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Unformatted text preview: ddress the physical nature of genes
During this time, biologists were studying the physical basis of heredity Physical Carriers of Genes
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o Wherever physical units of heredity came from they had to: be passed on from cell to cell, be passed from
generation to generation, and must be contained within a single cell
The cytoplasm and its contents divided randomly between daughter cells
Nuclear contents divided equally 1 o
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o During cell division, the nucleus organized into visible threads called chromosomes chromosomes are doubled
prior to cell division
Fertilization observed sperm and egg looked very different but had common chromosomes
Chromosomes are presented as pairs of homologous chromosomes
Chromosomal behaviour correlates with Mendel s laws of inheritance Chromosomes
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o Genes that are on the same chromosome do not assort independently
Genes on the same chromosomes are part of the same linkage group the traits analyzed by Mendel occur on
different chromosomes
Drosophila (flies) Morgan had wild type flies, but he developed his first mutant
This served as evidence for linking chromosomes as gene carriers to evolution
He eventually isolated 85 different mutations in 4 linkage groups (flies have 4 chromosomes)
He confirmed that genes reside on chromosomes Crossing over and Recombination
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o Mendel found that genes controlling different traits segregate independently of each other independent
assortment
Genes on the same chromosomes act as if they are linked linkage groups
But in reality, genes on the same chromosomes did now always act like they were linked they could be
inherited separately
Linkage between alleles on the same chromosome is incomplete
During meiosis homologous chromosomes wrap around each other proposed that this interaction allowed
breakage and exchange of chromosome pieces (genes)
Morgan called this crossing over or genetic recombination and explained incomplete linkage
Percentage of recombination between a pair of genes is constant
Percentage of recombination between different pairs of genes can be different suggests position of genes
along chromosome is fixed, the position of genes along the chromosome (loci) can be mapped frequency of
recombination indicates distance, and increases as the distance increases Chemical Nature of Genes
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o o Same time Mendel was working, Miescher isolated a new molecule from cell nuclei nuclein, later called nucleic
acid
Around the same time scientists were studying...
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- Spring '12
- Genetics, DNA
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