Why prokaryotic dna is circular

These SMC proteins induce positive supercoils. The region ahead of the polymerase complex will be unwound; this stress is compensated with positive supercoils ahead of the complex. Behind the complex, DNA is rewound and there will be compensatory negative supercoils. An open reading frame ORF is the part of a reading frame that varies in size and content in bacterial genomes. In molecular genetics, an open reading frame ORF is the part of a reading frame that contains no stop codons.

The transcription termination pause site is located after the ORF, beyond the translation stop codon, because if transcription were to cease before the stop codon, an incomplete protein would be made during translation. Normally, inserts which interrupt the reading frame of a subsequent region after the start codon cause frameshift mutation of the sequence and dislocate the sequences for stop codons.

Open reading frames are used as one piece of evidence to assist in gene prediction. Long ORFs are often used, along with other evidence, to initially identify candidate protein coding regions in a DNA sequence.

The presence of an ORF does not necessarily mean that the region is ever translated. For example, in a randomly generated DNA sequence with an equal percentage of each nucleotide, a stop-codon would be expected once every 21 codons. Even a long open reading frame by itself is not conclusive evidence for the presence of a gene.

If a portion of a genome has been sequenced e. In this sequence two out of three possible reading frames are entirely open, meaning that they do not contain a stop codon:. Thus, the last reading frame in this example contains a stop codon TAA , unlike the first two. Bacterial genomes display variation in size, even among strains of the same species. These microorganisms have very little noncoding or repetitive DNA, as the variation in their genome size usually reflects differences in gene repertoire.

Some species, particularly bacterial parasites and symbionts, have undergone massive genome reduction and simply contain a subset of the genes present in their ancestors. However, in free-living bacteria, such gene loss cannot explain the observed disparities in genome size because ancestral genomes would have had to contain improbably large numbers of genes.

Surprisingly, a substantial fraction of the difference in gene contents in free-living bacteria is due to the presence of ORFans, that is, open reading frames ORFs that have no known homologs and are consequently of no known function. The high numbers of ORFans in bacterial genomes indicate that, with the exception of those species with highly reduced genomes, much of the observed diversity in gene inventories does not result from either the loss of ancestral genes or the transfer from well-characterized organisms processes that result in a patchy distribution of orthologs but not in unique genes or from recent duplications which would likely yield homologs within the same or closely related genome.

It generates new knowledge that is useful in such fields as drug design and development of new software tools.

Bioinformatics also deals with algorithms, databases and information systems, web technologies, artificial intelligence and soft computing, information and computation theory, structural biology, software engineering, data mining, image processing, modeling and simulation, discrete mathematics, control and system theory, circuit theory, and statistics. Map of the human X chromosome : Assembly of the human genome is one of the greatest achievements of bioinformatics.

Development of this type of database involved not only design issues but the development of complex interfaces whereby researchers could access existing data as well as submit new or revised data.

In order to study how normal cellular activities are altered in different disease states, the biological data must be combined to form a comprehensive picture of these activities.

Therefore, the field of bioinformatics has evolved such that the most pressing task now involves the analysis and interpretation of various types of data. This includes nucleotide and amino acid sequences, protein domains and protein structures. In fact, a prokaryotic cell can undergo two rounds of DNA replication before the cell, itself, has divided.

This means that DNA replication can occur during cell division in prokaryotes. Since eukaryotic cells typically have multiple linear chromosomes, capped with telomeres, eukaryotic DNA replication and cell division mitosis and meiosis are a bit more complicated. In addition, the telomeres—repeating DNA sequences at the ends of each chromosome—limit the number of times a cell can divide before it dies or becomes senescent.

Each time a typical or somatic eukaryotic cell divides, the telomeres get shorter. The key difference between prokaryotic and eukaryotic cells is that eukaryotic cells have a membrane-bound nucleus and membrane-bound organelles , whereas prokaryotic cells lack a nucleus.

In eukaryotic cells, all the chromosomes are contained within the nucleus. In prokaryotic cells, the chromosome is located in a region of the cytoplasm called the nucleoid, which lacks a membrane. One interesting implication of this difference in the location of eukaryotic and prokaryotic chromosomes is that transcription and translation—the processes of creating an RNA molecule and using that molecule to synthesize a protein—can occur simultaneously in prokaryotes.

This is possible because prokaryotic cells lack a nuclear membrane, so transcription and translation occur in the same region. As the RNA is being transcribed, ribosomes can begin the translation process of stringing together amino acids. Supercoiling uses the application of tension to twist a DNA molecule, so it wraps around itself, creating loops. NAPs are proteins within the nucleoid that can bind to the DNA molecule, introducing bends and folds, and they are involved with processes such as DNA replication and transcription.

Most prokaryotic cells have just one chromosome, so they are classified as haploid cells 1n, without paired chromosomes. Even in Vibrio cholerae , which has two chromosomes, the chromosomes are unique from one another.

Many prokaryotes, such as bacteria, reproduce via binary fission. This is a method of asexual reproduction that is similar in its end result to mitosis—two daughter cells result, each with the same number of chromosomes as the parent cell. However, when bacteria undergo binary fission, no mitotic spindle forms. Although plasmids can occur in a variety of sizes ranging from around a thousand base pairs to hundreds of thousands , they usually only have a small number of genes.

Eukaryotic chromosomes are condensed in a membrane-bound nucleus via histones. In eukaryotes, transcription occurs in the nucleus, and translation occurs in the cytoplasm. Most eukaryotes contain two copies of each gene i. Some eukaryotic genomes are organized into operons, but most are not.

Extrachromosomal plasmids are not commonly present in eukaryotes. Eukaryotes contain large amounts of noncoding and repetitive DNA. References and Recommended Reading Abbott, A. Journal of Theoretical Biology , — Bendich, A. Bioessays 22 , — Bradley, M. Microbiology , — Cairns, J. Journal of Bacteriology , — Endy, D. Journal of Bacteriology , — Jacob, F. Journal of Molecular Biology 3 , — Lewin, B. Journal of Bacteriology 71 , — link to article Minsky, A.

Journal of Theoretical Biology , — Murphy, L. Journal of Structural Biology , — Rice, P. Cell 87 , — Robinow, C. Genome Biology 5 , Yasuzawa, K. Article History Close. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable.

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