Cell cycles, reproduction and development definitions

The cell cycle and mitosis

Chromatid

One of the two identical copies of a chromosome, joined to its sister at the centromere after DNA replication.

Homologous pair

Two chromosomes of the same size carrying the same genes at the same loci, one inherited from each parent. Not the same thing as a pair of chromatids.

Spindle

The framework of protein microtubules built across the cell before the chromosomes are moved, running from pole to pole and attaching to every centromere; shortening the fibres is what separates the chromatids.

Mitotic index

The proportion of cells in a sample that are visibly in mitosis.

Telomere

A cap of short non-coding sequences repeated many times at each end of a chromosome, which is eroded instead of the genes when replication fails to copy the end, and which stops the end being treated as a break in the DNA.

Telomerase

An enzyme carrying its own RNA template, which adds telomere repeats back onto the end of a chromosome; it is active in germ line cells, stem cells and most cancer cells.

Mitosis

Nuclear division that produces two daughter nuclei with the same number of chromosomes as the parent nucleus, genetically identical to it and to each other.

Stem cells and cell potency

Stem cell

An unspecialised cell that can divide repeatedly to produce more cells like itself, and can differentiate into one or more specialised types.

Self-renewal

Division that produces at least one daughter cell identical to the parent stem cell, so the population of stem cells is maintained.

Differentiation

The process by which a cell becomes specialised, through transcription of some of its genes and not others.

Potency

The range of cell types a stem cell is still able to become.

Gametes, fertilisation and early development

Acrosome

A vesicle at the front of a sperm head containing hydrolytic enzymes, which are released to digest a path through the layers around the egg.

Zona pellucida

The glycoprotein coat outside the egg's cell surface membrane, carrying the receptors a sperm binds to.

Cortical granules

Vesicles lying just inside the egg's cell surface membrane, whose contents are released at fertilisation and change the zona pellucida.

Polyspermy

Fusion of more than one sperm with a single egg, which gives a cell with too many chromosome sets to develop.

Sexual reproduction in flowering plants

Stamen

The male part of a flower: a filament carrying an anther, in which pollen grains are made.

Carpel

The female part of a flower: a stigma, a style and an ovary containing one or more ovules.

Pollination

The transfer of pollen grains from an anther to a stigma.

Micropyle

The small gap in the integuments at one end of an ovule, through which a pollen tube usually enters.

Tube nucleus

The larger of the two nuclei formed when a microspore's nucleus divides by mitosis; it controls the growth of the pollen tube.

Generative nucleus

The smaller nucleus of a pollen grain, which divides by mitosis to give the two male nuclei.

Embryo sac

The structure inside an ovule, formed from one megaspore by three mitotic divisions, holding the egg cell, two synergids, two polar nuclei and three antipodal cells.

Double fertilisation

The fusion, in the embryo sac of a flowering plant, of one male nucleus with the egg nucleus to form a zygote and of the other with the two polar nuclei to form a triploid endosperm nucleus.

Endosperm

The triploid food-storing tissue of a seed, formed by mitosis from the nucleus produced when a male nucleus fuses with the two polar nuclei.

Homeobox genes, Hox genes and apoptosis

Homeobox

A sequence of 180 base pairs found within many genes that control development, coding for the part of the protein that binds to DNA.

Homeodomain

The 60 amino acid section of a protein coded for by a homeobox, which folds into a shape that binds to a specific DNA sequence.

Transcription factor

A protein that binds to DNA near a gene and controls whether that gene is transcribed into mRNA.

Conserved sequence

A sequence of bases that has changed very little between species over long periods, because nearly every mutation in it is harmful and is not passed on.

Apoptosis

Programmed cell death, in which a cell is broken down in an ordered sequence by its own enzymes and the fragments are removed by phagocytosis, without the release of the cell's contents.

Necrosis

Death of a cell caused by injury or by conditions it cannot survive, in which the cell swells, the membrane breaks and the contents escape and cause inflammation.

Apoptotic body

A membrane-bound fragment of a cell undergoing apoptosis, engulfed whole by a phagocyte.

Blebbing

The bulging of the cell surface membrane during apoptosis, which changes the shape of the cell without the membrane breaking.

Cloning in plants and animals

Clone

A group of genetically identical organisms, or a cell genetically identical to the cell it was produced from.

Vegetative propagation

Asexual reproduction in plants, in which a new plant develops from a stem, root or leaf of the parent and is genetically identical to it.

Meristem

A region of a plant in which the cells remain unspecialised and continue to divide by mitosis.

Explant

The small piece of tissue cut from a parent plant to start a tissue culture, usually a shoot tip or a node.

Callus

The mass of undifferentiated dividing cells produced from an explant on a nutrient medium, from which whole plantlets are regenerated.

Micropropagation

The production of very large numbers of genetically identical plants from a small piece of tissue, grown on sterile nutrient medium containing plant hormones.

Somatic cell

Any body cell of a multicellular organism other than a gamete or the cells that produce gametes.

Enucleation

The removal of the nucleus from a cell, usually an egg cell, leaving its cytoplasm intact.

Somatic cell nuclear transfer

The transfer of a nucleus from a body cell into an enucleated egg cell, which is then stimulated to divide as an embryo genetically identical to the nucleus donor.

Reproductive cloning

Cloning in which the embryo produced is implanted into a surrogate and develops into a whole new individual.

Therapeutic cloning

Cloning in which the embryo produced is used as a source of stem cells and is never implanted.