Have you ever wondered why we age? What exactly is happening inside our
bodies to bring on the wrinkles, gray hair, and the other changes seen in
older people? Considering the universality of the process, you might be
surprised to know that there remain many unanswered questions about how
aging happens at the cellular level. However, theories abound, and the roles
played by various suspects in the aging process are beginning to take shape.
Cell death, on the other hand, is an area in which scientists have made
great leaps in understanding in recent years. Far from being strictly harmful,
scientists have found that cell death, when carefully controlled, is critical
to life as we know it. Without it, you wouldn't have your fingers and toes
or the proper brain cell connections to be able to read the words on this
If you'd like to know more about these fascinating processes, read on.
And thank cell death for it!
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(AK-tin) Part of the cytoskeleton. Actin filaments contract or lengthen
to give cells the flexibility to move and change shape. Together with myosin,
actin filaments are responsible for muscle contraction.
Adult stem cells Cells that
can renew themselves and differentiate into a limited number of specialized
cell types. They replace and renew damaged tissues.
Amino acid (uh-MEE-no) A chemical
building block of proteins. There are 20 standard amino acids. A protein
consists of a specific sequence of amino acids.
Anaphase (ANN-uh-faze) The fourth
of six phases of cell division, following metaphase and preceding telophase.
In anaphase, the chromosomes separate into two genetically identical groups
and move to opposite ends of the spindle.
The condition of having an abnormal number of chromosomes. See Down syndrome.
Antibody A protein produced by the
immune system in response to a foreign substance such as a virus or bacterium.
A substance that can neutralize dangerous compounds called reactive oxygen
species. Antioxidants are found naturally in our bodies and in foods such
as fruits and vegetables.
Apoptosis (ay-PAH-TOE-sis) Programmed
cell death, a normal process in which cells die in a controlled and predictable
way. See necrosis.
ATP, adenosine triphosphate (ah-DEH-no-seen
try-FOSS-fate) The major source of energy for biochemical reactions in all
bacteria) A one-celled microorganism that contains no nucleus. Some bacteria
are helpful, such as those in the intestines that help digest food, while
others cause disease. Bacteria are frequently used as model organisms to
study basic biological processes. See prokaryotic cell and model organism.
A molecule made up of one or more sugars. In the body, carbohydrates can
exist independently or be attached to proteins or lipids.
Cell The basic subunit of any living
organism; the simplest unit capable of independent life. Although there
are some single-celled organisms, such as bacteria, most organisms consist
of many cells that are specialized for particular functions. See prokaryotic
cell and eukaryotic cell.
Cell cycle The sequence of events
by which a cell duplicates its contents and divides in two.
Channel protein A hollow or
pore-containing protein that spans a cell membrane and acts as a conduit
for small molecules, such as charged particles (ions).
Checkpoint One of several points
in the cell cycle where the cycle can pause if there is a problem such as
incomplete DNA synthesis or damaged DNA. See cell cycle.
The movement of a cell toward or away from the source of a chemical.
Cholesterol A waxy lipid produced
by animal cells that is a major component of cell membranes. Cholesterol
is also used as a building block for some hormones.
Chromosome (KROH-muh-sohm) A cellular
structure containing genes. Excluding sperm and egg cells, humans have 46
chromosomes (23 pairs) in each cell.
Cilium (SILL-ee-um) (plural: cilia)
A hairlike projection from a cell surface. The rhythmic beating of cilia
can move fluid or mucus over a cell or can propel single-celled organisms.
Cilia are shorter than flagella.
A field of science that uses computers to study complex biological processes
that involve many molecular interactions.
Crossing over A process that
occurs during meiosis in which chromosome partners, one inherited from each
parent, physically swap sections with one another. This creates hybrid chromosomes
that are a patchwork of the original pair. Crossing over occurs in species
that reproduce sexually and increases the genetic variety of offspring.
The last of six phases of cell division. It occurs after the duplicated
genetic material has segregated to opposite sides of the cell. During cytokinesis,
the cell splits into two daughter cells.
Cytoplasm (SYE-toe-PLAZ-um) The
material found between the cell membrane and the nuclear envelope. It includes
the cytosol and all organelles except the nucleus. See cytosol.
A collection of fibers that gives a cell shape and support and allows movement
within the cell and, in some cases, by the cell as a whole. The three main
types of cytoskeletal fibers are microtubules, actin filaments, and intermediate
Cytosol (SYE-tuh-sol) The semi-fluid
portion of the cytoplasm, excluding the organelles. The cytosol is a concentrated
solution of proteins, salts, and other molecules. See cytoplasm.
The series of biochemical and structural changes by which an unspecialized
cell becomes a specialized cell with a specific function. During development,
embryonic stem cells differentiate into the many cell types that make up
the human body.
