0.1 Basic principles of genetics  (Page 10/19)

Most living organisms are composed of different kinds of cells specialized to perform different functions, which are called differentiated cells as opposed to stem cells. A liver cell, for example, does not have the same biochemical duties as a nerve cell. Yet every cell of an organism has the same set of genetic instructions, so how can different types of cells have such different structures and biochernical functions? Since biochemical function is determined largely by specific enzymes (proteins), different sets of genes must be turned on and off in the various cell types. This is how cells differentiate.

This notion of cell-specific expression of genes is supported by hybridization experiments that can identify the unique mRNAs in a cell type. More recently, DNA arrays and gene chips offer the opportunity to rapidly screen all gene activity of an organism. Co-expression of genes in response to external factors can thus be explored and tested, as shown in the figure to the left, kindly provided by Prof. Douglas J. Burks.

Lecture 10. genes move mostly together with chromosomes

The inheritance of genes is based on the behavior of chromosomes, on which genes are located, and how the chromosomes are distributed during cell divisions, mitosis and meiosis in eukaryotic organisms.

Mitosis produces genetically identical cells; meanwhile products of meiosis are genetically distinct because of independent assortment and crossing-over.

Mitosis is the process by which the contents of the eukaryotic nucleus are separated into 2 genetically identical packages. The result is 2 cells, each with an identical set of chromosomes.

Genetic information is reshuffled during meiosis, producing genetic diversity in populations. A diploid cell contains two sets of chromosomes. The maternal set was contributed by the mother, and the paternal set was contributed by the father. A pair of homologous chromosomes consists of one maternal and one paternal chromosome, which represent Mendel’s units of inheritance that show independent segregation and assortment. Homologous chromosomes carry the same genes but may have different forms or alleles of the genes. At the beginning of meiosis, homologous chromosomes pair and non-sister chromatids exchange sections of DNA through the process known as crossing-over or recombination.

The resulting chromosomes may now contain different combinations of alleles than were found in the chromosomes inherited from the parents. At the middle of meiosis I, the maternal and paternal chromosomes of one homologous pair align independently of the maternal and paternal chromosomes of the other homologous pairs. Genes that are located on different chromosomes undergo independent assortment because of the random alignment of the maternal and paternal chromosomes. Gametes produced by meiosis have different combinations of alleles as a result of both recombination and independent assortment.

Lecture 11. genes can transfer between species

Because of the universality of the genetic code, the polymerases of one organism can accurately transcribe a gene from another organism. For example, different species of bacteria obtain antibiotic resistance genes by exchanging small chromosomes called plasmids. In the early 1970s, researchers in California used this type of gene exchange to move a "recombinant" DNA molecule between two different species. By the early 1980s, other scientists adapted the technique and spliced a human gene into E. coli to make recombinant human insulin and growth hormone.