Chemical Composition of Chromosome and its Structure

What are Chromosomes ?

• Chromosomes are string-like structures located inside the nucleus of the cells.
• Chromosomes are thread-like structures in which DNA is tightly packaged within the nucleus.
• Chromosome is derived from the Greek words “chromo” meaning colour and “soma” meaning body.
• They got this name because Chromosomes become strongly stained when colourful dyes are applied to them.

Discovery of Chromosomes

• Hofmeister (1848) : First observed chromosomes in microsporocytes (microspore mother cells) of Tradescantia.
• Flemming (1879) : Observed splitting of chromosomes during cell division and coined the term, ‘chromatin’.
• Roux (1883) : He believed the chromosomes take part in inheritance.
• W.Waldeyer (1888) : He coined the term ‘chromosome’.
• Benden and Boveri (1887) : They found a fixed number of chromosomes in each species.

Chemical Composition of Chromosome

• The Chemical Composition of Chromosome is of Nucleoproteins i.e. Nucleic Protein DNA and basic protein Histones.
• Each chromosome contains one DNA molecule.
• The DNA is coiled tightly around proteins called histones.
• Histones provide structural support to a chromosome and allow the very long DNA molecule to form a compact shape and fit inside the nucleus of a cell.

Chromatin Fibres

• The nucleoplasm contains many thread-like, coiled and much elongated structures which are known as the chromatin substance or chromatin fibres.
• Observed only in the interphase nucleus. During the cell division (mitosis and meiosis) chromatin fibres become thick ribbon-like structures which are known as the Chromosomes.
• Chemically, chromatin consists of DNA and protein with small quantity of RNA (not more than about 5 %).
• Histones are constituents of the chromatin of all eukaryotes except fungi, which, therefore, resemble prokaryotes in this respect.
• The fibres of the chromatin are twisted and uniformly distributed in the nucleoplasm.
• Two types of chromatin material have been recognised, e.g., heterochromatin and euchromatin.
• Marked differences are observed in the staining behaviour of different regions of chromosomes during interphase. Some regions of chromosomes get dark stain while other get light stain. Hence, they are classified as –

Heterochromatin

• • The darkly stained regions are known as Heterochromatic regions.
  • The genes in heterochromatin become active for a short period.
  • Covers small regions of Chromosomes.
  • Contains small amount of the DNA and large amount of the RNA.
  • Usually found near Centromere and telomere. Found in eukaryotes

Euchromatin

• • Lightly stained portions are referred to as Euchromatin regions.
  • Contains comparatively large amount of DNA.
  • Usually found in middle region of Chromosomes.
  • Covers large regions of Chromosomes.
  • Found in eukaryotes & prokaryotes

 

• chromatin is composed of DNA and Histones with some other proteins.

1. DNA – Deoxyribo Nucleic acid

• • Chromosomes are composed of about 40 % of DNA.
  • DNA is a group of molecules that is responsible for carrying and transmitting the hereditary materials or the genetic instructions from parents to offsprings.
  • The DNA molecule is composed of materials called nucleotides, and each nucleotide is composed of three different components such as sugar, phosphate groups, and nitrogen bases.
  • The genetic material in most organisms is DNA or Deoxyribonucleic acid. A DNA molecule consists of two polynucleotide chains i.e. chains with multiple nucleotides which is known Polynucleotide Chain.

 

Structure Of Polynucleotide Chain

• • • A nucleotide is made of the following components –
    • Pentose sugar– A pentose sugar is a 5-carbon sugar. In case of DNA, this sugar is deoxyribose whereas, in RNA, it is ribose.
    • Phosphate group
    • Nitrogenous base– These can be of two types – Purines and Pyrimidines. Purines include Adenine and Guanine whereas pyrimidines include Cytosine and Thymine. In RNA, thymine is replaced by Uracil.
    • The backbone of a polynucleotide chain consists of pentose sugars and phosphate groups; whereas the nitrogenous bases project out of this backbone.

2. Histone

• • Histones are the proteins that facilitate the packaging of DNA into chromatin fibres.
  • Histone proteins are positively charged and have many arginine and lysine amino acids that bind to the negatively charged DNA.
  • They are the chief protein components of chromatin, acting as spools around which DNA winds, and playing a role in gene regulation.
  • Histones are proteins that help in the coiling and packaging of DNA into structural unit called Nucleosome.
  • DNA strands winds around a core of 8 histone proteins called Histone Octamer.
  • These helps to wind up long DNA molecule into a small condensed Chromosome.

