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Gregor Mendel Begins Experimentation on Peas
“The origins of genetics are to be found in Gregor Mendel's memoir on plant hybridization” (Gayon, 2016). Mendel chose to study variation in plants in the experimental garden of his monastery in the 1850s. He tested 34 varieties of pea plants for constancy of their traits. He carefully controlled pollination and traced the transmission of seven distinct traits, such as plant height, pod shape, and seed colour (Olby, 2024). -
Gregor Mendel Publishes Plant Heredity Findings
After years of studying pea plants, Mendel would publish his seminal paper “Versuche über Pflanzen-Hybriden,” or “Experiments on Plant Hybridization” (Ellis et al., 2011). In this paper, he would define the concepts of genes, alleles, and the segregation of alleles during reproduction (Mendel, 1866). He’s considered the father of genetics today because the legacy of his pioneering studies of hybridization in the pea continue to influence the way we understand modern genetics (Allen, 2003). -
Friedrich Miescher Isolates “Nuclein” (DNA)
While working in Felix Hoppe-Seyler's laboratory at the University of Tübingen, Miescher isolated a substance containing phosphorus and nitrogen from the nuclei of white blood cells found in pus (Dahm, 2005). He named this substance "nuclein" because it seemed to come from cell nuclei (The Editors of Encyclopaedia Britannica, 1998). He hypothesized that nuclein was a characteristic component of all nuclei and might be linked to the function of this organelle (Dahm, 2007). -
Friedrich Miescher Separates Nuclein into Protein and Acid Components
Miescher recognized that he had discovered a novel molecule, different from proteins, that was resistant to protein-digesting enzymes (Maderspacher, 2004). Nuclein later became known as nucleic acid after Miescher separated it into protein and acid components in 1874 (The Editors of Encyclopaedia Britannica, 1998). Today, we know it as deoxyribonucleic acid, or DNA. -
Walther Flemming Observes Threads of Genetic Material during Cell Division
Flemming was the first to observe and describe systematically the behavior of chromosomes in the cell nucleus during mitosis (The Editors of Encyclopaedia Britannica, 1998b). In 1879, he used aniline dyes to visualize cell structures, and found threadlike material in the nucleus. Applying these stains to cells killed at different stages of division, he prepared a series of slides that clearly established the sequence of changes occurring in the nucleus during cell division. -
Walther Flemming Names Cell Division Process “Mitosis”
Flemming would go on to show that the threads, called chromosomes, shortened and seemed to split longitudinally into two halves, each half moving to opposite sides of the cell (Paweletz, 2001). He would name the entire process “mitosis,” and describe it in his historic book "Zell-substanz, Kern und Zelltheilung" (Cell-Substance, Nucleus, and Cell-Division), published in 1882. He was also the first to realize that chromosomes have to double before they can divide! -
Phoebus Levine Isolates RNA’s d-Ribose
Phoebus Levene devoted his life to chemical research. Although his studies encompassed pretty much all organic compounds, his most valuable work was on nucleic acids (The Editors of Encyclopaedia Britannica, 1998b). Around 1909, while the entire scientific community was racing to learn more about the recently discovered nucleic acids, Levene was first to isolate d-ribose from RNA — a pentose sugar (A Dictionary of Scientists, 1999). Virtually nothing was known about DNA’s structure, until now. -
Phoebus Levene Establishes “Tetranucleotide Hypothesis”
In 1909, Levene’s findings on the pentose sugars would lead him to establish his tetranucleotide hypothesis for the structure of nucleic acids (Hargittai, 2009). According to Levene, each of the four bases occurred just once in each DNA/RNA molecule, and were joined together by sugar and phosphate groups (A Dictionary of Scientists, 1999). This was a remarkably simple structure… and obviously incorrect, but his work laid the foundation for understanding the chemical structure of nucleic acids. -
Frederick Griffith Discovers the Transforming Principle of DNA
Griffith’s 1928 experiment with pneumococcus bacteria was the first to reveal the "transforming principle", which led to the discovery that DNA acts as the carrier of genetic information (The Editors of Encyclopaedia Britannica, 2020). In his experiment, Griffith used two strains of bacteria that infect mice and demonstrated that the dead strain could transfer genetic information to the live through “transformation.” This laid the foundation for DNA as the carrier of heredity (Méthot, 2015). -
Phoebus Levene Discovers Deoxyribose Sugar
Twenty years after his discovery of d-ribose, Levene would discover 2-deoxyribose (a sugar derived from D-ribose by removing an oxygen atom), which is part of the deoxyribonucleic acid (DNA) molecule (The Editors of Encyclopaedia Britannica, 1998b). -
Tatum and Beadle Propose One Gene One Enzyme Hypothesis
Tatum and Beadle’s experiments with the bread mold Neurospora crassa revealed that genes control the production of enzymes, which in turn regulate specific chemical reactions in cells (Strauss, 2016). Each strain of mold that was X-rayed lacked a specific enzyme due to a mutation in the gene responsible for producing that enzyme (Pribadi, 2016). This led them to propose the "one gene-one enzyme hypothesis", which stated that each gene codes for the production of a single enzyme (Strauss, 2016) -
Avery, MacLeod, and McCarty Find DNA to be Carrier of Genetic Information
Avery, MacLeod and McCarty set out to identify the transforming principle discovered by Griffith in 1928. Through a series of biochemical purification steps, they isolated the transforming substance. They found that it was rich in DNA but not proteins. Enzymes that degrade DNA destroyed the transforming activity, while enzymes that degrade proteins and RNA didn’t (O’Connor, 2008).
