or ethically prohibited with humans.
encodes the cell’s genetic instructions. A form
of adenine called adenosine triphosphate (ATP)
antibody: An antibody is a protein component
serves as an energy storage molecule and is used
of the immune system that circulates in the
to power many chemical reactions within the cell.
blood and recognizes and neutralizes foreign
substances such as bacteria and viruses. After
allele: An allele is one of two or more versions
exposure to a foreign substance, called an
of a gene. An individual inherits two alleles
antigen, antibodies continue to circulate in
for each gene, one from each parent. If the
the blood, providing protection against future
two alleles are the same, the individual is
exposures to that antigen.
39
Human Genetic Variation
anticodon: An anticodon is a trinucleotide
autosomal dominant: Autosomal dominance is
sequence complementary to that of a corresponding
a pattern of inheritance characteristic of some
codon in a messenger RNA (mRNA) sequence.
genetic diseases. “Autosomal” means that the gene
An anticodon is found at one end of a transfer
in question is located on one of the numbered, or
RNA (tRNA) molecule. During protein synthesis,
non sex, chromosomes. “Dominant” means that
each time an amino acid is added to the growing
a single copy of the disease-associated mutation
protein, a tRNA forms base pairs with its
is enough to cause the disease. This is in contrast
complementary sequence on the mRNA
to a recessive disorder, where two copies of
molecule, ensuring that the appropriate
the mutation are needed to cause the disease.
amino acid is inserted into the protein.
Huntington’s disease is a common example of
an autosomal dominant genetic disorder.
antisense: Antisense is the noncoding DNA strand
of a gene. A cell uses antisense DNA as a template
autosome: An autosome is any of the numbered
for producing messenger RNA (mRNA), which
chromosomes, as opposed to the sex chromosomes.
directs the synthesis of a protein. Antisense can
Humans have 22 pairs of autosomes and one pair
also refer to a method for silencing genes. To
of sex chromosomes (the X and Y). Autosomes are
silence a target gene, a second gene is introduced
numbered roughly in relation to their sizes. That
that produces an mRNA complementary to that
is, chromosome 1 has about 2,800 genes, while
produced from the target gene. These two mRNAs
chromosome 22 has about 750 genes.
can interact to form a double-stranded structure
that cannot be used to direct protein synthesis.
bacteria: Bacteria are small single-celled
organisms. Bacteria are found almost everywhere
apoptosis: Apoptosis is the process of
on Earth and are vital to the planet’s ecosystems.
programmed cell death. It is used during early
Some species can live under extreme conditions of
development to eliminate unwanted cells—
temperature and pressure. The human body is full
for example, those between the fi ngers of a
of bacteria; in fact, it contains more bacterial cells
developing hand. In adults, apoptosis is used
than human cells. Most bacteria in the body are
to rid the body of cells that have been damaged
harmless, and some are even helpful. A relatively
beyond repair. Apoptosis also plays a role in
small number of species cause disease.
preventing cancer. If apoptosis is for some
reason prevented, it can lead to uncontrolled
bacterial artifi cial chromosome (BAC): A
cell division and the subsequent development
bacterial artifi cial chromosome (BAC) is an
of a tumor.
engineered DNA molecule used to clone DNA
sequences in bacterial cells (for example,
autism: Autism is a developmental brain disorder
Escherichia coli). BACs are often used in
characterized by impaired social interactions,
connection with DNA sequencing. Segments
communication problems, and repetitive
of an organism’s DNA, ranging from 100,000 to
behaviors. Symptoms usually appear before
about 300,000 base pairs, can be inserted into
the age of three. The exact cause of autism is
BACs. The BACs, with their inserted DNA, are
not known; however, it is likely infl uenced by
then taken up by bacterial cells. As the bacterial
genetics. Autism is one of a group of related
cells grow and divide, they amplify the BAC
developmental disorders called autism spectrum
DNA, which can then be isolated and used in
disorders (ASDs). Other ASDs include Asperger
sequencing DNA.
syndrome and Rett syndrome.
40
base pair: A base pair is two chemical bases
A cancerous tumor can spread to other parts of
bonded to one another forming a “rung of the
the body and, if left untreated, be fatal.
