7.8A: Chromosomes and DNA Replication in the Archaea
Archaea usually have a single circular chromosome.
LEARNING OBJECTIVES
Describe the characteristics of Archaeal chromosomes and their replication
Key Takeaways
Archaeal Genetics
Archaea usually have a single circular chromosome, the size of which may be as great as 5,751,492 base pairs in Methanosarcina acetivorans , which boasts the largest known archaean genome. One-tenth of this size is the tiny 490,885 base-pair genome of Nanoarchaeum equitans , which possesses the smallest archaean genome known; it is estimated to contain only 537 protein-encoding genes. Smaller independent pieces of DNA, called plasmids, are also found in archaea. Plasmids may be transferred between cells by physical contact, in a process that may be similar to bacterial conjugation.
Asexual Reproduction
The means of asexual reproduction that are used by Archaea include binary reproduction, multiple fission, fragmentation, or budding. The cell division process is controlled by the cell cycle; the chromosomes within the Archaea are replicated to produce two daughter chromosomes. Archaea typically have a single circular chromosome. The two daughter chromosomes are then separated and the cell divides. This process in Archaea appears to be similar to both bacterial and eukaryotic systems. The circular chromosomes contain multiple origins of replication, using DNA polymerases that resemble eukaryotic enzymes. However, the proteins involved that direct cell division are similar to those of bacterial systems.
DNA replication, similar in all systems, involves initiation, elongation, and termination. The replication of DNA, beginning at the origins of replication present on the circular chromosomes, requires initiator proteins. The recruitment of additional proteins by way of the initiator proteins allows the separation of the circular DNA and results in the formation of a bubble.
The DNA replication system in Archaea, similar to all systems, requires a free 3’OH group before synthesis is initiated. The primase used to synthesize a short RNA primer from the free 3’OH group varies in Archaea when compared to that of bacterial and eukaryotic systems. The primase used by archaea is a highly derived version of the RNA recognition motif (RRM). It is structurally similar to viral RNA dependent RNA polymerases, reverse transcriptases, cyclic nucleotide generating cyclases, and DNA polymerases involved in DNA replication and repair. Once the RNA primase has performed its job, DNA synthesis continues in a similar fashion by which the eukaryotic system and the DNA is replicated.
Key Points
- Archaea are genetically distinct from bacteria and eukaryotes, with up to 15% of the proteins encoded by any one archaeal genome being unique to the domain, although most of these unique genes have no known function.
- DNA replication in archaea requires a specific primase that shares similarities to the RNA recognition motif (RRM) and to viral RNA dependent RNA polymerases.
- The circular chromosomes of archaea contain multiple origins of replication for initiation of DNA synthesis.
Key Terms
- chromosome : A structure in the cell nucleus that contains DNA, histone protein, and other structural proteins.
- eukaryote : Any of the single-celled or multicellular organisms, of the taxonomic domain Eukaryota, whose cells contain at least one distinct nucleus.
- bacteria : A type, species, or strain of bacterium.