Chapter 13 Meiosis and Sexual Reproduction

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1 Biology 110 Sec. 11 J. Greg Doheny Chapter 13 Meiosis and Sexual Reproduction Quiz Questions: 1. What word do you use to describe a chromosome or gene allele that we inherit from our Mother? From our Father? 2. What is the name for the process that occurs during Prophase I of Meiosis, where homologous chromosomes swap parts to create recombinant chromosomes? 3. What is synapsis? What is its function, and when does it occur? 4. What is a recombinant chromosome, and how does it arise? 5. What is a recombination map unit and what does it represent? 6. What is the 1n number for humans? 7. What is the 2n number for humans? 8. How many chromosomes is one copy of the human genome distributed over? 9. For any diploid organism, how many chromatids does each chromosome have just after MITOTIC cell division? 10. For any diploid organism, how many chromatids does each chromosome have just BEFORE Mitotic cell division? 11. For any diploid organism, how many chromatids does each chromosome have during MITOTIC Prophase? 12. For any diploid organism, how many chromatids does each chromosome have during MITOTIC Telophase? 13. For any diploid organism, how many chromatids does each chromosome have during Prophase I or MEIOSIS? 14. For any diploid organism, how many chromatids does each chromosome have during Prophase II of MEIOSIS? 15. For any diploid organism, how many chromatids does each chromosome have during TELOPHASE I of MEIOSIS? 16. For any diploid organism, how many chromatids does each chromosome have during TELOPHASE II or MEIOSIS? NOTES The Advantage of Sexual Reproduction As mentioned in previous lectures, evolution was very slow during the first two billion years of life on earth, because the early life forms reproduced by simply splitting in two (binary fission) and making two identical copies of themselves. Evolution cannot work without variation, and selection for certain variants (birds with bigger or smaller beaks, pepper moths with lighter or darker wings etc.). Variation occurs when different variations of genes (different alleles) are created through random mutations. However, when organisms just make copies of themselves, the spread of those different alleles will be very slow. It would be much faster if all

2 the various gene alleles present in a population could be passed around and arranged in different combinations to create phenotypic variation. This is the purpose of sexual reproduction. Sexual reproduction allows whatever mutant alleles are present in a population to be passed around, and put together into various combinations much more quickly, thus allowing quicker responses to natural selection, and faster evolution. Two Types of Cell Division: Mitosis: Mitosis is how somatic cells divide. Simple cell division designed to create two daughter cells identical to the parent cell. One diploid somatic cell divides to form two diploid daughter cells. In human cells (for example), we have 23 pairs of chromosomes. One copy of each chromosome came from our mother (called the maternal set), and the other copy came from our father (called the paternal set). Just before cell division, each chromosome replicates from a single rod shape into an X shape consisting of two identical sister chromatids hooked together at their centromeres. At metaphase, all 46 X-shaped chromosomes line up at the metaphase plate (center) of the cell, and then each chromosome splits in half, with one of the two sister chromatids going to each side of the cell. The cell then divides (cytokinesis), and each of the daughter cells has one of each of the 23 maternal chromosomes and one of each of the 23 paternal chromosomes. Meiosis: Meiosis is how germ line cells divide to create haploid gametes. Germ line cells are the diploid cells inside the ovaries and testes that give rise to the haploid gametes (eggs and sperm respectively). One diploid germ line cell divides twice, to form four haploid cells. The two rounds of cell division are called Meiosis I and Meiosis II, and each consists of a Prophase, Metaphase, Anaphase and Telophase (Prophase I vs. Prophase II etc.). Meiosis also includes a mechanism (called synapsis, or crossing over) that facilitates re-arrangement of gene alleles on homologous chromosomes, thus increasing genetic diversity. Prior to the first cell division (Meiosis I), each of the 46 chromosomes (23 pairs) is replicated, forming an X shape consisting of two identical sister chromatids. (At this point, there are actually FOUR copies of each of the 23 chromosomes.) During Prophase I, all four of the homologous chromatids are joined together to form something called a synaptonemal complex, where structures called chiasmata (singular, chiasma) can form. This state is called synapsis. While the four homologous chromatids of each of the 23 chromosomes are together, the DNA strands will sometimes break at various points, and be re-joined to other chromatids, and vice versa (see Figures 13.8 and 13.9). Something called a crossover. Through such crossovers, it is possible for bits of one homologous chromosome to be swapped for bits from another homologous chromosome, thus increasing genetic diversity by rearranging the different gene alleles that are present. Each of the four homologous chromatids then moves to the metaphase plate, and the four chromatids break into two X shapes, which then move to opposite polls of the cell, and the cell divides. The two daughter cells then proceed to Meiosis II, and divide a second time (without DNA replication in between) to form a total of four haploid gametes.