DNA ladder.” The DNA molecule consists of
two strands that wind around each other like a
candidate gene: A candidate gene is a gene
twisted ladder. Each strand has a backbone made
whose chromosomal location is associated with a
of alternating sugar (deoxyribose) and phosphate
particular disease or other phenotype. Because of
groups. Attached to each sugar is one of four
its location, the gene is suspected of causing the
bases—adenine (A), cytosine (C), guanine (G), or
disease or other phenotype.
thymine (T). The two strands are held together by
hydrogen bonds between the bases, with adenine
carcinogen: A carcinogen is an agent with the
forming a base pair with thymine, and cytosine
capacity to cause cancer in humans. Carcinogens
forming a base pair with guanine.
may be natural, such as afl atoxin, which is
produced by a fungus and sometimes found on
bioinformatics: Bioinformatics is a subdiscipline
stored grains, or manmade, such as asbestos or
of biology and computer science concerned with
tobacco smoke. Carcinogens work by interacting
the acquisition, storage, analysis, and dissemination
with a cell’s DNA and inducing genetic mutations.
of biological data, most often DNA and amino
acid sequences. Bioinformatics uses computer
carrier: A carrier is an individual who carries
programs for a variety of applications, including
and is capable of passing on a genetic mutation
determining gene and protein functions, establishing
associated with a disease and may or may not
evolutionary relationships, and predicting the
display disease symptoms. Carriers are associated
three-dimensional shapes of proteins.
with diseases inherited as recessive traits. In
order to have the disease, an individual must
birth defect: A birth defect is an abnormality
have inherited mutated alleles from both parents.
present at birth. Also called a congenital defect,
An individual having one normal allele and one
it can be caused by a genetic mutation, an
mutated allele does not have the disease. Two
unfavorable environment during pregnancy, or a
carriers may produce children with the disease.
combination of both. The effect of a birth defect
can be mild, severe, or incompatible with life.
carrier screening: Carrier screening is a type of
genetic testing performed on people who display
BRCA1/BRCA2: BRCA1 and BRCA2 are the fi rst
no symptoms for a genetic disorder but may be at
two genes found to be associated with inherited
risk for passing it on to their children. A carrier
forms of breast cancer. Both genes normally act
for a genetic disorder has inherited one normal
as tumor suppressors, meaning that they help
and one abnormal allele for a gene associated with
regulate cell division. When these genes are
the disorder. A child must inherit two abnormal
rendered inactive due to mutation, uncontrolled
alleles for symptoms to appear. Prospective
cell growth results, leading to breast cancer.
parents with a family history of a genetic disorder
Women with mutations in either gene have a
are candidates for carrier screening.
much higher risk of developing breast cancer
than women without mutations in the genes.
cell: A cell is the basic building block of living
things. All cells can be sorted into one of two
cancer: Cancer is a group of diseases characterized
groups: eukaryotes and prokaryotes. A eukaryote
by uncontrolled cell growth. Cancer begins when
has a nucleus and membrane-bound organelles,
a single cell mutates, resulting in a breakdown
while a prokaryote does not. Plants and animals
of the normal regulatory controls that keep
are made of numerous eukaryotic cells, while
cell division in check. These mutations can be
many microbes, such as bacteria, consist of single
inherited, caused by errors in DNA replication,
cells. An adult human body is estimated to contain
or result from exposure to harmful chemicals.
between 10 and 100 trillion cells.
41
Glossary
Human Genetic Variation
cell cycle: A cell cycle is a series of events that
centrosome: A centrosome is a cellular structure
takes place in a cell as it grows and divides. A
involved in the process of cell division. Before
cell spends most of its time in what is called
cell division, the centrosome duplicates and
interphase, and during this time, it grows,
then, as division begins, the two centrosomes
replicates its chromosomes, and prepares for
move to opposite ends of the cell. Proteins called
cell division. The cell then leaves interphase,
microtubules assemble into a spindle between the
undergoes mitosis, and completes its division.
two centrosomes and help separate the replicated
The resulting cells, known as daughter cells,
chromosomes into the daughter cells.
each enter their own interphase and begin a
new round of the cell cycle.
chromatid: A chromatid is one of two identical
halves of a replicated chromosome. During cell
cell membrane (plasma membrane): The cell
division, the chromosomes fi rst replicate so that
membrane, also called the plasma membrane,
each daughter cell receives a complete set of
is found in all cells and separates the interior
chromosomes. Following DNA replication, the
of the cell from the outside environment. The
chromosome consists of two identical structures
cell membrane consists of a lipid bilayer that is
called sister chromatids, which are joined at
semipermeable. The cell membrane regulates
the centromere.
