DNA replication

• DNA polymerase III is the enzyme responsible for adding new nucleotides to the growing DNA strand during replication.
• If the template base is C and the enzyme inserts an A instead of a G into the new chain, the 3' → 5' exonuclease activity removes the misplaced nucleotide hydrolytically.
• The 5' → 3' polymerase adds activity to the template, then replaces it with the correct nucleotide.
• The 3' → 5' exonuclease and 5' → 3' polymerase are located on different subunits of DNA pol I.
• The 3'-hydroxyrye group of the RNA primer is the acceptor of the first deoxyribose nucleotide, DNA polymerase I begins to add nucleotides along the single-stranded template that specifies the sequence of bases in the new synthesis.
• DNA polymerase I is a highly processive enzyme that remains bound to the template as it moves along and does not diffuse away and then rebinds before adding each new nucleotide.
• The processivity of DNA polymerase I is the result of the β subunits of the holozyme forming a ring that encircles the template and moves along with it, thus serving as a siding DNA chain.
• DNA replication is initiated by binding to specific nucleotide sequences (DNA boxes) within oriC.
• Binding causes a change in the DNA unwinding element (AT-rich region) in the origin of melting, resulting in a short, localized region of single-stranded DNA.
• DNA helices require energy provided by ATP hydrolysis (see Figure 3010).
• Unwinding at the replication fork causes supercoiling in other regions of the DNA molecule.
• There are three major structural forms of DNA: the B form, the A form, and the Z form.
• The B form is a right-handed helix with 10 base pairs per 360° turn (or opposite), and with the planes of the bases perpendicular to the helical axis.
• Chromosomal DNA is primarily composed of B-DNA, as shown in Figure 307.
• The A form is produced by moderately dehydrating the B form.
• The A form is also a right-handed helix, but there are 11 base pairs per turn, and the planes of the bases are tilted 20° away from the helical axis.
• The Z form is a left-handed helix that contains 12 base pairs per turn.
• Lagging strand: The strand that is being copied in the direction away from the replication fork for continuously, with small fragments of DNA being copied near the replication fork.
• These short stretches of discontinuous DNA, termed Okazaki fragments, are eventually joined (lagged) by lagging strand.
• The new strand of DNA produced by this mechanism is termed the lagging strand.
• RNA primer is a short piece of RNA base-paired to the DNA template, forming a double-stranded DNA-RNA hybrid.
• The free hydroxyl group on the 3' end of the RNA primer serves as the first acceptor of a deoxy nucleotide by action of a DNA polymerase.
• Each chromosome is associated with no histone proteins that help compact the DNA to form an nucleoid.
• Most species of bacteria contain small, circular, extrachromosomal DNA molecules called plasmids.
• Plasmids carry genetic information and undergo replication independently of the chromosomes.
• Plasmids are used in genetic engineering to introduce new genetic information into the host bacterium and facilitate the transfer of genetic information from one bacterium to another.
• When the two strands of ds DNA are separated, each can serve as a template for the replication of a new complementary stand.
• The production of two daughter molecules, each containing two DNA strands, results in the formation of two double-stranded DNA molecules.
• DNA is written in a more styled form, emphasizing the deoxyribosome-phosphate backbone.
• The simplest representation of the nucleotide sequence is a double-helix, where the two chains are coiled around a common axis, called the helical axis.
• In the DNA helix, the hydrophilic deoxyribose-phosphate backbone is on the outside of the molecule, while the hydrophobic bases are stacked inside.
• The DNA helix structure resembles a ladder, with the two strands separated by a major groove and a minor groove.
• The major groove in the DNA helix is a wide groove, while the minor groove is a narrower groove.
• Stretches of Z- DNA can occur naturally in regions of DNA that have a sequence of altering purines and pyrimidines (for example, polG C).
• Transitions between the B- form and Z- form of DNA may play a role in regulating gene expression.
• B- DNA and Z- DNA have different structures, with B- DNA being linear and Z- DNA being circular.
• Each chromosome in the nucleus of a eukaryotic cell consists of one long, linear molecule of DNA, which is bound by a complex of proteins.
• Eukaryotic cells have closed, circular, Z- DNA molecules in their mitochondria.
• A typical prokaryotic organism contains a single, circular, Z- DNA molecule.
• Circulair DNA is "supercoiled," meaning it has double helix crosses over itself one or more times.