Relationship of DNA and Chromosomes

Key Concepts

■ DNA, Genes, Chromosomes – Relationship

■ DNA Packaging

■ Prokaryotic vs Eukaryotic DNA packaging

■ Levels of DNA packaging

■ Hetrochromatin and euchromatin

Introduction

The complete set of genetic instructions of any living organism is known as ‘genome’. The genome contains the complete set of information required for the growth and development of an organism. The characteristics of any living organism are determined by its genome. Every living organism has its own unique genome. No two genomes are found to be exactly alike. The genome of any living cell is composed of three important components,  

1. DNA – The genetic material 

2. Genes – Segments of DNA that form the functional segments of chromosome 

3. Chromosomes – Thread-like structures inside the living cell that contain genes 

DNA is arranged into functional units known as genes. In other words, DNA is the building block of genes. Many genes combine together to form thread-like structures known as chromosomes. Chromosome structures are formed only during cell division. When the living cell is not in the cell division phase, the genetic material is in the form of tangled treads known as chromatids. During cell division, chromatids undergo condensation to form chromosomes. 

DNA, Genes and Chromosomes: 

Chromosomes are thread-like structures present in the nucleus of plant and animal cells. They are complexes of DNA and protein. Chromosomes carry genetic material that is transferred from one generation to the other. Chromosomes are clearly visible during cell division. When the living cell is not in the cell division phase, the genetic material is in the form of tangled treads known as chromatids. 

Chromatids are tangled threads at a normal state. During cell division, chromatids undergo condensation and are arranged into chromosome structures. Chromosomes are made up of functional segments known as genes. Genes are made up of DNA. DNA is a double-stranded molecule made up of an antiparallel polydeoxyribonucleotide chain. The repeated monomeric units of DNA are known as nucleotides. Each nucleotide has three components – phosphate, sugar, and nitrogen-containing base (A, T, G, C). In short, DNA is made up of deoxyribose sugar, phosphate, and nitrogenous bases adenine (A), guanine (G), cytosine (C) and thymine (T). 

What are genes? 

Genes are segments of DNA that form the basic physical unit of inheritance. DNA is known as the ‘blueprint of life.’ Genes contain a set of instructions to produce specific biomolecules. Some genes carry instructions to produce protein molecules. Proteins are workhorses that control the growth, development, and characteristics of an organism. Some genes carry instructions to produce RNA molecules. Genes are also responsible for several characteristics such as skin color, eye color, blood type, height etc. Thousands of genes are stored in a chromosome. According to the requirements of an organism, different genes are read at different times.  

DNA Packaging: 

DNA packaging refers to the condensation of DNA to fit into the cell. The process of DNA packaging is different in prokaryotes and eukaryotes. 

DNA Packaging in Prokaryotes: 

Prokaryotic cells have a single circular chromosome that is located in the nucleoid region of the cell. Prokaryotic DNA is tightly packed to fit in the nucleoid region. The circular chromosomal DNA is compactly packed by the formation of a looped structure. The looped DNA is further compacted by the formation of supercoiled structure. Prokaryotic chromosomes do not have histone protein. DNA binding proteins, namely, HU and integration host factor (IHF), bind to the DNA and aid the formation of supercoiled structure. Eukaryotic cells have a linear chromosome that is present within the nucleus. 

DNA Packaging in Eukaryotes: 

The length of eukaryotic DNA is much greater than the length of prokaryotic DNA. Hence, the condensation of DNA is very important to fit into the nucleus. There are four levels of DNA packaging. 

1. First order of packaging – Nucleosome 

2. Second order of packaging – Solenoid fiber 

3. Third order of packaging – Chromatid 

4. Fourth order of packaging – Chromosome 

Nucleosome: 

In eukaryotic chromosomes, nucleosomes are the basic structural units. Nucleosomes are fundamental repeating subunits of eukaryotic chromosomes. Nucleosomes are composed of histone and non-histone proteins. Histones are positively charged proteins that play an important role in DNA packaging. Histones are classified into five different types, namely, H1, H2A, H2B, H3 and H4. DNA wraps around these histone proteins. As DNA is negatively charged and histones are positively charged, they easily bind with each other. A single nucleosome contains around 150 base pairs of DNA. In each nucleosome, 1.65 times of DNA is wound around eight histone proteins (histone octamer).  

Solenoid Fiber: 

Nucleosomes fold up to form 30 nanometers of chromatin fiber. Chromatosomes are the basic units of chromatin structure. They contain nucleosome and H1 histone protein. The solenoid model is also known as a secondary chromatin structure. It represents the formation of helical loops with an average length of 300 nm. Nucleosomes are assembled one over the other to form long chromatin fiber.  

Chromatid and Chromosome 

The 300 nm chromatin fiber is further compressed and folded to produce 250 nm wide fiber. These fibers are tightly coiled to form chromatids of the chromosome. The final packaging of chromosomes involves looping, scaffolding, and condensation of chromatids.  

Heterochromatin and Euchromatin: 

Based on the level of compaction, chromatins are classified into two types,  

1. Heterochromatin 

2. Euchromatin 

Heterochromatin chromatins have tightly packed DNA, whereas euchromatin chromatins have loosely packed DNA.  Density of DNA is high in heterochromatin, whereas it is low in euchromatin. Heterochromatins are not present in prokaryotes. 

Summary

• Genome refers to the complete set of genetic instruction present in any living organism.

• Each and every organism has a unique genome

• Genome of each organism is made up of DNA, genes and chromosomes

• Chromosomes are thread like structures that carry the genetic material

• In eulcaryo tic cell, chromosomes are linear and are located in the nucleus.

• In a prokaryotic cell, chromosome is circular and is located in the nucleoid region of the cytoplasm

• Chromosomes are clearly visible during cell division.

• Genes are segments of DNA which form the basic physical unit of inheritance

• Thousands of genes are stored in a chromosome.

• DNA packaging refers to the condensation of DNA to fit into the cell. The process of DNA packaging is different in prokaryotes and eukaryotes.

• Prokaryotic chromosomes do not have histone protein.

• In eukaryotes, histones are mainly involved in DNA packaging.

• DNA packaging in eukayotes takes place at four levels – nucleosome, solenoid fiber, chromatin, chromosome. • Based on the level of compaction, chromatins are classified into two types – heterochromatin, euchromatin

• Heterochromatin have tightly packed DNA whereas euchromatin have loosely packed DNA.