Module 5
1. Module 5
1.32. Module Glossary
Module 5—Cell Division: The Processes of Mitosis and Meiosis
Module Glossary
Consult the glossary in the textbook for other definitions that you may need to complete your work.
2n: the symbol referring to a diploid cell
alleles: different versions (base sequences) of a gene or trait that will code for slightly different proteins (e.g., sickle cell hemoglobin versus normal hemoglobin)
Increased types of alleles in the gene pool increases variation and diversity and protects the species from extinction.
alternation of generations: a plant life cycle where two distinct multicellular forms of a sporophyte and a gametophyte occur in one generation
A diploid zygote undergoes mitosis to form a diploid sporophyte, which undergoes meiosis to form haploid unicell spores, each of which undergo mitosis to form a multicellular haploid gametophyte, which undergoes mitosis to form unicellular gametes, which fuse with other gametes to produce a diploid zygote. Animals do not display alternation of generations.
amniocentesis: a needle sampling of fetal amniotic fluid to gather embryonic cells for karyotyping; determines chromosome number and gender, not presence of specific genes/alleles on chromosomes
anaphase: the third phase of mitosis where spindle fibres contract, pulling sister chromatids of each chromosome apart to opposite poles
anchorage dependence: a property of normal cells that only allows mitosis to occur when cells are attached to a substrate or surface, not floating freely
Anchorage dependence is lost in cancer, thereby allowing for metastasis to occur.
asexual reproduction: creation of a new organism without the input of cells from two separate organisms of opposite sexes; examples are binary fission, yeast and Hydra budding, and vegetative propagation of plants
autosomes: the 22 homologous pairs seen in a karyotype; have nothing to do with gender
binary fission: cell division in prokaryotes (bacteria); simple because there is only one circular chromosome so no spindle is needed
binary fission: asexual cell division in prokaryotes; replication of the circular chromosome and division of the DNA and cytoplasm without the use of a spindle; offspring are clones; rate is very rapid
blastocyst: the embryonic stage about one week after conception; a hollow ball of cells consisting of outer trophoblast that becomes the chorion and inner cell mass that becomes the embryo; implants in the wall of the endometrium
budding: a type of asexual reproduction
In yeast, unequal cytokinesis in mitosis forms a large cell and a tiny cell that buds off; in Hydra, a small multicell mini-polyp breaks off and forms an adult.
cancer: rapid proliferation (cell division) of cells that occurs when mutations result in disruption of the normal timing of mitosis; characterized by loss of density-dependent inhibition, loss of anchorage dependence, dedifferentiation of cell function, rapid metabolism, and short cell cycle
cell cycle: the period of time between one cell division and the next; consists of interphase, mitosis, and cytokinesis; may also be divided into interphase and M phase (mitosis and cytokinesis)
cell division: the period of the cell cycle where the cell is actively dividing; composed of mitosis and cytokinesis stages
cellular clock: a property of cells that allows them to go through a set number of cell divisions and then stop, whereupon the cell line dies out; sometimes called apoptosis
Cancer cells do not have a normal cell clock so they do not apoptose.
centrioles: organizing bodies of the spindle
As they move apart in prophase, spindle fibres stretch out between them, forming the spindle apparatus.
centromere: a ‘button’ that keeps the two identical sister chromatids together after the S phase of interphase and through mitosis until anaphase
chemotherapy: the use of cytotoxic drugs that inhibit cell division, usually by preventing DNA replication or interfering with the spindle mechanism of mitosis or by interfering with the supply of blood and nutrients to the tumour; applied systemically (into the bloodstream); targets cancerous cells but may also affect rapidly dividing normal cells to some degree
chorionic villus sampling (CVS): a sampling of chorionic villi through the vagina, used to obtain embryonic cells for karyotyping
chromatid: one-half or one of two threadlike strands into which a chromosome divides during cell division
chromatin: long fibres containing DNA, small amounts of RNA, and proteins
These fibres form chromosomes when they coil around histones.
chromosome: a thick, rod-shaped body in the nucleus that forms when chromatin (long, stringy DNA) supercoils around balls of histone proteins in prophase of mitosis and meiosis
clone: to create of an exact replica
A cell is a clone if it is the product of asexual reproduction (mitosis or binary fission) that produces two genetically identical cells. An organism can be a clone if it is genetically identical to another organism (e.g., animals can be cloned by taking a nucleus from one animal and inserting it into an empty egg of another, producing an offspring identical to the one that donated the nucleus). Cloning is used in agriculture and pharmaceutical industries to create uniform, consistent products.