the transport of materials entering and exiting
the cell.
chromatin: Chromatin is a substance within
a chromosome consisting of DNA and protein.
centimorgan: A centimorgan is a unit used
The DNA carries the cell’s genetic instructions.
to measure genetic linkage. One centimorgan
The major proteins in chromatin are histones,
equals a 1 percent chance that a marker on
which help package the DNA in a compact form
a chromosome will become separated from a
that fi ts inside the cell nucleus. Changes in
second marker on the same chromosome due to
chromatin structure are associated with DNA
crossing over in a single generation. It translates
replication and gene expression.
to approximately 1 million base pairs of DNA
sequence in the human genome. The centimorgan
chromosome: A chromosome is an organized
is named after the American geneticist Thomas
package of DNA found in the nucleus of the cell.
Hunt Morgan.
Different organisms have different numbers
of chromosomes. Humans have 23 pairs
centriole: Centrioles are paired barrel-shaped
of chromosomes—22 pairs of numbered
organelles located in the cytoplasm of animal
chromosomes, called autosomes, and one pair
cells near the nuclear envelope. Centrioles play
of sex chromosomes, X and Y. Each parent
a role in organizing microtubules that serve as
contributes one chromosome to each pair so
the cell’s skeletal system. They help determine
that offspring get half of their chromosomes
the locations of the nucleus and other organelles
from their mother and half from their father.
within the cell.
cloning: Cloning is the process of making
centromere: A centromere is a constricted region
identical copies of an organism, cell, or DNA
of a chromosome that separates it into a short arm
sequence. Molecular cloning is a process by
(p) and a long arm (q). During cell division, the
which scientists amplify a desired DNA sequence.
chromosomes fi rst replicate so that each daughter
The target sequence is isolated, inserted into
cell receives a complete set of chromosomes.
another DNA molecule (known as a vector), and
Following DNA replication, the chromosome
introduced into a suitable host cell. Then, each
consists of two identical structures called sister
time the host cell divides, it replicates the foreign
chromatids, which are joined at the centromere.
DNA sequence along with its own DNA. Cloning
can also refer to asexual reproduction.
42
codominance: Codominance is a relationship
crossing over: Crossing over is the swapping
between two versions of a gene. Individuals
of genetic material that occurs in the germ line.
receive one version of a gene, called an allele,
During the formation of egg and sperm cells, also
from each parent. If the alleles are different, the
known as meiosis, paired chromosomes from each
dominant allele will usually be expressed, while
parent align so that similar DNA sequences from
the effect of the other allele, called recessive, is
the paired chromosomes cross over one another.
masked. In codominance, however, neither allele
Crossing over results in a shuffl ing of genetic
is recessive, and the phenotypes of both alleles
material and is an important cause of the genetic
are expressed.
variation seen among offspring.
codon: A codon is a trinucleotide sequence of DNA
cystic fi brosis: Cystic fi brosis is a hereditary
or RNA that corresponds to a specifi c amino acid.
disease characterized by faulty digestion,
The genetic code describes the relationship between
breathing problems, respiratory infections from
the sequence of DNA bases (A, C, G, and T) in a
mucus buildup, and the loss of salt in sweat. The
gene and the corresponding protein sequence that
disease is caused by mutations in a single gene
it encodes. The cell reads the sequence of the gene
and is inherited as an autosomal recessive trait,
in groups of three bases. There are 64 different
meaning that an affected individual inherits two
codons: 61 specify amino acids, while the remaining
mutated copies of the gene. In the past, cystic
three are used as stop signals.
fi brosis was almost always fatal in childhood.
Today, however, patients commonly live to be
complex disease: A complex disease is caused
30 years or older.
by the interaction of multiple genes and
environmental factors. Complex diseases are
cytogeneticist: A cytogeneticist is a geneticist
also called multifactorial. Examples of complex
who specializes in the study of chromosomes
diseases include cancer and heart disease.
and the structure and function of the cell.
congenital: Congenital conditions are those
cytogenetics: Cytogenetics is a branch of genetics
present from birth. Birth defects are described as
that studies the structure and function of the
being congenital. They can be caused by a genetic
cell and the chromosomes. Early cytogenetic
mutation, an unfavorable environment in the
research was accomplished with a standard light
uterus, or a combination of both factors.
microscope. Today, more powerful techniques of
analysis, such as fl uorescent in situ hybridization
contig: A contig—from the word “contiguous”—
(FISH), are commonly used to examine cells
is a series of overlapping DNA sequences used to
and chromosomes.
make a physical map that reconstructs the original
DNA sequence of a chromosome or a region of a
cytoplasm: Cytoplasm is the gelatinous liquid
chromosome. A contig can also refer to one of
that fi lls the inside of a cell. It is composed
the DNA sequences used in making such a map.
of water, salts, and various organic molecules.