3 The Eight Stages of Meiosis: 1. Prophase I: Four copies of each chromosome are present! Two pairs of X shapes, each of which is composed of two identical sister chromatids. The two X shaped homologous chromosomes bind together to form a synaptonemal complex, where crossovers may (or may not) occur to form recombinant chromatids. 2. Metaphase I: The synaptonemal complexes dissolve, but the two X-shaped homologous chromosomes line up, SIDE BY SIDE at the metaphase plate. The centrosomes have now moved to opposite sides of the cell, and sent out microtubules to bind to the kinetochores of each chromosome. 3. Anaphase I: The microtubules from the meiotic spindle pull the two homologous X-shaped chromosomes to opposite sides of the cell. 4. Telophase I and Cytokinesis: The cell divides into two daughter cells, each of which has ONE SET of the 23 chromosomes. But note that each of these chromosomes is still composed of TWO CHROMATIDS, which will separate during Meiosis II. At this point, each daughter cell is technically diploid. If crossing over occurred in Prophase I, some of the individual chromatids may be recombinant chromatids. The chromosomes unravel, the nuclear envelope re-forms, and the cell divides; but the DNA does not replicate first. 5. Prophase II: WITHOUT DNA REPLICATION each daughter cell proceeds to divide again. The chromosomes compress to form two X-shaped chromosomes, each composed of two chromatids. 6. Metaphase II: The X-shaped chromosomes line up at the metaphase plate, and are grabbed (at the kinetochore) by meiotic spindles on opposite sides of the cell. 7. Anaphase II: The proteins holding the centromeres of the two chromatids that make up each of the X-shaped chromosomes break, and each chromatid is pulled to an opposite side of the cell. 9. Telophase II and Cytokinesis: The chromatids (now classified as individual chromosomes) unwind and the nuclear envelope re-forms, and the cell divides into two haploid gametes. Thus, one diploid cell has gone through two rounds of cell division to create four haploid daughter cells. Independent Assortment of Genes vs. Gene Linkage and Crossovers. The fact that synapsis occurs during meiosis allows for greater genetic diversity to be created. Suppose you had two different genes (Genes A and B) on the same chromosome, and each gene had two alleles in the general population (Aa and Bb). When you have alleles A and B in the same organism, that may create one phenotype, but having alleles A and b in the same organisms may create a slightly different phenotype (a bird with a bigger beak, for example). But if A and B are always inherited together, because they re on the same chromosome, and a and b are always inherited together for the same reason, you may never get the opportunity to

