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Home >> Genetics Dictionary >> Map Distance - Meiosis
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Map distance
The distance between genes expressed as map units or centiMorgans (cM).
Margulis, Lynn
A biologist who developed the serial endosymbiosis theory of origin of the eukaryotic cell. Although now accepted as a plausible theory, both she and her theory were ridiculed by mainstream biologists for a number of years.
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Marsupial mammal
A group (specifically, an order) of mammals whose females give birth to young at a very early stage of development. These newborns complete their development while sucking in a pouch, which is a permanent feature on the female. Examples include kangaroos and opossums.
Marker
genetic marker.
Mass spectrometry
An instrument used to identify chemicals in a substance by their mass and charge.
Mastodon
An extinct elephant-like mammal.
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Maternal serum screening
A blood test taken at 14-20 weeks gestation to determine whether there is an increased risk for a fetus having Down syndrome, trisomy 18, or spina bifida. The test is also known as the alpha-fetoprotein or AFP test, triple screen, or quad screen, depending on which combination of substances are measures on the blood sample.
Mayr, Ernst
Mayr's work has contributed to the synthesis of Mendelian genetics and Darwinian evolution, and to the development of the biological species concept. Mayr has been universally recognized and acknowledged as one of the leading evolutionary biologists of the 20th century
McGinnis, William
Professor of Biology, University of California at San Diego. Discover (with Mike Levine) of homeoboxes, the sequences of DNA that are characteristic of homeotic genes, which playa central role in specifying body development.
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His current research uses both genetics and biochemistry to examine such questions as how molecular variations in the Hox genes that specify the head-tail pattern of an organism can generate variety in animal shapes during evolution, and what the molecular changes were that allowed single celled animals to become multicellular.
Megabase (Mb)
Unit of length for DNA fragments equal to 1 million nucleotides and roughly equal to 1 cM. See also: centimorgan
Meiosis
The process of two consecutive cell divisions in the diploid progenitors of sex cells. Meiosis results in four rather than two daughter cells, each with a haploid set of chromosomes.
2. Two successive nuclear divisions (with corresponding cell divisions) that produce haploid gametes.
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3. A cell division process (in gametogenesis or sporogenesis) in which one replication of the chromosomes is followed by two nuclear divisions to produce four haploid cells.
4. A special kind of cell division that occurs during the reproduction of diploid organisms to produce the gametes. The double set-of genes and chromosomes of the normal diploid cells is reduced during meiosis to a single haploid set in the gametes. Crossing-over and therefore recombination occur during a phase of meiosis.
5. The nuclear process in diploid eukaryotes that results in gametes or spores with only one member of each original homologous pair of chromosomes per nucleus.
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6. The set of two successive cell divisions that serve to separate homologous chromosome pairs and thus reduce the total number of chromosomes by half. The meiotic process includes two sequential nuclear divisions that must occur prior to the formation of gametes (sperm and eggs). The major purpose of meiosis is the precise reduction in the number of chromosomes by one-half, so that a diploid cell can create haploid gametes. To accomplish this reduction, a single cell undergoes two meiotic divisions to produce four daughter cells, each with half the original chromosome complement. The nonmei6tic (or somatic) cells of humans, for instance, have 46 individual chromosomes, or 23 pairs of homologous chromosomes.
Simplified view of the two meiotic divisions
Simplified view of the two meiotic divisions
However, following meiosis, human eggs or sperm have only 23 chromosomes, one member of each pair. Reducing the number of chromosomes in the gametes to 23 allows the fusion of an egg with a sperm (fertilization), to result in an embryo with the requisite 46 chromosomes. Meiosis is therefore a critical component of sexual reproduction.
For example, consider an organism that contains only two pairs of chromosomes (Fig. 1). The chromosomes in each of these pairs are referred to individually as homologs; one is derived from the father of the organism and the other from the mother. Both homologs carry the same array of genes. As the cell begins meiosis, each chromosome has already duplicated its deoxyribonucleic acid (DNA) and carries two identical copies of the DNA molecule. These are visible as two lateral parts, called sister chromatids, which are connected by a centromere.
The basic events of meiosis are actually quite simple. Homologous pairs of chromosomes are first identified and matched. This process, which occurs only in the first of the two meiotic divisions, is called pairing. The matched pairs are then physically interlocked by recombination, which is also known as exchange or crossing-over. After recombination, the homologous chromosomes separate from each other, and at the first meiotic division are partitioned into different nuclei. As a consequence, the second meiotic division begins with half of the original number of chromosomes. During this second meiotic division, the sister chromatids of each chromosome separate and migrate to different daughter cells.
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