Meiosis Health Dictionary

An adjective describing organisms, cells or nuclei that have a single set of unpaired CHROMOSOMES. Human beings have haploid gametes (see GAMETE) following MEIOSIS.... haploid

See MEIOSIS.... reduction division

A type of cell division in which the chromosomes within the nucleus of a cell are exactly duplicated into each of 2 daughter cells.

Before cell division, the chromosomes duplicate themselves and coil up with the 2 copies joined together. The doubled chromosomes line up in the centre of the cell and are pulled apart to opposite ends of the cell, which then divides. Each daughter cell therefore has the same chromosome content as the original cell. (See also meiosis.)... mitosis

The processes by which cells multiply. Mitosis is the most common form of cell division, giving rise todaughter cells identical to the parent cells.

Meiosis produces egg (see ovum) and sperm cells that differ from their parent cells in that they have only half the normal number of chromosomes.... cell division

A group of drugs used in the treatment of certain forms of malignant disease. Chemically, they closely resemble substances (or METABOLITES) which are essential for the life and growth of CELLS. Antimetabolites are incorporated into new nuclear material in the cell or combine irreversibly with essential cellular enzymes, thus disrupting normal cellular division (see MITOSIS and MEIOSIS) and causing death of the cell. There is now a range of antimetabolites including CYTARABINE, METHOTREXATE, FLUOROURACIL and MERCAPTOPURINE.... antimetabolites

The rod-shaped bodies to be found in the nucleus of every cell in the body. They contain the GENES, or hereditary elements, which establish the characteristics of an individual. Composed of a long double-coiled ?lament of DNA, they occur in pairs – one from the maternal, the other from the paternal – and human beings possess 46, made up of 23 pairs. The number of chromosomes is speci?c for each species of animal. Each chromosone can duplicate an exact copy of itself between each cell division. (See GENETIC CODE; GENETICS; HEREDITY; MEIOSIS; SEX CHROMOSOMES.)... chromosomes

If non-dysjunction occurs after the formation of a ZYGOTE – that is, during a mitotic cell division and not a meiotic cell division (see MITOSIS; MEIOSIS) – some of the cells will have one chromosome constitution and others another. The term mosaicism describes a condition in which a substantial minority of cells in an individual’s body di?er from the majority in their chromosome content. How substantial this minority is will depend upon how early during cleavage the zygote undergoes nondysjunction. Mosaicism can cause disorders such as DOWN’S (DOWN) SYNDROME and TURNER’S SYNDROME. The proportion and type of abnormal cells affect the physical appearance of the affected individual. This may range from normal to the features typical of people with a chromosomal-abnormality syndrome.... mosaicism

An immature OVUM. When the cell undergoes MEIOSIS in the ovary it becomes an ovum and is ready for fertilisation by the spermatozoa. Only a small number of the many oöcytes produced survive until PUBERTY, and not all of them will become ova and be ejected into the FALLOPIAN TUBES.... oöcyte

The production of mature egg cells (ova – see OVUM) by the OVARIES. Germ cells in the ovary multiply to produce oogonia which divide by MEIOSIS to form oöcytes in the FETUS.... oögenesis

A unit of the material of heredity. A gene corresponds to a particular area of DNA within a chromosome. There are about 30,000 different genes arranged on the 23 pairs of chromosomes. These genes control the development and functioning of organs and body systems, providing an “instruction manual” for an individual’s growth, survival, reproduction, and possibly also for aging and death. Genes also play a part, together with environmental factors, in determining a person’s intelligence, personality, and behaviour.

Genes fulfil these functions by directing the manufacture of proteins. Many proteins have a structural or catalytic role in the body. Others switch genes “on” or “off”. The genes that make these regulatory proteins are called control genes. The activities of control genes determine the specialization of cells; within any cell some genes are active and others idle, according to its particular function. If the control genes are disrupted, cells lose their specialist abilities and multiply out of control; this is the probable mechanism by which cancers form (see carcinogenesis; oncogenes).Each of a person’s body cells contains an identical set of genes because all the cells are derived, by a process of division, from a single fertilized egg, and with each division the genes are copied to each offspring cell (see mitosis; meiosis). Occasionally, a fault occurs in the copying process, leading to a mutation. The gene at any particular location on a chromosome can exist in any of various forms, called alleles. If the effects of an allele mask those of the allele at the same location on its partner chromosome, it is called dominant. The masked allele is recessive. (See also genetic code; inheritance.)... gene

Mitochondria have their own DNA. In human mitochondria, the is a double-helical circle that codes for 13 proteins. Mitochondria have a distinctive genetic code, and their genomes are not changed by meiosis during reproduction, making the useful in genetic studies.

