DNA Packaging
Introduction to DNA Packaging
DNA is a long polymer of deoxyribonucleosides. The length of DNA is usually defined as number of nucleotides or base pairs (bp) present in it. This is the characteristic of an organism. For example, a bacteriophage known as Φ x 174 has 5386 nucleotides, bacteriophage lambda has 48502 bp, and haploid content of human DNA is 3.3 x 109 bp and diploid content is 6.6 x 109 bp. The distance between two consecutive base pairs is 0.34 nm. Total length of DNA can be calulated by:
Total DNA length = Total base pairs x distance between two base pairs
By above formula we if calculate length of human DNA, then it is 2.2 metres (6.6 x 109 bp x 0.34 x 10-9 m/bp = 2.2 m). This length is far greater than the dimension of a typical nucleus (10-6 m). How is such a long polymer packaged in a cell?
DNA packaging is the way in which the long DNA thread is folded and packed tightly with basic proteins so that it can fit inside the tiny nucleus of the cell.
DNA Packaging in Prokaryotes
Prokaryotes do not have a true nucleus. Their DNA is found in cytoploasm inside a region called the nucleoid. The DNA is present in a supercoiled loop form so that it can fit in the small cell. This compaction is achieved by positively charged non-histone proteins, such as polyamines. These proteins hold DNA in form of large loops. In this form DNA packaging in prokaryotes.
DNA Packaging in Eukaryotes
DNA packaging in eukaryotes occurs in a stepwise manner with the help of positively charged histone proteins. The long, negatively charged DNA wraps around histone octamers to form nucleosomes, which then undergo further folding and coiling to produce compact chromatin fibers and, finally, chromosomes. The steps are:
1. Histone Proteins
In eukaryotes, the negatively charged DNA molecule is packaged with the help of positively charged basic proteins called histones. These histone proteins are basic because they contain a high proportion of basic amino acids lysine and arginine. There are five main types of histone proteins: H1, H2A, H2B, H3, and H4.
2. Nucleosome
Histones (2 x H2A, H2B, H3 and H4) are organised to form a unit of eight molecules called histone octamer. The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called nucleosome. A typical nucleosome contains 200 bp of DNA helix (1.75 turns of DNA). H1 locks the DNA onto the nucleosome and binds linker DNA at the same time. Linker DNA is the stretch of DNA between two nucleosome.
3. Chromatin
Nucleosome repeats to form a thread like structure called chromatin. The nucleosomes in chromatin are seen as ‘beads-on-string’ structure when viewed under electron microscope. Number of nucleosome = 6.6 x 109 / 200 = 3.3 x 107.
| 1. Densely packed | 1. Loosely packed |
| 2. Dark stained | 2. Ligh stained |
| 3. Transcriptionally less active | 3. Transcriptionally more active |
4. Chromosome
During the metaphase stage of cell division, chromatin undergoes further coiling and condensation with the help of non-histone chromosomal (NHC) proteins, resulting in the formation of a highly compact structure known as the chromosome.
DNA Packaging NEET PYQ
- What are the structures called that give an appearance as ‘beads on string’ in the chromosomes when viewed under electron microscope?
- Nucleosomes
- Genes
- Base pairs
- Nucleotides
- Histones are rich in amino acids:
- Arginine and Leucine
- Argine and Lysine
- Arginine and Lysine
- Aspartate and Glutamate
- A typical nucleosome unit consists of about:
- 60 base pairs
- 140 base pairs
- 200 base pairs
- 400 base pairs
- Transcriptionally active chromatin is termed as:
- Heterochromatin
- Euchromatin
- Both are active
- Both are non active.
- The length of DNA in E.coli:
- 1.36 mm
- 2.2 mm
- 2.2 m
- 1.36 m
- During DNA packaging in eukaryotic cells, Non Histone Chromosomal proteins:
- Are not required
- Are required from the begining to the end of packaging
- Are required only for earlier stages of packaging
- Are required for higher level of packaging.