Meaning and structure of prokaryotic chromosomes

The genome of prokaryotes usually consists of one chromosome and plasmids. Eukaryotes, on the other hand, contain a larger number of chromosomes – both nuclear and extranuclear (mitochondrial) – and sometimes plasmids.

Most of what we know about prokaryotic chromosomes has been gained from studies of E. coli – the organism of choice for similar prokaryotic studies. A chromosome is made up of double-stranded circular DNA. Prokaryotes do not contain a nucleus or other membrane-bound organelles. The word "prokaryote" actually means "before the nucleus". The chromosome is stored in a special region called the nucleoid.

The genome of a prokaryote is often significantly larger than the cell itself. So how is it possible for genetic information to fit into a cell? Eukaryotes solve this problem by wrapping DNA around histones. However, prokaryotes do not contain histones (there are a few exceptions). A prokaryote compresses its DNA by coiling the filament into small coils (supercoiling). The fibers are twisted so tightly that eventually the loops overlap and form one large condensed ball. It distinguishes between two types of packing – positive (the turns of the DNA are in the same direction as the helix) or negative (the DNA is twisted in the opposite direction to the helix). Most bacteria during normal growth are negatively coiled.

 Specific properties of prokaryotes 

Prokaryotes most often reproduce asexually and are haploid (there is always only one copy of a gene). Prokaryotes often also contain several plasmids (extrachromosomally stored DNA molecules of a linear or circular nature). Compared to chromosomal DNA, they are typically smaller and encode genes that are not necessary for survival. However, they often provide their wearer with a certain advantage (eg: resistance to ATB,...). Plasmid replication occurs independently of chromosome replication.

Due to the need for prokaryotes to fit all their genes into a single chromosome, there is not much room left for non-coding sequences. Although in eukaryotes the proportion of non-coding DNA segments is around 98%, in prokaryotes it is only 12%.

Chromosomes of prokaryotes

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 * most prokaryotes contain a single circular chromosome
 * chromosomes are stored in the nucleoid, into which they fit due to the coiling of the DNA strand and binding to certain proteins
 * DNA communicating with the cytoplasm enables direct coupling of transcription and translation
 * contain only one copy of a gene (monoploid)
 * non-essential genes are usually stored in plasmids outside the chromosome
 * The prokaryote genome is very compact - containing very few non-coding DNA sequences

Prokaryotic and eukaryotic chromosomes: what's the difference?

Source

Genome Packaging in Prokaryotes

ReferencesALBERTS, B, D BRAY and A JOHNSON. Basics of cell biology. 2nd edition. Espero Publishing, 2005. 740 pp. ISBN 80-902906-2-0.