The significance of mitochondria having their own is that diseases can be inherited via abnormalities of mitochondrial , and inheritance of the is maternal, directly from the egg.... mitochondrial dna

n. the third stage of *mitosis and of each division of *meiosis. In mitosis and anaphase II of meiosis the chromatids separate, becoming daughter chromosomes, and move apart along the spindle fibres towards opposite ends of the cell. In anaphase I of meiosis the pairs of homologous chromosomes separate from each other. See disjunction.... anaphase

n. a star-shaped object in a cell that surrounds the *centrosome during mitosis and meiosis and is concerned with the formation of the *spindle.... aster

n. (in genetics) a structure consisting of homologous chromosomes attached to each other by *chiasmata during the first division of *meiosis. —bivalent adj.... bivalent

In humans there are 23 pairs of CHROMOSOMES. Male and female di?er in respect of one pair. In the nucleus of female cells, the two members of the pair are identical and are called X chromosomes. In the male nucleus there is one X chromosome paired with a dissimilar, di?erently sized chromosome called the Y chromosome. In the sex cells, after MEIOSIS, all cells in the female contain a single X chromosome. In the male, half will contain an X chromosome and half a Y chromosome. If a sperm with an X chromosome fertilises an ovum (which, as stated, must have an X chromosome) the o?spring will be female; if a sperm with a Y chromosome fertilises the ovum the o?spring will be male. It is the sex chromosomes which determine the sex of an individual.

Sometimes during cell division chromosomes may be lost or duplicated, or abnormalities in the structure of individual chromosomes may occur. The surprising fact is the infrequency of such errors. About one in 200 live-born babies has an abnormality of development caused by a chromosome, and two-thirds of these involve the sex chromosomes. There is little doubt that the frequency of these abnormalities in the early embryo is much higher, but because of the serious nature of the defect, early spontaneous ABORTION occurs.

Chromosome studies on such early abortions show that half have chromosome abnormalities, with errors of autosomes being three times as common as sex chromosome anomalies. Two of the most common abnormalities in such fetuses are triploidy with 69 chromosomes and trisomy of chromosome 16. These two anomalies almost always cause spontaneous abortion. Abnormalities of chromosome structure may arise because of:

Deletion Where a segment of a chromosome is lost.

Inversion Where a segment of a chromosome becomes detached and re-attached the other way around. GENES will then appear in the wrong order and thus will not correspond with their opposite numbers on homologous chromosomes.

Duplication Where a segment of a chromosome is included twice over. One chromosome will have too little nuclear material and one too much. The individual inheriting too little may be non-viable and the one with too much may be abnormal.

Translocation Where chromosomes of different pairs exchange segments.

Errors in division of centromere Sometimes the centromere divides transversely instead of longitudinally. If the centromere is not central, one of the daughter chromosomes will arise from the two short arms of the parent chromosome and the other from the two long arms. These abnormal daughter chromosomes are called isochromosomes.

These changes have important bearings on heredity, as the e?ect of a gene depends not only upon its nature but also upon its position on the chromosome with reference to other genes. Genes do not act in isolation but against the background of other genes. Each gene normally has its own position on the chromosome, and this corresponds precisely with the positon of its allele on the homologous chromosome of the pair. Each member of a pair of chromosomes will normally carry precisely the same number of genes in exactly the same order. Characteristic clinical syndromes, due to abnormalities of chromosome structure, are less constant than those due to loss or gain of a complete chromosome. This is because the degree of deletion, inversion and duplication is inconstant. However, translocation between chromosomes 15 and 21 of the parent is associated with a familial form of mongolism (see DOWN’S (DOWN) SYNDROME) in the o?spring, and deletion of part of an X chromosome may result in TURNER’S SYNDROME.

Non-disjunction Whilst alterations in the structure of chromosomes arise as a result of deletion or translocation, alterations in the number of chromosomes usually arise as a result of non-disjunction occurring during maturation of the parental gametes (germ cells). The two chromosomes of each pair (homologous chromosomes) may fail to come together at the beginning of meiosis and continue to lie free. If one chromosome then passes to each pole of the spindle, normal gametes may result; but if both chromosomes pass to one pole and neither to the other, two kinds of abnormal gametes will be produced. One kind of gamete will contain both chromosomes of the pair, and the other gamete will contain neither. Whilst this results in serious disease when the autosomes are involved, the loss or gain of sex chromosomes seems to be well tolerated. The loss of an autosome is incompatible with life and the malformation produced by a gain of an autosome is proportional to the size of the extra chromosome carried.

