2. Mutation and sexual reproduction lead to genetic variation in a population. As a basis for understanding this concept:

Transcription

1 2. Mutation and sexual reproduction lead to genetic variation in a population. As a basis for understanding this concept: 2a. Students know meiosis is an early step in sexual reproduction in which the pairs of chromosomes separate and segregate randomly during cell division to produce gametes containing one chromosome of each type. Haploid gamete production through meiosis involves two cell divisions. During meiosis prophase I, the homologous chromosomes are paired, a process that abets the exchange of chromosome parts through breakage and reunion. The second meiotic division parallels the mechanics of mitosis except that this division is not preceded by a round of DNA replication; therefore, the cells end up with the haploid number of chromosomes. (The nucleus in a haploid cell contains one set of chromosomes.) Four haploid nuclei are produced from the two divisions that characterize meiosis, and each of the four resulting cells has different chromosomal constituents (components). In the male all four become sperm cells. In the female only one becomes an egg, while the other three remain small degenerate polar bodies and cannot be fertilized. 2. b. Students know only certain cells in a multicellular organism undergo meiosis. Only special diploid cells, called spermatogonia in the testis of the male and oogonia in the female ovary, undergo meiotic divisions to produce the haploid sperm and haploid eggs. 2. c. Students know how random chromosome segregation explains the probability that a particular allele will be in a gamete. The steps in meiosis involve random chromosome segregation, a process that accounts for the probability that a particular allele will be packaged in any given gamete. This process allows for genetic predictions based on laws of probability that pertain to genetic sortings. 2. d. Students know new combinations of alleles may be generated in a zygote through the fusion of male and female gametes (fertilization). Once gametes are formed, the second half of sexual reproduction can take place. In this process a diploid organism is reconstituted from two haploid parts. When a sperm is coupled with an egg, a fertilized egg (zygote) is produced that contains the combined genotypes of the parents to produce a new allelic composition for the progeny. 2. e. Students know why approximately half of an individual s DNA sequence comes from each parent. Chromosomes are composed of a single, very long molecule of double-stranded DNA and proteins. Genes are defined as segments of DNA that code for polypeptides (proteins). During fertilization half the DNA of the progeny comes from the gamete of one parent, and the other half comes from the gamete of the other parent. 2. f. Students know the role of chromosomes in determining an individual s sex. The normal human somatic cell contains 46 chromosomes, of which 44 are pairs of homologous chromosomes and 2 are sex chromosomes. Females usually carry two X chromosomes, and males possess one X and a smaller Y chromosome. Combinations of these two sex chromosomes determine the sex of the progeny. 2. g. Students know how to predict possible combinations of alleles in a zygote from the genetic makeup of the parents. When the genetic makeups of potential parents are known, the possible assortments of alleles in their gametes can be determined for each genetic locus. Two parental gametes will fuse during fertilization, and with all pairwise combinations of their gametes considered, the possible genetic makeups of progeny can then be predicted. Overview of Meiosis: The object of meiosis is to generate a random assortment of chromosome combinations in cells that have half the normal number of chromosomes. In a diploid organism that means the formation of haploid reproductive cells

2 (germ cells) that carry a random collection of chromosomes. The process is very carefully regulated so that no cell receives less than the full complement of chromosomes. Each germ cell has one copy of each chromosome and none has more than one of any of those chromosomes. The only way to produce cells with half their normal number of chromosomes is for the cells to divide twice without allowing DNA replication. There are certainly several ways the cell could accomplish this end. One necessary part of such a scheme is that chromosomes would have to "pair up" before cell division could occur. In mitosis each pair of chromosomes is physically connected at the centromere so that when the package that we call a chromosome moves to the metaphase plate, the two identical chromosomes are held together. What would happen if chromosomes did not pair up? Would aneuploidy be increased? If chromosomes didn't pair it wouldn't be possible to faithfully segregate one copy to each of the daughter cells. The chromosomes would have no way of knowing which pole of the cell their homologue was going to. About half the time they would go to the same pole. In meiosis pairing occurs between duplicated chromosomes (those involving sister chromatids. Pairing of the duplicated chromosomes occurs on a metaphase plate. The spindle then pulls the pairs apart. As a result, each cell gets a pair of sister chromatids, not one each of the paternal and maternal chromosomes. This is the difference between meiosis and mitosis. The first division creates a pair of cells which are different. Each contains either the paternal or the maternal chromosomes, but not both. However, the direction that each of these chromosomes goes (that is to which cell they segregate) is random. As a result, there is a shuffling of the genes because a random selection of chromosomes is made at each first meiotic metaphase. In an organism with two chromosomes there are only four possible outcomes for a given cell after this division. Notes: Gametes such as eggs and sperm are produced by meiosis in special reproductive tissue. In females it is within the ovaries in cells called oogonia. In males it is within the testes in cells called spermatogonia. Meiosis has the same stages as mitosis but the stages are repeated a second time without the DNA making a copy of itself. This process reduces the number of chromosomes in half (from 2n to n). The beginning chromosomes are called homologous chromosomes, which are a matching pair of chromosomes, one from each parent, with the same gene loci in the same order. The key results of meiosis. 1. Meiosis I results in 2 diploid cells. This is known as reduction division because the homologous chromosomes are separated. 2. Meiosis II results in 4 gametes. Meiosis II is just like mitosis which separates sister chromatids and ends in cytokinesis. The four resulting cells are haploid because they only have single copies of each chromosome. The illustration and table below outline the key results of meiosis.

