Genetic Inheritance and DNA Structure

Intended Learning Outcomes

  • Explain why DNA is the basis of inheritance.

  • Describe the building blocks of DNA.

  • Understand the structure of DNA.

What Causes Inheritance?

  • Protein:

    • Complex structures.

    • Extremely variable from cell to cell.

    • Many (diverse) building blocks.

  • DNA:

    • Relatively simple structure.

    • Identical from cell to cell.

    • Four relatively simple building blocks.

Protein versus DNA

  • Protein:

    • 20 building blocks.

    • Potentially complex.

    • Variable from cell to cell.

  • DNA:

    • 4 building blocks.

    • Simple structure.

    • Identical from cell to cell.

Experiments Demonstrating DNA as the Genetic Material

  • Griffith's Experiment (using Streptococcus pneumoniae)

    • Demonstrated that a substance from dead bacteria can transform living bacteria.

  • Avery, MacLeod, and McCarty (1944)

    • Identified DNA as the substance responsible for transformation in Streptococcus pneumoniae.

    • Published in the Journal of Experimental Medicine: J. Exp. Med. 79 (2), 137-158.

  • Hershey and Chase Experiment

    • Showed that DNA, not protein, is the genetic material of bacteriophages.

    • Published in the Journal of General Physiology: Journal of General Physiology 36:39–56 (1952).

DNA - Part 2

  • DNA Lecture.

A Nucleotide Consists of 3 components:

  • Base

  • Sugar

  • Phosphate group

  • Important to note the 5’ and 3’ carbons

Chargaff's Rules

  • DNA composition varies between species.

  • The percentage of Adenine (A) and Thymine (T) are equal, and the percentage of Guanine (G) and Cytosine (C) are equal.

  • %A=%T\%A = \%T

  • %G=%C\%G = \%C

DNA Structure - Contributions

  • Rosalind Franklin and Maurice Wilkins:

    • Used X-ray crystallography to take a photo of DNA.

    • Determined DNA is helical in shape, with a width of 2nm2nm and a turn every 3.4nm3.4nm.

  • James Watson and Francis Crick:

    • Utilized Franklin and Wilkins' X-ray crystallography photo to discover that DNA is a double helix.

    • Received the Nobel Prize in Physiology or Medicine in 1962.

DNA Structure - Key Features

  • DNA strands are antiparallel (run in opposite directions).

  • Sugar-phosphate backbone is on the outside, with bases in the middle.

  • Two types of bases: purines and pyrimidines.

  • Strands are held together by hydrogen bonds.

  • DNA has a shallow (minor) and deep (major) groove.

DNA Structure - Dimensions

  • Diameter: 2nm2 nm

  • Bases are 0.34nm0.34 nm apart.

  • One full turn every 10 base pairs (3.4nm3.4 nm).

DNA Structure - Purine and Pyrimidine Pairing

  • Purine + purine: too wide.

  • Pyrimidine + pyrimidine: too narrow.

  • Purine + pyrimidine: width consistent with X-ray data.

    • Adenine (A) pairs with Thymine (T).

    • Guanine (G) pairs with Cytosine (C).

DNA Structure - Components

  • Phosphate group attached to the 5' carbon.

  • Nitrogenous base attached to the 1' carbon.

  • Covalent sugar-phosphate bonds link nucleotides.

  • Hydrogen bonds between bases hold strands together.

  • Van der Waals interactions between stacked base pairs help hold the molecule together.

DNA Structure - Polymorphic Forms

  • 3 crystallized structures: A, B, and Z.

DNA - Summary

  • Four building blocks.

  • Directional (5' -> 3').

  • Strands are complementary.

  • Double helix structure.

  • Base pairing:

    • Always T-A and C-G.

  • Hydrogen bonds:

    • 2 between A and T.

    • 3 between C and G.