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During DNA replication, the replisome will unwind the parental duplex DNA into a two single-stranded DNA template replication fork in a 5' to 3' direction. The leading strand is the template strand that is being replicated in the same direction as the movement of the replication fork.
The three-domain system adds a level of classification (the domains) "above" the kingdoms present in the previously used five- or six-kingdom systems.This classification system recognizes the fundamental divide between the two prokaryotic groups, insofar as Archaea appear to be more closely related to eukaryotes than they are to other prokaryotes – bacteria-like organisms with no cell nucleus.
By 1998, Cavalier-Smith had reduced the total number of kingdoms from eight to six: Animalia, Protozoa, Fungi, Plantae (including red and green algae), Chromista, and Bacteria. [44] Five of Cavalier-Smith's kingdoms are classified as eukaryotes as shown in the following scheme: Eubacteria; Neomura. Archaebacteria; Eukaryotes. Kingdom Protozoa
1938 [5] [6] Whittaker 1969 [7] Woese et al. 1977 [8] [9] Woese et al. 1990 [10] Cavalier-Smith 1993 [11] [12] [13] Cavalier-Smith 1998 [14] [15] [16] Ruggiero et al. 2015 [17] — — 2 empires: 2 empires: 2 empires: 2 empires: 3 domains: 3 superkingdoms 2 empires: 2 superkingdoms: 2 kingdoms 3 kingdoms — 4 kingdoms: 5 kingdoms: 6 kingdoms ...
This six-kingdom model is commonly used in recent US high school biology textbooks, but has received criticism for compromising the current scientific consensus. [13] But the division of prokaryotes into two kingdoms remains in use with the recent seven kingdoms scheme of Thomas Cavalier-Smith, although it primarily differs in that Protista is ...
[3] [4] In the three-domain model, the first two are prokaryotes, single-celled microorganisms without a membrane-bound nucleus. All organisms that have a cell nucleus and other membrane-bound organelles are included in Eukarya and called eukaryotes. Non-cellular life, most notably the viruses, is not included in this system.
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DNA replication. The two base-pair complementary chains of the DNA molecule allow replication of the genetic instructions. The "specific pairing" is a key feature of the Watson and Crick model of DNA, the pairing of nucleotide subunits. [5] In DNA, the amount of guanine is equal to cytosine and the amount of adenine is equal to thymine. The A:T ...