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Introducing the Chromosome Yr 12 Biology Linked to parts 3 – Chromosomal structure provides the key to inheritance outline the roles of Sutton and Boveri in identifying the importance of chromosomes describe the chemical nature of chromosomes and genes explain the relationship between the structure and behaviour of chromosomes during meiosis and the inheritance of genes explain the role of gamete formation and sexual reproduction in variability of offspring What is a chromosome? The word ‘chromosome’ comes from ‘chroma’ (meaning coloured) and ‘soma’ (meaning body). A chromosome is a single molecule of _____________ coiled around a series of __________________ called ______________. The DNA is compacted and wound up so much that is can actually be seen under a light microscope when stained with certain dyes What is a Karyotype? 1 Mutations A mutation caused by _____________, harmful ________________ or random ____________ can involve a change in the number of chromosomes, as shown here in a Down Syndrome karyotype with an extra chromosome 21. This karyotype is written as 47,XY,+21 A mutation can involve a rearrangement of the chromosomes already present. Chromosomes can be rearranged by section being inverted, duplicted, deleted or translocated. Chromosome Bands Each chromosome has a unique banding ___________________ that is used to identify it. You could think of it as a specific bar code. Naming the bands helps ________________ specific sections of the chromosome. The dark bands represent areas of the chromosome that are more ________________ and the pale bands show less condensed sections. The bands are used to identify parts of the chromosomes to know if there are any abnormalities within or between the chromosomes. Both ___________________ are from the same parent. The original sister chromatid ________________________ itself just before cell division. •This is a diagram of Chromosome 21. The band numbers are written down the side and next to that is a gene map. Genes have numbers and letters that make up their names. •During the Human Genome Project (which aimed to map the entire human genome) analysis of this chromosome revealed 127 known genes, 98 predicted genes, and 59 pseudogenes. Pseudogenes are sections of DNA that appear to be gene like, but do not function as genes. http://hgp.gsc.riken.go.jp/chr21/ Boveri’s Contribution In Germany at the turn of the 20th century, Theodor ____________________ studied sea urchins and discovered that the nucleus rather than the cytoplasm of a cell is concerned with __________________________. In 1896 he bred two different kinds of sea urchins and found that the characteristics mixed. Then he removed the nucleus from the female and noted that only the male traits were passed on into the offspring. In 1907 he fertilized sea urchin eggs with two sperm, 2 which developed into 4N tetraploid cells instead of 2N diploid cells. Suggested that chromosomes exchanged genes during meiosis – this was later known as crossing over. Sutton’s Contribution In 1902 Walter Sutton (an American geneticist) proposed that chromosomes were actually involved in inheritance. He suggested that the ‘factors’ (soon to be known as genes) that Mendel used to describe how character traits were inherited, were actually carried on _________________. Sutton was not famous for any single experiment, but instead he brought together the research of many other scientists – including Boveri – to explain how the chromosomes were involved in heredity. The Sutton Boveri Theory The Sutton-Boveri theory, otherwise known as the ‘chromosome theory of inheritance’, stated that chromosomes carried the units of inheritance and occurred in distinct pairs. The two scientists worked separately but came to the same conclusions. As there were more inheritable traits than there were chromosomes they suggested that each chromosome must carry many factors representing a particular trait. Their theory was initially discussed and debated, some embraced it while others strongly rejected it. By 1915 Thomas Hunt Morgan laid the controversy to rest with his genetic studies of the fruit fly Drosophila melanogaster.’ Crossing Over in Chromosomes Crossing over occurs when parts of _________________ chromosomes that are ______________ up next to each other during meiosis 1 break and then rejoin so that the parts of the two different chromosomes are _________________ over. The point where the chromosomes break and cross is known as the ____________________ . Does not occur every time chromosomes line up at meiosis. Introduces extra ________________ variation into the offspring. The _________________ genes are apart on a chromosome, the _______________ crossing over can occur between them, and the closer they are the less crossing over occurs. Crossing over at Meiosis List the 3 ways that genetic variation is introduced into the offspring during sexual reproduction: 1) 2) 3) 3