Glossary of Terms
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Select the first letter of the word you are seeking from the list above
to jump to the appropriate section of the glossary or scroll down to it
- A -
- alleles
- alternate forms or varieties of a gene. The alleles for a trait occupy the same locus or position on homologous chromosomes and thus govern the same trait. However, because they are different, their action may result in different expressions of that trait.
- amino acids
- organic molecules that are building blocks of proteins. There are at least 20 different kinds of amino acids in living things. Proteins are composed of different combinations of amino acids assembled in chain-like molecules. Amino acids are primarily composed of carbon, oxygen, hydrogen, and nitrogen.
- anatomy
- referring to the structural parts of organisms. See physiology.
-
antibodies
- proteins produced by the body to identify and neutralize or destroy alien antigens. Antibodies are involved in the rejection of mismatched blood transfusions and organ transplants. They are also responsible for recognizing and eliminating bacteria and viruses. Antibodies provide a major defense for our bodies against invasion by alien organisms.
- anticodons
- sequences of three nucleotide bases on a transfer RNA molecule that can pair with a specific messenger RNA Codon.
-
antigens
- proteins that provide the specific signature or identity to blood or other tissue cells. When alien antigens are introduced into the body, they stimulate the production and mobilization of antibodies. Antigens are found on the surface of blood and other tissue cells as well as bacteria and viruses.
- autosomes
- any chromosomes other than a sex chromosome.
Back to Top
- B -
Back to Top
- C -
- carbohydrates
- a class of organic molecules that include sugars and starches.
- cell
- a complex unit of protoplasm, usually with a nucleus, cytoplasm, and an enclosing membrane. All plants and animals are composed of one or more microscopic cells. The smallest organic unit capable of carrying out all of the functions normally attributed to life is a cell. See eukaryotic cell and prokaryotic cell.
- cell
membrane
- the selectively permeable membrane enclosing a cell.
- centriole
- small, dense bodies outside of a cell nucleus that contract and become visible early in mitosis and meiosis when the nuclear membrane breaks down. Spindle fibers eventually emanate from each centriole and connect to the centromeres of chromosomes.
-
- centromere
- a constriction in a chromosome where two or more chromatids come together.
-
chimera
(also spelled chimaera)
- an individual who is genetically two people. Fully chimeric individuals are usually the product of two fertilized ova fusing into a single embryo shortly after conception.
- chromatids
- one of two or more strands of a chromosome that are joined at a centromere.
Generalized animal cell
- chromosomes
- thread-like, gene-carrying bodies in the cell nucleus. Chromosomes are composed primarily of DNA and protein. They are visible only under magnification during certain stages of cell division. Humans have 46 chromosomes in each somatic cell and 23 in each sex cell.
-
- codon
- a sequence of three nucleotide bases in a DNA or RNA molecule that code for a specific type of amino acid that will be used in the synthesis of a protein.
-
(Simplified representation rather than actual molecular shape) - color blindness
- the inability to see certain colors as they normally appear to others. The most common form of this vision deficiency is X-linked genetically inherited red-green color blindness.
- crossing-over
-
- cytoplasm
- all of the material within a cell between the nuclear and cell membranes. The cytoplasm consists of semifluid water-rich viscous gel and contains numerous specialized structures, or organelles, involved with cell function.
the exchange of genetic material (DNA) between homologous chromosomes at the beginning of meiosis. This results in sperm and ova with greater genetic diversity due to a recombination of genes. Specifically, a portion of the end of a chromosome is broken and reattached on another chromosome.
Back to Top
- D -
- diploid
number
- the full component of chromosomes normally found in somatic cells. In humans, the number is 46. See haploid number.
- DNA
(deoxyribonucleic acid
)
- a large organic molecule that stores the genetic code for the synthesis of proteins. Each chromosome consists mostly of a DNA molecule. DNA is composed of sugars, phosphates and bases arranged in a double helix shaped molecular structure. Segments of DNA correspond to specific genes.
- dominant allele
- an allele that masks the presence of a recessive allele.
- double helix
- the twisted ladder shape that is characteristic of DNA molecules.
- Down
syndrome
- a genetically inherited form of mental retardation usually resulting from the inheritance of an extra autosome 21. Down syndrome individuals also typically are short and stocky in build with short appendages. They usually have broad round faces, saddle-shaped nose profiles, and thick tongues that are often stuck out of their mouths. The incidence of Down syndrome children goes up rapidly with the age of the mother, particularly after 40.
