Lecture 9: DNA and the Molecular Basis of Hereditary

10/6/2025

What is the hereditary chemical/universal blueprint?

Deoxyribonucleic acid (DNA)

What is DNA?

A molecule that carries genetic information for the growth, development, functioning, and reproduction for all known organisms and many viruses 

Where is DNA encoded?

In its sequence of four chemical building blocks/nucleotides

What are the four nucleotides/building blocks of DNA?

1. Adenine

2. Thymine

3. Cytosine

4. Guanine

What is the physical structure of DNA?

Double helix, held together by specific pairing of nucleotides 

What is the primary mechanism for heredity?

DNA replication

What are viruses made of?

Viruses are composed of a nucleic acid genome (DNA or RNA) surrounded by a protein cot (capsid)

How do viruses infect host cells?

Viruses inject their genetic material into host cells, changing how the host cell behaves. In bacteriophages, only the DNA enters the bacterial cell, while the protein coat stays outside

What are bacteriophages?

Viruses that infect and kill bacteria and archaea

What are the three main functions of DNA?

1. Providing the code for RNA and proteins

2. Encoding when and where RNA and proteins are expressed

3. Allowing duplication of DNA molecules for inheritance during cell division

How do children inherit genetic instructions from their parents?

Children inherit one set of chromosomes (and therefore DNA) from each parent. DNA is copied during reproduction, and each offspring receives a complete set of genetic instructions

How do daughter cells get identical genetic instructions during cell division?

Before a cell divides, its chromosomes (and DNA) are copied. Each daughter cell receives one identical set of chromosomes

What three key experiments proved DNA is the genetic material?

1. Griffith’s Experiment

2. Avery, MacLeod, and McCarty

3. Hershey and Chase 

What did Griffith discover in 1928?

A “transforming principle”

• Found that when harmless rough (R) Pneumococcus bacteria were mixed with heat-killed smooth (S) strain bacteria, the mixture became deadly - showing that some genetic material was transferred 

What question did Griffith’s experiment raise?

What is the “transforming principle” that transfers genetic information between bacteria?

Which large molecules could be the transforming principle?

• Carbohydrates

• Lipids

• Proteins

• Nucleic Acids

What did Avery, MacLeod, and McCarty discover in 1944?

That DNA is the transforming principle

• When they treated cells with deoxyribonuclease (DNase), transformation stopped - proving DNA carries genetic information

What did Hershey and Chase show in 1952 using bacteriophages?

They proved DNA is the genetic material

• When viruses were grown with 35S (sulfur) —> labeled proteins, radioactivity stayed in the liquid

• When viruses were grown with 32P (phosphorus) —> labeled DNA, radioactivity was in the pellet (inside cells)

Conclusion: DNA, not protein, carries genetic information

What did Chargaff discover in 1950?

The quantities of bases in DNA follow these rules:

• Amount of A = T

• Amount of G = C

This suggested base pairing

What did Rosalind Franklin contribute in 1951?

Using X-ray diffraction, Franklin and Gosling produced images showing that DNA has a helical structure with two or more strands arranged coaxially

What did Watson and Crick propose in 1953?

Using data from Franklin and Chargaff, Watson and Crick built a model of the DNA double helix

What did Watson and Crick’s DNA double helix have?

• Sugar-phosphate backbones on the outside

• Nitrogenous bases (A, T, G, C) paired on the inside

• A pairs with T, G pairs with C via hydrogen bonds

• Two strands are antiparallel (3’ —> 5’ and 5’ —> 3’)

What are the three parts of a DNA nucleotide?

1. A sugar (deoxyribose)

2. A phosphate group

3. A nitrogenous base (A, T, G, or C)

What is meant by the two DNA strands being “antiparallel”?

One strand runs in the 3’ —> 5’ direction, and the other runs 5’ — 3’, allowing proper base pairing

What does complementary base pairing mean?

The sequence on one strand determines the other:

• A ←→ T

• G ←→ C

If one strand reads AGTC, the complementary strand is GACT (opposite direction)

What did Meselson and Stahl’s 1958 experiment show?

DNA replication is semi-conservative: each new DNA molecule has one old (parental) strand and one new strand

How did Meselson and Stahl prove it?

They grew E. coli on heavy nitrogen (15N) so DNA was heavy, then switched to light nitrogen (14N). After generations, DNA samples had intermediate and light densities, proving one old + one new strand

How does DNA carry genetic instructions for proteins?

The sequence of bases (A, T, G, C) in DNA specifies the sequence of amino acids in proteins

What is a codon?

A triplet of bases in DNA (or mRNA) that encodes one amino acid

What is the “one gene-one protein” concept?

Each gene in DNA encodes one specific protein that carries out a particular function in the organism