Molecular Basis of Inheritance Quiz

The Molecular Basis of Inheritance

• James Watson and Francis Crick introduced a double-helical model for DNA structure in 1953.
• DNA, the substance of inheritance, is the most celebrated molecule.
• Hereditary information is encoded in DNA and reproduced in all cells.
• DNA directs biochemical, anatomical, physiological, and behavioral traits.

DNA is the Genetic Material

• Early 20th-century biologists faced challenges identifying molecules of inheritance.
• T.H. Morgan's group demonstrated gene location on chromosomes, suggesting DNA or protein as genetic material.

Evidence That DNA Can Transform Bacteria

• Frederick Griffith's 1928 research initiated the discovery of DNA's role in heredity.
• Griffith worked with two bacterial strains: pathogenic and harmless.
• Mixing heat-killed remains of the pathogenic strain with living harmless cells caused some living cells to become pathogenic.
• This phenomenon, called transformation, represents change in genotype and phenotype due to foreign DNA assimilation.

Evidence That Viral DNA Can Program Cells

• Studies of bacteriophages (viruses that infect bacteria) provided additional evidence for DNA as the genetic material.
• Bacteriophages are crucial in molecular genetic research.

Additional Evidence That DNA Is the Genetic Material

• DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group.
• Erwin Chargaff's 1950 observations indicated DNA composition varies between species; A=T and G=C.
• This diversity strengthens the case for DNA as the genetic material.

Additional Evidence: DNA Structure

• Two key findings are known as Chargaff's rules.
• Base composition of DNA varies between species.
• In any species, the number of adenine and thymine bases is equal, and the number of guanine and cytosine bases is equal.
• The basis for these rules was not understood until the discovery of the double helix.

Building a Structural Model of DNA: Scientific Inquiry

• After DNA's acceptance as genetic material, the task became determining how its structure facilitates heredity.
• Rosalind Franklin's X-ray crystallographic images of DNA were crucial in this effort.
• These images showed DNA was helical and had two strands

The Watson-Crick Model

• Watson and Crick developed models of a double helix that matched X-ray evidence and DNA chemistry.
• Franklin had identified two outer sugar-phosphate backbones, with bases paired in the interior.
• Watson's model featured antiparallel backbones (subunits run in opposite directions).

Base Pairing and Structure

• Watson and Crick initially proposed base pairing like with like (A with A, etc.).
• However, purine-pyrimidine pairings (A-T, G-C) create a uniform width consistent with X-ray data.
• Watson and Crick refined their model to propose specific base pairings dictated by base structures, with adenine pairing only with thymine and guanine pairing only with cytosine. The model explains Chargaff's rules (A=T and G=C).

Many Proteins Work Together in DNA Replication and Repair

• The relationship between DNA structure and function is clear in the double helix.
• The specific base pairing suggested a potential copying mechanism.

The Basic Principle: Base Pairing to a Template Strand

• DNA strands are complementary, allowing each strand to serve as a template for building a new strand during replication.
• In replication, the parent DNA molecule unwinds, and two new daughter strands are formed based on base pairing rules.

DNA Replication: A Closer Look

• DNA copying exhibits remarkable speed and accuracy, with participation from more than a dozen enzymes and proteins.

Getting Started

• Replication begins at origins of replication, where DNA strands separate to form replication forks.
• Replication proceeds in both directions from each origin until the entire molecule is duplicated.

Replication Mechanisms

• Replication bubbles open up at each replication origin, generating replication forks where new DNA strands elongate.
• Helicases untwist the DNA double helix at replication forks.
• Topoisomerase corrects overwinding ahead of the replication forks by breaking, swiveling, and rejoining DNA strands.

Synthesizing a New DNA Strand

• Enzymes called DNA polymerases catalyze new DNA strand elongation.
• DNA polymerases add nucleotides to the 3' end, making elongation in the 5' to 3' direction.
• Most DNA polymerases need a primer and a template strand.
• Primase starts an RNA chain. (Elongation is 500 nucleotides/second in bacteria and 50 nucleotides/second in humans.)

Nucleotide Structure

• Nucleotides added to the growing DNA strand are nucleoside triphosphates (e.g., dATP).
• dATP supplies adenine to DNA, resembling ATP in energy metabolism, but has deoxyribose sugar instead of ribose.

Antiparallel Elongation

• The antiparallel structure of the DNA double helix influences replication.
• In the 5' to 3' directions , DNA polymerases add nucleotides only to the 3' end of a growing strand.
• Along one template strand (leading strand), DNA polymerase synthesizes continuously toward the replication fork.
• Along the other template strand (lagging strand), DNA polymerase synthesizes discontinuously, in fragments called Okazaki fragments, which are eventually joined by DNA ligase.

The DNA Replication Complex

• Proteins involved in replication form a complex, essentially a DNA replication machine. The complex includes a number of different enzymes and proteins.

Proofreading and Repairing DNA

• DNA polymerases proofread freshly produced DNA, correcting errors.
• Mismatch repair enzymes correct base-pairing errors.
• DNA can be damaged, from chemical and physical agents (like cigarette smoke or X-rays), or through spontaneous changes.
• Nucleotide excision repairs damaged stretches of DNA by cutting and replacing them.

Evolutionary Significance of Altered DNA Nucleotides

• Errors occurring after proofreading/repair are low but not zero.
• Sequence changes can persist and be passed to future generations (mutations).
• These mutations are crucial to genetic variation, upon which natural selection operates.

Replicating the Ends of DNA Molecules

• DNA polymerase limitations pose problems for linear eukaryotic chromosomes.
• Prokaryotic chromosomes are typically circular (no similar issue).
• Eukaryotic chromosome ends possess telomeres, specialized nucleotide sequences that protect the genes.
• Shortening of telomeres could be a factor in aging or cell death.

A Chromosome

• A chromosome consists of a DNA molecule packaged with proteins.
• Eukaryotic chromosomes have linear DNA molecules, which are associated with a significant amount of protein.
• The complex structure of DNA and protein is called chromatin.
• Chromatin undergoes changes during interphase and cell mitosis.

Test your knowledge on the molecular basis of inheritance, focusing on the structure of DNA and its role in heredity as revealed by pioneers like Watson, Crick, and Griffith. Explore the concepts of DNA as genetic material and the transformational capabilities of DNA. This quiz will help reinforce your understanding of these key biological principles.