Back to Top
- E -
-
endoplasmic
reticula
(singular, endoplasmic reticulum )
- highly folded long membrane structures in the cytoplasm of cells. Ribosomes involved in protein synthesis are on the surface of these bodies.
- enzymes
- proteins that cause or regulate specific chemical reactions within cells.
- estrogen
- a class of feminizing hormones. Both men and women produce them, but females normally produce much more.
-
eukaryotic
cell
- a cell that has a true nucleus and that divides by mitosis. Complex single celled creatures such as protozoa as well as all multicelled plants and animals are primarily eukaryotes. With the exception of red blood cells, human cells are eukaryotic. See prokaryotic cell.
- exons
Generalized animal cell
Back to Top
- F -
- fertilization
- conception; the process of sexual reproduction by which the chromosomes from a sperm cell enter the nucleus of an ovum and combine with its chromosomes to create a zygote.
- flagellum
(plural, flagella
)
- a whip-like tail of a sperm and some single cell organisms that is used to achieve movement in surrounding liquid.
-
-
fragile-X syndrome
- a relatively common genetically inherited abnormality of the X chromosome which results in mental retardation. Since it is an X-linked trait, males more often have it expressed in their phenotypes. Most fragile-X males have large testes, big ears, narrow faces, and sensory integration dysfunctions that result in learning disabilities. It is likely to occur 1 in 1000 births. Approximately 1 in 700 females are carriers of the gene for this trait.
Back to Top
- G -
- gametes
- reproductive cells--sperm or unfertilized ovum cells produced in the testes and ovaries of animals. Gametes are produced by meiosis. They normally have half the number of chromosomes found in somatic cells.
- genes
- units of inheritance usually occurring at specific locations, or loci, on a chromosome. Physically, a gene is a sequence of DNA bases that specify the order of amino acids in a protein or, in some cases, a small RNA molecule referred to as a microRNA. A gene may be made up of hundreds to thousands of DNA bases. Genes are responsible for hereditary characteristics.
- genetics
- the study of gene structure and action and the patterns of inheritance of traits from parent to offspring. Genetic mechanisms are the underlying foundation for evolutionary change. Genetics is the branch of science that deals with the inheritance of biological characteristics.
- genome
- the full genetic complement of an individual (or a species). In humans, it is estimated that each individual possesses approximately 3 billion nucleotides in all of the DNA that makes up his or her genome. See Human Genome Project.
- genotype
Back to Top
- H -
- haploid
number
- the number of chromosomes in gametes. Human sperm and ova normally contain only 23 chromosomes, which is the result of the halving of chromosome pairs in meiosis. See diploid number.
- hemizygous
chromosomes
- The X and Y chromosomes of males are mostly not homologous--they only share a few genes. Those genes that are found on only the X or the Y chromosome are said to be hemizygous because there is only one copy of each gene. Regardless of whether that gene is a dominant or a recessive allele, it will be expressed in the phenotype of males.
- hemophilia
- an X-linked genetically inherited recessive disease in which one or more of the normal blood clotting factors is not produced. This results in prolonged bleeding from even minor cuts and injuries. Swollen joints caused by internal bleeding are a common problem for hemophiliacs. Hemophilia most often afflicts males.
-
hermaphrodite
(now more commonly referred to as "intersex")
- an individual who has one or more testes and ovaries and whose external genitalia are not clearly male or female. This condition can occur in chimeras who have inherited both a male and a female set of sex chromosomes. True hermaphrodites have one testes and one ovary. Male pseudohermaphrodites, or "merms," have testes but no ovaries and display some female genitalia tissue. Female pseudohermaphrodites, or "ferms," have ovaries but no testes and display some male genitalia tissue.
- heterozygous
- a genotype consisting of two different alleles of a gene for a particular trait (Aa). Individuals who are heterozygous for a trait are referred to as heterozygotes. See homozygous.
- homologous
chromosomes
- chromosomes that are paired during meiosis. Such chromosomes are alike with regard to size and also position of the centromere. They also have the same genes, but not necessarily the same alleles, at the same locus or location. All autosomes and the sex chromosomes of females occur in homologous pairs. The sex chromosomes of males are mostly hemizygous.