Diploid (DIP-loyd) Having two sets
of chromosomes, one inherited from each parent. All human cells except eggs
and sperm are diploid and have 46 chromosomes, 23 from each parent.
DNA, deoxyribonucleic acid (dee-AW-ksee-RYE-bo-new-CLAY-ick)
The substance of heredity. A long, helical, double-stranded molecule that
carries the cell's genetic information. See chromosome.
Down syndrome An inherited
condition caused by having an extra copy of chromosome 21. See aneuploidy.
A powerful microscope that uses beams of fast-moving electrons instead of
light to magnify samples. Powerful magnets focus the electrons into an image.
Embryonic stem cell A
cell found in early embryos that can renew itself and differentiate into
the many cell types that are found in the human body.
A process cells use to engulf particles or liquid from their surroundings.
It occurs when the cell surface membrane puckers inward, encircling the
material, then pinches off, producing a vesicle inside the cell.
(ER) (EN-doe-PLAZ-mik reh-TIK-yoo-lum) An organelle made up of interconnected
tubes and flattened sacs. There are two kinds of ER: rough (because it is
dotted with ribosomes) ER, which processes newly made proteins, and smooth
ER, which helps make lipid and neutralizes toxins.
Enzyme A protein that speeds up a
specific chemical reaction without being permanently altered or consumed.
Eukaryotic cell (YOO-kare-ee-AW-tick)
A cell that has a nucleus and other organelles not found in prokaryotes;
includes all animal and most plant cells.
A process cells use to send substances outside their surface membrane via
The material that surrounds and supports cells. It includes structural proteins
such as collagen and elastin.
(plural: flagella) A long, taillike structure extending from a cell. Sperm
and many microorganisms move using flagella.
G protein A protein
located on the inside of the cell membrane that helps transmit molecular
signals into cells.
Gene A unit of heredity; a segment
of DNA that contains the code for making a specific protein or RNA molecule.
Genome (JEE-nome) All of an organism's
Glial cell (GLEE-uhl) A kind of cell
in the nervous system that provides nutrition and support to a nerve cell.
The process of adding specialized chains of sugar molecules to proteins
or lipids; occurs in the ER and Golgi.
Golgi (GOLE-jee) Also called the Golgi
apparatus or Golgi complex; an organelle composed of membranous sacs in
which many newly made proteins mature and become functional.
Having a single set of chromosomes, as in egg or sperm cells. Haploid human
cells have 23 chromosomes.
Hormone A molecule that stimulates
specific cellular activity; made in one part of the body and transported
via the bloodstream to tissues and organs. Examples include insulin, estrogen,
Part of the cytoskeleton that provides strength. Some intermediate filaments
form nails, hair, and the outer layer of skin. Others are found in nerves
or other organs.
Interphase (IN-tur-faze) A period
in a cell's life cycle when it is not undergoing mitosis.
Lipid (LIP-id) A fatty,
waxy, or oily compound that will not dissolve in water. Lipids are a major
part of biological membranes.
Lysosome (LYE-so-sohm) A bubble-like
organelle that contains powerful enzymes that can digest a variety of biological
The type of cell division that makes egg and sperm cells. Meiosis generates
cells that are genetically different from one another and contain half the
total number of chromosomes in the parent cell. See haploid.
Membrane A semi-fluid layer of lipids
and proteins. Biological membranes enclose cells and organelles and control
the passage of materials into and out of them.
Metaphase (MET-uh-faze) The third
phase of cell division, following prometaphase and preceding anaphase. In
metaphase, the copied chromosomes align in the middle of the spindle.
Micrometer (MY-kroh-MEE-tur) One
micrometer is one millionth (10–6) of a meter or one thousandth of
a millimeter. The micrometer is frequently used to measure cells and organelles.
Part of the cytoskeleton; a strong, hollow fiber that acts as a structural
support for the cell. During cell division, microtubules form the spindle
that directs chromosomes to the daughter cells. Microtubules also serve
as tracks for transporting vesicles and give structure to flagella and cilia.
(plural: mitochondria) The cell's power plant; the organelle that converts
energy from food into ATP, fueling the cell. Mitochondria contain their
own small genomes and appear to have descended from free-living bacteria.
Mitosis (my-TOE-sis) The type of
cell division that eukaryotic cells use to make new body cells. Mitosis
results in two daughter cells that are genetically identical to the parent
Model system (or Model organism)
A cell type or simple organism—such as a bacterium, yeast, plant,
fruit fly, or mouse-used to answer basic questions about biology.
Mutation (myoo-TAY-shun) A change
in a DNA sequence.
