The third stage of mitosis when the kinetochores separate and the sister chromatids are pulled towards the opposite poles of the spindle. Each chromatid is now a chromosome in its own right. Cytokinesis (cell division) begins during late anaphase.
bivalent
A pair of homologous chromosomes during synapsis at the first meiotic prophase and metaphase.
cell cycle
The biochemical events that occur between one mitotic cell division and the next in a dividing population of eukaryotic cells. The cell cycle is divided into four phases: G1 (a preparation phase for DNA replication); S (during which DNA replicates); G2 (a short gap phase during which the DNA is checked for structural integrity), and M - mitosis. Cells may leave the cell cycle after mitosis and enter a state of quiescence called G0.
centromere
A region of the chromosome where the two sister chromatids are held together after DNA replication and chromosomal condensation and which is the attachment point for the mititoc spindle. The centromere contains highly repetitive, AT-rich DNA sequences.
centrosome
A region of the cell cytoplasm adjacent to the nucleus that duplicates during interphase and which contains the centrioles, which organize the spindle microtubules. The centrosomes move to either side of the nucleus during prophase and, as the nuclear envelope breaks down, form the two poles of the mitotic spindle.
chiasma (pl.) chiasmata
The visible signs of genetic crossing-over between chromosomes that have paired at meiosis (i.e. between bivalents) and therefore of homologous (non-random) recombination. Several chiasmata may occur per bivalent.
chromatid
Each of a pair of identical DNA molecules complexed with chromosomal proteins and joined at the centromere after DNA replication and before mitosis.
chromatin
The name for the protein/DNA complex that makes up the chromosomes.
chromosomes
Molecules of DNA complexed with specific proteins responsible in eukaryotes for storage and transmission of genetic information.
cytokinesis
The process of cell division following mitotic nuclear divisoin and chromosome segregation.
diakinesis
The fifth and final stage of meiotic prophase I. Further contraction and condensation of the diplotene chromosomes take place to form compact and easily moved structures.
diplotene
The fourth stage of meiotic prophase I. The fact that each chromosome in a synapsed pair is itself made up of two identical DNA molecules (sister chromatids) now becomes more obvious as the synaptonemal complex dissolves and the synapsed structures now appear as bundles of four chromatids - the tetrad. As the four strands separate, crossover points (chiasmata) become visible between non-sister chromatids resulting in genetic recombination.
gamete
A haploid cell specialised for fertilisation.
histones
Small, very basic proteins forming complexes with eukaryotic DNA. There are five types - H1, H2A, H2B, H3 and H4. There are also other "non-histone" proteins associated with DNA.
interphase
The period between one mitosis and the next. Consists of the combined G1, S and G2 phases of the cell cycle.
kinetochore
Structures forming at the centromeres of each chromosome during mitosis for binding the spindle microtubules.
leptotene
The first stage of meiotic prophase I. After DNA replication, the chromatin condenses to form visible chromosomes which appear as long thin threads comprising pairs of replicated DNA molecules. Small areas of thickening (chromomeres) appear along the length of the chromosomes. Condensation continues throughout the whole of prophase I.
meiosis
The process whereby a nucleus divides by two divisions (meiosis I and meiosis II) into four nuclei, each containing half the original number of chromosomes (haploid). Each meiotic division is divided into four phases: meiotic prophase, meiotic metaphase, meiotic anaphase and meiotic telophase with the first meiotic prophase comprising five stages: leptotene, zygotene, pachytene, diplotene and diakinesis.
meiotic anaphase I
The third phase of the first meiotic division. As in mitosis, the two members of each bivalent are pulled towards opposite poles by the spindle fibres.
meiotic anaphase II
The third phase of the second meiotic division. The centromeres split and the sister chromatids are pulled apart to opposite poles by the spindle fibres.
meiotic metaphase I
The second phase of the first meiotic division. The nuclear membrane disappears and the homologous pairs position themselves in the equatorial plane. The chromatids are still joined at the centromere.
meiotic metaphase II
The second phase of the second meiotic division. The pairs of sister chromatids become arranged pon the equatorial plane
meiotic prophase I
The first phase of the first meiotic division. Comprises five stages: leptotene, zygotene, pachytene, diplotene and diakinesis.
meiotic prophase II
The first phase of the second meiotic division. The nuclear membrane disappears and the chromosomes condense again. This stage is characterised by the presence of the haploid number of sister chromatid pairs.
meiotic telophase I
The fourth phase of the first meiotic division. The nuclear membrane reforms and the chromosomes become diffuse again. May be followed by a short interphase (interkinesis) before the second meiotic division commences. In some organisms, meiotic telophase I and interkinesis do not exit and cells proceed straight from meiotic anaphase I to meiotic prophase II without reformation of a nuclear membrane.
meiotic telophase II
The fourth and last phase of the second meiotic division. The nuclear membranes reform around the chromosomes at the opposite poles.
metaphase
The second stage of mitosis after prophase and the transitional prometaphase when the chromosomes move back and forth until they all lie in one plane (the metaphase plate) halfway between the two spindle poles. Each chromosome lies with its long axis at right angles to the axis of the spindle.
mitosis
The process of nuclear division (or karyokinesis) during which the chromosomes condense, the nuclear envelope breaks down (in most cells), the cytoskeleton reorganizes to form the mitotic spindle, and the chromosomes move to opposite poles. Chromosome segregation is then usually followed by cell division (cytokinesis). Mitosis is conventionally divided into four stages - prophase, metaphase, anaphase and telophase with the transitional prometaphase between prophase and metaphase.
nucleosomes
Basic structural unit of eukaryotic chromatin forming "beads on a string." Made up of DNA and histones.
pachytene
The third stage of meiotic prophase I. Chromosomes are now fully synapsed forming (in humans ) 46 homologous pairs.
prometaphase
A transitional stage of mitosis between prophase and metaphase when the microtubules of the mitotic spindle attach to the kinetochores of the condensed chromosomes
prophase
The first stage of mitosis when the chromosomes condense and the centrosomes move to opposite sides of the nucleus, initiating formation of the mitotic spindle. In higher eukaryotes, the nuclear envelope breaks down at the end of prophase.
spindle
A complex of microtubules that appears during mitosis and meiosis and which attach at one end to the spindle poles formed by the centrosomes and at the other to the kinetochores on the condensed chromosomes.
synapsis
Tight pairing of chromosomes along their entire lengths during meiosis. Formed by a zip-like DNA-protein complex between the chromosomes - the synaptonemal complex.
telophase
The fourth and final stage of mitosis when nuclear envelopes reform round the two groups of daughter chromosomes which then decondense again to form interphase chromatin. Cytokinesis (cell division) is completed.
zygotene
The second stage of meiotic prophase I. Being diploid, the cell has two copies of each chromosome (each of which has also now replicated, so four copies of genetic information). At zygotene, each replicated chromosome pairs up closely with its partner along their whole length (they are now said to be synapsed). This paring involves a DNA-protein structure called the synaptonemal complex that "glues" them together. Each pair is called a bivalent.
