Biology - ch. 7 section 7.2 From biology book

chiasmata

(singular = chiasma) the structure that forms at the crossover points after genetic material is exchanged

If a muscle cell of a typical organism has 32 chromosomes, how many chromosomes will be in a gamete of that same organism? A. 8 B. 16 C. 32 D. 64

A. 8

Meiosis produces ________ daughter cells. A. two haploid B. two diploid C. four haploid D. four diploid

C. four haploid

The part of meiosis that is similar to mitosis is ________. A. meiosis I B. anaphase I C. meiosis II D. interkinesis

C. meiosis II

At which stage of meiosis are sister chromatids separated from each other? A. prophase I B. prophase II C. anaphase I D. anaphase II

D. anaphase II

Explain how the random alignment of homologous chromosomes during metaphase I contributes to variation in gametes produced by meiosis.

Random alignment leads to new combinations of traits. The chromosomes that were originally inherited by the gamete-producing individual came equally from the egg and the sperm. In metaphase I, the duplicated copies of these maternal and paternal homologous chromosomes line up across the center of the cell to form a tetrad. The orientation of each tetrad is random. There is an equal chance that the maternally derived chromosomes will be facing either pole. The same is true of the paternally derived chromosomes. The alignment should occur differently in almost every meiosis. As the homologous chromosomes are pulled apart in anaphase I, any combination of maternal and paternal chromosomes will move toward each pole. The gametes formed from these two groups of chromosomes will have a mixture of traits from the individual's parents. Each gamete is unique.

section summary

Sexual reproduction requires that diploid organisms produce haploid cells that can fuse during fertilization to form diploid offspring. The process that results in haploid cells is called meiosis. Meiosis is a series of events that arrange and separate chromosomes into daughter cells. During the interphase of meiosis, each chromosome is duplicated. In meiosis, there are two rounds of nuclear division resulting in four nuclei and usually four haploid daughter cells, each with half the number of chromosomes as the parent cell. During meiosis, variation in the daughter nuclei is introduced because of crossover in prophase I and random alignment at metaphase I. The cells that are produced by meiosis are genetically unique. Meiosis and mitosis share similarities, but have distinct outcomes. Mitotic divisions are single nuclear divisions that produce daughter nuclei that are genetically identical and have the same number of chromosome sets as the original cell. Meiotic divisions are two nuclear divisions that produce four daughter nuclei that are genetically different and have one chromosome set rather than the two sets the parent cell had. The main differences between the processes occur in the first division of meiosis. The homologous chromosomes separate into different nuclei during meiosis I causing a reduction of ploidy level. The second division of meiosis is much more similar to a mitotic division.

In what ways is meiosis II similar to and different from mitosis of a diploid cell?

The two divisions are similar in that the chromosomes line up along the metaphase plate individually, meaning unpaired with other chromosomes (as in meiosis I). In addition, each chromosome consists of two sister chromatids that will be pulled apart. The two divisions are different because in meiosis II there are half the number of chromosomes that are present in a diploid cell of the same species undergoing mitosis. This is because meiosis I reduced the number of chromosomes to a haploid state.

reduction division

a nuclear division that produces daughter nuclei each having one-half as many chromosome sets as the parental nucleus; meiosis I is a reduction division

interkinesis

a period of rest that may occur between meiosis I and meiosis II; there is no replication of DNA during interkinesis

somatic cell

all the cells of a multicellular organism except the gamete-forming cells

recombinant

describing something composed of genetic material from two sources, such as a chromosome with both maternal and paternal segments of DNA

meiosis I

the first round of meiotic cell division; referred to as reduction division because the resulting cells are haploid

synapsis

the formation of a close association between homologous chromosomes during prophase I

meiosis II

the second round of meiotic cell division following meiosis I; sister chromatids are separated from each other, and the result is four unique haploid cells

fertilization

the union of two haploid cells typically from two individual organisms

tetrad

two duplicated homologous chromosomes (four chromatids) bound together by chiasmata during prophase I