Prokaryotic cells and eukaryotic cells have some obvious differences in size and complexity, but we’ll explore additional differences in how their DNA is structured and functions. Test your knowledge with a brief quiz.
You probably learned about the basic differences between prokaryotic cells and eukaryotic cells in high school biology:
- Prokaryotes don’t have a membrane-bound nucleus or major organelles and eukaryotes do
- Prokaryotes are usually single-celled individuals where eukaryotes can be single or multi-celled
- Prokaryotes are bacteria, and archaea and eukaryotes are pretty much everything else
However, we’ll see in this lesson that the differences go deeper by exploring the differences in how DNA is structured and functions in prokaryotes and eukaryotes.
The physical differences between the two cell types are pretty obvious. But their real distinction is at the DNA level. Remember that DNA is organized into chromosomes, bundles of DNA and protein that contain gene sequences that code for the physical and behavioral characteristics of organisms.
Biologists refer to an organism’s complete set of DNA, including its genes, as its genome.The gene sequences that are used to code for proteins are called exons, which act as protein blueprints. But not all of the DNA is actually used in coding; some of it consists of non-coding sequences, including segments called introns, which are cut out from the ‘final blueprint’ before proteins are made. These non-coding regions are nicknamed ‘junk DNA,’ and while it may seem strange that they even exist, they actually do have a purpose.Some non-coding regions facilitate the protein-making process by providing a ‘start’ or ‘stop’ message or serving as genes of RNA molecules in the cell, which are also crucial in the process of DNA replication, transcription, and translation. Basically, these little non-coders act like the stage crew of a play: supporting the exon ‘actors’ from behind the scenes!
Prokaryotes are haploid, meaning they have one set of chromosomes.
Because there is no nucleus to confine the DNA, prokaryotic DNA can be found anywhere in the cell. The prokaryotic genome is almost entirely made up of coding DNA; no introns! Some of those coding regions include the repetitive repeat sequences, duplicates of certain sequences throughout the entire genome.Genes that serve a related function are often clustered closely together on the genome. Prokaryotes, mostly bacteria, also often have short, usually circular, double-stranded segments of DNA called plasmids. These compact, little strands can replicate independently from the main prokaryotic DNA.
Prokaryotes primarily undergo asexual reproduction, simply cloning themselves and dividing into two individual cells; however, sometimes bacteria are capable of conjugation, in which two cells swap plasmids, essentially sharing genes. This can even include adaptive genes, making some harmful bacteria even stronger and more resistant to antibiotics!To successfully adapt to changes in their external environment, prokaryotes have to reproduce very quickly, and so their genomes are all about coding for proteins and are very efficient. Only 2% of prokaryotic DNA is non-coding!
Unlike the neat and concise prokaryotic plasmid, eukaryotic DNA is a bit more complex. It is held within the cell nucleus, so it cannot wander in the cell. There are many linear strands of DNA jumbled together much like a ball of yarn and consists mostly of introns. For example, about 98% of the human genome consists of non-coding DNA! But it’s not a waste; most of these sequences are regulatory sequences, directing and managing gene expression of that tiny 2% that does code for proteins.
These coding sequence genes are spread out across the long strands of DNA, and each gene is managed and regulated separately.Eukaryotes are typically diploid, meaning they have two sets of their chromosomes. This is because they usually reproduce sexually, and so contain a set of chromosomes from each parent, making their appearances and behaviors a blend of both mom and dad. This process is more time-consuming than asexual reproduction but is certainly advantageous in ensuring a good mix of genes in the offspring.Different species have different numbers of chromosomes, and closely related species have not only similar chromosome numbers but also similar DNA! Chimpanzees, for example, have 48 chromosomes whereas humans have 46. But when we compare the genes on those chromosomes, we see that chimp and human genomes are 98% identical!
Prokaryotes are physically more simple cells than eukaryotes, and their genomes are a simple, single circle of almost entirely coding sequences of DNA, which is replicated and coded into proteins very rapidly.
These organisms are haploid with a single chromosome, and usually undergo asexual reproduction by cloning themselves but can also swap DNA with other prokaryotic cells, giving them an adaptive advantage.Eukaryotic cells are more complex, with many strands of DNA bundled in the nucleus and lots of non-coding regions, which are actually useful in arranging and managing protein synthesis. These organisms are diploid with two sets of chromosomes, the total number of which varies from species to species. Both cell types have amazing strategies for passing on their DNA and turning the genomic code into form and function!
Once you’ve finished with this lesson, you should have the ability to:
- Define genome
- Differentiate between haploid and diploid
- Identify the differences in prokaryotic and eukaryotic DNA structure
- Explain how prokaryotes and eukaryotes reproduce differently