conjugation: a type of sexual reproduction that occurs when two cells form a cytoplasm bridge through which they exchange genetic material, producing variation; occurs only in unfavourable environments
cordiocentesis: a sampling of umbilical cord blood of fetus for karyotyping
crossing over: an occurance during meiosis I when homologous pairs and their attached sister-chromatids form tetrads, entwine in synapsis, and may be chopped by enzymes into pieces
When chromosomes are reassembled, sections of the mother’s homologue may be exchanged with the father’s, forming chromosomes with new combinations of mother’s and father’s alleles. This increases variation in gametes and offspring, improving the species' chances of survival if the environment changes.
cutting: type of vegetative propagation when a stem of a plant is cut off and produces roots, stems, leaves, and flowers; an asexual form of reproduction
cytokinesis: the phase of the cell cycle after mitosis when the cytoplasm divides into two separate daughter cells
A cleavage furrow forms in animal cells; a division plate forms in plant cells.
daughter cell: a cell that is the product of cell division
In mitosis, daughter cells are identical to the mother cell; in meiosis, they are not identical to the parent cell.
density-dependent inhibition: a property of normal cells that allows mitosis to occur only until cells touch each other
Density-dependent inhibition is lost in cancer cells; therefore, cells begin to form on top of one another, forming masses of cells called tumours.
diploid: the chromosome number of a somatic (body) cell; both chromosomes of each homologous pair are present; two sets of chromosomes are present, one from each parent
DNA: the genetic material found contained in the nucleus in eukaryotes (also in mitochondria and chloroplasts) and loose in the cytoplasm in prokaryotes, such as bacteria
Down syndrome: typically characterized by some impairment of physical growth, unique physical features, and below average cognitive ability
If an n + 1 gamete that results from nondisjunction of a twenty-first chromosome is fertilized by a normal sperm, a Trisomy 21 (2n + 1) offspring is produced with Down syndrome.
eukaryotic cell: a cell with membrane-bound organelles and nucleus
fertilization: fusion of an egg and sperm (gametes) to produce a zygote; occurs in sexual reproduction only
fragmentation: an asexual form of reproduction in animals similar to cutting in plants (e.g., a flatworm with tail cut off can regenerate the lost section)
fraternal twins: two siblings born at the same time, resulting from the accidental ovulation of two eggs, which are fertilized by two sperm
Fraternal twins are as different as any two siblings.
G1 phase: the first part of interphase where the cell is actively growing and undergoing metabolism and protein synthesis
G2 phase: the third part of interphase where the cell continues growing, metabolizing, and carrying out protein synthesis
gametes: sex cells (sperm and egg); have half the normal chromosome number so they can participate in fertilization
gametophyte: a multicellular stage in alternation of generations that consists of haploid cells that split off to produce haploid gametes
gene: the unit of hereditary information that can be passed on to offspring; includes the specific DNA sequence encoding or regulating the sequence of a protein, tRNA, or rRNA molecule; determines the expression of a trait
genetic material: DNA; contains the genes that direct the synthesis of proteins needed by the cell; exists as chromatin or chromosomes
genetic variation: the permutations and combinations of genes and alleles possible; refers to different combinations of mother’s and father’s alleles in gametes; increases variation in the offspring and translates into better odds of offspring survival in changing environments
haploid: the term describing a cell containing half the chromosomes that a diploid parent cell contains
This condition occurs in gametes, either in the egg or the sperm.
haploid: chromosome number of a gamete (sex cell—egg or sperm), resulting from meiosis; only one chromosome of each homologous pair is present; one set of chromosomes present
histones: proteins found in chromosomes that provide scaffolding for DNA to twine around so that the DNA can fit within the confined space of the nucleus
Hodgkin’s disease: a blood cancer of lymph tissue
identical twins: result of one egg fertilized by one sperm; occurs when the morula splits into two masses that develop independently in the uterus; offspring are genetically identical
independent assortment: one of Mendel’s Laws of heredity
There are two uses of this term1) In meiosis I, tetrads line up randomly on the metaphase plate so that either the mother’s or the father’s sister chromatids are on one side. When all the tetrads are pulled apart in anaphase I, the chromosomes that collect at each pole are a unique combination/permutation of mother’s and father’s chromosomes/alleles. The orientation of one tetrad is independent of the others. (2) in reference to whether or not two genes are on the same chromosome or on different ones: If on different chromosomes, one gene (allele) is independent of the other gene in the way their tetrad lines up on the metaphase plate of meiosis I, and much gamete variation can result. If genes are linked on the same chromosome, the two genes cannot line up independently of each other—they are tied together, limiting the variety of permutations/combinations and variation in gametes that can be formed.
