Meiosis

= = =Chapter 18: Meiosis=

By: Brian Vu

[[image:http://panda.trailing-edge.com/panda.gif]]Related activities

 * Lab 3: Mitosis and Meiosis
 * AP Biology: Meiosis and Medelian Genetics Outline Ch 13 & 14

[[image:http://panda.trailing-edge.com/panda.gif]]Vocabulary

 * alteration of generations: third type of life cycle exhibited by plants and some species of algae.
 * asexual reproduction: a single individual is the sole parent and passes copies of all its genes on to its offspring.
 * autosomes: all chromosomes besides the X and Y chromosomes.
 * chiasmata: X-shaped regions that represent a crossing of nonsister chromatids, which are two chromatids belonging to separate but homologous chromosomes.
 * clones: a group of genetically identical individuals.
 * crossing over: individual chromosomes that combine genes inherited from their two parents.
 * diploid cells: the zygote and all other cells having two sets of chromosomes.
 * fertilization (syngamy): the union of gametes.
 * gametes: reproductive cells.
 * genes: coded information in the form of hereditary unit.
 * genetics: the scientific study of heredity and hereditary variation.
 * haploid cell: a cell with a single chromosome set.
 * heredity: the transmission of traits from one generation to the next.
 * homologous chromosomes (homologues): the chromosomes that make up a pair.
 * karyotype: a method of organizing the chromosomes of a cell in relation to number, size, and type
 * life cycle: the generation-to-generation sequence of stages in the reproductive history of an organism, from conception to production of its own offspring.
 * locus: a gene's specific location on a chromosome.
 * meiosis: production of gametes
 * sex chromosomes: the X and Y chromosomes.
 * sexual reproduction: two parents give rise to offspring that have unique combinations of genes inherited from the two parents.
 * somatic cell: any cell other than a sperm or ovum.
 * spores: meiosis in the sporophyte produces haploid cells
 * sporophyte: the multicellular diploid stage.
 * synapsis: process where the during prophase I of meiosis, the duplicated chromosomes pair with their homologues.
 * tetrad: the four closely associated chromatids of a homologous pair.
 * zygote: a fertilized egg

=Objective 1= Parents endow their offspring with coded information in the form of hereditary units called genes. Genes program the specific traits that emerge as we develop from fertilized eggs into adults. Genes are segments of DNA. Most genes program cells to synthesize specific enzymes and other proteins that produce an organism's inherited traits. The cellular vehicles that transmit the genes from one generation to the next are sperm and ova. The DNA of a eukaryotic cell is subdivided into chromsomes within the nucleus. Every living species has a characteristic number of chromsomes. A gene's specific location along the length of a chromsome is called the gene's locus. =Objective 2= Asexual reproduction is where a single individual is the sole parent and passes copies of all it's genes on to its offspring. Occasionally genetic differences are due to relatively rare changes in the DNA calle mutations. Asexual reproduction gives rise to clones, a group of genetically indentical individuals. Sexual reproduction usually results in greater variation: two parents give rise to offspring that have unique combinations of genes inherited from two parents. Their offspring vary genetically from their siblings and both parents. =Objective 3= In humans, each somatic cell has 46 chromsomes. Examining these chromosomes reveal that there are two of each type. They are arranged in pairs starting with longest chromosomes. The resulting display is called a karyotype. The paired chromosomes are called homologues. The two chromosomes of each pair carry genes controlling the same inherited characters. There is one exception to the rule of homologous chromosomes for human somatic cells: the X and Y chromosomes. Human females have a homologous pair of X chromosomes, while human males have one X chromosome and one Y chromosome. Gametes have a different chromosome count from somatic cells. Each gamete has 22 autosomes and 1 sex chromosome. A cell with a single chromosome set is call a haploid cell. By sexual intercourse, a haploid sperm cell fuses with a haploid ovum cell. The union of gametes is called fertilzaiton. The fertlized egg is now a zygote, which contains the two haploid sets of chromosomes. The zygote and all somatic cells having two sets of chromosomes are called diploid cells. The zygote's genes are passed on with precision to all somatic cells of the body thru mitosis. The only cells of the human body not produced by mitosis are the gametes, which develop in the gonads. Instead gametes go through a process where that halves the chromosome number called meiosis. Mitosis conserves chromosome number, while meiosis reduces the chromosome number by half. The timing of meiosis and fertilization in the life cycle vary from different species. Refer to Figure 13.4 on page 230 in the AP Biology textbook for an overview of the three sexual life cycles. =Objective 4= Meiosis resembles mitosis in that it is preceded by the replication of chromsomes. But in meiosis, this single replication is followed by two consecutive cell divisions called meiosis I and meiosis II, resulting in the formation of four daughter cells, each with half as many chromosomes as the parent cell. For a comparison between mitosis and meiosis, refer to figure 13.7 on page 234 in the AP Biology textbook. Three events unique to meiosis all occur during the first division in meiosis I: Since the chromosomes do not replicate between meiosis I and meiosis II, the final outcome of meiosis is a halving of the number of chromosomes per cell. =Objective 5= The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation. One way sexual reproduction produces genetic variation is the alternative arrangements of two homologous chromosome pairs on the metaphase plate in meiosis I as seen in figure 13.8 on page 235 in the AP Biology textbook. The orientations of the homologous pairs relative to the poles of the cell are random; there are two alternative possibilities for each pair. Another way creating genetic variation is through the process called crossing over. A diagram showing crossing over can be seen in figure 13.9 on page 236. Crossing over occurs during prophase of meiosis I, taking place when homologous portions of two nonsiste chromatids trade places. The random nature of fertilization adds to the genetic variation arising from meiosis. =Objective 6= Darwin recognized the importance of genetic variation in the eveolutionary mechanism he called natural selection. Those best individuals besy suited to the local environment leave the most offspring, transmitting their genes in the process. This natural selection results in adaptation, the accumulation of the genetic variations favored by the environment. Sex and mutations are the two sources of this variation. Useful links:
 * 1) During prophase I of meiosis, the duplicated chromosomes pair with their homologues, a process called synapsis. The four closely associated chromatids of a homologous pair are visible in the light microscope as a tetrad. An X-shaped region called the chiasmata represents a crossing of nonsister chromatids, which are two chromatids belonging to separate but homologous chromosomes
 * 2) At metaphase I of meiosis, homologous pairs of chromosomes, rather than individual chromosomes, align on the metaphase plate.
 * 3) At anaphase I of meiosis, sister chromatids do not separate, as they do in mitosis. Rather, the two sister chromatids of each chromosome remain attached and go to the same pole of the cell. Meiosis I separates the homologous pairs of chromosomes, not sister chromatids of individual chromosomes.
 * [|Interactive tutorial on meiosis]
 * http://www.youtube.com/v/rOpKwlnBYsI

Helpful pages in the AP Bio book:
 * pg. 230 Figure 13.4- Three sexual life cycles differing in the timing of meiosis and fertilization.
 * pg. 231 Figure 13.5- Overview of meiosis: how meiosis reduces chromosome number.
 * pg. 232-233 Figure 13.6- The stages of meitotic cell division.
 * pg. 234 Figure 13.7- A comparison of mitosis and meiosis.
 * pg. 235 Figure 13.8 -The results of alternative arrangements of two homologous chromosome pairs on the metaphase plate in meiosis I.
 * pg. 236 Figure 13.9- The results of crossing over durig meiosis.