similar way to prokaryotes.

The endosymbiotic theory proposes that mitochondria and chloroplasts may have once been prokaryotes.

Early eukaryotes may have ingested an aerobic or photosynthetic prokaryote. This could have formed a symbiotic relationship, where the eukaryotic cell provides nutrients to the prokaryote and the prokaryote provides ATP by cellular respiration or photosynthesis. Over billions of years, these prokaryotes may have lost many genes that were no longer needed since they were provided for by the eukaryote and eventually became known as mitochondria or chloroplasts. You can see this below.

Now, let’s look at the possible answers for how the endosymbiotic theory may explain why mitochondria have their own circular loop of DNA.

In A, “mitochondria have had many mutations in their DNA, so they now resemble the structure of bacterial DNA” is not accurate. The endosymbiotic theory states that a prokaryotic cell was ingested by a eukaryote that formed a close symbiotic relationship and the prokaryote eventually evolved into a mitochondrion. Therefore, the DNA from the mitochondrion was prokaryotic originally, not eukaryotic as this answer is implying. Also, although many mutations could have occurred over time in the DNA of the symbiotic prokaryote, these would have made the resulting mitochondrion much simpler compared to the prokaryotic chromosome. Many genes were lost over time as they were no longer needed since they were provided by the symbiotic eukaryotic cell.

In B, “mitochondria have evolved to be separate, parasitic organelles within the cells of a eukaryote” is not true. The endosymbiotic theory states that a prokaryote and eukaryote entered in a symbiotic relationship, where both members benefited. A parasitic relationship, where one member benefits and another (the host) is harmed is not a symbiotic relationship.

In C, “mitochondria were previously separate, prokaryotic organisms before being incorporated in eukaryotes” is accurate. The endosymbiotic theory proposes that mitochondria may have once been a prokaryote that was ingested by a eukaryote. They formed a symbiotic relationship and the prokaryote ultimately evolved into the mitochondrion.

Therefore, the correct answer is C: mitochondria were previously separate, prokaryotic organisms before being incorporated in eukaryotes.

Let’s recap some of the key points we have covered in this explainer.

Key Points

  • Most prokaryotic DNA is found in their single, circular chromosome.
  • Additional genetic information, like antibiotic resistance, may be found on tiny circular extrachromosomal pieces of DNA called plasmids.
  • There are many differences between prokaryotic DNA and eukaryotic DNA.
  • Mitochondria and chloroplasts may have evolved from prokaryotes based on the endosymbiotic theory.

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