Bio Ch 9 Glossary

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Glossary

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CHAPTER 9

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allele

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One of two or more molecular forms of a gene that arise by mutation and code for different versions of the same trait.

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asexual reproduction

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Any reproductive mode by which offspring arise from a single parent and inherit the genes of that parent only.

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chromosome number

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All chromosomes in a given type of cell. See haploidy; diploidy.

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crossing over

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At prophase I of meiosis, an interaction in which nonsister chromatids of a pair of homologous chromosomes break at corresponding sites and exchange segments; genetic recombination is the result.

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diploidy

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Presence of two of each type of chromosome (i.e., pairs of homologous chromosomes) in a cell nucleus at interphase. Compare haploidy.

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egg

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Mature female gamete; an ovum.

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fertilization

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[L. fertilis, to carry, to bear] The fusion of a sperm nucleus with the nucleus of an egg, thus forming a zygote.

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gamete

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[Gk. gametes, husband, and gamete, wife] Haploid cell, formed by meiotic cell division of a germ cell; required for sexual reproduction. Eggs and sperm are examples.

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gene

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[German pangan, after Gk. pan, all; genes, to be born] Unit of information for a heritable trait, passed from parents to offspring.

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germ cell

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Animal cell of a lineage set aside for sexual reproduction; gives rise to gametes.

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haploidy

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Presence of only half of the parental number of chromosomes in a spore or gamete, as brought about by meiosis.

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homologous chromosome

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One of a pair of chromosomes that are identical in size, shape, and the genes they carry, and that line up with each other at meiosis I. The X and Y chromosomes differ in these respects but still function as homologues.

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life cycle

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A recurring pattern of genetically programmed events from the time individuals are produced until they themselves reproduce.

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meiosis

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[Gk. meioun, to diminish] Two-stage nuclear division process that halves the chromosome number of a parental germ cell nucleus, to the haploid number. Basis of gamete formation (and meiospore formation).

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oocyte

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Type of immature egg.

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oogenesis

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Process by which a germ cell develops into a mature oocyte.

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prophase I

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First stage of meoisis I. Each duplicated chromosome starts to condense. It pairs with its homologue; nonsister chromatids usually undergo crossing over and are attached to spindle. One pair of duplicated centrioles (if present) is moved to the opposite spindle pole.

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prophase II

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First stage of meiosis II. Spindle microtubules attach to each chromosome and move them to spindle's equator. A centriole pair (if present) is already at each spindle pole.

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sexual reproduction

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Production of offspring by meiosis, gamete formation, and fertilization.

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sister chromatid

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Of a duplicated chromosome, one of two DNA molecules (and associated proteins) attached at the centromere until they are separated from each other at mitosis or meiosis; each is then a separate chromosome.

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sperm

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[Gk. sperma, seed] Mature male gamete.

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spermatogenesis

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Formation of mature sperm from a germ cell.

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spore

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A reproductive or resting structure of one or a few cells, often walled or coated; used for resisting harsh conditions, dispersal, or both. May be nonsexual or sexual, as formed by way of meiosis. Some bacteria as well as sporozoans, fungi, and plants form spores.