* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download P.324doc
Survey
Document related concepts
Zinc finger nuclease wikipedia , lookup
DNA sequencing wikipedia , lookup
DNA repair protein XRCC4 wikipedia , lookup
Homologous recombination wikipedia , lookup
DNA profiling wikipedia , lookup
Eukaryotic DNA replication wikipedia , lookup
DNA nanotechnology wikipedia , lookup
United Kingdom National DNA Database wikipedia , lookup
Microsatellite wikipedia , lookup
DNA replication wikipedia , lookup
DNA polymerase wikipedia , lookup
Transcript
P. 324 #13,14,16,27,28,30,34 13. Hydrogen bonds, which are individually weak but collectively strong enough to ensure the stability of the DNA molecule, hold together the two DNA strands in the double helix. It is important that these bonds are relatively weak because of the function of DNA, which requires the constant “breaking” of these hydrogen bonds for replication and protein synthesis. If the bonds between the strands were stronger, replication and protein synthesis would require much more energy, which, over an evolutionary period, would likely alter the manner in which the processes occur in organisms. 14. Maintaining the correct nucleotide sequence in newly formed and existing strands of DNA is integral to the functioning of an organism, because mistakes can lead to the mutation of genes that control vital living processes. Ensuring the quality control of new DNA strands is the job of DNA polymerases I and III. During replication, they proofread the newly synthesized strands for missing or mismatched bases. If they find a mismatched nucleotide, they will backtrack over the strand, excise it, and replace it with the appropriate nucleotide. In existing strands, DNA, which can be damaged by such hazards as radiation and chemicals, are “inspected” by certain enzymes. If any mutations are found in the DNA, another enzyme will excise the damaged region, and then DNA polymerase and DNA ligase will replace the damaged nucleotides with correct ones. In some instances, mutations can be beneficial for organisms if they provide a competitive advantage; in fact, mutations are a major cause of evolution because they allow organisms to physically adapt to the changing environment in which they live. 16. The seven stages, which break down the events of DNA replication, are listed below: 1) The enzyme gyrase relieves any tension from the unwinding double helix. 2) The enzyme helicase breaks the hydrogen bonds holding the two complementary parent strands together, resulting in an unzipped helix that terminates at the replication fork. 3) Single-stranded binding proteins anneal to the newly exposed template strands, preventing them from reannealing. 4) The enzyme primase lays down RNA primers that will be used by DNA polymerase III as a starting point to build the new complementary strands. 5) DNA polymerase III adds the appropriate deoxyribonucleic triphosphates to the 3' end of the new strand, using the template strand as a guide. The energy in the phosphate bonds is used to drive the replication process. The leading strand is built continuously toward the replication fork. A lagging strand comprising short segments of DNA, known as Okazaki fragments, is built discontinuously, away from the replication fork. 6) DNA polymerase I excises the RNA primers and replaces them with the appropriate deoxyribonucleotides. DNA ligase joins the gaps in the Okazaki fragments by the creation of a phosphodiester bond. 7) DNA polymerase I and DNA polymerase III proofread by excising incorrectly paired nucleotides at the end of the complementary strand and adding the correct nucleotides 27. If one strand contains the nucleotide proportions 15% A, 30% T, 20% G, and 35% C, the other strand in the double helix must contain the bases in the following proportions: 30% A, 15% T, 35% G, and 20% C, since bases are complementary. 28. The roles of the following enzymes in DNA replication are outlined below: DNA ligase: joins DNA fragments by catalyzing the formation of a bond between the 3' hydroxyl group and the 5' phosphate group on the sugar-phosphate backbones. DNA gyrase: relieves tension produced by the unwinding of DNA. DNA helicase: unwinds double-helical DNA by disrupting hydrogen bonds. DNA polymerase I: removes RNA primers and replaces them with the appropriate deoxyribonucleotides during DNA replication. DNA polymerase III: synthesizes complementary strands of DNA during DNA replication in the 5' to 3' direction. 30. Silent mutations do not lead to deleterious effects in an organism because they either occur in introns, noncoding regions of DNA, or have no effect on the translation of proteins, because of the redundant nature of the genetic code. Introns are regions are cut out of the mRNA in the process of transcription, thus preventing the mutation from manifesting itself in the organism. Silent mutations that do occur in coding regions, exons, do not affect the translation process and thus the organism as a whole, because many codons can be coded for with multiple base sequences. Thus, even if one base is mutated, the codon may still code for the same amino acid. 34. In a double-stranded DNA molecule complementary bases (A–T, C–G) must exist in the same proportion. On this basis, the following results were determined: Sample A: Single stranded, the proportion of adenine is not equal to the proportion of thymine, and the proportion of guanine is not equal to the proportion of cytosine. Sample B: Double stranded, the proportion of adenine is equal to the proportion of thymine and the proportion of guanine is equal to the proportion of cytosine. Sample C: Single stranded, the proportion of guanine is not equal to the proportion of cytosine.