4 have the A and b or a and B phenotypes. Synapsis gives you the opportunity to do this! Thus, if one bird is homozygous for AB; AB, and it mates with a bird that is homozygous for ab; ab, the offspring will be heterozygous at both loci (AB; ab). If not for synapsis, this offspring bird could only produce gametes AB or ab, and we d have no opportunity to pass on the alleles in the combinations Ab or ab. Synapsis allows for this to happen! If there is a crossover between the two chromosomes in Prophase I or Meiosis, you will create gametes with allele combinations Ab and ab. Thus, a bird with this genotype may have inherited only chromosomes with the AB and ab allele combinations, but it can create chromosomes with all four combinations of AB; ab; Ab; and ab. Combinations AB and ab are said to be the parental combinations, because they are like the two parent chromosomes. Combinations Ab and ab are said to be recombinant combinations because they are like neither of the parent chromosomes. When two genes are on the same chromosome (like the example above), they are said to be linked. If they are not on the same chromosome, they are said to sort independently. Therefore, suppose genes A and C are on separate chromosomes, and you again create an offspring bird that is genotypically Aa and Cc. If genes A and C assort independently because they are on separate chromosomes, this bird will produce all four of the possible gamete combinations AC, ac, Ac, and ac in equal frequency. Using linkage mapping to tell which genes are on the same chromosomes, and how far apart they are: Again taking the above example, genes A and B are linked, and therefore they usually sort together (they do not sort independently). However, because the offspring bird is heterozygous for the chromosome having AB and ab, there could be a crossover between the two, giving rise to two recombinant chromosomes that look like this: (Ab) and (ab). Recombinant gametes that are created as a result of crossing over are much less frequent than recombinant gametes that are created as a result of independent assortment, but they do happen! Recombination of non-linked genes as a result of independent assortment occurs at a frequency of exactly 50%. Recombination of linked genes occurs at a frequency of LESS than 50%. In fact, for most of the history of genetics, geneticists used the frequency of recombination between gene alleles to determine whether two genes were on the same chromosome or not. Genes with a recombination frequency of less than 50% were assumed to be on the same chromosome. Furthermore, the frequency of recombination (as long as it was less than 50%) was used as an indirect measurement of how far apart two genes on the same chromosome were. The frequency of recombination between two linked genes was assumed to be a function of how far apart they were, such that the odds of having a crossover between two genes that are close together is less than that of two genes that are far apart! Geneticists therefore drew up what were called recombination maps showing the distances between genes on the same chromosomes in recombination map units. One map unit was a recombination frequency of 1%.

5 Practice Questions: Short Answer and Essay Questions: 1. What is a recombinant chromosome, and how does it occur? (10 points) 2. What is a crossover and why does it occur? (5 points) 3. What is synapsis, when does it occur, and what is its function? (10 points) 4. What is a recombination map, what is a map unit, and what does it represent? (10 points) 5. Is a chromatid actually a chromosome? Why or why not? (10 points) 6. What is a trans-heterozygote, and how is it different from a cis-heterozygote? (5 points) (See question below for the explanation!) Essay Question: In 200 words or less, explain how sexual reproduction allows evolution to occur at a faster rate than asexual reproduction (such as binary fission)? (20 points) Mitosis, Meiosis and Chromosome Number: One copy of the genome of the genome of the fruit fly (Drosophila melanogaster) is distributed over four chromosomes numbered 1 through 4. The fruit fly is diploid. (Once you ve answered these questions for the fruit fly, repeat the same questions for HUMANS! They will probably be on the final exam.) 1. During synapsis in Meiotic Prophase I, is it possible (under normal circumstances) for a crossover to occur between the two chromosome 1 homologs? 2. During synapsis, is it possible (under normal circumstances) for a crossover to occur between chromosome 1 and chromosome 3? 3. What is the 1n number for fruit flies? 4. What is the 2n number for fruit flies? 5. How many chromosomes are in a fruit fly sperm cell? 6. How many chromosomes are in any fruit fly GAMETE? 7. How many chromatids does each chromosome have in a fruit fly sperm cell? 8. How many chromosomes are in an UNFERTILIZED fruit fly egg? 9. How many chromosomes are in a fertilized fruit fly egg (a fruit fly zygote)? 10. How many chromosomes in total are in a 2n fruit fly somatic cell? 11. How many chromosomes in total are in a 1n fruit fly gamete? 12. How many chromatids does each chromosome have in a somatic cell just after cell division? What is the n number for a somati 13. How many pairs of homologous chromosomes are in a 2n fruit fly somatic cell? 14. How many pairs of homologous chromosomes are in a 1n fruit fly sperm cell? 15. How many chromatids does each chromosome consist of just after MITOTIC cell division? 16. How many chromatids does each chromosome consist of just BEFORE MITOTIC cell division? 17. Is a chromatid actually a chromosome? Why or why not?

