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Prokaryotic and Eukaryotic Chromosomes
- The genome of an organism encompasses all of the genes of that organism.
- Gene is a distinct sequence of nucleotides forming part of a chromosome, the order of which determines the order of monomers in a polypeptide or nucleic acid molecule.
- Thus a protein-coding gene is defined as a region of DNA that encodes a single polypeptide or a set of closely related polypeptides.
- Genes are contained in chromosomes.
- Chromosomes are thus structures within cells that contain hundreds to thousands of genes.
- The DNA of a bacterial cell, such as Escherichia coli, is a circular double-stranded molecule often referred to as the bacterial chromosome.
- The circular DNA is packaged into a region of the cell called the nucleoid where it is organized into 50 or so loops or domains that are bound to a central protein scaffold, attached to the cell membrane.
- The DNA is negatively supercoiled, that is, it is twisted upon itself.
- It is complexed with several DNA-binding proteins, the most common of which are proteins HU, HLP-1 and H-NS. These are histone-like proteins.
- The large amount of genomic DNA in a eukaryotic cell is tightly packaged in chromosomes contained within a specialized organelle, the nucleus.
- With the exception of the sex chromosomes, diploid eukaryotic organisms such as humans have two copies of each chromosome, one inherited from the father and one from the mother.
- Chromosomes contain both DNA and protein.
- Most of the protein on a weight basis is histones, but there are also many thousands of other proteins found in far less abundance and these are collectively called non- histone proteins (NHP).
- This nuclear DNA–protein complex is called chromatin.
- In the nucleus, each chromosome contains a single linear double-stranded DNA molecule.
- The length of the packaged DNA molecule varies. In humans, the shortest DNA molecule in a chromosome is about 1.6 cm and the longest is about 8.4 cm.
- The extensive packaging of DNA in chromosomes results from three levels of folding involving nucleosomes, 30 nm filaments and radial loops.
- The first level of packaging involves the binding of the chromosomal DNA to histones.
- Overall, in chromosomes, the ratio of DNA to histones on a weight basis is approximately 1:1.
- There are five main types of histones called H1, H2A, H2B, H3 and H4.
- Histones are very basic proteins; about 25% of their amino acids are lysine or arginine so histones have a large number of positively charged amino acid side-chains.
- These positively charged groups therefore bind to the negatively charged phosphate groups of DNA
30 nm fiber
- If nuclei are lysed very gently, the chromatin is seen to exist as a 30 nm diameter fiber.
- The fiber is formed by a histone H1 molecule binding to the linker DNA of each nucleosome at the point where it enters and leaves the nucleosome.
- The histone H1 molecules interact with each other, pulling the nucleosomes together.
- When chromosomes are depleted of histones, they are seen to have a central fibrous ‘protein scaffold’ (or nuclear matrix) to which the DNA is attached in loops.
- Therefore, in vivo it seems likely that the next order of packaging involves the attachment of the 30 nm fiber to multiple locations on this central protein scaffold in a series of radial loops.
- The mitochondria and chloroplasts of eukaryotic cells also contain DNA but, unlike the nuclear DNA, this consists of double-stranded circular molecules resembling bacterial chromosomes.
- David Hames and Nigel Hooper (2005). Biochemistry. Third ed. Taylor & Francis Group: New York.
- Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1 ed.). S .Chand and company Ltd.