Prokaryotic and Eukaryotic DNA: Structure, Function, and Examples
Eukaryotic and prokaryotic cells are present in organisms, depending on whether they include membrane-bound organelles. Whether a nuclear membrane protects its genetic components is a matter of debate. In this article, we will be looking at the differences between eukaryotic and prokaryotic cells. Let us start by understanding what is DNA, in the coming section.
What is DNA?
Deoxyribonucleic acid (DNA) is the hereditary material in most living organisms. It is a molecule comprising two helices that remain coiled around each other and has the genetic information to be transmitted to the RNA or ribonucleic acid through a process called transcription.
Human DNA is located in the nucleus as a chromatin reticulum where DNA remains associated with histone and non-histone proteins. In prokaryotes like in bacteria, the DNA remains in the nucleoid region and does not remain associated with proteins. The DNA contains the code for protein synthesis as nucleotides with four types of nitrogen bases, Adenine, Guanine, Cytosine and Thymine. The double-helical structure of DNA is typical for both eukaryotic and prokaryotic cells. However, DNA coiling is different.
What Are Prokaryotic and Eukaryotic Cells?
Prokaryotic cells were formed before eukaryotic cells and have a much simpler structure than the latter. Both eukaryotic and prokaryotic cells have diverse DNA structures and chemical compositions. There is no nucleus, no organelles, and just a minimal amount of DNA in prokaryotic cells. Eukaryotic cells, on the other hand, have a nucleus and cell organelles, as well as a large amount of DNA. The cytoplasm contains prokaryotic DNA, which is smaller and circular. Inside the nucleus, eukaryotic DNA is large and linear.
The Differences Between Prokaryotic and Eukaryotic Cells
Differences Between Eukaryotic and Prokaryotic DNA
Prokaryotic DNA: Prokaryotic DNA is double-stranded circular DNA that remains diffused in a dense region of cytoplasm called the nucleoid. There is no nuclear membrane surrounding the DNA in prokaryotes. The single circular DNA represents a single chromosome. This DNA is not a supercoiled structure as in eukaryotes as they do not have histone proteins in their structure. They form loops with the help of nucleoid-associated proteins. Due to the absence of the histone scaffolding, the prokaryotic DNA is often referred to as “naked DNA”.
The size of the DNA is around 160000 to 12.2 million base pairs. The genes present in this kind of DNA is less in number and is present in the form of operons. Operons are a group of genes with a common promoter and a common terminator sequence as a result of which these genes are expressed simultaneously. Since the number of base pairs is less, it does not keep much scope for “junk DNA”.
This means that most of the genes are without introns or non-functional DNA. There is a single origin of replication as a result of which the entire genome is replicated at once. During this type of replication, a single replication fork is formed. The rate of replication is close to 2000 nucleotides per second. Transposons or mobile genetic elements are small segments of DNA that can jump from one place to another in the genome. Such elements are present in prokaryotes and help in bringing in genetic variation. During transcription, the RNA formed is polycistronic, which contains information for more than one protein. When ribosomes attach to each of these cistrons, it forms a beaded structure called polyribosomes.
Apart from the DNA present in the nucleoid, prokaryotes also have extranuclear, double-stranded, circular DNA in the cytoplasm. These are known as plasmids. The genes present in the plasmids help in the survival of the bacteria and contains antibiotic-resistant genes. They replicate autonomously and are transferred from one bacterium to another during conjugation.
Eukaryotic DNA: The DNA in eukaryotes is found in the nucleus enclosed in the nuclear membrane. It is linear in shape. This DNA is present in the form of chromatin reticulum when the cell is not dividing and condenses to form rod-shaped structures called chromosomes during cell division. The DNA molecule remains tightly coiled and supercoiled against basic proteins called histone proteins. Non-histone proteins are also present in this DNA. The DNA forms a coiled structure called nucleosome which supercoils many folds to form the condensed structure of the eukaryotic DNA. This coiling allows the large bulk of DNA to be incorporated into the nucleus.
The number of chromosomes varies from species to species and is unique for each one of them. The human genome consists of 23 pairs of chromosomes, consisting of 2.9 billion base pairs. There is a large amount of non-coding or junk DNA present in eukaryotes. Hence introns need to be cut and removed from the RNA during gene expression. This process is known as RNA splicing. There is more than one origin of replication allowing different genes to be transcribed separately. The rate of replication is 100 nucleotides per second and is hence much slower than that in prokaryotes. Transposons are inactive in eukaryotes. The mRNA formed during transcription are monocistronic, that is codes for only one protein. Polyribosomes are absent in eukaryotes.
Comparison of Eukaryotic DNA with Prokaryotic DNA
Prokaryotic cells are much more complicated than eukaryotic cells because eukaryotic cells are considered to exist in the later stages of evolution. It is probable that eukaryotic cells developed from prokaryotic cells. At the cellular level, the difference in complexity can be seen. The necessary and sufficient feature to define an organism as a eukaryote is that a nuclear membrane surrounds the nucleus with nuclear pores.
All existing eukaryotes have cells with a nucleus; most of the genetic material of eukaryotic cells is contained in the nucleus. In contrast, prokaryotic DNA is not contained in the nucleus, but attached to the plasma membrane and contained in the form of a nucleoid. The nucleoid is an irregularly shaped area that is not surrounded by the nuclear membrane. Eukaryotic DNA is packaged into a bunch of chromosomes, and each chromosome is composed of a linear DNA molecule that is wound around basic proteins called histones, which coils the DNA into a more compact structure.
