Chapter 15- DNA and the Gene: Synthesis and Repair
15.1 What Are Genes Made Of?
- During the infection, the exterior protein coat, or capsid, of the virus is left behind on the exterior of the host cell
- ==Each deoxyribonucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base==
- Deoxyribonucleotides link together into a polymer when a hydroxyl group on the 3' carbon of one deoxyribose and the phosphate group attached to the 5’ carbon of another deoxyribose are joined by a covalent bond called a phosphodiester linkage, or phosphodiester bond
- Watson and Crick realized that these antiparallel strands will twist around each other into a double-stranded spiral-or double helix-and that only certain bases fit together snugly in pairs within the helix by complementary base pairing.
- The double-helical DNA molecule is stabilized in two ways:
- complementary base pairing
- interactions between the stacked base pairs inside the helix.
15.2 Testing Early Hypotheses about DNA Synthesis
- Alternative hypotheses about DNA replication include:
- Semiconservative replication: If the parental strands of DNA separated, each one could then be used as a template for the synthesis of a new daughter strand.
- Conservative replication: If the bases of both strands temporarily turned out from the helix, they could serve as a template for the synthesis of an entirely new double helix all at once.
- Dispersive replication This proposed that the parental double helix was fragmented into small pieces before replication and then each piece was replicated by either a conservative or semi conservative mechanism.
- Like all organisms, bacterial cells copy their entire complement of DNA,their genome, before every cell division.
15.3 A Model for DNA Synthesis
- The initial breakthrough on DNA replication came with discovery of an enzyme called DNA polymerase.
- The potential energy of the deoxyribonucleotide monomers is first raised by reactions that add two phosphate groups to form deoxyribonucleoside triphosphates (dNTPs).
- Initially, the replication bubble forms at a specific sequence of bases called the origin of replication
- A replication fork is the Y-shaped region where the parental DNA double helix is separated into single strands and copied.
- Single-strand DNA-binding proteins (SSBPs) attach to the separated strands to prevent them from snapping back into a double helix
- A topoisomerase is an enzyme that cuts DNA, allows it to unwind, and rejoins it.
- A short strand of RNA called a primer that is base-paired to 1e DNA template
- The other strand, appropriately called the lagging strand, or discontinuous strand, must be synthesized in a direction αwαy from the moving replication fork.
- These short DNAs, which were attached to RNA primers came to be known as Okazaki fragments
- Once the RNA primer is removed and replaced by DNA, an enzyme called DNA ligase catalyzes the formation of a phosphodiester bond between the 3’ and 5’ ends of adjacent Okazaki fragments, closing up the backbone
- The proteins and enzymes work together in a large macromolecular machine called the replisome
15.4 Replicating the Ends of Linear Chromosomes
- The region at the end of a eukaryotic chromosome is called a telomere
- An enzyme called telomerase replicates telomeric DNA. ==Telomerase catalyzes the synthesis of DNA using an RNA template that is an integral part of the enzyme.==
- Any cell not involved in gamete formation is a somatic cell.
15.5 Repairing Mistakes and DNA Damage
- The ability of DNA polymerase to recognize and remove an incorrect deoxyribonucleotide is called proofreading.
- Mismatch repair is a form of error correction that cleans up errors introduced during DNA synthesis
- ==Mismatch repair is the final layer of error detection and correction for errors introduced during DNA synthesis.==
- Xeroderma pigmentosum (XP) is a rare autosomal recessive disease in humans.