Question

Briefly describe the structure of chromatin.

Short answer

Chromatin is a complex of DNA and proteins. Its basic unit is the nucleosome, and it forms higher-order structures like the 30-nm fiber. Chromatin exists as euchromatin and heterochromatin.

Step-by-step solution

Define Chromatin

Chromatin is a complex of DNA and proteins found in the nucleus of eukaryotic cells. It functions to efficiently package DNA into a small volume to fit into the nucleus of a cell and protect the DNA structure and sequence.

Describe the Basic Units

The fundamental unit of chromatin is the nucleosome. A nucleosome is made up of a segment of DNA wound in sequence around eight histone protein cores. This structure resembles 'beads on a string' when observed under a microscope.

Explain Higher-Order Structures

Nucleosomes are further folded into higher-order structures. This compact structure is called the 30-nm fiber, which then forms looped domains. These looped domains are further organized to form the metaphase chromosome.

Discuss Chromatin States

Chromatin can exist in two forms: euchromatin and heterochromatin. Euchromatin is less condensed and associated with active transcription, while heterochromatin is highly condensed and typically transcriptionally inactive.

Key concepts

Nucleosome

The nucleosome is the fundamental unit of chromatin, acting like the 'basic building block' of DNA packaging. Essentially, it consists of DNA wrapped around histone proteins. To be specific, about 147 base pairs of DNA coil around a core of eight histones, forming a structure similar to 'beads on a string'.
These histones are proteins that help stabilize and package DNA. Histones are classified into five types: H1, H2A, H2B, H3, and H4.
Four pairs of histones (two of each type: H2A, H2B, H3, and H4) assemble to create the histone core around which DNA wraps itself. The surrounding DNA and histone core make up the nucleosome. Histone H1 then acts as a linker, aiding in the binding between nucleosomes.
The primary role of nucleosomes is to condense the DNA so that it can fit within the nucleus, while also playing crucial roles in how genes are accessed and expressed. Changes to this structure can influence gene regulation and other cellular processes.

Euchromatin and Heterochromatin

Chromatin can be categorized into two distinct forms: euchromatin and heterochromatin. Understanding these forms is vital as they play key roles in DNA accessibility and gene expression.
Euchromatin is the less condensed type of chromatin. In this state, it appears lighter under a microscope. Euchromatin is more accessible for transcription machinery, meaning it is generally associated with active gene expression. This chromatin region holds genes that are frequently being transcribed into RNA.
On the other hand, heterochromatin is highly condensed, making it appear darker under the microscope. Because of its tightly packed structure, it is transcriptionally inactive or silent. Heterochromatin usually contains repetitive DNA sequences and regions such as telomeres and centromeres.
Aside from structural differences, the dynamic shifting between euchromatin and heterochromatin also helps control cellular processes like replication, repair, and transcriptional regulation. The balance of these two states helps maintain the proper functioning of the genome.

30-nm Fiber

Beyond the nucleosome level, chromatin compacts even further into higher-order structures like the 30-nm fiber. This level of organization involves the nucleosomes coiling into a more compact form to facilitate even tighter packaging.
The next level of folding turns the 'beads on a string' structure into the 30-nm fiber, which is approximately 30 nanometers in diameter. Though the exact structure is still a subject of ongoing research, it is believed that this fiber organizes into either a solenoid or zigzag model—both of which enhance the density of DNA packing.
This higher-order structure is essential for fitting about 2 meters of DNA into the microscopic nucleus of a cell. The 30-nm fiber forms looped domains that anchor to a protein scaffold within the nucleus, facilitating chromosome organization and accessibility for cellular processes.
During cell division, these fibers further compact, eventually forming the visible metaphase chromosomes. The ability to compact and expand as needed plays a key role in genetic regulation and expression.