Login processing...

Trial ends in Request Full Access Tell Your Colleague About Jove

5.1: DNA Packaging

JoVE Core
Molecular Biology

A subscription to JoVE is required to view this content. Sign in or start your free trial.

DNA Packaging

5.1: DNA Packaging


Eukaryotes have large genomes compared to prokaryotes. In order to fit their genomes into a cell, eukaryotes must pack their DNA tightly inside the nucleus. To do so, DNA is wound around proteins called histones to form nucleosomes, the main unit of DNA packaging. Nucleosomes then coil into compact fibers known as chromatin.

You Have Enough DNA to Stretch to the Sun and Back Hundreds of Times

Most cells in the human body contain about 3 billion base pairs of DNA packaged into 23 pairs of chromosomes. It is hard to imagine exactly how much DNA these numbers represent. So how much packing has to happen to fit the genome 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 human cell, like a skin cell, into a straight line, it would be two meters long–over 6.5 feet. The human body contains around 50 trillion human cells. This means that each person has a total of about 100 trillion meters of DNA. In other words, each person has enough DNA to stretch from the Earth to the Sun 300 times!

And humans do not have particularly large genomes–those of many fish, amphibians, and flowering plants are much larger. For example, the genome of the flowering plant Paris japonica is 25 times larger than the human diploid genome. These figures emphasize the astonishing task that eukaryotes must accomplish to pack their DNA inside cells.

Nucleosomes Are Central Players in DNA Packaging

Each nucleosome consists of DNA wrapped around a core of eight histone proteins. Each core is composed of four different types of histones—H2A, H2B, H3, and H4—that are each present in two copies. Another type of histone—H1—binds to both the nucleosome and the linker DNA, stabilizing the structure.

DNA becomes more compact as nucleosomes and linker DNA coil into chromatin fibers. Uncondensed chromatin fibers, or euchromatin, are approximately 10 nm in diameter. Nucleosomes resemble beads on a string in these fibers. As DNA continues to condense, the 10-nm fibers coil into strands that are approximately 30 nm thick, which in turn form loops that make 300-nm thick fibers. When chromatin is fully compacted it is known as heterochromatin.

The loosely packed structure of euchromatin allows enzymes, such as RNA polymerase, access to the DNA. Transcription, therefore, tends to occur predominantly in euchromatic regions of the genome, which are rich in genes. By contrast, the tightly packed structure of heterochromatin blocks access to the DNA, preventing transcription. Heterochromatin predominates in the centromeres and telomeres of chromosomes, where highly repetitive DNA sequences are much more common than genes. Furthermore, organisms can dynamically adjust the level of DNA packing in response to cellular and external environmental cues, de-condensing DNA when genes need to be turned on, and re-condensing it to turn them off.

Lectura sugerida


DNA Which Stands For Deoxyribonucleic Acid Is The Genetic Material Found In All Living Organisms. It Contains The Instructions Necessary For The Development Functioning And Reproduction Of An Organism. However DNA Is A Long And Complex Molecule That Needs To Be Packaged In A Way That Allows It To Fit Within The Nucleus Of A Cell. DNA Packaging Refers To The Process By Which DNA Is Organized And Condensed Into A More Compact Form Called Chromatin. Chromatin Consists Of DNA Tightly Wound Around Proteins Called Histones. This Packaging Allows DNA To Be Efficiently Stored And Protected Within The Nucleus. The Packaging Of DNA Is Crucial For Several Reasons. Firstly It Helps To Prevent DNA Damage And Breakage By Providing Physical Protection. Secondly It Plays A Role In Gene Regulation By Determining Which Genes Are Accessible For Transcription And Which Are Not. Additionally DNA Packaging Facilitates The Process Of DNA Replication And Cell Division. The Packaging Of DNA Is A Hig

Get cutting-edge science videos from JoVE sent straight to your inbox every month.

Waiting X
Simple Hit Counter