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A
gene is a segment of DNA containing the code used to synthesize a protein.
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A
chromosome contains hundreds to thousands of genes.
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Every
human cell contains 23 pairs of chromosomes, for a total of
46 chromosomes.
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A
trait is any gene-determined characteristic and is often determined by more
than one gene.
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Some
traits are caused by abnormal genes that are inherited or that are the
result of a new mutation.
Proteins are probably the most important class of material in
the body. Proteins are not just building blocks for muscles, connective
tissues, skin, and other structures. They also are needed to make enzymes.
Enzymes are complex proteins that control and carry out nearly all chemical
processes and reactions within the body. The body produces thousands of
different enzymes. Thus, the entire structure and function of the body is
governed by the types and amounts of proteins the body synthesizes. Protein
synthesis is controlled by genes, which are contained on chromosomes.
The genotype is a person's unique combination
of genes or genetic makeup. Thus, the genotype is a complete set of
instructions on how that person's body synthesizes proteins and thus how
that body is supposed to be built and function.
The phenotype is the actual structure
and function of a person's body. The phenotype typically differs somewhat
from the genotype because not all the instructions in the genotype may be
carried out (or expressed). Whether and how a gene is expressed is
determined not only by the genotype but also by the environment (including
illnesses and diet) and other factors, some of which are unknown.
The karyotype is the full set of chromosomes
in a person's cells.
Genes consist of
deoxyribonucleic acid (DNA). DNA contains the code, or blueprint, used to
synthesize a protein. Genes vary in size, depending on the sizes of the
proteins for which they code. Each DNA molecule is a long double helix that
resembles a spiral staircase containing millions of steps. The steps of the
staircase consist of pairs of four types of molecules called bases
(nucleotides). In each step, the base adenine (A) is paired with the base
thymine (T), or the base guanine (G) is paired with the base cytosine (C).
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Structure of
DNA
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DNA
(deoxyribonucleic acid) is the cell's genetic material, contained in
chromosomes within the cell nucleus and mitochondria.
Except for
certain cells (for example, sperm and egg cells and red blood cells),
the cell nucleus contains 23 pairs of chromosomes. A chromosome contains
many genes. A gene is a segment of DNA that provides the code to
construct a protein.
The DNA
molecule is a long, coiled double helix that resembles a spiral
staircase. In it, two strands, composed of sugar (deoxyribose) and
phosphate molecules, are connected by pairs of four molecules called
bases, which form the steps of the staircase. In the steps, adenine is
paired with thymine and guanine is paired with cytosine. Each pair of
bases is held together by a hydrogen bond. A gene consists of a sequence
of bases. Sequences of three bases code for an amino acid (amino acids
are the building blocks of proteins) or other information.
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Proteins are
composed of a long chain of amino acids linked together one after another.
There are 20 different amino acids that can be used in protein
synthesis—some must come from the diet (essential amino acids), and some
are made by enzymes in the body. As a chain of amino acids is put together,
it folds upon itself to create a complex three-dimensional structure. It is
the shape of the folded structure that determines its function in the body.
Because the folding is determined by the precise sequence of amino acids,
each different sequence results in a different protein. Some proteins (such
as hemoglobin) contain several different folded chains. Instructions for
synthesizing proteins are coded within the DNA.
Information is coded
within DNA by the sequence in which the bases (A, T, G, and C) are
arranged. The code is written in triplets. That is, the bases are arranged
in groups of three. Particular sequences of three bases in DNA code for
specific instructions, such as the addition of one amino acid to a chain.
For example, GCT (guanine, cytosine, thymine) codes for the addition of the
amino acid alanine, and GTT (guanine, thymine, thymine) codes for the
addition of the amino acid valine. Thus, the sequence of amino acids in a
protein is determined by the order of triplet base pairs in the gene for
that protein on the DNA molecule. The process of turning coded genetic
information into a protein involves transcription and translation.