interphase: the longest period of the cell cycle when the cell is actively growing and metabolizing; consists of G1, S, and G2 phases; DNA is in loose, stringy chromatin form not visible under the microscope
karyotype: a pictorial representation of all the chromosomes of a cell arranged in homologous pairs according to size, centromere position, and banding pattern; used to diagnose abnormalities in chromosome number (non-disjunction) and to determine sex chromosomes
kinetochore: another word for centromere; the small body holding the sister chromatids together as one chromosome; attaches to a spindle fibre at the metaphase plate
Klinefelter syndrome: born with primary male sex characteristics but develops female secondary sex characteristics
When an XX egg due to nondisjunction is fertilized by a Y sperm, the offspring (XXY) has Klinefelter syndrome.
life cycle: the stages an organism goes through to grow, reproduce, and die
locus: a specific location on a chromosome
meiosis: cell division that results in cells that have half the normal chromosome number (haploid gametes); also called reduction division
meiosis I: the first division of meiosis; preceded by DNA replication in interphase; results in one secondary oocyte and first polar body in females and two secondary spermatocytes in males
Because homologous pairs are separated from each other in anaphase, cell products are already considered haploid.
meiosis II: the second division of meiosis; no DNA replication in interphase; results in one haploid ootid and second polar body in females and four haploid spermatids in males
metaphase: the second phase of mitosis where chromosomes line up on the equator (metaphase plate) and attach via their centromeres to a spindle fibre
Each centromere replicates so each sister chromatid has its own to allow spindle fibre to attach.
metastasis: the tendency of some cancer cells to break off from a primary tumour and move through the blood or lymphatic systems to other locations in the body where secondary tumours form; sometimes referred to as the “spreading” of cancer
mitosis: cell division that results in identical cells; used for growth and repair of organisms
monosomy: offspring that only have one of a particular chromosome rather than two; occurs when a normal n gamete fuses with an n − 1 gamete, producing a 2n − 1 zygote; in humans, an offspring that has a diploid number of 45 rather than 46
M phase: mitosis and cytokinesis together
mutagenic agent: a chemical or physical agent that has the ability to mutate DNA, affecting the timing of the cell cycle; increasing the rate of mitosis
mutation: a permanent change in a cell's genetic structure, often resulting in the expression of a new trait or feature in the affected organism; usually due to random errors occurring during DNA replication or protein synthesis, but can also be caused by chemical or physical mutagens
n: symbol referring to a haploid cell
nondisjunction: an error in meiosis that results in non-separation of chromosomes; results in two chromosomes entering one gamete; produces gametes with an extra chromosome (n + 1), or gametes that are missing a chromosome (n – 1)
nondisjunction: non-separation of chromosomes during meiosis; one pair gets dragged to one pole, and no representative of that pair is pulled to the other pole; if occurs in meiosis I, gametes will be two n + 1 gametes and two n – 1 gametes; if occurs in meiosis II, gametes will be n + 1, n – 1, n, and n, results in offspring that have a trisomy or monosomy for that chromosome
oocyte: a cell undergoing oogenesis
The primary oocyte undergoes meiosis I, producing a large secondary oocyte and the first polar body, which is reabsorbed. The secondary oocyte undergoes meiosis II, forming a large ootid and a second polar body, which is reabsorbed.
parent cell: a diploid somatic cell about to enter cell division
parthenogenesis: a rare type of asexual reproduction; an unfertilized haploid egg divides by mitosis, producing a complete multicellular organism in which all cells are haploid; seen in amphibians, reptiles, and birds
ploidy: refers to the chromosome number of a cell or how many sets of chromosomes are present; haploid cells have one set, diploid two, tetraploid four, octoploid eight
polar body: a tiny cell that results from each division of oogenesis
Meiosis I results in large secondary oocyte and a tiny polar body that is reabsorbed. Meiosis I results in a huge ootid and a tiny second polar body that is reabsorbed. Polar bodies are “garbage cans” that extra nuclear material is dumped into to fulfill the need for one large haploid ovum.
