Question

Describe the "folded-fiber" model of the mitotic chromosome.

Short answer

Short answer: The folded-fiber model is a model of chromosomal organization that describes how DNA is condensed and packaged within a mitotic chromosome during cell division. It emphasizes linear folding and intertwining of chromatin fibers, creating organized patterns crucial for maintaining chromosome integrity. The model consists of components such as nucleosomes, 30-nm fiber, higher-order folding, and chromatid structure. It has been influential in understanding chromosome organization and histone protein roles; however, it is only one way to describe chromosome organization, and other models continue to be investigated as our knowledge of chromatin structure advances.

Step-by-step solution

Introduction to the mitotic chromosome

Mitotic chromosomes are the condensed form of DNA that is observed during cell division (mitosis). Each chromosome consists of two identical chromatids, which are joined together at the centromere. Chromosomes are composed of DNA and proteins, with histones being the primary proteins involved in the packaging of DNA.

The folded-fiber model concept

The folded-fiber model is a model of chromosomal organization that describes how DNA is condensed and packaged within a chromosome during mitosis. This model proposes that the DNA is wrapped around histone proteins to form nucleosomes, which are then further coiled and folded into higher-order structures. The folding is not random but is organized into specific patterns that help maintain chromosome integrity and aid in the proper distribution of genetic material during cell division.

Comparison to other models

Other models that attempt to explain chromosomal organization include the radial loop model and the hierarchical model. The radial loop model suggests that loops of DNA are anchored to a central scaffold, while the hierarchical model proposes a series of coiling and folding events to organize the chromosome. The folded-fiber model, on the other hand, emphasizes the linear folding and intertwining of chromatin fibers.

Major components of the folded-fiber model

The folded-fiber model consists of several key components: 1. Nucleosomes: These are the basic units of chromatin and consist of a stretch of DNA wrapped around a core of eight histone proteins. 2. 30-nm fiber: The nucleosomes are coiled together to form a solenoid-like structure called the 30-nm fiber, which serves as the basis of higher-order folding. 3. Higher-order folding: The 30-nm fibers interact with each other and fold upon themselves, creating loops and intertwining that compacts the DNA even further. 4. Chromatid structure: Each chromatid in a mitotic chromosome contains a single, long DNA molecule organized according to the folded-fiber model.

Implications and limitations of the folded-fiber model

The folded-fiber model has been influential in understanding chromosome organization and has guided many studies on the role of histone proteins and chromatin structure. However, it's important to note that this model is just one way to describe chromosome organization and that evidence for other models exists as well. Moreover, advances in microscopy and molecular biology techniques continue to provide new insights into chromatin structure, resulting in ongoing refinement and development of chromosome organization models.

Key concepts

Mitotic Chromosome

When a cell prepares to divide, the chromosomal structure changes significantly to ensure the genetic material is accurately passed to the daughter cells. During mitosis, chromosomes are highly condensed structures. This condensation allows chromosomes to be visible under a microscope. In this state, each chromosome comprises a pair of sister chromatids, which are identical copies of DNA joined at a central region called the centromere.
The condensation of the chromosomal structure is essential for accurate segregation during cell division. Without this compact state, the long strands of DNA could become tangled or broken.

• Condensed form of DNA facilitates easy movement.
• Centromere is crucial for the attachment of spindle fibers.

Mitotic chromosomes ensure that each daughter cell receives the correct genetic information.

Nucleosomes

Nucleosomes are fundamental units of DNA packaging in eukaryotic cells. Each nucleosome consists of DNA wrapped around a core of eight histone proteins. This structure looks somewhat like beads on a string, where the beads are nucleosomes and the string is the DNA.
The wrapping of DNA around histones reduces its length, making it more compact and organized within the cell nucleus. This arrangement plays a critical role in regulating gene expression, as only unpacked DNA can be accessed to make proteins.

• Each nucleosome contains two copies of four different histones: H2A, H2B, H3, and H4.
• The "linker" DNA between nucleosomes is bound by histone H1, helping to further pack the chromatin.

Nucleosomes help prevent damage to DNA and regulate access to genetic information.

Chromatin Structure

Chromatin is the material that makes up chromosomes, composed of DNA and proteins, primarily histones. The chromatin structure is dynamic and varies depending on the cell's functional state.
During the interphase, before mitosis, chromatin is generally in a less condensed form called euchromatin, which is rich in active genes. In contrast, heterochromatin is more compact and is found in regions where gene expression is low.

• Euchromatin ensures that genes can be accessed and transcribed as needed.
• Heterochromatin maintains structure and regulates genes that need to be "switched off."

The folding and arrangement of chromatin allow efficient packaging while making specific sections accessible for gene expression.

Histone Proteins

Histone proteins are essential components of chromatin and play a critical role in the packaging of DNA into chromosomes. Histones are positively charged, which allows them to bind tightly to the negatively charged DNA.
These proteins are not just structural; they also influence gene regulation. By altering how tightly or loosely DNA is wrapped around histones, cells can control which genes are expressed.

• Histones undergo various chemical modifications such as acetylation and methylation, impacting gene activity.
• Histone modifications contribute to the epigenetic regulation of genes.

Learning about histone proteins provides insight into how cells manage vast amounts of DNA within the confined space of the nucleus.