Molecular Basis of Inheritance

Lesson

Part 1 — what Mendel’s “factors” are made of: the structure of DNA, how it is packed, and how scientists proved it is the genetic material.

In this lesson you will

Describe the structure of a polynucleotide and of the DNA double helix
Explain how DNA is packaged in prokaryotes and eukaryotes
Explain the experiments of Griffith, Avery and colleagues, and Hershey and Chase
Compare DNA and RNA as genetic material

Concept

DNA sizes

DNA is a long polymer of deoxyribonucleotides. Its length is usually given as the number of nucleotides, or base pairs (bp) for double-stranded DNA.

Table of genome sizes of two bacteriophages, E. coli and humans
Organism	Genome size
Bacteriophage φX174	5386 nucleotides
Bacteriophage lambda	48 502 bp
Escherichia coli	4.6 × 10⁶ bp
Human (haploid)	3.3 × 10⁹ bp

Concept

Building a polynucleotide

A nucleotide = nitrogenous base + pentose sugar + phosphate. Base + sugar (joined by an N-glycosidic linkage) = nucleoside. A phosphate on the 5′ carbon makes a nucleotide. Nucleotides join by 3′–5′ phosphodiester linkages, giving a strand with a free 5′ phosphate end and a free 3′ OH end.

Table comparing the sugars and bases of DNA and RNA
Component	In DNA	In RNA
Sugar	Deoxyribose	Ribose (extra 2′ OH)
Purines	Adenine, guanine	Adenine, guanine
Pyrimidines	Cytosine, thymine	Cytosine, uracil
Thymine	5-methyl uracil	Absent

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