6 LINKAGE and CROSSOVERS: Consider the following example: Two genes (genes A and B) are linked on chromosome 2 of the fruit fly. Each has a dominant and a recessive allele (Aa and Bb). 1. Is a fly that carries chromosomes AB and AB a heterozygote or a homozygote? 2. Is a fly that carries ab and ab a heterozygote or a homozygote? 3. Is a fly that carries Ab and Ab a heterozygote or a homozygote? 4. Is a fly that carries ab and AB a heterozygote or a homozygote? 5. Consider the example of a fly that is heterozygous for both genes, and carries chromosomes (Ab) and (ab). This is called a TRANS-HETEROZYGOTE. Another fly is also heterozygous for both alleles, and carries chromosomes (AB) and (ab). This is called a CIS-HETEROZYGOTE. Can you suggest why? 6. Can a trans-heterozygotic fly (Ab; ab) have a crossover between the A and B genes? If so, what recombinant chromosomes will be created? 7. Can a cis-heterozygotic fly (AB; ab) have a crossover between the A and B genes? If so, what recombinant chromosomes will be created? 8. Can a fly that is homozygous at both loci (AA; BB) have a crossover between the A and B genes? 9. Is a (AA; bb) fly a heterozygote or a homozygote? 10. Is a (aa; BB) fly a heterozygote or a homozygote? 11. Is a (Aa; BB) fly a heterozygote or a homozygote? 12. Is a (aa; Bb fly a heterozygote or a homozygote? LINKAGE and RECOMBINATION MAPPING: Consider three genes in the mouse (Genes A, B and C), each of which has a dominant and a recessive allele (Aa, Bb, and Cc). It is not known whether these genes are linked or not. You are a geneticist who wishes to find out, so you do the following experiments. A homozygous dominant female (AA; BB; CC) is mated to a homozygous recessive male (aa; bb; cc). All of their offspring will be heterozygotes (Aa; Bb; Cc). You then cross these heterozygotes to a tester strain of mouse that is homozygous recessive (aa; bb; cc). You then get the following offspring showing the following phenotypes. Part I) Ignoring gene C, you look to see if genes A and B are linked. You get the following offspring: 40 mice are phenotypically AB 40 mice are phenotypically ab 10 mice are phenotypically Ab 10 mice are phenotypically ab a. Which are parental phenotypes? b. Which are recombinant phenotypes? c. Are genes A and B linked? If so, how far apart are they in recombination map units?

7 Part II) Ignoring gene B, you look to see if genes A and C are linked. You get the following offspring: 25 mice are phenotypically AC 25 mice are phenotypically ac 25 mice are phenotypically Ac 25 mice are phenotypically ac a. Which are the parental phenotypes? b. Which are the recombinant phenotypes? c. Are genes A and C linked? If so, how far apart are they in recombination map units? Part III) Based on these results, can you say anything about linkage between genes B and C? Match the description to the Meiotic Stage in HUMAN Mitosis: (One copy of the human genome is distributed over 23 chromosomes.) A. Prophase I: B. Metaphase I: C. Anaphase I: D. Telophase I and Cytokinesis: E. Prophase II: F. Metaphase II: G. Anaphase II: H. Telophase II and Cytokinesis. 1. The nuclear envelope reforms around 23 single chromosomes consisting of only one chromatid. 2. Synapsis occurs 3. The nuclear envelope reforms around 23 chromosomes, each consisting of two chromatids, some of which may be recombinant. 4. Chiasmata are formed pairs of chromosomes line up at the metaphase plate. Each chromosome consists of two chromatids (some of which may be recombinant), and each chromosome is lined up NEXT TO its homologous partner.