Biology Lessons Part 2: Population Biology


 Lesson 2.5: How Do Organisms Reproduce?


Meiosis (Production of Haploid Sex Cells)

alleles - alternative forms of genes having a distinct genotype and often, a distinct phenotype.

centromere - the point of the chromosome where the two sister chromatids are joined; this is also where the spindle attaches during metaphase.

chromatid - a daughter strand of a replicated chromosome. Replication of a chromosome produces two chromatids joined together at the centromere.

chromosome - a self-duplicating body in the cell nucleus made up of DNA and proteins and containing genetic information in the form of genes. Chromosomes are transmitted from one generation to the next via gametes. In humans, there are 23 pairs of chromosomes (46 chromosomes in all).

crossing-over - the reciprocal exchange of DNA between homologous chromosomes during meiosis.

cytokinesis - the process of division of the cytoplasm accompanying division of the nucleus.

cytoplasm- gel-like substance in which all cellular components outside of the nucleus are immersed.

diploid cells - a cell that contains two copies of each type of chromosome in its nucleus.

DNA replication - the process of DNA copying that accompanies cell division. The DNA double-helix is unwound and two complimentary strands are produced.

fertilization - the fusion of two haploid gamete nuclei and cells (egg and sperm) which forms a diploid zygote.

flagellum - long whip-like tail protruding from the surface of a cell that propels the cell, acting as a locomotive device.

gamete - a specialized sex cell such as an egg or a sperm, which is haploid. A male gamete and a female gamete fuse and produce a diploid zygote which develops into a new individual.

gene - the biological unit of inheritance that transmits hereditary information from parent to offspring and controls the appearance of a physical, behavioral or biochemical trait.

genome - the total compliment of genes in a single cell.

genotype - the genetic makeup of an individual.

haploid cells - cells that contain only one copy of a chromosome set. A human haploid cell has 23 chromosomes.

homologous chromosomes - chromosomes that pair up and separate during meiosis and generally have the same size, shape, and genetic information (the same genes but not necessarily the same alleles). One member of each pair of homologous chromosomes comes from the mother and the other comes from the father.

meiosis - the type of cell division that occurs in sex cells during gamete formation; the diploid parent cell undergoes two rounds of division, generating a total of four daughter cells, each of which is haploid and genetically unique.

mitosis - cell division that takes place in somatic cells; the diploid parent cell undergoes one round of division to generate two diploid daughter cells, each of which is genetically identical to the parent cell.

ovum - an egg cell (female gamete). The largest of the four female gamete cells produced by a single meiotic division, it receives the majority of the cytoplasm and organelles from the parent cell.

polar bodies - three of the four daughter cells produced during gamete formation in females; they are deficient in cytoplasm and organelles compared to the larger ovum.

sex cells- a sperm cell or ovum (egg cell); the haploid gametes which will fuse to form a new individual.

sister chromatid - one half of a replicated chromosome; one of the two daughter DNA strands produced by semiconservative replication.

somatic cell - a cell in the body of an animal that is not a sex cell.

tetrad - the structure that results when two replicated homologous chromosomes pair during prophase I of meiosis; tetrad refers to the fact that the structure contains 4 chromatids.