Circular, non-chromosomal DNA is found in prokaryotic cells. Prokaryotes also have plasmids, which are tiny circular DNA fragments that can replicate independently of prokaryotic genomic DNA. Because eukaryotic DNA is linear, telomeres, or repeating non-coding DNA sequences, are present on both ends of chromosomes to preserve them from deterioration.
Mitosis, a nuclear division process wherein replicated chromosomes are divided and separated via cytoskeleton components, is present in all eukaryotes. Actin microfilaments and microtubules are structural and motility components of the cytoskeleton. These cytoskeletal elements are found in all existing eukaryotes. Prokaryotes, on the other hand, go through a binary fission process in which the DNA is copied, splits into two poles of the cell, and ultimately divides completely.
The existence of mitochondrial DNA is a major difference between eukaryotes and prokaryotes. As eukaryotes have mitochondria and prokaryotes do not have. As a result, we may split cells into prokaryotes and eukaryotes based on genetic materials enclosed by a nuclear envelope. Prokaryotes do not have membrane-bound organelles, but eukaryotes do.
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FAQs on Difference Between Prokaryotic and Eukaryotic DNA
1. What are the main differences between prokaryotic and eukaryotic DNA?
The primary differences between prokaryotic and eukaryotic DNA lie in their structure, location, and organisation. Key distinctions include:
- Location: Prokaryotic DNA is found in the cytoplasm in a region called the nucleoid, whereas eukaryotic DNA is enclosed within a membrane-bound nucleus.
- Structure: Prokaryotic DNA is typically a single, circular chromosome, while eukaryotes have multiple, linear chromosomes.
- Packaging: Eukaryotic DNA is tightly coiled around proteins called histones to form chromatin. Prokaryotic DNA is considered 'naked' as it is not associated with histones, though some proteins are involved in its condensation.
- Genetic Content: Eukaryotic DNA contains large non-coding regions called introns, which are absent in prokaryotic DNA.
- Amount: Eukaryotic cells contain significantly more DNA than prokaryotic cells.
2. How does the location of DNA differ in prokaryotic and eukaryotic cells?
In eukaryotic cells, the DNA is housed inside a well-defined, membrane-bound organelle called the nucleus. This compartmentalisation protects the DNA and separates transcription from translation. In contrast, prokaryotic cells lack a true nucleus. Their genetic material is located in a dense, irregularly shaped region of the cytoplasm known as the nucleoid, which is not enclosed by a membrane.
3. Are histone proteins found in both prokaryotic and eukaryotic DNA?
No, histone proteins are a defining feature of eukaryotic DNA packaging. In eukaryotes, the long, linear DNA molecules are wrapped around these basic proteins to form a compact structure called chromatin. This allows the vast amount of DNA to fit inside the nucleus. Prokaryotic DNA is not associated with histones and is often referred to as 'naked DNA', although it is compacted into loops by other nucleoid-associated proteins.
4. What is the significance of eukaryotic DNA being linear while prokaryotic DNA is circular?
The structural difference impacts key cellular processes, especially replication. The single, circular DNA in prokaryotes has one origin of replication, allowing for rapid and simple DNA duplication. Eukaryotic linear chromosomes have multiple origins of replication, which is necessary to replicate their much larger genomes efficiently during the cell cycle. However, the linear structure of eukaryotic DNA presents a challenge, as the ends (telomeres) can shorten with each replication cycle, a process linked to cellular ageing.
5. Why is there a significant size difference between eukaryotic and prokaryotic genomes?
The eukaryotic genome is much larger than the prokaryotic genome primarily due to two factors. First, eukaryotes are generally more complex organisms, requiring a larger number of genes to control their development and functions. Second, a substantial portion of eukaryotic DNA consists of non-coding sequences, including introns and regulatory elements, which are largely absent in the highly streamlined and gene-dense prokaryotic genome. This 'extra' DNA in eukaryotes plays crucial roles in complex gene regulation.
6. How does the presence of introns in eukaryotic DNA affect protein synthesis?
The presence of introns in eukaryotic genes means that the initial messenger RNA (mRNA) transcribed from the DNA, known as pre-mRNA, is not immediately ready for translation. It must undergo a processing step called splicing, where the non-coding introns are removed and the coding exons are joined together. This process allows for 'alternative splicing,' where different combinations of exons can be used to produce multiple proteins from a single gene. Prokaryotic DNA lacks introns, so their mRNA can be translated directly as it is being transcribed, a process known as coupled transcription and translation.
7. What are plasmids, and do they alter the basic understanding of prokaryotic DNA?
Plasmids are small, circular, extrachromosomal DNA molecules found in many bacteria, separate from the main bacterial chromosome. They do not alter the basic understanding of the main prokaryotic chromosome but add another layer to it. Plasmids carry non-essential genes that can provide advantages like antibiotic resistance or virulence. They can replicate independently of the main chromosome and can be transferred between bacteria, which is a key mechanism for spreading genetic traits. You can learn more about the difference between plasmid DNA and chromosomal DNA for further clarity.
8. How is human DNA different from the DNA of a bacterium like E. coli?
Human DNA is a classic example of eukaryotic DNA, while E. coli DNA is prokaryotic. The key differences are:
- Structure: Human DNA consists of 46 linear chromosomes, whereas E. coli has a single circular chromosome.
- Location: Human DNA is in the nucleus; E. coli DNA is in the nucleoid.
- Size: The human genome contains about 3 billion base pairs, while the E. coli genome has only about 4.6 million base pairs.
- Packaging: Human DNA is extensively packaged with histone proteins, forming complex chromatin structures. E. coli DNA is not associated with histones.