- homozygous
- having the same allele at the same locus on both members of a pair of homologous chromosomes. Homozygous also refers to a genotype consisting of two identical alleles of a gene for a particular trait. An individual may be homozygous dominant (AA) or homozygous recessive (aa). Individuals who are homozygous for a trait are referred to as homozygotes. See heterozygous.
- hormones
- substances (usually proteins) that are produced by specialized cells and that travel to other parts of the body, where they influence chemical reactions and regulate various cellular functions. Hormones include the secretions of the endocrine glands that affect metabolism and behavior. Testosterone, estrogen, and insulin are examples of such hormones.
- Human Genome Project
- a
multinational research effort designed to identify and map the location of
all human genes. The idea of a
Human Genome Project began at a 1984 international conference in Utah.
Research to decode the human genome began in earnest in 1986, funded by the
U.S. Department of Energy. The initial phase of discovering all human DNA
codons was completed early in 2001 at a cost of 2.7
billion dollars. The next
phase of research will be to identify the proteins
for which these genes code. This decoding of the
human proteome
will be an even more daunting task than the original Human Genome Project. See genome.
- I -
- Intersex
see hermaphrodite.-
introns
- sections of
DNA molecules
that do not code for proteins
but still perform important functions. Apparently, some are
enhancers or suppressors of genes.
About a fourth
of DNA consists of introns. The remaining "junk" DNA, that makes up three fourths of
a DNA molecule, very likely also has subtle functions. For
instance, It helps determine
the shape of chromosomes and also buffers against change by absorbing
the mutagenic
effect of radiation and viruses. See exons.
-
in vitro
fertilization
(IVF)
- a set of medical procedures to help a couple have children if they have not been able to in the past due to low sperm count, blocked fallopian tubes, and some other causes of infertility. The woman is given fertility inducing drugs to cause her to ovulate numerous ova which are surgically removed and fertilized outside of her body with sperm from her mate or another donor. Two or more of the fertilized ova are then placed into her uterus in the hope that one or more will attach and develop into viable embryos. As a result, multiple births are relatively common with IVF.
Back to Top
- J -
Back to Top
- K -
- karyotype
- a standardized arrangement of pictures of an individual’s chromosomes cut out from a microphotograph of a cell and rearranged into homologous pairs according to size and other physical characteristics. The standardized arrangement of karyotypes allows medical researchers to discover if an individual is a male or female and if he/she has any gross chromosomal abnormalities.
Back to Top
- L -
- linked genes
- genes that are on the same chromosome and subsequently are inherited together as a package unless crossing-over separates them.
- lipids
- a class of organic molecules that include fats, oils, and waxes.
- locus
(plural, loci
)
- the position on a chromosome where a given gene occurs. The term is sometimes used interchangeably with gene, but this usage is technically incorrect.
Back to Top
- M -
- meiosis
- the cell division process in specialized tissues of female ovaries and male testes which results in the production of sex cells, or gametes. Meiosis involves two divisions and produces four sperm cells in males and one ovum in females from a starting cell. Each sperm and ovum contains only half the original number of chromosomes--23 in the case of humans. Subsequently, meiosis is also called "reduction division." Spermatogenesis is the term used for meiosis in males and oögenesis refers to the same process in females. See mitosis.
- messenger RNA (mRNA)
- micro RNA (miRNA)
- a very short form of RNA in the nucleus of cells. Micro RNA molecules are typically only 20-25 base units long. They are transcribed from DNA, but are not involved directly in protein synthesis like other forms of RNA. They perform important functions similar to enzymes in regulating chemical reactions in our cells, especially in the embryonic stage at the beginning of life. It is thought that 1/3 or more of human genes are controlled in some way by micro RNA molecules. At least 200-255 human DNA genes code for the production of micro RNA molecules.
-
mitochondria
(singular, mitochondrion )
- small rod-like structures in the cytoplasm that produce fuel for the cell in the form of adenosine triphosphate (ATP). A small amount of DNA in a circular looping chromosome is located in mitochondria. This DNA is normally inherited only from mothers and is distinct from DNA that makes up the chromosomes within the cell nucleus.
- mitochondrial
DNA
(mtDNA)
- DNA located in the mitochondria. Since it is normally inherited only from mothers, an offspring's mtDNA will be identical to its mother's, unless a mutation has occurred.
