Genome
A genome is all of a living thing's genetic material.
It is the entire set of hereditary instructions for building, running,
and maintaining an organism, and passing life on to the next generation.
The whole shebang.
In most living things, the genome is made of a
chemical called DNA. The genome contains genes, which are packaged in chromosomes
and affect specific characteristics of the organism.
Imagine these relationships as a set of Chinese
boxes nested one inside the other. The largest box represents the genome.
Inside it, a smaller box represents the chromosomes. Inside that is a box
representing genes, and inside that, finally, is the smallest box, the
DNA.
In short, the genome is divided into chromosomes,
chromosomes contain genes, and genes are made of DNA.
The word " genome " was coined in about 1930,
even though scientists didn't know then what the genome was made of. They
only knew that the genome was important enough, whatever it was, to have
a name.
Each one of earth's species has its own distinctive
genome: the dog genome, the wheat genome, the genomes of the cow, cold
virus, bok choy, Escherichia coli (a bacterium that lives in the human
gut and in animal intestines), and so on.
So genomes belong to species, but they also belong
to individuals. Every giraffe on the African savanna has a unique genome,
as does every elephant, acacia tree, and ostrich. Unless you are an identical
twin, your genome is different from that of every other person on earth—in
fact, it is different from that of every other person who has ever lived.
Though unique, your genome is still recognizably
a human genome. The difference is simply a matter of degree: The genome
differences between two people are much smaller than the genome differences
between people and our closest relatives, the chimpanzees.
Genomes are found in cells, the microscopic structures
that make up all organisms. With a few exceptions, each of your body's
trillions of cells contains a copy of your genome: the cells in your muscles,
the cells in your brain, the cells in your blood, and so on.
Imagine all the trillions of genomes in your body,
in other people's bodies, in cedar and apple trees, in walruses, forest
mushrooms, and migrating birds: The whole world is full of genomes.
But if the genome is a commonplace thing, it is
also quite powerful. A genome is information that affects every aspect
of our behavior and physiology. Cooking dinner, digesting your food, talking,
singing, sleeping—your genome has a hand in all these things.
A genome alone can't make a person, because we
are also influenced by where we live, the human culture that surrounds
us, and hundreds of other aspects of our environment. But the fact remains
that you can't make a person without a genome.
Studying the human genome, therefore, is likely
to give us insights into why some people die of heart disease and others
die of cancer, why some people are comfortable schmoozing with a crowd
of strangers and others are paralyzed by shyness, why some people have
trouble keeping weight on while others have trouble keeping it off, and
so on.
It's not difficult for scientists to get their
hands on the human genome. They draw a bit of blood from people who volunteer
to have their genomes studied, then use some simple laboratory procedures
to break open the cells in the blood sample and extract the DNA. The body
is constantly making more blood cells, so blood is a renewable genome resource.
Each human being contains a slightly different
version of the human genome, but all human genomes are similar enough that
we can learn about the human genome in general by studying the genomes
of one or a few individual people.
Studying the genome can mean many different things.
You can study a very small part of the genome or the genome as a whole.
You can study the sequence of a gene, the function of a gene, the parts
of the genome that regulate genes, or the DNA outside of genes. You can
observe where genes are located in the genome, or investigate how different
genes work
Chromosomes
A chromosome is a package containing a chunk of
a genome—that is, it contains some of an organism's genes. The important
word here is "package": chromosomes help a cell to keep a large amount
of genetic information neat, organized, and compact.
Chromosomes are made of DNA and protein. Most
living things have chromosomes that are linear, like bits of fat thread,
and are kept in the nucleus, a sphere-shaped sac within the cell.
Humans have 46 chromosomes.
What makes one chromosome different from
another?
Though similar in basic appearance, different chromosomes
vary slightly in size and shape. In addition, when chromosomes are stained
with fluorescent dyes they develop distinctive patterns of bright and dark
bands. These subtle differences enable cell biologists to distinguish different
chromosomes from one another, much as field biologists learn to distinguish
members of a pod of whales by the marks and scars on their fins.
The largest chromosome of an organism is generally
referred to as chromosome 1, the next largest as chromosome 2, and so on.
Different chromosomes contain different genes. That is, each chromosome
contains a specific chunk of the genome. For example, in humans the gene
for alpha globin, a part of the hemoglobin protein that carries oxygen
in red blood cells, is found on chromosome 16. The gene for beta globin,
the other part of the hemoglobin protein, is found on chromosome 11.
Stained chromosomes can be photographed and arranged
in order of size to produce a karyotype, a chart scientists use to study
chromosomes. A karyotype isn't detailed enough to tell you about the individual
genes on a chromosome, but it can tell you whether the chromosomes as a
whole are in working order and it may help doctors diagnose and understand
diseases. For example, people with Down's syndrome have too many chromosomes,
and chromosome rearrangements (when a part of a chromosome breaks off and
reattaches to a different chromosome) are associated with certain cancers.
A quick glance at any karyotype will tell you
one of the most important facts about chromosomes: They come in pairs.
The members of a pair, or "homologous" chromosomes, are the same size and
shape, and they have the same banding patterns. In other words, each person
actually possesses two copies of chromosome 1, two copies of chromosome
2, and so on. Human cells contain 23 pairs of chromosomes.
Most of an organism's chromosomes—generally all
except for one pair—are called autosomes, which are the same in males and
females. Humans have 22 pairs of autosomes.
Many organisms also have a pair of sex chromosomes,
which differ between males and females. In humans, a female has two identical
sex chromosomes. A male has one sex chromosome that is like those of females,
and one that is smaller and differently shaped. In scientific shorthand,
the female's sex chromosomes are referred to as XX, and the male's as XY.