Only a few instances of a gain of an autosome are known. An additional chromosome 21 (one of the smallest autosomes) results in mongolism, and trisomy of chromosome 13 and 18 is associated with severe mental, skeletal and congenital cardiac defects. Diseases resulting from a gain of a sex chromosome are not as severe. A normal ovum contains 22 autosomes and an X sex chromosome. A normal sperm contains 22 autosomes and either an X or a Y sex chromosome. Thus, as a result of nondisjunction of the X chromosome at the ?rst meiotic division during the formation of female gametes, the ovum may contain two X chromosomes or none at all, whilst in the male the sperm may contain both X and Y chromosomes (XY) or none at all. (See also CHROMOSOMES; GENES.)... sex chromosomes

(kinetochore) n. the part of a chromosome that joins the two *chromatids to each other and becomes attached to the spindle during *mitosis and *meiosis. When chromosome division takes place the centromeres split longitudinally.... centromere

n. (pl. chiasmata) (in genetics) the point at which homologous chromosomes remain in contact after they have started to separate in the first division of *meiosis. Chiasmata occur from the end of prophase to anaphase and represent the point at which mutual exchange of genetic material takes place (see crossing over).... chiasma

n. one of the two threadlike strands formed by longitudinal division of a chromosome during *mitosis and *meiosis. They remain attached at the *centromere. Chromatids can be seen between early prophase and metaphase in mitosis and between diplotene and the second metaphase of meiosis, after which they divide at the centromere to form daughter chromosomes.... chromatid

The presence of an extra chromosome within a person’s cells, making 3 of a particular chromosome instead of the usual 2. A fault during meiosis to form egg or sperm cells leaves an egg or sperm with an extra chromosome. When the egg or sperm takes part in fertilization, the resulting embryo inherits an extra chromosome in each of its cells.

The most common trisomy is of chromosome 21 (Down’s syndrome). Trisomy 18 (Edward’s syndrome) and trisomy 13 (Patau’s syndrome) are less common; trisomy 8 and trisomy 22 are very rare. Partial trisomy, with only part of a chromosome in triplicate, also occurs. Full trisomies cause abnormalities such as skeletal and heart defects and learning difficulties. Except in Down’s syndrome, babies usually die in early infancy. The effects of partial trisomies depend on the amount of extra chromosomal material present.

Diagnosis is made by chromosome analysis of cells, which may be obtained from the fetus by amniocentesis or after the birth. There is no specific treatment. Parents of an affected child should seek genetic counselling.... trisomy

n. the basic unit of all living organisms, which can reproduce itself exactly (see mitosis). Each cell is bounded by a cell membrane of lipids and protein, which controls the passage of substances into and out of the cell. Cells contain *cytoplasm, in which are suspended a *nucleus and other structures (*organelles) specialized to carry out particular activities in the cell (see illustration).

Complex organisms are built up of millions of cells that are specially adapted to carry out particular functions. The process of cell differentiation begins early on in the development of the embryo and cells of a particular type (e.g. blood cells, liver cells) always give rise to cells of the same type. Each cell has a particular number of *chromosomes in its nucleus. The sex cells (sperm and ova) always contain half the number of chromosomes of all the other cells of the body (see meiosis); at fertilization a sperm and ovum combine to form a cell with a complete set of chromosomes that will develop into the embryo.... cell

n. one of the threadlike structures in a cell nucleus that carry the genetic information in the form of *genes. It is composed of a long double filament of *DNA coiled into a helix together with associated proteins, with the genes arranged in a linear manner along its length. It stains deeply with basic dyes during cell division (see meiosis; mitosis). The nucleus of each human somatic cell contains 46 chromosomes, 23 of which are of maternal and 23 of paternal origin (see illustration). Each chromosome can duplicate an exact copy of itself between each cell division (see interphase) so that each new cell formed receives a full set of chromosomes. See also chromatid; centromere; sex chromosome. —chromosomal adj.... chromosome

(in genetics) the exchange of sections of chromatids that occurs between pairs of homologous chromosomes, which results in the recombination of genetic material. It occurs during *meiosis at a *chiasma.... crossing over

n. the final stage in the first prophase of *meiosis, in which homologous chromosomes, between which crossing over has occurred, are ready to separate.... diakinesis

n. the fourth stage in the first prophase of *meiosis, in which *crossing over occurs between the paired chromatids of homologous chromosomes, which then begin to separate.... diplotene

n. the separation of pairs of homologous chromosomes during meiosis or of the chromatids of a chromosome during *anaphase of mitosis or meiosis. Compare nondisjunction.... disjunction

n. the fusion of a spermatozoon and an ovum. Rapid changes in the membrane of the ovum prevent other spermatozoa from penetrating. Penetration stimulates the completion of meiosis and the formation of the second polar body. Once the male and female pronuclei have fused the zygote starts to divide by cleavage.... fertilization

