Chapter 16: The Molecular Basis of Inheritance

Concept 16.1: DNA is the Genetic Material

This section explores the experimental evidence that established DNA as the hereditary material, the discovery of DNA structure, and the significance of base pairing.

• Frederick Griffith's Experiment: In 1928, Griffith discovered the phenomenon of transformation while working with Streptococcus pneumoniae. He found that non-virulent (R) bacteria could be transformed into virulent (S) bacteria when mixed with heat-killed S cells, suggesting a "transforming principle" carried genetic information.

• Avery, McCarty, and MacLeod: In 1944, these scientists identified DNA as the "transforming principle" by demonstrating that only DNA, not proteins or RNA, could transform R cells into S cells.

• Hershey and Chase: In 1952, using bacteriophages labeled with radioactive isotopes, they showed that DNA (not protein) entered bacterial cells and directed viral replication, confirming DNA as the genetic material.

• Erwin Chargaff: Chargaff discovered that in DNA, the amount of adenine (A) equals thymine (T), and the amount of guanine (G) equals cytosine (C), leading to the base-pairing rules.

• Watson and Crick's Discovery: Using data from Rosalind Franklin's X-ray diffraction images and Chargaff's rules, Watson and Crick deduced the double helix structure of DNA in 1953. Their model explained how genetic information is stored and replicated.

• Rosalind Franklin's Contribution: Franklin's X-ray crystallography provided critical evidence for the helical structure of DNA, particularly the regular spacing and dimensions of the helix.

• Structure of DNA: DNA is a double helix composed of two antiparallel strands of nucleotides. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base (A, T, G, or C).

• Base-Pairing Rule: Adenine pairs with thymine (A-T) via two hydrogen bonds, and guanine pairs with cytosine (G-C) via three hydrogen bonds. This specificity is crucial for accurate DNA replication and genetic fidelity.

Example: If a DNA sample contains 30% adenine, it must also contain 30% thymine, and the remaining 40% is equally divided between guanine and cytosine (20% each).

Concept 16.2: Many Proteins Work Together in DNA Replication and Repair

This section describes the mechanisms of DNA replication, the roles of key enzymes, and the semiconservative nature of the process.

• Semiconservative Model of Replication: Each new DNA molecule consists of one old (parental) strand and one newly synthesized strand. The Meselson-Stahl experiment (1958) used isotopic labeling to confirm this model.

• Replication Fork and Origin: DNA replication begins at specific sites called origins of replication. The replication fork is the Y-shaped region where the DNA is unwound and new strands are synthesized.

• Leading and Lagging Strands: DNA polymerase synthesizes the leading strand continuously in the 5' to 3' direction. The lagging strand is synthesized discontinuously as short Okazaki fragments, also in the 5' to 3' direction, but away from the replication fork.

• Antiparallel Structure: The two DNA strands run in opposite directions (one 5' to 3', the other 3' to 5'), which is why continuous synthesis is only possible on one strand.

• Okazaki Fragments: Short DNA segments synthesized on the lagging strand. They are later joined by DNA ligase to form a continuous strand.

• Key Enzymes and Proteins:

  • DNA Polymerase: Adds nucleotides to the 3' end of a growing DNA strand; also proofreads and corrects errors.

  • Primase: Synthesizes short RNA primers needed to start DNA synthesis.

  • Helicase: Unwinds the DNA double helix at the replication fork.

  • Topoisomerase: Relieves tension ahead of the replication fork by cutting and rejoining DNA strands.

  • Single-Stranded Binding Proteins (SSBs): Stabilize unwound DNA strands and prevent them from reannealing.

  • DNA Ligase: Joins Okazaki fragments on the lagging strand by forming phosphodiester bonds.

Example: During replication, helicase unwinds the DNA, primase lays down an RNA primer, DNA polymerase extends the new strand, and DNA ligase seals the fragments.

Additional info: The semiconservative model can be illustrated by showing that after one round of replication, each daughter DNA molecule contains one parental and one new strand.

Concept 16.3: A Chromosome Consists of a DNA Molecule Packed Together with Proteins

This section explains how DNA is organized and packaged within eukaryotic cells.

• Nucleosome Structure: The nucleosome is the basic unit of DNA packaging in eukaryotes. It consists of a segment of DNA wrapped around a core of eight histone proteins (two each of H2A, H2B, H3, and H4).

• Chromatin Structure: Chromatin is the complex of DNA and proteins (mainly histones) that forms chromosomes. It exists in two forms:

  • Euchromatin: Less condensed, transcriptionally active.

  • Heterochromatin: Highly condensed, transcriptionally inactive.

Example: During interphase, most chromatin is in the euchromatin state, allowing gene expression. During mitosis, chromatin condenses into visible chromosomes.

Summary Table: Key Proteins in DNA Replication

Key Equation: Base Pairing

The base pairing rule can be summarized as:

where [A], [T], [G], and [C] represent the molar concentrations of adenine, thymine, guanine, and cytosine, respectively.