`Important words and concepts from Chapter 13, Campbell & Reece, 2002 (1/29/2005):
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(1) Chapter title: Meiosis and Sexual Life Cycles
QUICKIE INTRO TO GENETICS
(2) Genetics (see also genetics)
(3) Genes (see also gene)
(a) Genes are sections (linear segments) of DNA
(b) Typically genes specify the construction of specific proteins
(c) Together these proteins control an organism's inherited traits
(4) Heredity (see also heredity)
(b) Copies of these sequences are passed from parents to offspring in the process of heredity
(5) Variation (see also genetic variation)
(b) These sequence variations are also passed on to offspring
(c) The existence of these alternative nucleotide sequences forms the root of quite a bit of the phenotypic variation one observes among individuals of the same species
GENETICS OF CHROMOSOMES
(b) Eukaryote chromosomes have certain properties and structures including
(ii) telomeres at their ends
(iii) they are linear (rather than closed circular like bacterial chromosomes)
(iv) they are propelled to daughter cells during anaphase, etc.
(c) Note that your text is using the more global definition of chromosome at this point, i.e., more than just something that is visible during mitosis
(7) Locus (plural, loci) (see also locus)
(8) Karyotype (see also karyotype)
(a) A karyotype is an arrangement of chromosomes, in order from largest to smallest, on a photographic plate
(b) Karyotypes separate chromosomes into different types based on
(ii) Staining (i.e., banding) patterns
(iii) location of the centromere (e.g., near one end versus near middle)
(a) A cell that contains two or more of a given type of chromosome is said to contain two or more homologous chromosomes
(c) Alls cells from normal humans contain at least 22 pairs of homologous chromosomes (cells from human females contain 23)
(10) Haploidy (see also haploid)
(a) A cell that contains only one of each type of chromosome that is typical for its species is said to be haploid
(b) For humans, a haploid cell possesses 23 chromosomes
(c) That is, a total of 23 distinguishable types of chromosomes
(d) Another way of saying this is that n = 23
(11) Diploidy (see also diploid)
(a) Most human cells are diploid
(c) That is, a total of 46 chromosomes
(d) 2n = 46
(b) Such chromosomes are termed autosomes
(c) Cells from normal humans contain 44 autosomes (male or female)
(13) Sex chromosomes (see also sex chromosome)
(a) Exceptions to the two-of-each-type-of-chromosome rule are the sex chromosomes
(b) In particular, in humans and other mammals the sex chromosomes are found as a non-homologous pair in males
(c) That is, a haploid of the chromosome termed X and a haploid of the chromosome termed Y are found in males
(d) Females possess a homologous pair of X chromosomes (and hence human females possess 23 pairs of homologous chromosomes)
ORGANISM LIFE CYCLES, ESPECIALLY SEXUAL
(a) Asexual reproduction involves
(i) No change in ploidy going from parent to offspring
(ii) Little genetic variation between parent and offspring
(iii) Only mitotic division between parent and offspring (among eukaryotes at least)
(15) Sexual reproduction (see also sexual reproduction)
(a) Sexual reproduction involves
(i) Changes in ploidy over the course of going from parent to offspring
(ii) Significant genetic variation between parent and offspring
(b) [sexual reproduction, "sex" alone had nearly 35 million google hits 2/2/2001, over 87 million hits as of 2/13/02, and about 262 million hits on 2/9/04--that's an approximate doubling per year! (Google Search)] [index]
(16) Life cycle (see also life cycle)
(a) A life cycle is a description of an organism as it passes from conception to production of progeny (offspring)
(c) See Figure, The human life cycle
(17) Soma (somatic cells) (see also soma)
(a) The soma, or somatic cells of a human are mostly diploid (or possess a higher ploidy)
(b) These are the cells that constitute the majority of your body
(c) The evolutionary goal of your soma is the make gametes and then to assure their fertilization and subsequent development, thereby contributing genetically to the next generation
(18) Germ line (see also germ line)
(b) These cells are called your germ-line cells
(c) Note that the majority of your cells are not capable of making gametes, only cells from your germ line are
(19) Gametes (see also gamete)
(a) Gametes are haploid cells
(b) They are called sperm and ova (coming from the male and the female, respectively)
(c) Gametes do not divide
(d) In humans, gametes are generated by meiosis
