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Unicellular Organisms


DNA DNA is formed by joining together nucleotides with the phosphate groups link to the sugars at the 3' and 5' carbons. This is the backbone held up by covalent bonds. The nitrogen bases are attached to the 1' carbon in the sugar. The complementary DNA strand has the same kind of construction but running in opposite direction (with the 5-sugar pointing upside down). The two strands are joined by weaker hydrogen bonds (H-O or H-N). The pairing of the bases can occur only between Adenine (A) and Thymine (T) or Guanine (G) and Cytosine (C) (See Figure 11-19). DNA replication occurs when the complementary strands of DNA break apart and unwind. This is accomplished with the help of enzymes called helicases. Additional enzymes and proteins attach to the individual strands, holding them apart and preventing them from coiling upon themselves. The strand with the 5-sugar pointing down is called the 5' to 3' strand, while the complementary pair with the 5-sugar pointing up is the 3' to 5' strand. They are said to be anti-parallel, the difference has consequence in DNA replication.

Figure 11-19 DNA Structure
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DNA Replication The point at which the helicase cleaves the double helix, is called the replication fork, because of the shape of the molecule. At this site another enzymes called DNA polymerases move along each of the separated DNA strands, adding nucleotides to the exposed bases according to the base pairing rules. The ribose-phosphate bonds form between the new nucleotides to hold the new strand together. The synthesis acquires energy via the removal of two phosphates from the triphosphate. The process continues until the original double helix is completely unwound and two new double helices have been formed. Each new double helix is composed of one old DNA strand and one new strand (See Figure 11-20). There is a small variation for the processing on the 3' to 5' strand because the polymerase runs only in one direction (from 5' to 3') Thus the polymerase on the lagging (3' to 5') strand has to copy a small section of the DNA, moving back to copy another short segment. Then uses the ligase to splice the sections together, and so on to complete the replication. See also DNA organization.

Figure 11-20 DNA Replication
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A mutation is a change in the DNA nucleotide sequence that alters the sequence of amino acids, which would alter the structure and function of a protein in a cell. Some mutations are known to result from X-rays, UV light, chemicals called mutagens, and possibly some viruses. If a change in DNA occurs in a somatic cell, the altered DNA will be limited to that cell and its daughter cells. If there is uncontrolled growth, the mutation could lead to cancer. If the mutation occurs in germ cell DNA, then all the DNA produced in a new individual will contain the same genetic change. If the genetic change greatly affects the catalysis of metabolic reactions or the formation of important structural proteins, the new cells may not survive or the person may exhibit a genetic disease (see Hallmarks of Cancer).

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