|Genome and Environment||Back to Top||Syllabus||
The character of
living organisms result from the interaction of environment
For example, the risk of colon cancer is increased by dietary factors
as high fat and low fiber. But a certain genetic allele
(version of a gene sequence) in certain individuals confers a high risk
of colon cancer, even with a low-risk diet.
|A SAMPLING OF ENVIRONMENTAL GENES|
|Polymorphism||Class||Environmental exposure||Associated disease|
|NAT2||Detoxification||Smoking||Bladder, breast cancer|
|GSTT1 (null)||Detoxification||Chlorinated solvents||Cancer, toxicity|
|Paraoxonase||Detoxification||Nerve agents, pesticides||Nervous system damage|
|HLA-H||Nutritional factors||Iron in diet||Hemochromatosis|
|TGF-||Growth factor||Maternal smoking||Cleft lip & palate|
|Locus on chrom. 17 in mice||Immune/inflammatory response||Ozone||Lung inflammation|
|HLA-DP bet1 marker||Immune response||Beryllium||Chronic beryllium disease (lung disorder)|
Experiments designed to demonstrate the genetic or environmental component of a trait generally aim to keep all components constant except the one tested. As a result, someone who studies Down's Syndrome might conclude that the basis of intelligence is genetics; whereas someone studying lead poisoning would conclude that the basis of intelligence is environmental. Actually, all traits -- appearance, development, behavior -- depend on BOTH genes and environment.
Chance also determines the development of an organism. For example individuals with the chromosome abnormality of Down's Syndrome may develop by chance to a wide range of levels -- some develop so badly they die before birth; others have severe heart defects and other physical problems; mental delays vary from severe to minor; some show few physical problems, and can attend college. None of this variation can be predicted from the genes, only from chance effects in development.
Cell - Organism
- Population - Ecosystem - Biosphere
Yet for most of
time, humans had no idea how traits were inherited. Why?
Offspring resemble parents (or don't) in bewilderingly complex ways. That is because individuals in nature contain many genes, and many different versions (alleles) of each gene. Consider these three individual orchids:
But Mendel's work
lost. Only in the past century did humans learn the fundamental
How and why organisms resemble their parents; and how the inherited information functions to make organisms look and behave as they do.
Walter Sutton and Theodore Boveri, using dyes synthesized by
the German organic chemistry industry, observed that "colored bodies"
cells behaved in ways parallel to the hypothetical agents of heredity
by Mendel. These bodies were called chromosomes.
1905 -- Nettie Stevens observed in Tenebrio beetles that all pairs of homologous chromosomes are the same size, except for one pair which determines sex -- X, Y.
1909 -- Thomas H. Morgan correlates the X chromosome with sex-linked inheritance of the white eye trait in Drosophila -- a strain of flies discovered by an undergraduate lab assistant, cleaning out old bottles of flies in Morgan's lab. Morgan went on to make many important discoveries in fly genetics and linkage analysis that apply to all diploid organisms.
1941 -- Beadle and Tatum determined in Neurospora that each gene encodes one product (protein). (Later, we learned that RNA can be a product, not always transcribed to protein; for example, a ribosomal RNA.)
1944 -- Oswald Avery identified DNA as the genetic material. Pieces of DNA can transfer genes into bacteria cells, and transform them genetically.
Thomas Watson and Frances Crick determined the double-helical
of DNA, and the structure of the base pairs which enable replication
two identical daughter helices.
1961 -- Jacob and Monod figured out regulation of the lac operon.
1960's -- Barbara McClintockdiscovered transposable elements in corn; later found in bacteria and animals.
1970 -- Temin and Balitimore discovered reverse transcriptase in retroviruses; an enzyme later used to clone genes based on the RNA encoding the product.
1977 -- Maxam, Gilbert, Sanger, others -- developed methods to sequence DNA.
first transgenic mammals were made.
