Nucleosomes, Chromatin, and Chromosomes
A nucleosome is a structure in your chromosomes, or bundled DNA. Each nucleosome has a core particle, DNA, and a linker protein. The proteins in the core particle and linker proteins are called histones. The DNA will wrap around the core particle about 1.65 times and is secured by the linker protein. This figure shows a drawing of a nucleosome.
About 200 bases of DNA are involved with each nucleosome. This includes the portion that is wrapped around the core and a bit of a tail region that connects to the next nucleosome. This arrangement is said to look like beads on a thread. Several nucleosomes together are called chromatin. Chromosomes are bundles of tightly packed chromatin. Humans have 23 pairs of chromosomes.
The protein portion of a nucleosome is made of histones. There are five major families of histones, which include H1, H2A, H2B, H3, H4, and H5. The core particle has eight total histones. One H2A and H2B bind together to form a dimer, or two proteins bound together. An H3 and an H4 will bind to also form a dimer. Next, one H3/H4 dimer binds to another H3/H4 dimer to form a tetramer, or four proteins bound together. Finally, an H2A/H2B dimer will bind an H3/H4 tetramer to create the core particle.
After DNA wraps around the core particle either H1 or H5 will bind. The purpose of H1 and H5 is to secure the DNA strand to the core particle. Either H1 or H5 can do this function.
Nucleosomes and Types of Chromatin
DNA arranged in nucleosomes is called chromatin. The histones in nucleosomes interact to form more complex structures of chromatin. As a result, there are several types of chromatin. The type depends on the phase of the cell cycle and how much the genes in a particular region of DNA are needed for cell function.
When a cell prepares to divide it must first compact its DNA into chromosomes. This ensures that the DNA is not broken and helps ensure transfer into daughter cells. DNA in this form is called chromosomal chromatin.
When a cell is not undergoing cell division, its DNA is not packaged into chromosomes. This part of the cell cycle is called interphase. Thus, DNA in this form is called interphase chromatin. The DNA is spread out throughout the nucleus and looks like a plate of spaghetti.
Interphase chromatin can be of two types. The first is a tightly bound version called heterochromatin. This is also called silent chromatin because the DNA is not accessible to the protein-making machinery of the cell. Therefore, the genes in heterochromatin are not expressed, or silent.
The second type of interphase chromatin is called euchromatin. This version of chromatin is more loosely packed than heterochromatin. This is also called active chromatin because the genes are accessible to the protein-making machinery of the cell. Thus, the genes in euchromatin are expressed, or active. This figure shows these different types of chromatin.
Let's review. DNA must be bundled to fit inside the nucleus. The smallest DNA bundle is called a nucleosome and it is made of DNA and protein. The protein portion is made of small units called histones. The core particle is made of four types of histones (H2A, H2B, H3, and H4). DNA wraps around the core particle. Either the H1 histone or the H5 histone will be used to bind the DNA to the core particle.
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In this activity, students are going to be applying their knowledge of the nucleosome to create two three dimensional models and comparing them. In order to do this, you should have a variety of supplies on hand that students can use for their models, such as clay, wire, beads, pipe cleaners, string, glue, tape, pom poms, and anything else you'd like to include. The more supplies are offered, the more creative your models will be.
For example, a student might use the beads on a string analogy, and actually create a tetramer of histone proteins with beads. The wire could be twisted to look like the DNA helix, and then wrapped around the tetramer twice, as shown in the lesson. Students could also create a more involved double helix shape from pipe cleaners or paper, and wrap them around clay histone proteins.
Now that you're familiar with the shape of nucleosomes, it's time to build your own with craft supplies. In this activity, you'll be building two different models from craft supplies that represent a nucleosome, then comparing the accuracy of your models. Check out the criteria for success on how to create your models. Then, once you've created two different models, answer the questions below.
Criteria For Success for Nucleosome Models:
- Two different models are created, each with different combinations of craft supplies
- Models include an 8-histone core with the appropriate histone proteins and H1
- Model includes a double-stranded helix of DNA
- Compare your two models of the nucleosomes. What was similar and what was different between the two models?
- Which model was a more accurate representation of a nucleosome and why do you think that?
- Is there anything you could do to make your model more accurate? Why or why not?
Students should be able to create two models using different supplies. This shows students that scientific concepts can be represented in a variety of ways and encourages creativity and critical thinking. To evaluate the more realistic model, students should look for details like the different histones included in the model, the double helix shape of DNA, and accuracy compared to the image in the lesson. Students might think that they could include more nucleosomes to make the model more accurate or house them in a nucleus, as in a real cell.
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