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The Bacterial Genome: Structure & Organization

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  • 0:06 Genome
  • 2:04 Supercoiling
  • 3:56 Genome Size
  • 5:25 Plasmids
  • 6:08 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

All organisms have DNA. While the basic structure of DNA is the same, the organization of the DNA in bacterial cells is very different than in human or animal cells. In this lesson we will explore the basics of the bacterial genome.

Genome

DNA is the language of life. Just like you, bacteria have DNA that stores genetic information. The basic Watson and Crick structure of DNA is identical in you, your dog, and the bacteria living on and inside of you and your dog. But the genome of an organism is much more than just the DNA bases. Once we really dig into the bacterial genome, we can see some major differences between the genomes of you and your dog and a bacterial genome.

A genome is the complete set of genes in an organism. A bacterial genome is generally composed of a single, circular chromosome. You probably learned that your genome is diploid, meaning that you have two copies of each chromosome, one from each parent. Unlike humans, though, bacterial cells reproduce by making clones of themselves. The mother cell copies its DNA chromosome, then splits her cell in half, keeping one chromosome and giving one to the new daughter cell. Since there is only one copy of the chromosome, bacterial cells are considered haploid.

In eukaryotic cells, like your own cells, the chromosomes are contained within a membrane called the nucleus. Bacteria are considered prokaryotes, or cells that lack a nucleus. In bacteria, the chromosome is not enclosed by a membrane but is instead located in the nucleoid. The nucleoid is the cytoplasmic location of the bacterial genetic material. It is not a nucleus because it lacks a nuclear membrane, but it still succeeds in packing the chromosome into a small space within the cytoplasm. Proteins aid in holding the chromosome in this nucleoid space, which is filled with genetic material and devoid of ribosomes. This nucleoid area generally takes up about 1/3 the interior volume of the bacteria.

Supercoiling

Why would bacteria need to pack the chromosome into a small area within the cytoplasm? Why not just let it float freely? Well, if you took out the circular chromosome, cut it open, and laid it out in a straight line, it would measure about 1.5 millimeters. That might sound small, but the average bacterial cell is only 1-2 micrometers in length! That means the chromosome is over 500 times the length of the cell. If the chromosome was not packed tightly into the nucleoid, the entire cytoplasm could be taken up by the chromosome, leaving little room for other important processes like cell metabolism and protein synthesis. So the question is: how can the cell fit a chromosome that is over 500 times the length of the cell itself?

The answer is supercoiling. A supercoiled chromosome has been twisted and wound around itself very tightly. I know it might be hard to imagine, but back in the day before cell phones, everybody had a house phone with a long, typically tangled, cord. These phone cords are coiled into a helical shape, similar to DNA. If you took one end of a cord and started to twist it, tightening the coils, eventually the coils would get so tight the cord would kink. By continuing to twist, the cord would continue to kink and shorten until the entire cord was in a small, tight ball. The bacterial chromosome does this as well, but it requires some help to achieve this tight twisting. Proteins are able to bind to the DNA and twist the chromosome into loops of about 10,000 base pairs. These loops are called looped domain structures, and as the loops pile up, they give the chromosome a 'flower' shape instead of an open, circular arrangement.

Genome Size

So why does the bacterial chromosome have to be so large in the first place? The short answer is that the chromosome has to contain all of the genes needed for cell survival. We have just examined where the bacterial genome resides in the cell and its conformation. Now let's look at some general characteristics of the average bacterial genome.

An average bacterial chromosome is between 0.16 Mb and 10 Mb. One megabase (Mb) is equal to one million base pairs. That means the chromosome is a string of up to 10 million As, Gs, Cs, and Ts! E. coli, for example, has about 5 Mb in its genome, with about 4,400 different genes that code for a few thousand different proteins. So the chromosome is so long because it has to encode all of these genes.

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