1987 -- Kary Mullis invented the polymerase chain reaction (PCR), using a thermostable enzyme from a thermophilic bacterium discovered by Thomas Brock at a geyser in Yellowstone. Mullis sold the process to a pharmaceutical company, and earned very little. Brock didn't earn a cent.
1995 -- The first bacterial genome sequence, Haemophilus influenzae, was completely determined.
1996 -- Ian Wilmut cloned the lamb Dolly from adult mammary gland tissue.
1999 -- Completion of the first sequence of a human chromosome, number 22.
2002 --Completion of the human genome?
2010 -- Whole organs grown in culture?
2020 -- Chimp/human hybrids demand human rights?
2050 -- Self-aware computers demand human rights?
The genome of Escherichia coli contains 4.6 million base pairs, encoding 4,400 genes.
The human genome contains 3 billion base pairs in the nucleus, but only 60,000 genes (estimated), taking up 3% of the sequence. The rest includes regulator regions and large stretches of repetitive sequence of unknown function.
The entire genome
been sequenced for several microbes, and for one simple animal, Caenorhabditis
Take a closer look at three genomes:
T. A. Brown, Genomes, BIOS
|Chromosome Structure||Back to Top||Syllabus||
The bacterial DNA is packaged in loops back and forth. The bundled DNA is called the nucleoid. It concentrates the DNA in part of the cell, but it is not separated by a nuclear membrane (as in eukaryotes.) The DNA does form loops back and forth to a protein core, attached to the cell wall.
|The DNA double helix is bound to proteins called histones. The histones have positively charged (basic) amino acids to bind the negatively charged (acidic) DNA. Here is an SDS gel of histone proteins, separated by size (those migrating down farthest are smaller).|
The DNA is wrapped around the histone core of eight protein subunits, forming the nucleosome. The nucleosome is clamped by histone H1. About 200 base pairs (bp) of DNA coil around one histone. The coil "untwists" so as to generate one negative superturn per nucleosome.
form of DNA is active chromatin;
it can be "expressed" (transcribed and translated) to make RNA and
(Week 4, 5).
After DNA has been replicated for mitosis (cell division), the chromatin condenses.The nucleosomes zig-zag back and forth to form a flat ribbon:
The ribbon forms a coil, which then loops back and forth attached to a nuclear matrix -- similar to the protein core of bacteria, but greatly extended. During mitosis, several more layers of coiling result in fully condensed chromatin (see textbook Ch. 9).
mitosis, the chromosomes
appear as the thick rod-shaped bodies which can be stained and
under light microscopy.
The modern way to visualize condensed chromosomes is by FISH -- fluorescence in situ hybridization. In this method, fluorescent antibody-tagged DNA probes hybridize to their complementary sequences in the chromosomes. By using FISH probes with different colored fluorophores, one can color each human chromosome independently, and thus identify all 23 chromosomes. This is called chromosome painting.
Two different mechanisms do this in cells:
Bacterial cell fission, in which the circular chromosome is replicated.
Eukaryotic cell cycle, including Mitosis, in which multiple linear chromosomes are separated and passed on.
The big problem with eukaryotes is that they have to replicate linear chromosomes with special ends called telomeres. To do this, they need to use a special enzyme called telomerase, actually related to the reverse transcriptase of HIV virus. Telomerase activity may play a crucial role in human aging; if the chromosome ends fail to replicate properly, the chromosomes gradually lose parts of their end sequence.
on telomerase, go
Telomerase gene can extend life of human cells, perhaps preventing aging! (Or will it cause virulent cancer?)
on picture for stages:
Trent University, Biology Department
Bacteria can divide TWICE in the time it takes to complete replication
their entire circle of DNA. (This is one reason kids get sick so fast after
eating E.coli-contaminated hamburger.)
How is this possible? Can animal cells do the same thing? Why or why not?
(2) Suppose that in a field of cells in tissue culture, about five percent of cells show the condensed chromosomes of mitosis. If the duration of mitosis is five minutes, what is the overall generation time of the cells?