3 2 homologous chromosomes 2 sister chromatids 1 chromosome 2n n - n n 1 cell 2 cells 4 cells diploid diploid haploid Mendel's Law of Segregation: During formation of egg or sperm (meiosis), the 2 genes (alleles) for a trait separate (each goes to separate eggs or sperm). Meiosis results in essentially an infinite genetic variety in the sex cells, partly by crossing over (where sister chromatids exchange chromosome pieces) or by randomly lining up along the equator. The number of possible chromosome orientations is 2 raised to the power of the number of chromosome pairs. With human cells there are 2 to the 23rd power possibilities, which is 8.4 million possible ways to mix the 23 chromosome pairs! Meiosis I (takes place after the 3 stages of Interphase) 1. Prophase I a. chromatin condenses into well defined chromosomes (already replicated during interphase) b. synapsis (intimate pairing of homologous chromosomes) happens forming a tetrad c. crossing over occurs (exchange of genetic material) d. nucleoli and nuclear envelope break up e. spindle fibers begin to form f. centrioles move to poles 2. Metaphase I a. tetrads line up on equatorial plane b. line up double file ---- different than mitosis c. tetrads attach to spindle fiber at centromere 3. Anaphase I a. homologous pairs pull apart (centromeres do NOT divide) b. a whole duplicated chromosome moves to each pole c. random assortment of chromosomes ( maternal may go to one pole for one homolog while the next chromosome the paternal may go to that pole) This is dependent on how the tetrads line up during prophase I. 4. Telophase I a. new nuclear membrane forms around each group sometimes

4 b. cytokinesis may occur now making each of the 2 cells haploid at this point (1 chromosome from each pair of chromosome is present (reduction in # of chromosomes) c. may or may not have interphase, but if there is there is no S stage of interphase B. Meiosis II 1. Prophase II a. if a new nuclear membrane formed during telophase I then it would disappear during prophase II 2. Metaphase II a. chromosomes line up single file much like metaphase of mitosis except now the cell is haploid instead of diploid 3. Anaphase II a. centromeres divide with the chromatids pulling toward the opposite poles (now each chromatid is a chromosome) b. spindle fibers shorten 4. Telophase II a. new nuclear membrane forms around each set of chromosomes b. nucleolus reappears c. spindle fibers disappear d. chromosomes decondense into chromatin C. Cytokinesis takes place leaving 4 daughter cells E. Spermatogenesis 1. Primary spermatocyte is diploid cell that begins meiosis 2. Meiosis I results in 2 haploid secondary spermatocytes 3. Meiosis 2 results in 4 haploid spermatids 4. After maturation result is 4 haploid sperm D. Oogenesis 1. Primary oocyte is diploid cell that begins meiosis 2. Meiosis I results in 1 haploid secondary oocyte and 1 haploid 1 st polar body. This is the result of unequal cytokinesis and the polar body usually disintegrates and doesn t continue through meiosis II. 3. Meiosis II results in 1 haploid ovum and 1 haploid 2 nd polar body. Again the polar body is formed from unequal cytokinesis and doesn t survive. 4. The ovum matures into a haploid egg.

5 DNA recombination and meiosis There is another level at which the collection of genes in the genome are randomized-dna recombination. During prophase I the chromosomes pair with each other so that the maternal and paternal chromosomes are in close contact. Enzymes in the cell recognize regions of identical sequence in the chromosomes. They catalyze the breakage and rejoining of DNA strands to exchange sequences between non-sister chromatids. This changes the linkage of genes on the chromosome. Remember that the two homologous chromosomes are not identical in every way. They carry sequence differences between the genome of the mother and the father. These differences are called alleles. The father's chromosome will have a set of alleles on each chromosome many of which will be different from those on the mother's chromosome. Recombination puts some of the mother's alleles and some of the father's alleles on the same DNA molecule

6 So, since in Anaphase I the paternal and maternal chromosomes separate to the two poles of the cell, what is the effect of recombination on segregation of maternal and paternal alleles? How does this influence the amount of variability in the products of meiosis? The take-home-lesson for meiosis is that random segregation of chromosomes at Anaphase I and the shuffling of sequences by recombination during Prophase I provides an immense variability in the products of meiosis. The segregation provides over 8 million possible ways chromosomes can segregate into the gametes. Recombination greatly increases this number since each chromosome probably undergoes at least one recombination per chromosome arm per meiotic prophase. In combination with the great diversity of alleles present in a population, this random assortment of alleles provides for an immense number of possible gametes. Fusion of two such gametes creates a new individual who is almost certainly unique genetically