Chapter No.6 : Chromosomes And DNA - Botany - For HSC Part 2 (XII) - Science Group

GO TO INDEX

Chapter No.6
Chromosomes And DNA

Botany

For HSC Part 2 (XII) Science Group

Q.No.7: Explain ultrastructure of Chromosomes.
Ans: Ultrastucture of Chromosomes:
The chromosome of a eukaryotic cell has 40%. DNA and 60% protein. A significant amount of RNA is also associated with chromosome because DNA is responsible for RNA formation. The DNA of a chromosomes is very, long double stranded fiber that stands, a duplex which extends unbroken through the entire length of chromosome.
A typical human chromosome has 140 million nucleotide in its DNA. The amount of information Coded in one chromosome fills about 280 books of 1000 pages. If a strand of a chromosome is laid in straight line, it would be 5 cm long.
Under electron microscope DNA of a eukaryotic cell show following Ultrastucture of Chromosomes:

Nucleosome:
The coiling of DNA is like string of beads. DNA duplex is coiled around a complex histone, which are small, very basic polypeptides, rich in the amino acids, arginine and lysine. Eight of these histone form the core of an assembly called a 'NUCLEOSOME'. Because so many of their amino acids are basic histones are very positively charged. The DNA duplex, which is negatively charged, is strongly attracted to histone and wraps tightly around the histone core of each nucleosome. The core thus acts as a magnetic forms that promotes and guides the coiling or DNA. Nucleoprotein (histone) gives DNA fiber as a beaded structure. The nucleosome is repeated after every 200 nucleotides of DNA.

Spacer DNA:
The DNA present from one nueleosome (bend) to an other is called Spacer DNA.

Super Coil:
Further coiling of DNA occurs when the string of nucleosome wraps into higher-order coils called Super Coil.

Chromatin:
The super coil network is called chromatin (Chromatin network) Chromatin is a loose network of threads which is seen in non living nucleus. Chromatin is again classified into Euchromatin and heterpchromatin on the basis of staining properties.

Heterochromatin:
Stains deeply because it is more coiled and compact it is inactive and never expressed itself.

Euchromatin:
Stains more during nuclear division and is involved in transcription (Protein Synthesis).