Transcription and translation:
Transcription is the
process in which information coded in DNA is transferred (transcribed) to
ribonucleic acid (RNA). RNA is a long chain of bases just like a strand of
DNA, except that the base uracil (U) replaces the base thymine (T). Thus,
RNA contains triplet-coded information just like DNA.
When transcription is initiated, part of the DNA double helix
splits open and unwinds. One of the unwound strands of DNA acts as a
template against which a complementary strand of RNA forms. The
complementary strand of RNA is called messenger RNA (mRNA). The mRNA
separates from the DNA, leaves the nucleus, and travels into the cell
cytoplasm (the part of the cell outside the nucleus—see Fig. 1: Inside a Cell ). There, the RNA attaches to a ribosome,
which is a tiny structure in the cell where protein synthesis occurs.
With translation, the mRNA code (from the DNA) tells the
ribosome the order and type of amino acids to link together. The amino
acids are brought to the ribosome by a much smaller type of RNA called
transfer RNA (tRNA). Each molecule of tRNA brings one amino acid to be
incorporated into the growing chain of protein, which is folded into a
complex three-dimensional structure under the influence of nearby molecules
called chaperone molecules.
Control of gene expression:
There are many types
of cells in a person's body, such as heart cells, liver cells, and muscle
cells. These cells look and act differently and produce very different
chemical substances. However, every cell is the descendant of a single
fertilized egg cell and as such contains essentially the same DNA. Cells
acquire their very different appearances and functions because different
genes are expressed in different cells (and at different times in the same
cell). The information about when a gene should be expressed is also coded
in the DNA. Gene expression depends on the type of tissue, the age of the
person, the presence of specific chemical signals, and numerous other
factors and mechanisms. Knowledge of these other factors and mechanisms
that control gene expression is growing rapidly, but many of these factors
and mechanisms are still poorly understood.
The mechanisms by which genes control each other are very
complicated. Genes have markers to indicate where transcription should
begin and end. Various chemical substances (such as histones) in and around
the DNA block or permit transcription. Also, a strand of RNA called
antisense RNA can pair with a complementary strand of mRNA and block
translation.
Cells reproduce by
splitting in two. Because each new cell requires a complete set of DNA
molecules, the DNA molecules in the original cell must reproduce
(replicate) themselves during cell division. Replication happens in a
manner similar to transcription, except that the entire double-strand DNA
molecule unwinds and splits in two. After splitting, bases on each strand
bind to complementary bases (A with T, and G with C) floating nearby. When
this process is complete, two identical double-strand DNA molecules exist.
To prevent mistakes
during replication, cells have a “proofreading” function to help ensure
that bases are paired properly. There are also chemical mechanisms to
repair DNA that was not copied properly. However, because of the billions
of base pairs involved in and the complexity of the protein synthesis
process, mistakes can happen. Such mistakes can occur for numerous reasons
(including exposure to radiation, drugs, or viruses) or for no apparent
reason. Minor variations in DNA are very common and occur in most people.
Most variations do not affect subsequent copies of the gene. Mistakes that
are duplicated in subsequent copies are called mutations. Mutations that
affect the reproductive cells may be passed on to offspring. Mutations that
do not affect reproductive cells affect the descendants of the mutated cell
(for example, becoming a cancer) but are not passed on to offspring.
Mutations may be unique to an individual or family, and most mutations are
rare. Mutations that become so common that they affect more than 1% of a
population are called polymorphisms (for example, the human blood types A,
B, AB, and O). Most polymorphisms have no effect on the phenotype
(see Genes and Chromosomes).
Mutations may involve small or large segments of DNA.
Depending on its size and location, the mutation may have no apparent
effect or it may alter the amino acid sequence in a protein or decrease the
amount of protein produced. If the protein has a different amino acid
sequence, it may function differently or not at all. An absent or
nonfunctioning protein is often harmful or fatal. For example, in
phenylketonuria, a mutation results in the deficiency or absence of the
enzyme phenylalanine hydroxylase. This deficiency allows the amino acid
phenylalanine (absorbed from the diet) to accumulate in the body,
ultimately causing severe intellectual disability. In rare cases, a
mutation introduces a change that is advantageous. For example, the sickle
cell gene causes sickle cell anemia but also provides protection against
malaria. Although the protection against malaria can help a person survive,
sickle cell anemia causes symptoms and complications that may shorten life
span, so that survival is not always prolonged.
