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| Genetics
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| Overview of Genetics |
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By
Amy Adams, MS
Reviewed By Amanda
Toland, PhD and Adam
Lowe
Last updated: October 19, 2011
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The
field of genetics is in the news daily, with researchers
mapping the human genome, cloning animals, and identifying
new disease genes. For many people, the problem is putting
this information into context and understanding what
recent genetic "breakthroughs" really mean in terms
of our health. To help answer these questions, we've
created a primer to help you understand how advances
in genetics can help in the treatment of common diseases.
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What
is DNA?
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image for larger
representation
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DNA
(deoxyribonucleic acid) is a long molecule that is contained
within almost all of our cells in
a compartment called the nucleus. It is composed of individual units called bases. There are four types of bases, designated A (adenine), T (thymine), G (guanine), and C (cytosine).
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image for larger
representation
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Each
DNA molecule is made of two individual strands paired
together. Each strand consists of a series of the four
bases. When the two strands
pair up, an A on one strand is always across from a
T on the other strand, and a C always pairs with a G.
These A/T and G/C combination
are called base pairs. The
double-stranded molecule then twists like a coiled ribbon
into a shape called a double helix. A piece of DNA millions
of base pairs long — in conjunction with
some proteins — is a chromosome.
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What are genes?
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| The sequence of bases in each gene contains instructions for making a single protein. |
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Each
chromosome contains thousands of genes,
each of which is several thousand bases long. The sequence
of bases in each gene contains instructions for making
a single protein. Each protein serves a particular function in the body. For example, enzymes help us digest food, structural elements give our cells shape, and signaling molecules help the cells communicate with each other.
Additional
bases that come before the genes on a chromosome tell
cells when each gene should be used. For example, these
sequences might contain instructions that a protein
for making hair should only be made in certain skin
cells, and not by other cells of the body.
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How
do we inherit our genes?
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Humans inherit 23 chromosomes from each of their parents
for a total of 46 chromosomes. Of these, 44 are identical
in men and women — these are called autosomes.
The remaining two chromosomes are called sex chromosomes,
which are designated X and Y. Women inherit two X chromosomes,
whereas men inherit one X chromosome from their mother
and one Y chromosome from their father. Because of the
way we inherit our chromosomes, we all have two copies
of every gene that is contained on the autosomes. Depending
on the combination of the genes we inherit, we end up
with some traits that resemble our mother and others
that resemble our father. Women have two copies of each
gene on the X chromosome, while men have only the genes
that they inherit from their mother on the X chromosome
and only genes that they inherit from their father on
the Y chromosome.
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What
is a mutation?
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Each gene is made up of a series of bases, and those
bases provide instructions for making a single protein.
Any change in the sequence of bases — and therefore
in the protein instructions — is a mutation.
Just like changing a letter in a sentence can change
the sentence's meaning, a mutation can change the instruction
contained in the gene. Some mutations have little or
no effect on the protein, while others cause the protein
not to function at all.
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What
kind of problems can mutations cause?
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Some
mutations result in proteins that do not function normally,
and may end up causing disease. There are several ways
that gene mutation can change the way a protein functions,
including:
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Altered function: Some mutations result in a protein that cannot carry out its normal function in the cell, or cannot carry out that function very well. One example of this type of mutation is sickle cell anemia. In this disorder, an altered protein in red blood cells alters the shape of the red blood cell, which causes the cell to become stuck in blood vessels. This prevents cells from carrying sufficient oxygen to the rest of the body.
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Lack of protein: Some mutations prevent the protein from being made. One example of this type of mutation is hemophilia. In this condition, a mutation results in the absence of a protein that causes blood to clot. The result is uncontrolled bleeding in response to injury.
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Change in how much protein is made:
Some mutations cause too much or too little of a normal
protein to be made. Although the protein itself functions
properly, it is not present in quantities that are
appropriate. One example of this is in the development
of some cancers. In this case, a protein that prevents
additional mutations from building up can become turned
off. Without this protein, the cell accumulates mutations
and becomes increasingly cancerous.
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Can
we overcome our genetics?
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Our
risk for almost any medical condition is a function
of both our genes and our environment. While we can't
change our genes, we can apply our knowledge of our
family medical history to predict
our risk for specific problems. This, in turn, allows
us to focus on the things we can change — diet,
lifestyle, screening, treatment — to ensure a long,
healthy life.
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