Big 100: Genetics
1. Rules of Heredity (1850s)
Austrian monk and botanist Gregor Mendel discovers how genetic information is passed down through generations. In experiments performed on pea plants, he notices that characteristics of a plant's offspring, such as height, exhibit recessive and dominant behavior. Mendel's findings are ridiculed during his lifetime and he dies never knowing that he would come to be known as the "father of genetics."
2. Genes Are Located on Chromosomes (1910 – 1920s)
Thomas Hunt Morgan discovers that genes are located on chromosomes. Working on fruit flies, he concludes that certain traits are linked to gender and that those traits are probably carried on one of the sex chromosomes (X or Y). He hypothesizes that other genes are also carried on specific chromosomes. Using chromosome recombination, he and his students map the locations of genes on chromosomes. Morgan and his students write the seminal book The Mechanism of Mendelian Heredity.
3. Genes Control Biochemical Events (1930)
George Beadle and Edward Tatum discover through experiments on neurospora, a bread mold, that genes are responsible for the production of enzymes. Their report is the genesis of the "one gene-one enzyme" concept.
4. Some Genes Can Jump (1940)
Barbara McClintock discovers transposons — genes that can jump on a chromosome — while seeking to explain color variations in corn. Transposons are segments of DNA that can move to different positions in the genome of a single cell. In the process, they may cause mutations and increase (or decrease) the amount of DNA in the genome. These mobile segments of DNA are sometimes called "jumping genes."
5. DNA Is the Genetic Material (1928, 1944, 1952)
Several scientists prove that DNA is the chemical basis of genetic information. Oswald Avery proves that DNA carries genetic information. Linus Pauling discovers that many proteins take the shape of a spiral, like a spring coil. Finally, biochemist Erwin Chargaff finds the arrangement of certain nitrogen bases in DNA always occurs in a 1-to-1 ratio, forming base pairs.
6. DNA Is a Double Helix (1953)
James Watson and Francis Crick describe the DNA molecule. The scientists suggest that the DNA molecule is made of two chains of nucleotides, each in a helix, one going up and the other going down. Crick adds the idea that matching base pairs interlock in the middle of the double helix to keep the distance between the chains constant. They show that each strand of the DNA molecule is a template for the other, and that DNA can reproduce itself without changing its structure, except for occasional errors or mutations.
7. Cracking the Genetic Code (1960s)
Marshall Nirenberg leads the team that discovers the genetic code, showing that a sequence of three nucleotide bases (a codon) determines each of the 20 amino acids.
8. RNA Conveys Genetic Information (1960s)
A number of scientists discover ribonucleic acid, or RNA, a chemical found in the nucleus and cytoplasm of cells with a structure similar to DNA. They find that RNA plays an important role in protein synthesis and other chemical activities in the cell.
9. Restriction Enzymes (1950s – 1960s)
Several scientists discover restriction enzymes — biological scissors that recognize and cut specific DNA sequences.
10. RNA Splicing (1976)
Several groups of scientists discover RNA splicing. They learn that for cells to produce protein, DNA is first transcribed into pre-messenger RNA. For reasons that remain unclear, pre-messenger RNA molecules are then spliced to create mature messenger RNA. In many genetic diseases, gene mutations cause errors in the RNA splicing process. Improperly spliced messenger RNA molecules create altered proteins and result in disease.
11. DNA Polymorphism (1985)
Alec Jeffreys discovers that some DNA sequences are unique to each individual, leading to the birth of DNA forensics. His DNA technique is first used to hunt down a child molester who killed two girls. The suspect, Colin Pitchfork, is convicted of murder after DNA samples taken from him match semen samples taken from the two dead girls.
12. Humans Have 20,000 to 25,000 Genes (2003)
Upon sequencing the human genome, it's discovered that humans have approximately 20,000 to 25,000 genes, far fewer than most scientists had predicted. It is hoped that understanding the genome will boost the fields of medicine and biotechnology, eventually leading to cures for diseases such as cancer and Alzheimer's disease.
13. RNA Interference (1998)
Andrew Fire and Craig Mello discover RNA interference (RNAi), in which the presence of small fragments of double-stranded RNA (dsRNA) whose sequence matches a given gene interferes with the expression of that gene. Scientists believe that dsRNAs that trigger RNAi may be usable as drugs.