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Comparative Genomics: Prokaryotes vs Eukaryotes

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  • 0:01 Basic Differences
  • 0:39 DNA Defined
  • 2:00 Prokaryotic DNA
  • 3:28 Eukaryotic DNA
  • 5:03 Lesson Summary
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Lesson Transcript
Instructor: Meredith Mikell
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.

Basic Differences

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.

DNA Defined

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!

Prokaryotic DNA

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!

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