(20) Meiosis (see also meiosis)
(a) All sexual life cycles are characterized by an occurrence of meiosis
(c) In humans, only germ line cells undergo meiosis
(d) Anatomically, this occurs in the sperm- or ova-generating testes or ovaries
(e) See Figure, Overview of meiosis: how meiosis reduces chromosome number
(21) Fertilization (syngamy) (see also syngamy)
(a) Fertilization (a.k.a., syngamy) is also involved in all sexual life cycles
(c) That is, the haploid to diploid conversion during a life cycle in most organisms is a result of fertilization
(a) The human life cycle is only one of a variety of sexual cycles
(c) In humans (and most though not all other animals) only the diploid stage is capable of undergoing mitosis; the haploid cells do not divide
(d) See Figurea, Three sexual life cycles in the timing of meiosis and fertilization (syngamy)
(b) Instead they undergo mitosis only during their haploid stage
(c) See Figureb, Three sexual life cycles in the timing of meiosis and fertilization (syngamy)
(d) Note that this does not mean that gametes are replicating
(e) Instead it means that gametes (i.e., cells capable of undergoing fertilization) are generated by mitosis rather than by meiosis (that is, if the immediate cellular parent of a gamete is itself haploid then the gamete must have been the product of mitosis)
(f) Depending on the organism, the haploid stage may or may not remain unicellular, i.e., there exist multicellular haploid organisms
(b) This is termed alternation of generations since both the haploid and the diploid organisms are considered separate generations
(c) See Figurec, Three sexual life cycles in the timing of meiosis and fertilization (syngamy)
(25) Sporophyte (see also sporophyte)
(a) The multicellular diploid organism given alternation of generations is termed the sporophyte
(26) Gametophyte (see also gametophyte)
(a) The multicellular haploid organism given alternation of generations is termed the gametophyte
(27) Meiosis in detail (see also meiosis)
(a) See Figure, The stages of meiotic cell division
(b) See Figure, A comparison of mitosis and meiosis
(d) Exceptions include:
(f) Note that the diploid to haploid transition in sexual life cycles occurs via meiosis
(a) Interphase I is essentially similar to that which occurs prior to mitosis
(30) Prophase I (see also prophase I)
(b) Chiasmata form between the homologous chromosomes
(c) Otherwise Prophase I resembles prophase and prometaphase of mitosis
(31) Synapsis (see also synapsis)
(32) Tetrad (see also tetrad)
(a) Upon synapsis two paired-up chromosomes form a tetrad
(33) Chiasmata (singular, chiasma)
(b) See Figure, The results of crossing over during meiosis
(34) Metaphase I (see also metaphase I)
(37) Cytokinesis (I) (see also cytokinesis)
(a) Two cells are present following a round of cytokinesis
(a) Interphase II may or may not be present
(b) Note that synthesis of DNA does not occur during interphase II
(b) Phases include: prophase II, metaphase II, anaphase II, telophase II
(c) Don't worry about the lack of a prometaphase II; that is contained within prophase II (just as mitosis used to be taught)
(40) Cytokinesis (2) (see also cytokinesis)
GENETIC CONSEQUENCES OF MEIOSIS AND SEXUAL CYCLES
(41) Genetic variation (see also genetic variation)
(ii) Crossing over
(iii) Random fertilization
(b) It is important to keep in mind, however, that the ultimate root of all variation is mutation (which will be considered in a subsequent chapter)
(b) One set came from your father
(c) The other set came from your mother
(d) The products of meiosis contain, with high probability, some mixture of the chromosomes from your father and your mother, rather than a complete set from one and an absence of the other
(f) See Figure, The results of alternative arrangements of two homologous chromosome pairs on the metaphase plate in meiosis I
(i) Thus, for any given pair of chromosomes, one pair of sister chromatids, coming from your mother, will go to one daughter cell and the other pair of sister chromatids, coming from your father, will go to the other daughter cell
(j) Multiplied over all of the types of chromosomes in your body, the end result is a random mixing of the chromosomes coming from each of your parents during anaphase I
(k) Considering 23 pairs of chromosomes and no recombination, the likelihood that any one product of meiosis I will contain chromosomes only coming from your father is [(½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½ * ½) = (½)23 = 0.00000012] and the same value (probability) for those coming only from your mother.