Natural selection refers to the concept that mutations that impair
survival in a given environment are less likely to be passed on to
offspring (and thus become less common in the population), whereas
mutations that improve survival progressively become more common. Thus,
beneficial mutations, although initially rare, eventually become common.
The slow changes that occur over time caused by mutations and natural
selection in an interbreeding population collectively are called evolution.
A chromosome is made of a very long strand of DNA and contains
many genes (hundreds to thousands). The genes on each chromosome are
arranged in a particular sequence, and each gene has a particular location
on the chromosome (called its locus). In addition to DNA, chromosomes
contain other chemical components that influence gene function.
Except for certain
cells (for example, sperm and egg cells or red blood cells), the nucleus of
every human cell contains 23 pairs of chromosomes, for a total of 46
chromosomes. Normally, each pair consists of one chromosome from the mother
and one from the father.
There are 22 pairs of nonsex (autosomal) chromosomes and one
pair of sex chromosomes. Paired nonsex chromosomes are, for practical
purposes, identical in size, shape, and position and number of genes.
Because each member of a pair of nonsex chromosomes contains one of each
corresponding gene, there is in a sense a backup for the genes on those
chromosomes.
The 23rd pair is the sex chromosomes (X and Y).
The pair of sex
chromosomes determines whether a fetus becomes male or female. Males have
one X and one Y chromosome. A male's X comes from his mother and the Y
comes from his father. Females have two X chromosomes, one from the mother
and one from the father. In certain ways, sex chromosomes function
differently than nonsex chromosomes.
The smaller Y chromosome carries the genes that determine male
sex as well as a few other genes. The X chromosome contains many more genes
than the Y chromosome, many of which have functions besides determining sex
and have no counterpart on the Y chromosome. In males, because there is no
second X chromosome, these extra genes on the X chromosome are not paired
and virtually all of them are expressed. Genes on the X chromosome are
referred to as sex-linked, or X-linked, genes.
Normally, in the nonsex chromosomes, the genes on both of the
pairs of chromosomes are capable of being fully expressed. However, in
females, most of the genes on one of the two X chromosomes are turned off
through a process called X inactivation (except in the eggs in the
ovaries). X inactivation occurs early in the life of the fetus. In some
cells, the X from the father becomes inactive, and in other cells, the X
from the mother becomes inactive. Thus, one cell may have a gene from the
person's mother and another cell has the gene from the person's father.
Because of X inactivation, the absence of one X chromosome usually results
in relatively minor abnormalities (such as Turner syndrome—see see Turner Syndrome). Thus, missing an X chromosome is far less
harmful than missing a nonsex chromosome.
If a female has a disorder in which she has more than two X
chromosomes, the extra chromosomes tend to be inactive. Thus, having one or
more extra X chromosomes causes far fewer developmental abnormalities than
having one or more extra nonsex chromosomes. For example, women with three
X chromosomes (triple X syndrome) are often physically and mentally normal
(see see Triple X Syndrome). Most males who have more than one Y
chromosome (see XYY Syndrome) are not physically and mentally normal.
A genetic disorder is a detrimental trait caused by an
abnormal gene. The abnormal gene may be inherited or may arise
spontaneously as a result of a mutation. Abnormalities of one or more genes
are fairly common. Humans carry an average of 100 to 400 abnormal genes.
However, most of the time the corresponding gene on the other chromosome in
the pair is normal and prevents any harmful effects. In the general population,
the chance of a person having two copies of the same abnormal gene (and
hence a disorder) is very small. However, in children who are offspring of
close blood relatives, the chances are higher. Chances are also high among
children of parents who have married within an isolated population, such as
the Amish or Mennonites.
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