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Ch. 12 - DNA Organization in Chromosomes
Chapter 12, Problem 10

Describe the transitions that occur as nucleosomes are coiled and folded, ultimately forming a chromatid.

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Start by understanding that nucleosomes are the basic unit of DNA packaging in eukaryotic cells, consisting of a segment of DNA wound around a core of histone proteins.
Recognize that the first level of DNA packaging involves the formation of the 10-nm fiber, where DNA is wrapped around histone proteins to form nucleosomes, resembling 'beads on a string.'
Next, these nucleosomes are further coiled into a 30-nm fiber, which involves the nucleosomes folding into a more compact structure, often facilitated by the H1 histone, creating a solenoid or zigzag pattern.
The 30-nm fiber is then organized into looped domains, which are attached to a protein scaffold, forming a structure that is approximately 300 nm in diameter. This level of organization is crucial for further compaction.
Finally, during cell division, these looped domains are further coiled and folded to form the highly condensed structure known as a chromatid, which is one half of a duplicated chromosome, ensuring efficient segregation during mitosis or meiosis.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Nucleosome Structure

Nucleosomes are the fundamental units of chromatin, consisting of DNA wrapped around a core of histone proteins. This structure allows for the compaction of DNA, making it possible to fit the long strands of genetic material into the nucleus of a cell. Each nucleosome is connected by a short stretch of linker DNA, creating a 'bead-on-a-string' appearance that is essential for higher-order folding.
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07:10
Chromosome Structure

Chromatin Organization

Chromatin is organized into two forms: euchromatin, which is loosely packed and accessible for transcription, and heterochromatin, which is tightly packed and generally inactive. The transition from nucleosomes to higher-order structures involves the coiling and folding of chromatin fibers, which can further condense into loops and domains, ultimately leading to the formation of a chromatid during cell division.
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Chromatid Formation

A chromatid is one half of a duplicated chromosome, formed during the S phase of the cell cycle. As nucleosomes coil and fold, they create a 30-nanometer fiber, which further loops and condenses to form the characteristic X-shaped structure of chromatids. This organization is crucial for the accurate segregation of genetic material during mitosis and meiosis.
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