n. the process by which spermatozoa and ova are formed. In both sexes the precursor cells undergo *meiosis, which halves the number of chromosomes. However, the timing of events and the size and number of gametes produced are very different in the male and female. See oogenesis; spermatogenesis.... gametogenesis

n. 1. the resting stage between the two divisions of *meiosis. 2. see interphase.... interkinesis

n. the first stage in the first prophase of *meiosis, in which the chromosomes become visible as single long threads.... leptotene

n. (in genetics) the situation in which two or more genes lie close to each other on a chromosome and are therefore very likely to be inherited together. The further two genes are apart the more likely they are to be separated by *crossing over during meiosis.... linkage

n. a cell that undergoes meiosis to form mature female sex cells (macrogametes) of the malarial parasite (see Plasmodium). Macrogametocytes are found in human blood but must be ingested by a mosquito before developing into macrogametes.... macrogametocyte

n. the second stage of *mitosis and of each division of *meiosis, in which the chromosomes line up at the centre of the *spindle, with their centromeres attached to the spindle fibres.... metaphase

rules of inheritance based on the breeding experiments of the Austrian monk Gregor Mendel (1822–84), which showed that the inheritance of characteristics is controlled by particles now known as *genes. In modern terms they are as follows. (1) Each body (somatic) cell of an individual carries two factors (genes) for every characteristic and each gamete carries only one. It is now known that the genes are arranged on chromosomes, which are present in pairs in somatic cells and separate during gamete formation by the process of *meiosis. (2) Each pair of factors segregates independently of all other pairs at meiosis, so that the gametes show all possible combinations of factors. This law applies only to genes on different chromosomes; those on the same chromosome are affected by *linkage. See also dominant; recessive.... mendel’s laws

n. a cell that undergoes meiosis to form 6–8 mature male sex cells (microgametes) of the malarial parasite (see Plasmodium). Microgametocytes are found in human blood but must be ingested by a mosquito before developing into microgametes.... microgametocyte

n. a condition in which pairs of homologous chromosomes fail to separate during meiosis or a chromosome fails to divide at *anaphase of mitosis or meiosis. It results in a cell with an abnormal number of chromosomes (see monosomy; trisomy).... nondisjunction

n. a cell in the ovary that undergoes *meiosis to form an ovum. Primary oocytes develop from *oogonia in the fetal ovary as they enter the early stages of meiosis. Only a fraction of the primary oocytes survive until puberty, and even fewer will be ovulated. At ovulation the first meiotic division is completed and a secondary oocyte and a polar body are formed. Fertilization stimulates the completion of the second meiotic division, which produces a second polar body and an ovum.... oocyte

(egg cell) n. the mature female sex cell (see gamete). The term is often applied to the secondary *oocyte although this is technically incorrect. The final stage of meiosis occurs only when the oocyte has been activated by fertilization.... ovum

n. the third stage of the first prophase of *meiosis, in which *crossing over begins.... pachytene

n. the first stage of *mitosis and of each division of *meiosis, in which the chromosomes become visible under the microscope. The first prophase of meiosis occurs in five stages (see leptotene; zygotene; pachytene; diplotene; diakinesis).... prophase

n. a cell produced as an intermediate stage in the formation of spermatozoa (see spermatogenesis). Spermatocytes develop from spermatogonia in the walls of the seminiferous tubules of the testis; they are known as either primary or secondary spermatocytes according to whether they are undergoing the first or second division of meiosis.... spermatocyte

n. the process by which mature spermatozoa are produced in the testis (see illustration). *Spermatogonia, in the outermost layer of the seminiferous tubules, multiply throughout reproductive life. Some of them divide by meiosis into *spermatocytes, which produce haploid *spermatids. These are transformed into mature spermatozoa by the process of spermiogenesis. The whole process takes 70–80 days.... spermatogenesis

n. a collection of fibres seen in a cell when it is dividing. The fibres radiate from the two ends (poles) and meet at the centre (the equator) giving a structure shaped like two cones placed base to base. It plays an important part in chromosome movement in *mitosis and *meiosis and is also involved in division of the cytoplasm.... spindle

n. the final stage of *mitosis and of each of the divisions of *meiosis, in which the chromosomes at each end of the cell become long and thin and the nuclear membrane reforms around them. The cytoplasm begins to divide.... telophase

n. (in genetics) 1. the four cells resulting from meiosis after the second telophase. 2. the four chromatids of a pair of homologous chromosomes (see bivalent) in the first stage of meiosis.... tetrad

n. the second stage of the first prophase of *meiosis, in which the homologous chromosomes form pairs (bivalents).... zygotene