- mitosis
- the simple cell division process that occurs in somatic cells. One cell divides into two offspring cells that are identical to each other in their chromosome complement. Mitosis produces cells with diploid numbers of chromosomes--46 in the case of humans. See meiosis.
-
monozygotic
twins
- identical twins. Twins that come from the same zygote and are, subsequently, the same genetically in terms of their nuclear DNA. Any differences between monozygotic twins later in life are mostly the result of environmental influences rather than genetic inheritance. Fraternal twins may look similar but are not genetically identical. Monozygotic twins may not share all of the same sequences of mitochondrial DNA. This is due to the fact that the mitochondria in a cell may have somewhat different versions of DNA, and the mitochondria can be dispersed unequally when a zygote fissions. Female monozygotic twins can also differ because of differences between them in X-chromosome inactivation. Subsequently, one female twin can have an X-linked condition such as muscular dystrophy and the other twin can be free of it.
- muscular
dystrophy
(MD)
- any of a group of inherited progressive muscle disorders caused by a defect in one or more genes that control muscle function. MD is characterized by gradual irreversible wasting of skeletal muscle. It is an X-linked trait most often passed on to sons by their mothers. The most common form, Duchenne MD, begins to weaken the legs of boys by age 3 and inevitably gets worse with each passing year. There is no cure for this disorder, and it usually results in death before the age of 30.
- mutagen
- an agent that can cause a mutation to occur. Various kinds of chemicals, viruses, and radiation have been identified as mutagens.
- mutation
- an alteration of genetic material such that a new variation is produced. For instance, a trait that has only one allele (A) can mutate to a new form (a). A mutation can be a change in DNA bases or changes in chromosome number and /or structure. Mutation is the only mechanism of evolution that can produce new alleles of a gene. See mutagen.
Back to Top
- N -
-
neurotransmitters
- specialized proteins that relay, amplify and modulate electrical signals between neurons in the brain and nervous system.
- nuclear
membrane
- the selectively permeable membrane enclosing the nucleus of a cell.
- nucleic acids
- the largest of the molecules in living organisms. It is composed of a chain of nucleotides that code for the synthesis of specific proteins. DNA and RNA are types of nucleic acid.
- nucleotide
- the basic building block of nucleic acid. It consists of any one of four specific purine or pyrimidine bases attached to a ribose or deoxyribose sugar and phosphate group.
- nucleus
- a comparatively large structure found in all eukaryotic cells. It contains the chromosomes (nuclear DNA) and is enclosed by a nuclear membrane.
Back to Top
- O -
- oöcyte
- oögenesis
- organelles
- a general term referring to any of the discrete structures within a cell that perform specific functions (e.g., mitochondria).
-
ovulation
- the release of a secondary oöcyte from an ovary to begin its path down a fallopian tube (or oviduct) to the uterus.
- ovum
(plural, ova
)
Back to Top
- P -
- phenotype
- the observable or detectable characteristics of an individual organism. The detectable expression of a genotype.
- physiology
- referring to the organic or bodily processes of an organism. See anatomy.
the marriage of one woman to several men at the same time. This is a rare type of polygamy. It usually takes the form of "fraternal polyandry", which is brothers sharing the same wife.
the generic term for marriage to more than one spouse at the same time. It occurs as polygyny or, more rarely, polyandry.
the marriage of one man to several women at the same time. This is the most common form of polygamy. It often takes the form of "sororal polygyny", which is two or more sisters married to the same man.
- primary oöcyte
- see oöcyte.
- prokaryotic
cell
- a cell that lacks a true nucleus and divides by simple fission rather than mitosis. Bacteria and blue-green algae are prokaryotes. See eukaryotic cell.
- proteins
- any of a large number of organic molecules that are composed of one or more chains of amino acids. These chains are twisted and folded back on themselves in complex patterns. Proteins can serve a wide variety of functions through their ability to bind to other molecules. Proteins may be transporting molecules in blood, structural components, enzymes, hormones, antibodies, or neurotransmitters.
-
proteome
- the full complement of proteins produced by an individual (or a species). It is estimated that each human produces approximately 90,000 types of proteins. See Human Genome Project.
- puberty
- the stage of maturation of an individual when secondary sexual characteristics begin to develop and sexual reproduction first becomes possible. These changes are genetically controlled and triggered by hormones.