polyploid: the term describing a cell that contains more than two homologous chromosomes
prenatal genetic testing: sampling and testing of embryonic or fetal cells to determine chromosome number and gender
prokaryotes: single-celled organisms without a nuclear membrane, such as bacteria; have one circular chromosome; divide asexually by binary fission and “sexually” by conjugation
prophase: the first phase of mitosis where visible chromosomes appear scattered through a cell; nuclear membrane dissolves; centrioles move to opposite poles, forming a spindle between them
radiation treatment: cancer treatment in which high-energy radiation from radioactive isotopes is directed at a cancerous tumour in an effort to destroy it without destroying surrounding normal tissue
replication: the copying of the cell’s DNA prior to mitosis so that each daughter cell has an exact copy of the mother cell’s genetic material; results in sister chromatids; occurs in the S phase of interphase
resistance: occurs when a drug removes susceptible bacteria or viruses from a population and leaves those variants (mutants) that are resistant to the drug
Rapid cell division ensures that the whole population becomes resistant quickly.
sex chromosomes: the last (twenty-third) pair of chromosomes seen in a karyotype that determines the gender of an organism
X and Y sex chromosomes are not homologous to each other in terms of shape, size, or genetic information.
sexual reproduction: creation of offspring through input of genetic material from two different organisms of opposite sexes (sperm from male and egg from female); increases variation
sister chromatids: two pieces of DNA that are identical to each other as a result of DNA replication in S phase; lie side-by-side and are buttoned together by a centromere; together make up one chromosome
somatic cell: the name given to any of the cells of a multicellular organism, including humans
The exception is those cells that form gametes, which are not somatic cells.
spermatids: haploid cells that result from meiosis; swim to epididymis for maturation to a spermatazoan and storage until ejaculation
spermatocyte: cells undergoing spermatogenesis; primary spermatocyte undergoes first meiotic division, forming two secondary spermatocytes; each secondary spermatocyte undergoes second meiotic division, forming two spermatids; four spermatids formed from one primary spermatocyte
spermatogenesis: the process of producing gametes in males; occurs in walls of seminiferous tubules of testes
spermatogonium: initial diploid germ cell in spermatogenesis; undergoes mitosis, forming many primary spermatocytes that each carry out meiosis
S phase: the second part of interphase where DNA replication occurs in preparation for upcoming mitosis; produces sister chromatids
spindle apparatus: a structure composed of spindle fibres; forms during prophase in mitosis to facilitate separation and movement of chromosomes in cell division
spore: a haploid cell produced by meiosis in the sporophyte; each is capable of dividing to form a multicellular gametophyte consisting of haploid cells
sporophyte: the diploid multicellular stage in plants that show alternation of generations; produces haploid spores by meiosis
staining: a technique used in slide preparation to make the chromosomes of a dividing cell visible and dark
stem cell: an undifferentiated “generic” cell that can be coaxed into producing a number of different kinds of cells; present in embryonic blastocysts and also recently found in adults; potentially used to repair tissue and build replacement organs
Embryonic stem cell research is controversial due to the fact that the creation and termination of human blastocysts is used as a method of obtaining stem cells.
super bugs: bacteria that are immune to many antibiotics
Super bugs develop because of an overuse of antibiotics and antibacterials that have destroyed susceptible bacteria, leaving only those bacteria that are resistant to these drugs.
synapsis: the entwining of the homologous pair and attached sister chromatids in prophase I of meiosis; crossing-over between non-sister chromatids may occur
telomere: a section on each end of a chromosome that shortens with each mitotic division
If the telomere is too short, the cell no longer divides.
telophase: the fourth phase of mitosis where nuclear membranes form around the two groups of chromosomes; spindle apparatus dissolves; chromosomes decondense to become chromatin
tetrad: formed in prophase of meiosis I when homologous pairs and their attached sister chromatids find each other and entwine in synapsis; may undergo crossing-over with non-sister chromatids, increasing variation in the gametes that result
totipotent: cells that have not specialized or differentiated (e.g., zygote, morula); all genes in the cell have the potential to be expressed (turned on)
Totipotent cells have the ability to form a complete organism.
trisomy: offspring that have three of one kind of chromosome rather than two; occurs when an n + 1 gamete fuses with a normal n gamete, producing a 2n + 1 zygote; in humans an offspring that has a diploid number of 47 rather than 46
variation: the existence of many combinations of genes/traits in a population; improves the probability that some members will survive if environmental conditions change; is high in sexual reproduction
X chromosome: the longer sex chromosome
Females are XX.
Y chromosome: the shorter sex chromosome; determines maleness; has much fewer genes on it than the X
Males are XY.