Organization of prokaryotic, eukaryotic and mitochondrial genome
Genome refers to complete set of genetic material present in a cell or organism.
Prokaryotic Genome[edit | edit source]
The word prokaryote means "before the nucleus". Prokaryotes usually have a single, circular chromosome that contains their genetic information. It is present in a region of the cell called the nucleoid. The DNA is compressed into a smaller space by supercoiling, which is essentially twisting the DNA strands. The DNA can be negatively supercoiled meaning twisted in the direction opposite of the double helix or positively supercoiled meaning in the same direction as the DNA helix.
During transcription, small regions of the chromosome project away from the nucleoid and unwind, so they can be accessed by the proteins needed. Since prokaryotes don't have a nucleus, translation and transcription occur simultaneously.
Prokaryotes still show variation in their genomes. For instance, come cells contain plasmids, which are extrachromosomal linear or circular DNA containing non essential genes, for example that of antibiotic resistance. [1]
Eukaryotic Genome[edit | edit source]
Eukaryotes have nuclei which contain all of their genetic information. DNA is organized into chromosomes.
The amount of DNA is very large and thus has to be effectively packaged into the nucleus. The DNAs base pairs are associated with basic proteins called histones. This interaction forms the basic unit of DNA organization called nucleosomes. The nucleosomes are are connected to each other by pieces of linker DNA, resembling beads on a string.
Two molecules of each of the histone protein H2A, H2B, H3, H4 forms the base of the nucleosome around which the DNA is wrapped twice. The linker DNA is approximately 50 base pairs long. The histone protein H1 binds to this linker DNA.
Nucleosomes eventually form filaments and fibers and eventually the individual chromosome. Thus each chromosome is made of one continuous length of DNA. [2]
Mitochondrial DNA[edit | edit source]
Mitochondrial DNA, or mtDNA is a circular chromosome found in mitochondria. It contains 37 genes that code for proteins required in the mitochondria or give transfer or ribosomal RNA.
Since mitochondria are transferred from the maternal egg cell to the fetus, the DNA can be used to trace back a person's ancestry though the maternal linkage, by looking at the mitochondrial haplogroup. [3]
- ↑ Griswold, A. (2008) Genome packaging in prokaryotes: the circular chromosome of E. coli. Nature Education 1(1):57
- ↑ Lippincott's Illustrated Reviews: Biochemistry. Fifth edition. ISBN-13: 978-1-60831-412-6
- ↑ National Human Genome Research Institute - Mitochondrial DNA Medline Plus - Mitochondrial DNA
