Quick check:

1. List the ingredients Kornberg used to make DNA in the test tube.

2. During DNA replication, what is the function of:

a) helicases

b) DNA binding proteins

c) DNA polymerase

d) DNA ligase?

3. Suppose DNA replication were conservative. What results would Meselsohn and Stahl have obtained in the first generation?

4. Describe how DNA can be made in the laboratory.

5. Interpret Meselsohn and Stahl’s experiment on semiconservative replication.

6. Describe how semiconservative replication takes place.

7. Divide the text into an introduction, principal part and conclusion.

8. Express the main idea of each part.

9. Give a title to each paragraph of the text.

10. Summarize the text in brief.

■ Text 7. The Chemical Nature Of Genes

Acapsuleis an outer coat covering a bacterial cell. Unlike a slime layer, it is not easily washed off. Although capsules are not essential for bacterial growth and reproduction in laboratory conditions, they can make the difference between life and death in natural situations. For example, Streptococcus pneumoniae (a member of the pneumococci, the group of pneumonia-causing bacteria used in Griffith's experiment; see text) has non-capsulated and capsulated strains. Those lacking a capsule are easily destroyed by the host and do not cause disease. However, the capsulated strain kills mice quickly. The capsule helps the bacterium resist phagocytosis by host cells. It contains a great deal of water, protecting the bacterium from desiccation; it keeps out detergents which could destroy the cell surface membrane; and it helps bacteria attach to host cells.

We know today that DNA is the chemical in which information is from parent to offspring. This spread looks at how researchers established this link between DNA and inheritance. In the 1860s, nearly 100 years before Watson and Crick's work on the structure of DNA, Gregor Mendel established that inheritance depends on factors that are transmitted from parents to offspring. In 1909 it was found that patterns of inheritance were reflected in the behaviour of chromosomes. Wilhelm Johannsen referred to these factors as genes. Genes were assumed to be located on the chromosomes because genes that are inherited together (linked genes) were found to be carried on the same chromosome. However, the chemical composition of genes was not known.

Protein or DNA: which is the genetic material?

Chromosomes were known to contain both protein and DNA. Most biologists assumed that proteins, with their highly complex and infinitely variable structure, were the inherited material. The nucleic acids were thought to be too simple to carry complex genetic information. This view was reinforced by the work of Phoebus Aaron Levene. Levene made major contributions to the chemistry of nucleic acids but believed, mistakenly, that DNA was a very small molecule, probably only four nucleotides long.

In 1928 Fred Griffith, an English medical bacteriologist, published a paper describing experiments on pneumococci. His results set the stage for the research that finally showed that DNA is the genetic material.