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10.4:

Genomic DNA in Eukaryotes

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Biology
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JoVE Core Biology
Genomic DNA in Eukaryotes

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In eukaryotes, double stranded DNA is specially organized within a membrane bound nucleus to accommodate the cell's limited space.

At the first level of compaction, DNA is wrapped tightly around specific proteins called histones. A histone core and the DNA wrapped around it are together known as a nucleosome. The basic unit of DNA packaging. Nucleosomes are joined together by linker DNA to resemble beads on a string. In the next level of packaging, nucleosomes and linker DNA, coil to form Chromatin fibers.

Finally, additional fiber's proteins compact the chromatin even further allowing such long lengths of DNA in to such tightly condensed units, recognized as chromosomes depending on the phase of cell division.

10.4:

Genomic DNA in Eukaryotes

Eukaryotes have large genomes compared to prokaryotes. To fit their genomes into a cell, eukaryotic DNA is packaged extraordinarily tightly inside the nucleus. To achieve this, DNA is tightly wound around proteins called histones, which are packaged into nucleosomes that are joined by linker DNA and coil into chromatin fibers. Additional fibrous proteins further compact the chromatin, which is recognizable as chromosomes during certain phases of cell division.

The Human Genome Measured in Meters

Most cells in the human body contain about 6 billion base pairs of DNA packaged into 23 pairs of chromosomes. It is hard to imagine exactly how much DNA these numbers represent, and therefore it is difficult to grasp how densely packed DNA must be to fit into a cell. We can gain some insight by expressing the genome in terms of length. If we were to arrange the DNA of a single diploid cell into a straight line, it would be about two meters long!

Note that humans do not have unusually large genomes. Many fish, amphibians, and flowering plants have much larger genomes than humans. For example, the haploid genome of the Japanese flowering plant Paris japonica contains about 50 times more DNA than the human haploid genome. These figures emphasize the astonishing work that histones and other chromatin remodeling proteins must do to package DNA.

Suggested Reading

Li, Guohong, and Danny Reinberg. “Chromatin Higher-Order Structures and Gene Regulation.” Current Opinion in Genetics & Development 21, no. 2 (April 2011): 175–86. [Source]