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基因管理學 -- R. Nixon
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Epigenetics: A Revolutionary Look at How Humans Work

Robin Nixon, Special to LiveScience

Scientists are now pinpointing exactly how nurture affects nature. Life experiences -- from toxin exposure to physical affection -- can alter gene expression in dynamic and sometimes reversible ways.

These insights -- the result of a burgeoning field called
epigenetics -- were aided by the sequencing of the human genome, completed in 2003
. However, the genome itself

turns out to have limited value for understanding disease and human characteristics.

Most traits and diseases involve a
multitude of gene combinations, and while "you can identify certain associated genes, they don't explain
that much," said Dennis Grayson, a researcher at the University of Illinois at Chicago. But the epigenome may provide countless explanations.

The most surprising thing about this revolutionary field, Grayson said, is that
no one is talking about it yet
.

The real boss

It is now common knowledge that humans share the majority of their genes with monkeys and apes, even worms and mice. It is something called the
epigenome that keeps us from "having a long tail and a snout," said Randy Jirtle, an epigenetics research pioneer at Duke University in North Carolina.

Epigenome literally means "
above the genome." It is a molecular marking system that controls gene expression without altering the DNA sequence. In a sense, the epigenome is the genome's boss
.

In the initial weeks after an egg is fertilized -- when its cells are rapidly multiplying and developing into something like a full-fledged body -- the epigenome acts as an
overbearing micro-manager. Each new cell has identical DNA, but the epigenome orders some cells to work as, say, skin cells and others to become neurons
.

New view

It was once thought this nitpicking backed off after birth, except for a brief resurgence around
puberty. But recently scientists have realized the boss is always watching -- taking stock of not only the genome but the world at large.

"The fascinating thing about it is that the epigenome is the
mediator between the genes and the environment
," said Liang Liu, a researcher at the University of Alabama at Birmingham

Food, pollution, toxic chemicals (such as those found in hard clear plastics), drugs, stress, even exercise and social interaction can all affect the epigenome and alter its attitude towards DNA
, Liu said.

Sometimes, the epigenome responds to environmental cues by asking a gene, via a charged molecular messenger, either
to start working or to work harder. For example, experiments by Michael Meaney and colleagues at McGill University have shown that when rat mothers are affectionate to their young, the baby’s epigenome promotes a gene that helps the pup remain calm even in stressful situations. (Conversely, neglected rat pups grow up observably anxious
.)

Other times, the epigenome
demotes
a gene. Many cancers, in fact, involve an irrational epigenetic firing of the gene that fights off tumors. The delivery of a molecular pink slip -- often a methyl group -- to the gene spurs a tight recoiling of the DNA spiral. The contraction makes it harder, or impossible, for transcription factors to interact with the gene -- like stripping an employee of her Blackberry, computer and contact list.

Wishy-washy


The epigenome, however, is occasionally a wishy-washy supervisor. In some cases, epigenomic "
decisions" can be reversed, Jirtle said.

Many medical treatments, Grayson said, such as those being developed for cancer and schizophrenia, try to coax the epigenome into changing its mind. Similarly,
healthy nutrition and lifestyle choices can positively sway gene expression. "Food is truly medicine when you are talking about epigenetic changes
," Jirtle said.

Even
interpersonal relationships
can persuade the epigenome to have a change of heart. For example, when neglected rat babies were adopted by affectionate mothers, the talented stress response gene was put back to work. And the pups calmed down.

The epigenome’s
malleability
highlights the power we can have over the health of ourselves and our children, said Jirtle. And gives hope that complicated diseases will someday have cures.

You Decide: What's the Greatest Mystery in Science? 

Top 10 Worst Hereditary Diseases 

All About DNA and Genes

轉貼自︰       

http://www.livescience.com/health/090427-epigenetics-overview.html



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基因理論ABC -- C. Binns
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Genes: The Instruction Manuals for Life

Corey Binns, Special to LiveScience

A gene is a how-to book for making one product -- a protein.

Proteins perform most life functions, and make up almost all cellular structures. Genes control everything from hair color to blood sugar by telling cells which proteins to make, how much, when, and where.

