Problem 174 - Objective Biology for NEET

Question

Assertion: DNA is associated with proteins. Reason: DNA binds around histone protein that form a pool and the entire structure is called a nucleosome.

Step-by-Step Solution

Answer

Both the assertion 'DNA is associated with proteins' and the reason 'DNA binds around histone protein to form a structure known as a nucleosome' are correct and the reason accurately explains the assertion.

Step 1: Understand the Components

First, it is essential to know what DNA, proteins, histone proteins, and nucleosomes are. DNA (Deoxyribonucleic acid) is a molecule that carries genetic instructions in all living organisms and many viruses. Proteins are large, complex molecules that are vital for the body’s structure, function, and regulation of tissues and organs. Histone proteins are basic proteins that package and order the DNA into structural units called nucleosomes.

Step 2: Verify the Assertion

The assertion is 'DNA is associated with proteins.' From the definition in the previous step, we can confirm that this is correct. DNA is indeed associated with proteins. In the chromatin of eukaryotic cells, DNA is wrapped around histone proteins, a form of which is a nucleosome.

Step 3: Verify the Reason

The reason given is that 'DNA binds around histone protein that forms a unit, and the whole structure is called a nucleosome'. The histones help order DNA into a structured form, and the DNA wound around the histones is called a nucleosome. Hence, the reason is also correct and is a valid explanation for the assertion.

Key Concepts

DNA structure

DNA, or deoxyribonucleic acid, is the blueprint of life. Imagine DNA as a long, twisted ladder, where the sides of the ladder are made up of sugar and phosphate molecules. The rungs of the ladder consist of bases paired together to form what we know as base pairs. There are four types of bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). In DNA structure, these bases pair in a specific way: A with T, and C with G.
This pairing is essential for the DNA molecule to maintain its structure and carry out its functions. DNA is found in every cell and contains instructions needed to construct proteins and consequently, every part of our body. Furthermore, strands of DNA coil tightly to form structures known as chromosomes, which reside in the nucleus of the cell.

Histone proteins

Histone proteins are like spools around which DNA is wound. They play a crucial role in DNA packaging within the nucleus of the cell. Without histones, the lengthy DNA molecules would be too long to be housed within the confines of a cell nucleus.
There are five main types of histone proteins: H1, H2A, H2B, H3, and H4. These proteins are not only structural, but they also help regulate genes. The DNA wraps around histone proteins, forming a structure called a nucleosome.
Nucleosomes are the building blocks of chromatin, and are composed of about 146 base pairs of DNA wrapped around a core of eight histone proteins. This arrangement helps to compact the DNA, aiding in its efficient organization and access during processes such as replication and transcription.

Chromatin organization

Chromatin is the term used to describe the complex of DNA and proteins found in the nucleus of eukaryotic cells. It plays a key role in DNA packaging, allowing the long DNA molecules to fit within the cell nucleus.
The primary structural unit of chromatin is the nucleosome, which is like a bead on a string of DNA. Chromatin can be compacted into higher-order structures to form chromosomes during cell division. Two main types of chromatin exist: **euchromatin** and **heterochromatin**.

• Euchromatin is less tightly packed, making it accessible for transcription and therefore, considered active in terms of gene expression.
• Heterochromatin is more tightly packed, often considered inactive in terms of gene expression, since it is less accessible for transcription.

Understanding chromatin organization helps explain how cells manage to replicate accurately or respond to various signals that might trigger gene expression.