Prokaryotic and Eukaryotic Chromosomes

Prokaryotic and Eukaryotic Chromosomes

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.

Prokaryotic and Eukaryotic Chromosomes

Image Source: https://image.slidesharecdn.com/3-150517031851-lva1-app6892/95/ib-biology-32-slides-chromosomes-3-638.jpg and http://bio1100.nicerweb.com/Locked/media/SAVE/ch06/chromosome.html

Prokaryotic Chromosomes

  • 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.

Eukaryotic Chromosomes

  • 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.
  1. Nucleosomes

  • 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
  1. 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.
  1. Radial loops

  • 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.

References

  1. David Hames and Nigel Hooper (2005). Biochemistry. Third ed. Taylor & Francis Group: New York.
  2. Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1 ed.). S .Chand and company Ltd.
  3. https://www.msdmanuals.com/home/fundamentals/genetics/genes-and-chromosomes
  4. https://www2.le.ac.uk/projects/vgec/highereducation/topics/dna-genes-chromosomes.

Prokaryotic and Eukaryotic Chromosomes

(Visited 1,875 times, 1 visits today)

2 thoughts on “Prokaryotic and Eukaryotic Chromosomes”

Leave a Comment