The sex chromosomes of many other species have
a similar pattern, but it is not the only possibility. In grasshoppers,
females have two identical sex chromosomes, while males have only one sex
chromosome and are said to be XO. In birds, butterflies, and moths, it
is the males that have two identical sex chromosomes—they are XX—and females
are XY or XO.
Genes
A gene is a small piece of the genome. It's the
genetic equivalent of the atom: As an atom is the fundamental unit of matter,
a gene is the fundamental unit of heredity.
Genes are found on chromosomes and are made of
DNA. Different genes determine the different characteristics, or traits,
of an organism. In the simplest terms (which are actually too simple in
many cases), one gene might determine the color of a bird's feathers, while
another gene would determine the shape of its beak.
The number of genes in the genome varies from
species to species. More complex organisms tend to have more genes. Bacteria
have several hundred to several thousand genes. Estimates of the number
of human genes, by contrast, range from 25,000 to 30,000.
What do genes do?
Genes tell a cell how to make proteins. Roughly
speaking, each gene is a set of instructions for making one specific protein.
Proteins are a diverse group of large, complex
molecules that are crucial to every aspect of the body's structure and
function. Collagen, which forms the structural scaffolding of skin and
many other tissues, is a protein. Insulin, a hormone that regulates blood
sugar, is a protein. Trypsin, an enzyme involved in digestion, is a protein.
So is the pigment melanin, which gives hair and skin its color. Still other
proteins regulate the body's production of proteins.
Genes sometimes affect characteristics in indirect
ways. For example, genes affect the size and shape of your nose, even though
there's no such thing as a "nose size" protein. But directly or indirectly,
the way genes influence your traits is by telling your cells which proteins
to make, how much, when, and where.
DNA
DNA is the molecule that is the hereditary material
in all living cells.
Genes are made of DNA, and so is the genome itself.
A gene consists of enough DNA to code for one protein, and a genome is
simply the sum total of an organism's DNA.
DNA is long and skinny, capable of contorting
like a circus performer when it winds into chromosomes. It's skinny as
a whip and smart as one too, containing all the information necessary to
build a living organism. In a very real sense, DNA is information.
How much DNA is in a gene? How much is in
a genome?
Both genes and genomes come in a variety of sizes.
About 1,000 base pairs would be enough DNA to
encode most proteins. But introns—"extra" or "nonsense" sequences inside
genes—make many genes longer than that. Human genes are commonly around
100,000 base pairs long, and some are up to 2 million base pairs.
Very simple organisms tend to have relatively
small genomes. The smallest genomes, belonging to primitive, single-celled
organisms, contain just over half a million base pairs of DNA.
But among multicellular species, the size of the
genome does not correlate well with the complexity of the organism. The
human genome contains 3 billion base pairs of DNA, about the same amount
as frogs and sharks. But other genomes are much larger. A newt genome has
about 15 billion base pairs of DNA, and a lily genome has almost 100 billion.
GENOME SEQUENCING
Genome sequencing is figuring out the order of
DNA nucleotides, or bases, in a genome—the order of As, Cs, Gs, and Ts
that make up an organism's DNA. The human genome is made up of over 3 billion
of these genetic letters.
Today, DNA sequencing on a large scale—the scale
necessary for ambitious projects such as sequencing an entire genome—is
mostly done by high-tech machines. Much as your eye scans a sequence of
letters to read a sentence, these machines "read" a sequence of DNA bases.
A DNA sequence that has been translated from life's
chemical alphabet into our alphabet of written letters might look like
this:
That is, in this particular piece of DNA, an adenine
(A) is followed by a guanine (G), which is followed by a thymine (T), which
in turn is followed by a cytosine (C), another cytosine (C), and so on.
What is genome sequencing?
By itself, not a whole lot. Genome sequencing is
often compared to "decoding," but a sequence is still very much in code.
In a sense, a genome sequence is simply a very long string of letters in
a mysterious language.
When you read a sentence, the meaning is not just
in the sequence of the letters. It is also in the words those letters make
and in the grammar of the language. Similarly, the human genome is more
than just its sequence.
Imagine the genome as a book written without capitalization
or punctuation, without breaks between words, sentences, or paragraphs,
and with strings of nonsense letters scattered between and even within
sentences.
Even in a familiar language it is difficult to
pick out the meaning of the passage if the sentences are garbled.
For example pass your mouse over the above image, you will see the difference. And the genome is "written"
in a far less familiar language, multiplying the difficulties involved
in reading it.
So sequencing the genome doesn't immediately lay
open the genetic secrets of an entire species. Even with a rough draft
of the human genome sequence in hand, much work remains to be done. Scientists
still have to translate those strings of letters into an understanding
of how the genome works: what the various genes that make up the genome
do, how different genes are related, and how the various parts of the genome
are coordinated. That is, they have to figure out what those letters of
the genome sequence mean.
Why is genome sequencing so important?
Sequencing the genome is an important step towards
understanding it.
At the very least, the genome sequence will represent
a valuable shortcut, helping scientists find genes much more easily and
quickly. A genome sequence does contain some clues about where genes are,
even though scientists are just learning to interpret these clues.
Scientists also hope that being able to study
the entire genome sequence will help them understand how the genome as
a whole works—how genes work together to direct the growth, development
and maintenance of an entire organism.
Finally, genes account for less than 5 percent
of the DNA in the genome, and so knowing the entire genome sequence will
help scientists study the parts of the genome outside the genes. This includes
the regulatory regions that control how genes are turned on an off, as
well as long stretches of "nonsense" or "junk" DNA—so called because we
don't yet know what, if anything, it does.
OK, we all got enough information on genome, recently
http://www.celera.com has released
the fully sequenced human genome for the first time. We all have around
30,000 genes. It is worth reading or even just glancing this research article at Science
Magazine.
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