Chapter 10, Problem 1P

Give descriptions for the following terms:

histone proteins

nucleosome core particle

scaffold proteins

G bands

euchromatin

heterochromatin

nucleosome

chromosome territory

Summary Introduction

To review:

Describe the following terms:

a) Histone proteins

b) Nucleosome core particle

c) Scaffold proteins

d) G bands

e) Euchromatin

f) Heterochromatin

g) Nucleosome

h) Chromosome territory

Introduction:

The genome of the species is referring to the complete set of genes as well as the total of all heredity information carried in the DNA of the species. The genetic information is present in the chromosome. The eukaryotic genome and their organization are quite larger and complex than that of the prokaryotes. The prokaryotes carry $\text{1 to 2}$ chromosome in their genome and as there is only one copy of each chromosome present, they are known as haploid (n) whereas eukaryotes have at least two copies of each chromosome and are therefore referred to as diploid and denoted as $2$ n. Many animal species and plant species are diploid; however, those who have more than $2$ copies of the chromosome are called as polyploid.

Explanation of Solution

A chromosome is a threadlike structure, made up of log DNA associated with the protein. The DNA combined with the protein is referred to as the chromatin. Chromatin stabilizes the DNA, protects the DNA from damage and compacts the chromosome efficiently so that they can fit into the nucleus of the cell.

a) Histone protein

Histone protein is the main protein in the chromatin. Histone proteins wrap themselves around the DNA to pack the DNA into the structural unit. These units are called nucleosomes. There are eight histone proteins (Octamer). Each octamer is made up of two copies of each histone protein that packed the DNA to form a nucleosome, namely $\text{H2A, H2B, H3, and H4}$.

b) Nucleosome core particle

Using DNA and histone protein, the nucleosome core particle is made. It plays a very important role in the packaging of DNA in the chromosome. It consists of fourteen turns of B-form DNA around the octomer of histone protein.

c) Scaffold protein

In the chromosome, the non-histone protein remaining, after all the DNA and histone proteins are removed from a chromosome is called DNA scaffold. During the chromosome condensation DNA scaffold occurs.

d) G- the band

Chromosomes are stained with the Giemsa stain to generate the chromosome bands. This method of banding is called the Giemsa banding, and the bands appearing on the chromosomes are called the G –bands. The G- banding nomenclature uses letters and numbers to identify the major and minor bands on the chromosome. The numbering begins from the centromere of the chromosome and progresses outward along each arm of the chromosome towards the telomere. Each band on the chromosome has a specific designation given by the chromosome numbers, arm, and the band location.

e) Euchromatin

The DNA combined with the protein is referred to as the chromatin. The chromosome contains DNA which is made up of about $\text{1}$ million to $\text{10}$ million base pairs. The DNA in the nucleus is present in two forms, euchromatin, and heterochromatin.

Euchromatin is a loosely packed form of DNA, which is active in the transcription of many cells. The DNA which in the heterochromatin form, i.e., tightly packed, converts themselves in euchromatin at the time of transcription in the cell. Euchromatin form of the DNA is found in both prokaryotes and eukaryotes; it contains less DNA than the heterochromatin. Therefore, it shows lightly stained region.

f) Heterochromatin

In contrast to euchromatin, heterochromatin is the tightly packed form of DNA. Heterochromatin is found only in the eukaryotes. Unlike euchromatin, heterochromatin is the transcriptionally inactive form of the DNA. Heterochromatin form of the DNA stains dark as it has a high density of DNA than euchromatin.

g) Nucleosome

The DNA wrapping itself by histone protein is referred to as the nucleosome. The nucleosome is made up of eight histone proteins (Octamer), and each octamer is made up of two copies of each histone protein that packed the DNA to form a nucleosome, namely $\text{H2A}$, $\text{H2B}$, $\text{H3}$, and $\text{H4}$.

h) Chromosome territory

The region of the nucleus which is preferentially occupied by the particular chromosomes is the chromosome territory.

Conclusion

The eukaryotic genome organization has a very complicated structure in which various protein forms and structures of DNAs are involved.