Histones: Function & Types

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  • 0:01 What Are Histones?
  • 0:44 Types of Histones
  • 1:34 Nucleosome Structure
  • 2:07 Modifications & Gene…
  • 3:15 Lesson Summary
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Lesson Transcript
Instructor: Shannon Compton

Shannon teaches Microbiology and has a Master's and a PhD in Biomedical Science. She also researches cancer and neurodegenerative diseases.

Did you know that you have almost six feet of DNA in each of your cells? The diameter of the largest human cell is about the width of a hair. So, how does all that DNA fit inside a cell without spikes of DNA sticking out all over? The answer is histones.

What Are Histones?

Every cell in the body has about six feet of DNA. This shortens to 0.09 millimeters once the DNA is wrapped around histones. While it somewhat commonly known that DNA is the molecule that holds the genetic information of an organism. What are histones?

Histones are alkaline (basic pH) proteins. They are found inside the nucleus of eukaryotic cells. Their function is to package DNA into structural units called nucleosomes. Histones are the main proteins in chromatin. Chromatin is a combination of DNA and protein which makes up the contents of a cell nucleus. Because DNA wraps around histones, they also play a role in gene regulation.

Types of Histones

There are five families of histones (H1 through H5). H2A, H2B, H3, and H4 are the core histones, and H1 and H5 are the linker histones. The core histones form the center of the nucleosome, hence the term 'core.' The linker histones are found at the entrance and exit sites of the nucleosome and lock the DNA in place, hence the term 'linker.'

A strand of DNA will wrap around the core histones 1.65 times. Interactions between nucleosomes allow for higher-order structures to form. These higher-order structures can condense the chromatin to the point where chromosomes form. Chromosomes are found in the nucleus and contain genes. The familiar shape of a chromosome is that of an X. It is histones that make this familiar structure possible.

Specific Nucleosome Structure

Histones form dimers and tetramers. Dimers have two histones, and tetramers have four histones. Each nucleosome has two identical dimers, each comprised of one H2A and one H2B histone. This is called a H2A-H2B dimer. Each nucleosome also has one tetramer comprised of two H3 and two H4 histones. This is then called a H3-H4 tetramer. The individual histones and DNA form a nucleosome.

Modifications of Histones and Gene Regulation

Often histones are modified. In fact, there have been a large number of modifications identified by researchers. One example is that sometimes histones have a phosphate molecule added to them. Other times they have long tails added. It has been proposed that these modifications can collectively be called a histone code. Overall, however, the purposes of the modifications remain largely unknown.

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