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Chapter 11: Nucleic Acid Structure, DNA Replication, and Chromosome Structure

Page history last edited by Derek Weber 10 years, 9 months ago


Learning Objectives 

11.1 Biochemical Identification of the Genetic Material

  • Describe the experiments that demonstrated that DNA is the genetic material.

11.2 Nucleic Acid Structure

  • Explain how the contributions of Wilkins and Franklin, Watson and Crick, and Chargaff resulted in understanding the structure of DNA.
  • Describe the importance of covalent bonds and hydrogen bonds to the structure of a DNA molecule.

11.3 An Overview of DNA Replication

  • Explain the results of the Meselson-Stahl experiment and describe the predicted results if DNA replication followed the other possible models.
  • Describe the relationship between the structure of a DNA molecule and the means by which DNA is replicated.

11.4 Molecular Mechanism of DNA Replication

  • Outline the basic steps involved in DNA replication, including major differences between eukaryotes and bacteria.
  • Explain how eukaryotes overcome the difficulty of replicating the ends of linear chromosomes.

11.5 Molecular Structure of Eukaryotic Chromosomes

  • Describe the various strategies employed by eukaryotes to compact their genomes into a nucleus.
  • Explain the significance of histone proteins, including their charge and amino-terminal tails.

 

Chapter Summary

Scientific advances are a result of proper experimental design mixed with insight and a little luck. The events leading to the discovery of DNA as the material of heredity are especially good examples of how individual experiments build upon one another to answer a larger scientific question. Among the first experiments were those that indicated that the hereditary material was stored within the nucleus of every cell. Although this now seems intuitive, there are many structures within a cell that segregate during meiosis other than the chromosomes. The role of the nucleus was further clarified by observing embryonic development after physical manipulation of the nucleus. Several different kinds of experiments were performed to prove that the hereditary material was nucleic acid rather than protein. Among these were the Griffith and Avery experiments in which nonvirulent bacteria were made virulent by a nonprotein-transforming principle. The Hershey Chase experiments indicated that it was the DNA within viruses and not their protein exteriors that was the infecting material that killed bacteria.

 

Chemical analysis of nucleic acids illustrated their structure but did not hint as to how these units were assembled into a working blueprint. Chargaff determined that DNA was not a simple repeating polymer and that the proportions of the adenine and thymine nitrogenous bases were always equal as were the proportions of guanine and cytosine. X-ray diffraction of impure samples of DNA by Rosalind Franklin gave Watson and Crick sufficient information to construct their three-dimensional model of the DNA molecule. A key point of the model was the complementarity of the DNA strands, a result of the bonding of their bases, adenine to thymine and guanine to cytosine.  The Watson-Crick DNA model consists of two complementary phosphodiester strands wound around each other forming a double helix.  The two phosphodiester strands are anti-parallel with the bases oriented within the molecule.  The two strands are held together by hydrogen bonds forming between the complementary bases.

 

The Meselson Stahl experiments began to explain DNA replication by determining that it was a semiconservative process; each strand served as a template for the production of a new one and each old and new strand then intertwined to become a new helix.  DNA replication is a complex process involving many enzymes (DNA polymerases, primase, helicase, ligase, etc.).  At the replication fork, several of these enzymes form a complex assemblage known as the replisome.  Furthermore, double-stranded DNA replication is complicated since new nucleotides must be added to both the 5’ to 3’ strand and the 3’ to 5’ strand at the same time, but DNA polymerase can only add onto the 3’ end. The 5’ to 3’ or leading strand is replicated simply by adding nucleotides as the old strands unzip. The 3’ to 5’ lagging strand is replicated in batches via discontinuous synthesis. Segments called Okazaki fragments are made in the usual way. These fragments are then connected by phosphodiester bonds by DNA ligase. Since one strand is processed continuously and the other discontinuously, replication as a whole is semidiscontinuous.

 

Eukaryotic chromosomes are linear structures composed of chromatin, mostly DNA and protein with a small amount of RNA. Eukaryotic DNA is a long double-stranded fiber. Every 200 nucleotides it coils around a core of eight histone polypeptides forming a nucleosome. The string of nucleosomes is further wrapped into supercoils. Heterochromatin is highly condensed chromatin while euchromatin is relatively uncondensed. Some portions of the DNA are permanently heterochromatic to prevent DNA expression; the remainder is uncondensed at the proper time to facilitate transcription.

 

PowerPoint Presentations

Chapter 11 PowerPoint (.pdf)

  

Reading Assignments and Homework

Please access the ConnectPlus site for Health Science Academy to access our reading assignments and homework.

 

 

 

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