Difference between Chromatin and Chromosomes

Chromatin	Chromosome
In the nucleus, the DNA double helix is packaged by histones to form a complex called chromatin. The chromatin undergoes further condensation to form the chromosome.	A compact structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes.
Composed of nucleosomes-a complex of DNA and proteins	Chromosomes are condensed Chromatin Fibers.
Chromatin Fibers are Long and thin.	Chromosomes are compact, thick and ribbon-like.
Found throughout the cell cycle.	Distinctly visible during cell division (metaphase, anaphase)
Allows DNA Replication, gene recombination etc.	Donot show metabolic activities.

Difference between Chromatid and Chromosomes

Chromosome	Chromatid
They carry the genetic material	Their main function is to enable the cells to duplicate
A chromosome occurs throughout the cell’s life cycle	They occurs when cell undergoes mitosis or meiosis
Chromosomes are not exact copies of each other. One copy of gene comes from each parent	Sister Chromatids are identical copies of each other
Chromosomes have centromeres	Only Sister Chromatids have centromeres
DNA is used during macromolecule synthesis  (production of complex proteins)	DNA is not used during macromolecule synthesis

Structure of Chromosome

(Morphology of Chromosome)

• Each chromosome consists of seven parts which are –

1. 1. 1. Chromatids
      2. Centromere
      3. Telomere
      4. Chromomere
      5. Secondary constriction and satellite
      6. Chromonema

 

1. Chromatids

• • One of the two distinct longitudinal subunits of a chromosome is called chromatid. In other words, Each chromosome consists of two symmetrical structures, called chromatids.
  • Each chromatid contains a single DNA molecule.
  • Both chromatids are attached to each other only by the centromere.
  • They separate during anaphase, when the sister chromatids of a chromosome migrate to the opposite poles.
  • Chromatids are of two types –
    • • Sister Chromatids are derived from one and the same chromosome.
      • Non-sister Chromatids originate from homologous chromosomes.

2. Centromere

• • The region of chromosome with which spindle fibres are attached during metaphase is known as centromere or primary constriction or kinetochore.
  • Centromere has four important functions –
    • • Orientation of chromosomes at metaphase,
      • Movement of chromosomes during anaphase,
      • Formation of chromatids, and
      • Chromosome shape.
  • Centromere may occupy various positions on the chromosome, viz., terminal, sub-terminal, median, etc.
  • Depending upon the position and number of centromere, chromosomes are given various names.

3. Telomere

• • (telo=for; meros=part)
  • Each extremity of the chromosome has a polarity and therefore, it prevents other chromosomal segments to be fused with it.
  • The terminal region of a chromosome on either side is known as telomere.
  • Each chromosome has two telomeres.
  • The telomere of one chromosome cannot unite with the telomere of another chromosome due to polarity effect.

4. Chromomere

• • The linearly arranged bead like structures of chromatin material found on the chromosomes is known as chromomeres.
  • Clearly visible in the polytene chromosomes.
  • Chromomeres are regions of tightly folded DNA.
  • The Chromomere bearing chromatin has an appearance of a necklace in which several beads occur on a string.
  • At metaphase the chromosomes are tightly coiled and the chromomeres are no longer visible.

5. Chromonema

• • Thread like coiled structures are found in the chromosomes and chromatids which are called chromonema.
  • Main Functions of Chromonema includes –
    • • It controls size of chromosomes,
      • results in duplication of chromosomes and
      • It is the gene bearing portion of chromosomes.
  • Chromonema is a structure of sub-chromatid nature.

6. Secondary Constriction & Satellite

• • The constricted or narrow region other than that of centromere on Chromosome is called secondary constriction.
  • It has constant position and, therefore, can be used as useful marker.
  • A chromosome segment separated from the main body of chromosome by one secondary constriction is known as satellite.
  • A chromosome with secondary constriction is referred to as satellite chromosome or Sat-chromosome.

So, this was all about the Chemical Composition of Chromosome and its Structure.

In the Next Post Click here), We  will discuss about Shapes of Chromosome and its Function, Karyotype.