For an explanation, see the main splats page
The principles of chromosomes
- Most people who have stopped to think about it has always known that animals and plants inherit their appearance and nature from their parent organisms.
- A complication for early scientists was the belief in spontaneous generation, which assumed that parents were not needed to provide inherited information.
- In 1831, botanist Robert Brown announced the discovery that each living cell contains a nucleus. Others had seen nuclei, but now they were seen as universal.
- Once people realised that we are made of cells, and that cells reproduced by dividing, it became fairly obvious there must be something inheritable in the cell.
- In 1879, Walther Flemming discovered a thread-like material in the nucleus of cells. These threads were later named the chromosomes, because they took stains.
- In 1883, Edouard van Beneden announced the principles of genetic continuity of chromosomes and reported the occurrence of what we would now call meiosis.
- In 1888 Theodor Boveri verified August Weismann's predictions of chromosome reduction (which we call meiosis today) by direct observation in the worm Ascaris.
- In 1900, Walter Sutton observed homologous pairs in the chromosomes of a grasshopper, and he reported in 1902 that they separate to opposite ends in meiosis.
- In 1903, Walter Sutton and Theodor Boveri independently confirmed that the chromosomes behave in a way that matched what was known of Mendelian inheritance.
- By the careful study of genetics, Sutton and Boveri were able to recognise that the only thing in the cell behaving like Mendel's genes was the chromosomes.
- Alfred Sturtevant worked in the Columbia 'fly room' and actually constructed the first genetic map of a chromosome in 1913, using linkage data.
- In 1927, H. J. Muller used X-rays to cause artificial gene mutations in Drosophila, showing that the mutation rate was 1500 times higher when X-rays were used.
- It was not enough to know that the chromosomes carry the genes in some way, because chromosomes are made up of both protein and nucleic acids.
- In 1956, Joe-Hin Tjio and Johan Albert Levan revised Walther Flemming's 1898 estimate of the human chromosome count from 24 pairs to 23 pairs.
- In 1973, Bruce Ames published details of the 'Ames Test' to identify DNA-damaging chemicals. The test has since become a widely used to screen for carcinogens.
- All living things are made up of small units called cells. Each cell usually contains a nucleus with coded information that the cell uses to operate.
- Genes were first located as existing on the chromosome by the Sutton-Boveri theory which has stood the tests of time. The genetic code is further proof.
- The gender in animals is usually determined by the sex chromosomes, although different groups such as birds and insects do this differently from mammals.
- In mammals, normal males have an X-chromosome and a Y-chromosome, females have two X chromosomes. A sperm cell carries an X or a Y, an ovum carries an X only.
- The 'plans' for a living thing are found in chemical strings in the nucleus of the cell, and mistakes sometimes get made when these plans are copied.
- Each gene in the nucleus codes for a single unique protein. Most proteins work as enzymes that catalyse the conversion of one biochemical into another.
- A biochemical pathway involves a large number of conversions, each managed by an enzyme. If any enzyme is changed, the end product may never be produced.
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