Back to Top
- Q -
Back to Top
- R -
- recessive allele
- an allele that is masked in the phenotype by the presence of a dominant allele. Recessive alleles are expressed in the phenotype when the genotype is homozygous recessive (aa).
- recombination
- The creation of a new combination of genes on a chromosome that results from crossing-over.
- red-green color blindness (or red-green color deficiency)
- an X-linked genetically inherited color vision deficiency in which an individual cannot perceive red and green in the same way as people with normal vision. This condition most often afflicts males.
- regulator gene
- a gene
that can initiate or block the functions of other genes. Regulator
genes control the timing of production of a variety of chemicals in
humans and other organisms. Shortly after conception, regulator
genes work as master switches orchestrating the timely development
of our body parts. They are also responsible for changes that occur
in our bodies as we grow older. Regulator genes are also called
homeotic
genes. - replication (of DNA)
- the process that occurs during the rest period (interphase) at the outset of mitosis and meiosis by which a DNA molecule is duplicated or copied. One DNA molecule becomes two identical ones. This is accomplished by the DNA molecule unwinding and unzipping along its base pairs so that both sides can be copied by free nucleotides. Replication is triggered by an enzyme.
-
-
ribosomal
RNA (rRNA)
- ribosomes
- small structures usually near the surface of endoplasmic reticula in the cytoplasm of cells. Ribosomes are the sites where proteins are assembled. Each ribosome is composed of 3 RNA strands and 54 proteins woven into 2 separate, but entangled lumps.
-
RNA (ribonucleic acid
)- a type of nucleic acid that is found in both the nucleus and the cytoplasm of cells. Unlike DNA, RNA is single stranded. Messenger RNA (mRNA) carries the genetic code from the DNA in the chromosomes and translates it with the help of transfer RNA (tRNA) at the site of the ribosomes in the cytoplasm in order to assemble, or synthesize, proteins.
Back to Top
- S -
- secondary oöcyte
- see oöcyte.
- selectively permeable
- the characteristic of allowing only certain things to pass through. Cell and nuclear membranes are selectively permeable.
- sex cell
- a gamete, either a sperm or an ovum. Sex cells are produced by the meiosis process and have a haploid number of chromosomes. See somatic cell.
- sex chromosome
- the X and Y chromosomes which are responsible for determining whether an individual is a male or a female. Normal males inherit an X from their mother and a Y from their father. Normal females get an X chromosome from both parents. See SRY gene.
- sex-linked
- referring to a gene that is part of a sex chromosome. Since all of the genes on a chromosome are inherited as a package, they are essentially linked together.
- somatic
cell
- all the cells in the body except those directly involved with reproduction. Most cells in multicellular plants and animals are somatic cells. They reproduce by mitosis and have a diploid number of chromosomes. See sex cell.
- sperm
- a male sex cell or gamete.
- spermatogenesis
-
SRY gene
- sex-determining region Y gene. A gene on the Y chromosome which, if present and functioning, causes the undifferentiated sex organs of an animal embryo to become testes. These then produce the hormone testosterone which, in turn, causes the development of masculine physical traits.
Back to Top
- T -
- testosterone
- a class of masculinizing hormones. Both men and women produce them, but males normally produce much more. Testosterone is mainly produced in the testes of males. Smaller amounts are produced by the cortex of the adrenal glands in both males and females.
- transfer RNA (tRNA)
- a form of RNA that binds to specific amino acid molecules and transports them to the ribosomes for the assembly of proteins. Transfer RNA molecules temporarily bond with corresponding messenger RNA codons at the ribosomes in this process of protein synthesis.
Back to Top
- U -
Back to Top
- V -
Back to Top
- W -
Back to Top
- X -
-
X-linked
- referring to a gene that is carried by an X chromosome. See sex-linked gene.
Back to Top
- Y -
-
Y-linked
- referring to a gene that is carried by a Y chromosome. See sex-linked gene.
Back to Top
- Z -
- zygote
- a "fertilized" ovum. More precisely, this is a cell that is formed when a sperm and an ovum combine their chromosomes at conception. A zygote contains the full complement of chromosomes (46 in humans) and has the potential of developing into an entire organism.
Back to Top
Copyright © 1997-2014 by Dennis
O'Neil.
All rights reserved.
illustration credits