Genetics
Geneticsis the branch of science that studies heredity and how traits (expressed
characteristics) are passed to new generations of species and between microor-
ganisms. Scientists who study genetics are called geneticistsand are interested in
how traits are expressed within a cell and how traits determine the characteristics
of an organism.
Think of a trait as an instruction that tells an organism how do something,
such as how to form a toe. Each instruction is contained in a gene. As you can
imagine, there are thousands of genes (instructions) necessary for an organism
to grow and flourish. This is why if a youngster looks and behaves like her
mother, family members tend to say she has her mother’s genes—that is, she has
more genes (instructions) from her mom than from her dad.
Genes are actually made up of segments or sections of deoxyribonuclear acid
(DNA), or in the case of a virus, ribonucleic acid (RNA) molecules. These seg-
ments are placed in a specific sequence that code for a functional product.DNA Replication: Take My Genes, Please!
In 1868, Swiss biologist Friedrich Miescher carried out chemical studies on the
nuclei of white blood cells in pus. His studies led to the discovery of DNA. DNA
was not linked to hereditary information until 1943 when work performed by
Oswald Avery, Colin MacLeod, and Maclyn McCarty at the Rockefeller Institute
revealed that DNA contained genetic information. These studies also revealed
that genetic information is passed from “parent cells” to “daughter cells,” creat-
ing a pathway through which genetic information is passed to the next genera-
tion of an organism.
Scientists were baffled about how the exchange of DNA occurred. The answer
came in 1953 when American geneticist James Watson and English physicist
Francis Crick discovered the double-helical structure of DNA at the University
of Cambridge in England. Discovery of the double-helical structure was the key
that enabled Watson and Crick to learn how DNA is replicated.
In the late 1950’s, Mathew Meselson and Franklin Stahl first described the
DNA molecule and how DNA replicates in a process called semiconservative(^120) CHAPTER 7 Microbial Genetics