Plants case study: Barbara McClintock and maize
Much of our understanding of how chromosomes work is due to one researcher’s life’s work into the secrets of maize
In the early 20th century, scientists believed that chromosomes held genetic information in a static, unchanging fashion. Today, we now know that chromosomes are much more dynamic – a discovery that greatly affects our understanding of molecular, cell and developmental biology, medicine, and agriculture.
We know this because of Barbara McClintock, who would go on to win a Nobel Prize for her research. Even though McClintock’s findings held revolutionary implications for human biology, her research was done on a plant.
Maize, which is also known as Indian corn, has the most extensively mapped genome of all plant species, and its inheritance patterns of genes controlling seed and plant colours can be easily observed.
It is also highly varied. Some types of maize are as genetically different as two entirely different species might be.
On a microscopic scale, maize was also of genetic interest to McClintock due to the nature of the chromosomes in different varieties. For example, some maize cells were polyploid (having multiple sets of chromosomes), which appeared to trigger breakage, fusion, and thus restructuring of the chromosomes. The study of both the chromosomes and the resultant traits was essential in connecting the dots between our genetic material and how it is expressed.
McClintock was a pioneering figure in cytogenetics (the study of inheritance in relation to the structure and function of chromosomes). She made major breakthroughs despite facing sexism throughout her career.
Born in 1902 in Connecticut, she pursued a PhD in botany from Cornell University, where she flourished both academically and socially.
Despite demonstrating a great flair for the sciences with her advanced understanding and independent curiosity, McClintock was not permitted to be taught genetics in the plant-breeding department, as the staff did not accept female graduates. She instead registered in the botany department, where she majored in cytology (the study of the structure and function of plant and animal cells) and obtained a minor in genetics and zoology. McClintock became an instructor at Cornell after completing her PhD in 1927.
McClintock researched the chromosomes of maize, working with Harriet Creighton to describe how homologous chromosomes swap sections of DNA during cell division in regulated recombination, a process known as crossing over.
She produced the first genetic map of maize, demonstrating the link between gene loci and consequent phenotypes. She also outlined the functions of the telomere and centromere, chromosomal regions that are essential for the conservation of genetic information.
Her early work provided the first experimental proof that genes are located on chromosomes, and brought her global prominence in the scientific community.
Unfortunately, she was denied the chance to become a professor at Cornell because she was a woman. But she didn’t give up – she obtained a grant from the National Research Council to use her own lab and field space to fulfill her scientific passion.
There, she made many important discoveries that are fundamental to our understanding of genetics today. She observed chromosomal alterations – deletions, inversions, and translocations (in which a chromosome breaks and a portion of it reattaches to a different chromosome) – which can consequently alter phenotypes and even result in genetic disorders. She also described ring-shaped chromosomes in organisms which usually only display linear configuration.
McClintock was the first scientist to accurately propose epigenetic theory – heritable changes in gene expression that are caused by ancestral experience as opposed to alteration of DNA sequences. She produced this idea before the molecular structure of DNA was published, and 40 years before formal research into epigenetics began.
In 1944, at the relatively young age of 42, McClintock became a member of the National Academy of Sciences – only the third woman to have ever been elected. Four years later, she made yet another vital observational breakthrough – the detachment and movement of maize DNA between chromosomes (transposition).
Publishing her theory on transposition in 1948, she used vast quantities of data to support her discovery, yet many scientists at the time were unwilling to accept her work as valid. “It didn’t bother me,” she later said. “I just knew I was right.”
It would be almost 20 more years before her work was fully recognised. Eventually, similar mechanisms were found in other organisms, such as the 1967 discovery of mobile genetic elements in viruses that infect bacteria.
She received the Kimber Genetics Award in 1967, the National Medal of Science in 1970, and in 1983, she became the only woman to be awarded a solo Nobel Prize in Physiology or Medicine for her discovery of transposition, 35 years after her original publication.Lead image:
Caribb/Flickr CC BY NC
- Barbara McClintock – by Nina Federoff [PDF]
- Nature: Barbara McClintock – the secret of maize
- Science: Maize genome poster [PDF]
- North Dakota State University: Characteristics of organisms used for genetics studies
- The polyploid origin of maize – by Joachim Messing
Questions for discussion
- What is the significance of McClintock’s discoveries on the dynamic nature of the human genome in relation to genetic variation?
- How might the concept of translocation be relevant to gene therapy?