DNA Packaging

DNA – Deoxyribonucleic acid is a hereditary material which is found in the nucleus of the cell and is mainly involved in carrying the genetic information used in the growth, reproduction, development, and functioning of all living organisms.

DNA is an organic, complex, molecular structure, found in both prokaryotic and eukaryotic cells and also in many viruses.

DNA Structure

DNA Structure

In the year 1953, the DNA structure was suggested by Watson and Crick. According to their discovery, the DNA is a double helix, the long molecule made up of a double helical structure of two polynucleotide chains or strands running anti-parallel to each other. The two strands are connected by Hydrogen bonds between the complementary nitrogenous bases. The Adenine pairs up with Thymidine with the help of two H-Bonds and Guanine pairs up with Cytosine with the help of three H-bonds.

DNA is a polymer of nucleotides, namely the deoxyribonucleotides. The term deoxyribonucleotides can be divided into two words i.e., deoxyribose + nucleotides, where deoxyribose is a sugar and nucleotide is made up of a nitrogenous base, a sugar molecule, and phosphate group.

DNA Packaging

Eukaryote DNA structure

As mentioned above:

  • The strands of the DNA are helically wounded, every single strand forms a right-handed coil.
  • The pitch of each helix is 3.32 nm and about 10 nucleotides make up one turn.
  • The distance between two succeeding base pairs is 0.34nm
  • The total length of a DNA is the distance between two succeeding base pairs and the product of a total number of base pairs.
  • A typical DNA has an extent around 2.2 meters which is much longer than a nucleus.

Have you ever wondered how a DNA is present in a nucleus which smaller than it?

Prokaryotes are distinguished from eukaryotes for lacking a well-defined nucleus. However, their DNA which is negatively charged are arranged properly in a region called nucleoid. They appear as a loop wrapped around a protein molecule having positive charges.

 

Packaging of DNA helix

All eukaryotes have well-structured nucleus and organelles. Hence, their DNAs are arranged systematically. DNA is a negatively charged polymer which is compactly packed inside the chromatin, around a ball of positively charged proteins known as histone proteins. The octamer of these histone proteins is wrapped with DNA helix giving rise to a structure called nucleosome. The nucleosomes makeup to a repeating structure which results in the formation of chromatin. Chromatin is a stained thread-like structure whereas nucleosomes are beads on that. These chromatin fibers later, during cell division, condense to form chromosomes.

Why is DNA Packaging required?

The length of the DNA  which is around 3 meters long and it has to be packed within the nucleus which is few micrometers in diameter.Therefore, in order to fix the huge DNA molecules into the minute nucleus, it need be packed into an extremely compressed or compacted structure called as chromatin. During the initial stage of the DNA packaging, the DNA is reduced into 11 nm fiber which denotes an approximate 5-6 –folds of compaction. This is achieved through nucleosome order of packaging.

There are  There are two order of packaging

  1. First order of packaging – Nucleosome.
  2. Second order of packaging – Solenoid.

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Practise This Question

The number of hydrogen bonds between adenine and thymine in a DNA molecule is