And so, they confirmed that DNA was the transforming principle, and carrier of genetic information (Cobb, 2014). -
Erwin Chargaff Develops Chargaff Rules for Nucleotides
In the 1940s, Chargaff analyzed the chemical composition of DNA from various species and found that, in any given DNA sample, the amounts of adenine (A) and thymine (T) are always equal, and the amounts of guanine (G) and cytosine (C) are always equal, and that the overall base composition of DNA varies considerably between species (Kresge et al., 2005). These findings, now known as "Chargaff's rules," suggested that the four DNA bases are paired in a specific way (MIT Technology Review, 2011). -
McClintock Discovers Transposons in Maize
Through her studies of genetic mutation in maize, McClintock discovered that genes could move from one location to another on the chromosome. She found that these transposable elements could control the behaviour of neighbouring genes and cause new mutations (L. Pray & Zhaurova, n.d.). She also produced the first genetic map for maize, and
established that chromosomes form the basis of genetics through her work with Harriet Creighton in 1931 (The Editors of Encyclopaedia Britannica, 1998a). -
Hershey and Chase Conclude that DNA, Not Protein, is Genetic Material
Hershey and Chase used bacteriophages (viruses that infect bacteria) to determine that viral phage DNA, not protein, enters the host bacterial cell and is responsible for transferring genetic information (Hernandez, 2019). Bacteria infected with phages containing radioactive DNA became radioactive themselves and passed on the label to the next generation of phages, but bacteria infected with phages labeled with radioactive PROTEIN showed little radioactivity (Kimball, 2016). -
Rosalind Franklin Images DNA’s Helical Structure
Franklin took X-ray images of DNA while working at King's College London in 1951, one of which was the famous "Photo 51" which provided crucial evidence of the helical structure of DNA (Elkin, 2003) This X-ray diffraction image taken by Franklin and her student Raymond Gosling laid the foundation for the identification of DNA as the carrier of genetic information.
(Elkin, 2003) But Franklin didn’t fail to grasp DNA’s structure; she was an equal contributor to solving it (Cobb & Comfort, 2023) -
Watson and Crick Propose Double Helix Structure of DNA
Watson and Crick brought together data from a number of researchers, including Rosalind Franklin's X-ray images, to assemble their celebrated model of the 3D structure of DNA (Pray, 2008). The model showed that DNA consists of two complementary strands wound into a double helix, with the four DNA bases (adenine, thymine, guanine, and cytosine) paired between the strands (Watson & Crick, 1953). It elegantly explained how genetic information could be replicated and passed on to offspring. -
Arthur Kornberg Discovers DNA Polymerase
Arthur Kornberg made several groundbreaking discoveries in biochemistry. Most notably, he discovered and purified DNA polymerase, the enzyme that catalyzes the synthesis of DNA molecules, from E. coli in 1956 (Baker, 2007). He also discovered enzymes responsible for the initiation and elongation of DNA chains during replication and repair, which were crucial for the development of recombinant DNA technology (The Editors of Encyclopaedia Britannica, 1998a). -
Meselson and Stahl Prove Semi Conservative Model of DNA Replication
Meselson and Stahl conducted an elegant experiment that provided evidence for the semi-conservative model of DNA replication proposed by Watson and Crick (Pray, 2008a). It involved growing E. coli bacteria for many generations with isotopes of nitrogen (Hernandez, 2017). Their results were consistent with the semi-conservative model, where each strand of the original DNA molecule serves as a template for a new complementary strand, resulting in two DNA molecules. -
Syndney Brenner Assists in Identifying mRNA, Demonstrates Triplet Nature of Genetic Code
Sydney Brenner was a South African biologist who made significant contributions to the field of molecular biology. For example, he made important contributions to the identification of messenger RNA (mRNA) in 1961. As well as that, he demonstrated the triplet nature of the genetic code in the 1960s while working in Cambridge, England (White & Bretscher, 2020). -
Frederick Sanger Develops First Method for DNA Sequencing
