Examples of DNA in the following topics:
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- Prokaryotic DNA is replicated by DNA polymerase III in the 5' to 3' direction at a rate of 1000 nucleotides per second.
- In prokaryotes, three main types of polymerases are known: DNA pol I, DNA pol II, and DNA pol III.
- DNA pol III is the enzyme required for DNA synthesis; DNA pol I and DNA pol II are primarily required for repair.
- DNA polymerase III uses this primer to synthesize the daughter DNA strand.
- DNA polymerase I replaces the RNA primer with DNA.
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- DNA replication uses a semi-conservative method that results in a double-stranded DNA with one parental strand and a new daughter strand.
- Watson and Crick's discovery that DNA was a two-stranded double helix provided a hint as to how DNA is replicated.
- In dispersive replication, after replication both copies of the new DNAs would somehow have alternating segments of parental DNA and newly-synthesized DNA on each of their two strands.
- DNA from cells grown exclusively in 15N produced a lower band than DNA from cells grown exclusively in 14N.
- So each "daughter" DNA actually consists of one "old" DNA strand and one newly-synthesized strand.
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- Eukaryotic DNA is packed into bundles of chromosomes, each consisting of a linear DNA molecule coiled around basic (alkaline) proteins called histones, which wind the DNA into a more compact form.
- In addition, prokaryotes have plasmids, which are smaller pieces of circular DNA that can replicate separately from prokaryotic genomic DNA.
- A major DNA difference between eukaryotes and prokaryotes is the presence of mitochondrial DNA (mtDNA) in eukaryotes.
- The mtDNA is composed of significantly fewer base pairs than nuclear DNA and encodes only a few dozen genes, depending on the organism.
- Eukaryotic DNA is stored in a nucleus, whereas prokaryotic DNA is in the cytoplasm in the form of a nucleoid.
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- Noncoding DNA are sequences of DNA that do not encode protein sequences but can be transcribed to produce important regulatory molecules.
- In genomics and related disciplines, noncoding DNA sequences are components of an organism's DNA that do not encode protein sequences.
- The amount of noncoding DNA varies greatly among species.
- For example, over 98% of the human genome is noncoding DNA, while only about 2% of a typical bacterial genome is noncoding DNA.
- The amount of total genomic DNA varies widely between organisms, and the proportion of coding and noncoding DNA within these genomes varies greatly as well.
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- Each helicase unwinds and separates the DNA helix into single-stranded DNA.
- Once RNA primer has been synthesized at the template DNA, primase exits, and DNA polymerase extends the new strand with nucleotides complementary to the template DNA.
- DNA polymerase halts when it reaches a section of DNA template that has already been replicated.
- Once the primers are removed, a free-floating DNA polymerase lands at the 3' end of the preceding DNA fragment and extends the DNA over the gap.
- The DNA fragments are joined by DNA ligase (not shown).
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- Both the packaging of DNA around histone proteins, as well as chemical modifications to the DNA or proteins, can alter gene expression.
- Histones package and order DNA into structural units called nucleosome complexes, which can control the access of proteins to the DNA regions.
- The tags do not alter the DNA base sequence, but they do alter how tightly wound the DNA is around the histone proteins .
- The DNA molecule itself can also be modified.
- Nucleosomes can slide along DNA.
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- DNA has a double-helix structure, with sugar and phosphate on the outside of the helix, forming the sugar-phosphate backbone of the DNA.
- This antiparallel orientation is important to DNA replication and in many nucleic acid interactions.
- This is known as the base complementary rule because the DNA strands are complementary to each other.
- During DNA replication, each strand is copied, resulting in a daughter DNA double helix containing one parental DNA strand and a newly synthesized strand.
- Native DNA is an antiparallel double helix.
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- After DNA replication, each newly synthesized DNA strand is shorter at its 5' end than at the parental DNA strand's 5' end.
- Enzymes RNase H and FEN1 remove RNA primers, but DNA Polymerase will add new DNA only if the DNA Polymerase has an existing strand 5' to it ("behind" it) to extend.
- However, there is no more DNA in the 5' direction after the final RNA primer, so DNA polymerse cannot replace the RNA with DNA.
- Each daughter DNA would become shorter than the parental DNA, and eventually entire DNA would be lost.
- Parental DNA strands are black, newly synthesized DNA strands are blue, and RNA primers are red.
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- The DNA of a prokaryote exists as as a single, circular chromosome.
- If a nonpathogenic bacterium takes up DNA for a toxin gene from a pathogen and incorporates the new DNA into its own chromosome, it, too, may become pathogenic.
- The DNA transferred can be in the form of a plasmid or as a hybrid, containing both plasmid and chromosomal DNA.
- The DNA may remain separate as plasmid DNA or be incorporated into the host genome.
- In (b) transduction, a bacteriophage injects DNA into the cell that contains a small fragment of DNA from a different prokaryote.
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- DNA replication is a highly accurate process, but mistakes can occasionally occur as when a DNA polymerase inserts a wrong base.
- The segment of DNA is removed and replaced with the correctly-paired nucleotides by the action of DNA pol.
- Errors during DNA replication are not the only reason why mutations arise in DNA.
- A special enzyme, DNA ligase (shown here in color), encircles the double helix to repair a broken strand of DNA.
- DNA ligase is responsible for repairing the millions of DNA breaks generated during the normal course of a cell's life.