Myelin (MY-eh-lin) A fatty covering
that forms a protective sheath around nerve fibers and dramatically speeds
the transmission of nerve signals.
One billionth (10–9) of a meter or one thousandth of a micrometer.
The nanometer is frequently used to measure organelles and small structures
Necrosis (neh-CROH-sis) Unplanned
cell death caused by outside circumstances, such as traumatic injury or
infection. See apoptosis.
Neuron A cell in the nervous system
that is specialized to carry information through electrical impulses and
chemical messengers. Also called a nerve cell.
Neurotransmitter A chemical
messenger that passes signals between nerve cells or between a nerve cell
and another type of cell.
Nuclear envelope A barrier
that encloses the nucleus and is made up of two membranes perforated by
Nuclear pores An opening in
the nuclear envelope that allows the passage of small molecules such as
salts, small proteins, and RNA molecules.
Nucleus The organelle in eukaryotic
cells that contains genetic material.
The developing female reproductive cell; an immature egg.
Organ A group of tissues that perform
a particular job. Animals have more than a dozen organs, including the heart,
brain, eye, liver, and lung.
Organelle (OR-gun-EL) A specialized,
membrane-bounded structure that has a specific function in a cell. Examples
include the nucleus, mitochondria, Golgi, ER, and lysosomes.
(PRO-kare-ee-AW-tick) A cell that lacks a nucleus. Bacteria are prokaryotes.
See eukaryotic cell.
The second of six phases of cell division, following prophase and preceding
metaphase. In prometaphase, the nuclear membrane breaks apart and the spindle
starts to interact with the chromosomes.
Prophase (PRO-faze) The first of
six phases of cell division. In prophase, chromosomes condense and become
visible and the spindle forms.
Proteasome (PRO-tee-uh-some) A
cellular machine that digests proteins that have been tagged with ubiquitin
Protein A molecule composed of amino
acids lined up in a precise order determined by a gene, then folded into
a specific three-dimensional shape. Proteins are responsible for countless
biological functions and come in a wide range of shapes and sizes.
species One of several types of small molecules containing oxygen
with an unstable number of electrons. Reactive oxygen species can damage
many kinds of biological molecules.
Ribosome (RYE-bo-sohm) A molecular
complex in which proteins are made. In eukaryotic cells, ribosomes either
are free in the cytoplasm or are attached to the rough endoplasmic reticulum.
RNA, ribonucleic acid (RYE-bo-new-CLAY-ick)
A molecule very similar to DNA that plays a key role in making proteins.
There are three main types: messenger RNA (mRNA) is an RNA version of a
gene and serves as a template for making a protein, ribosomal RNA (rRNA)
is a major component of ribosomes, and transfer RNA (tRNA) transports amino
acids to the ribosome and helps position them properly during protein production.
RNAi (RNA interference) The process
of using small pieces of double-stranded RNA to reduce the activity of specific
genes. The process occurs naturally in many organisms and is now commonly
used in basic research. It has the potential to be therapeutically useful.
RNA polymerase (puh-LIH-mer-ase)
An enzyme that makes RNA using DNA as a template in a process called transcription.
Spindle A football-shaped
array of fibers made of microtubules and associated proteins that forms
before cells divide. Some of the fibers attach to the chromosomes and help
draw them to opposite ends of the cell.
An enzyme that adds telomeres to the ends of eukaryotic chromosomes, preventing
the chromosome from shrinking during each cell division.
Telomere (TEE-lo-meer) A repetitive
segment of DNA at the ends of eukaryotic chromosomes. Telomeres do not contain
genes and, in the absence of telomerase, they shorten with each cell division.
Telophase (TEE-lo-faze) The fifth
of six phases of cell division, following anaphase and preceding cytokinesis.
In telophase, nuclear membranes form around each of the two sets of chromosomes,
the chromosomes begin to spread out, and the spindle begins to break down.
Tissue A group of cells that act together
to carry out a specific function in the body. Examples include muscle tissue,
nervous system tissue (including the brain, spinal cord, and nerves), and
connective tissue (including ligaments, tendons, bones, and fat). Organs
are made up of tissues.
Transcription The process of
copying information from genes (made of DNA) into messenger RNA.
Translation The process of making
proteins based on genetic information encoded in messenger RNA. Translation
occurs in ribosomes.
A small protein that attaches to and marks other proteins for destruction
by the proteasome.
A small, membrane-bounded sac that transports substances between organelles
as well as to and from the cell membrane.
Virus An infectious agent composed
of proteins and genetic material (either DNA or RNA) that requires a host
cell, such as a plant, animal, or bacterium, in which to reproduce. A virus
is neither a cell nor a living organism because it can not reproduce independently.
A cell resulting from the fusion of an egg and a sperm.