Genes exist in most cells. Inside a cell is a long strand of the chemical DNA (deoxyribonucleic acid). A DNA sequence is a specific lineup of chemical base pairs along its strand. The part of DNA that determines what protein to produce and when, is called a gene.

Inside genes

The term gene, first created by Danish botanist Wilhelm Johannsen in 1909, comes from the Greek word for origin, genos.

The number of genes in an organism's complete set of DNA, called a genome, varies from species to species. More complex organisms have more genes. A virus has a few hundred genes. Honeybees have about 15,000 genes. Scientists estimate that humans have around 25,000 genes.

Each gene has many parts. The protein-making instructions come from short sections called exons. Longer "nonsense" DNA, known as introns, flank the exons. Genes also include regulatory sequences. Although scientists don't fully understand their function, regulatory sequences help turn genes on.

Each gene helps determine different characteristics of an individual, such as nose shape. Full of information, genes pass similar traits from one generation to the next. That's how your cousin inherited grandpa's nose.

Peas in a pod

The "Father of Genetics," Gregor Mendel, was an Austrian monk who experimented with plants growing in his monastery. He studied inheritance in pea plants during the 1860s.

Mendel observed that when he bred plants that had green pea pods with plants that had yellow pea pods, all of the offspring had green pods. When Mendel bred the second generation with one another, some of the baby pods had green pods and some had yellow pods.

He discovered that a trait, or phenotype, could disappear in one generation and could reappear in a future generation.

Individuals have two copies of each gene, one inherited from each parent. Mendel explained how these copies interact to determine which trait is expressed.

In all peas there is a gene for pod color. The pod color gene has green and yellow versions, or alleles. Mendel's green pod alleles are dominant, and the yellow pod alleles are recessive. In order to express a recessive form of the trait (yellow), individuals must inherit recessive alleles from both parents.

A plant that inherits one green allele and one yellow allele will be green. But it can still pass the recessive yellow allele onto its offspring. That's how some of Mendel's pea pods came out yellow.

More to it

Human diseases such as sickle cell anemia are passed down in a similar way.

However, genetics don't always work so simply. Most genetics and instances of heredity are more complex than what Mendel saw in his garden.

It often takes more than a single gene to dictate a trait; and one gene can make instructions for more than trait. The environment, from the weather outside to an organism's body chemistry, plays a large role in dictating traits too.

Related Stories

Scientists Begin Reconstructing Neanderthal Genome 

Cracked Genome Shows Chicken DNA a Lot Like Yours 

Scientists Decode DNA of Extinct Animal

轉貼自︰       

http://www.livescience.com/health/060529_mm_genes.html



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基因概念ABC -- LiveScience
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LiveScience Topics: DNA and Genes

Genes are the blueprints of life. Genes control everything from hair color to blood sugar by telling cells which proteins to make, how much, when, and where. Genes exist in most cells. Inside a cell is a long strand of the chemical DNA (deoxyribonucleic acid). A DNA sequence is a specific lineup of chemical base pairs along its strand. The part of DNA that determines what protein to produce and when, is called a gene.

First established in 1985 by Sir Alec Jeffreys, DNA testing has become an increasingly popular method of identification and research. The applications of DNA testing, or DNA fingerprinting within forensic science is often what most people think of when they hear the phrase. Popularized by television and cinema, using DNA to match blood, hair or saliva to criminals is one purpose of testing DNA. It is also frequently used for other benefits, like wildlife studies, paternity testing, body identification, and in studies pertaining to human dispersion.

While most aspects of DNA are identical in samples from all human beings, concentrating on identifying patterns called microsatellites reveals qualities specific and unique to the individual. During the early stages of this science, a DNA test was performed using an analysis called restriction fragment length polymorphism. Because this process was extremely time consuming and required a great deal of DNA, new methods like polymerase chain reaction and amplified fragment length polymorphism have been employed.

The benefits of DNA testing are ample. In 1987, Colin Pitchfork became the first criminal to be caught as a result of DNA testing. The information provided with DNA tests has also helped wrongfully incarcerated people like Gary Dotson and Dennis Halstead reclaim their freedom.

http://www.livescience.com/topic/dna-and-genes



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