Some intracellular organelles, such the nucleus
copy number variation (CNV): A copy number
and mitochondria, are enclosed by membranes
variation (CNV) is when the number of copies of
that separate them from the cytoplasm.
a particular gene varies from one individual to
the next. Following the completion of the Human
cytosine: Cytosine (C) is one of four chemical
Genome Project, it became apparent that the
bases in DNA, the other three being adenine (A),
genome experiences gains and losses of genetic
guanine (G), and thymine (T). Within the DNA
material. The extent to which CNV contributes
molecule, cytosine bases located on one strand
to human disease is not yet known. It has long
form chemical bonds with guanine bases on the
been recognized that some cancers are associated
opposite strand. The sequence of four DNA bases
with elevated copy numbers of particular genes.
encodes the cell’s genetic instructions.
43
Glossary
Human Genetic Variation
deletion: Deletion is a type of mutation involving
sample from a suspect. If the two DNA profi les
the loss of genetic material. It can be small,
are a match, then the evidence came from that
involving a single missing DNA base pair, or
suspect. Conversely, if the two DNA profi les
large, involving a piece of a chromosome.
do not match, then the evidence cannot have
come from the suspect. DNA fi ngerprinting
diabetes (diabetes mellitus): Diabetes mellitus
is also used to establish paternity.
is a disease characterized by an inability to make
or use the hormone insulin. Insulin is needed
DNA replication: DNA replication is the process
by cells to metabolize glucose, the body’s main
by which a molecule of DNA is duplicated. When
source of chemical energy. Type I diabetes, also
a cell divides, it must fi rst duplicate its genome
called insulin-dependent diabetes mellitus, is
so that each daughter cell winds up with a
usually caused by an autoimmune destruction
complete set of chromosomes.
of insulin-producing cells. Type II diabetes, also
called non-insulin-dependent diabetes mellitus,
DNA sequencing: DNA sequencing is a laboratory
occurs when cells become resistant to the effects
technique used to determine the exact sequence
of insulin.
of bases (A, C, G, and T) in a DNA molecule. The
DNA base sequence carries the information a cell
diploid: Diploid is a cell or organism that has
needs to assemble protein and RNA molecules.
paired chromosomes, one from each parent. In
DNA sequence information is important to
humans, cells other than human sex cells are
scientists investigating the functions of genes.
diploid and have 23 pairs of chromosomes. Human
The technology of DNA sequencing was made
sex cells (egg and sperm cells) contain a single
faster and less expensive as a part of the Human
set of chromosomes and are known as haploid.
Genome Project.
DNA (deoxyribonucleic acid): DNA is the
dominant: Dominant refers to the relationship
chemical name for the molecule that carries
between two versions of a gene. Individuals
genetic instructions in all living things. The
receive two versions of each gene, known as
DNA molecule consists of two strands that wind
alleles, from each parent. If the alleles of a gene
around one another to form a shape known
are different, one allele will be expressed; it is
as a double helix. Each strand has a backbone
the dominant gene. The effect of the other allele,
made of alternating sugar (deoxyribose) and
called recessive, is masked.
phosphate groups. Attached to each sugar is one
of four bases—adenine (A), cytosine (C), guanine
double helix: Double helix is the description
(G), and thymine (T). The two strands are held
of the structure of a DNA molecule. A DNA
together by bonds between the bases; adenine
molecule consists of two strands that wind
bonds with thymine, and cytosine bonds with
around each other like a twisted ladder. Each
guanine. The sequence of the bases along the
strand has a backbone made of alternating
backbones serves as instructions for assembling
groups of sugar (deoxyribose) and phosphate
protein and RNA molecules.
groups. Attached to each sugar is one of four
bases: adenine (A), cytosine (C), guanine (G),
DNA fi ngerprinting: DNA fi ngerprinting is a
or thymine (T). The two strands are held
laboratory technique used to establish a link
together by bonds between the bases, with
between biological evidence and a suspect in
adenine forming a base pair with thymine and
a criminal investigation. A DNA sample taken
cytosine forming a base pair with guanine.