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Chapter 24: Problem 33

What are the higher-order structures of DNA that eventually make up a chromosome?

Short Answer

Expert verified
DNA wraps around histones to form nucleosomes, coils into chromatin fibers, organizes into looped domains, and finally condenses into chromatids that make up chromosomes.

Step by step solution

01

Basic Unit of DNA - Nucleosome Formation

The DNA molecule is wrapped around histone proteins to form a structure called a nucleosome. Each nucleosome consists of 146 base pairs of DNA wrapped around an octamer of histone proteins. This is the basic unit of DNA packaging in eukaryotic cells.
02

Chromatin Fiber Formation

Nucleosomes further coil into a more compact structure known as chromatin. The "beads on a string" form of chromatin is about 10 nm in diameter, while further coiling makes a 30-nm chromatin fiber. This involves additional histone proteins interactions and DNA looping.
03

Formation of Loop Domains

The 30-nm chromatin fiber forms looped domains, which are attached to a non-histone protein scaffold. These loops contribute to a more compact and structured organization that facilitates efficient use of genetic information.
04

Chromatid Formation

The looped domains further compact to form a chromatid. In preparation for cell division, each chromatid condenses further to form one arm of the chromosome structure. These chromatids are held together by a centromere.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Nucleosome Formation
Nucleosome formation is the first step in the packaging of DNA into higher-order structures. In eukaryotic cells, DNA cannot simply float about freely due to its long and fragile structure. To begin its organization, the DNA wraps itself around special proteins called histones.

  • A nucleosome is approximately 146 base pairs of DNA wound around an octamer composed of eight histone proteins.
  • This setup resembles "beads on a string", where each nucleosome is a "bead."
This forms the most fundamental packing unit of DNA. The formation of nucleosomes not only condenses the DNA but also plays a role in regulating gene expression. This tightly packed DNA ensures that only certain areas are active at a given time, safeguarding cellular efficiency.
Chromatin Fiber
Once the DNA is organized into nucleosomes, the next level of higher-order structure formation is the creation of chromatin fiber. This step creates a more compact and structured arrangement of the nucleosomes.

  • The "beads on a string" arrangement, initially about 10 nm wide, coils further to produce a 30-nm fiber.
  • This coiling is aided by interactions among histones and additional proteins that assist DNA looping.
The resultant chromatin fiber is more condensed and plays a crucial role in regulating gene accessibility. Depending on needs such as DNA replication or transcription, chromatin can undergo remodeling which allows certain segments to unravel or tighten accordingly.
Chromatid
The next phase in DNA higher-order structure is the formation of chromatids. The 30-nm chromatin fiber forms looped domains by anchoring to a protein scaffold.

  • These looped domains help organize chromatin into more efficient and compact forms.
  • Prior to cell division, these loops coil tightly to form chromatids.
Each chromatid represents one "arm" of a chromosome, and during cell division, chromatids pair up to ensure accurate DNA distribution.
Chromatids are connected at a central region called the centromere, which is vital for holding sister chromatids together until they are separated and assigned to daughter cells, ensuring each cell receives the correct genetic material.

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