Frederick Sanger, like pretty much everyone on this list, made many groundbreaking contributions to the field of molecular biology. Most notably, in 1977, he published the first method for rapid DNA sequencing (“Sanger method”) (Jeffers, 1998). This allowed him to sequence the genome of the bacteriophage φX174, the first complete genome of a living organism to be sequenced. This method was used to sequence the human mitochondrial genome in 1981 (Walker, 2014). -
Maxam and Gilbert Develop a New Method for Sequencing DNA
In 1977, Allan Maxam and Walter Gilbert developed another method for DNA sequencing known as Maxam-Gilbert sequencing. Their chemical sequencing technique was one of the first widely adopted methods for determining the nucleotide sequence of DNA molecules (Maxam & Gilbert, 1977). It was more popular than the Sanger dideoxy method initially, as it allowed direct sequencing of purified DNA without the need for cloning (Eren et al., 2023). Improvements to the Sanger method have made it obsolete. -
Kary Mullis Develops the PCR (Polymerase Chain Reaction) Method
In 1983, Kary Mullis invented the polymerase chain reaction technique (The Editors of Encyclopaedia Britannica, 1999). This technique allows for the amplification of a specific DNA sequence, making it possible to generate millions of copies of from a small initial DNA segment. It involves heating the DNA to separate its two strands, cooling it to allow primers to attach to the complementary sequences, and then letting DNA polymerase extend the primers to form new strands (Kossakovski, 2024) -
Carol Greider Co-discovers Telomerase Enzyme
In 1984, while a graduate student working with Elizabeth Blackburn, Greider discovered the enzyme telomerase. Telomerase adds DNA sequence repeats to the ends of chromosomes, counteracting the natural shortening of telomeres that occurs with each cell division (Rogers, 2009). Greider cloned and characterized the RNA component of telomerase while a fellow at Cold Spring Harbor Laboratory from 1988-1990. She later determined the secondary structure of the human telomerase RNA (Feng et al., 1995). -
Ian Wilmut and Keith Campbell Cloned Sheep
In 1995, Wilmut and Campbell generated the first cloned sheep, Megan and Morag, using nuclei from nine-day-old embryonic cells, showing that the age or stage of differentiation of the donor cell did not matter for successful cloning (Bradford, 2023). In 1996, they cloned Dolly the sheep, the first mammal cloned from an adult somatic cell, overturning the belief that development is a one-way street and that differentiated cells cannot be reprogrammed (Franklin, 2023). -
Smith and Hutchison Sequence the Smallest Genome
Smith and Hutchison were both accomplished scientists in their own right — Hamilton discovered the first Type II restriction enzyme, HindII (Sheahan, 2010). After sequencing the M. genitalium genome (published in 1995), the two began work on the “minimal genome project,” to see how many genes were necessary to sustain life (Hutchison et al., 2016). This work was published in Science in 1999. They were also involved in the creation of the first synthetic cell in 2010, Mycoplasma laboratorium. -
Elaine Mardis Develops Next Generation DNA Sequencing Methods
Dr. Mardis is a globally recognized expert in cancer genomics (Nationwide Children’s Hospital, n.d.). Her expertise was a pivotal contribution to the sequencing and completion of the Human Genome Project (Lander et al., 2001). She also co-led the research team that first sequenced and analyzed a whole cancer genome (Berger & Mardis, 2018). Some of her most notable work includes co-developing methods for next-generation high-throughput DNA sequencing, improving past science (Mardis, 2008). -
Venki Ramakrishnan Builds First Map of Ribosome’s 30S Subunit
Venki Ramakrishnan is a Nobel Prize-winning structural biologist. In 2009, he made groundbreaking discoveries about the structure and function of ribosomes (Rogers, 2009b). Using X-ray crystallography, Ramakrishnan determined the atomic structure of the small ribosomal subunit from the bacterium Thermus thermophilus at high resolution, revealing how the ribosome reads the genetic code to synthesize proteins (Rogers, 2009b). -
Jennifer Doudna Co-discovers CRISPR-Cas9 Gene Editing
In 2012, Doudna and French microbiologist Emmanuelle Charpentier discovered that the CRISPR-Cas9 system could be reprogrammed to target any desired DNA sequence by changing a short RNA sequence (Rogers, 2015). This provided the foundation for using CRISPR-Cas9 as a powerful and versatile genome editing technology.
Doudna's work has enabled researchers to make precise changes to DNA sequences in a much more efficient and technically simpler way than previous method's (Rogers, 2015).