(b) This is because, in addition to the separation from each other of chromosomes coming from each parent, meiosis I additionally mixes together individual chromosomes
(c) In this way, each chromosome is no longer descended only from one of each parent but instead is descended from a combination of both parents
(d) This mixing up occurs randomly
(e) Consequently, different cells independently assort a randomly-varying combination of chromosomes, thus greatly increasing the possible genetic variation seen in offspring
(f) See Figure, The results of crossing over during meiosis
(g) Note that all of this crossing over occurs during prophase I
(i) Within tetrads, chromosomes form chiasmata
(j) Chiasmata are a physical manifestation of the process of crossing over
(44) Random fertilization (see also crossing over)
(b) Fertilization then occurs as a random coming together of genetically unique gametes
(c) Note that some fertilizations are more unique than others: A process that can lead to a reduction in the random variation present between randomly fertilizing gametes is inbreeding (but we will consider that at a later date)
(45) (If there is time, I will introduce probability theory which otherwise is found in chapter 14)
(46) Vocabulary [index]
(b) Anaphase I
(h) Crossing over
(i) Cytokinesis (1)
(j) Cytokinesis (2)
(r) Germ line
(x) Interphase I
(y) Interphase II
(aa) Life cycle
(ee) Meiosis I
(ff) Meiosis II
(gg) Meiosis in detail
(hh) Metaphase I
(ii) Prophase I
(jj) Random fertilization
(ll) Replicating haploid
(mm) Sex chromosomes
(nn) Sexual reproduction
(pp) Somatic cells
(tt) Telophase I
(47) Practice questions [index]
(a) The actual location of a gene on a chromosome is its __________.
(b) Distinguish, in terms of ploidy and mitosis, between the human life cycle and the alternation of generations seen in plants.
(c) How many pairs of autosomal chromosomes does a somatic cell from a "normal" human male possess?
(d) During what two steps (phases) of meiosis is genetic variation generated? (neither "meiosis I" nor "meiosis II" are sufficient answers; by variation I mean other than the actual occurrence of mutation events as well as not chromosomal rearrangements)
(e) Approximately what is the probability that one of your gametes will contain the following (Assume no crossing over):
(i) Chromosomes 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and 13 from your father
(ii) Chromosomes 14, 15, 16, 17, 18, 19, 20, 21, 22, and X from your mother
(f) Assume that for a given species the nuclear membrane reforms during telophase I, nucleoli reappear, and chromosomes decondense to chromatin. In addition, assume that cytokinesis occurs. Contrast the subsequent interphase with that which occurs prior to mitosis (I'm looking for one crucial difference). (Assume additionally that there is no difference in the duration of these interphases.)
(g) Describe a typical human sexual cycle in terms of diploidy, haploidy, gametes, meiosis, fertilization, mitosis, and multicellularity. Note: It is possible to answer this question using a flow chart-type diagram.
(h) Draw and label a cell undergoing (i.e., in) anaphase II of meiosis. The diploid parent of this cell (i.e., pre-meiotic) contained six double helices during G1 of the pre-meiotic interphase and was, of course, diploid. Be as thorough and as detailed as you can. Relative points will be gained for any and all relevant detail. Relative points will be lost for incorrect information.
(i) What is an autosome?
(j) What is the ploidy of the typical, pre-meiotic, animal germ-line cells (e.g., such as those found in our own bodies), i.e., that are capable of dividing to produce gametes?
(k) Organisms which are capable of producing gametes that divide (i.e., divide by mitosis and do so prior to the occurrence of fertilization) include:
(v) all of the above are capable of producing gametes that divide
(vi) none of the above are capable of producing gametes that divide
(l) What is a gametophyte?
(m) What phase of meiosis does/do:
(i) sister chromatid pairs separate: __________________
(ii) synapsis occurs: __________________
(iii) tetrads line up at the center of the cell: __________________
(iv) chiasmata occur: __________________
(n) What occurs during anaphase I that contributes significantly to the variation among offspring?
(o) In a diploid organism in which 2n = 8 chrosomes, what is the likelihood that a given gamete produced by that organism will contain only chromosomes inherited from the organism's mom? Assume, for this example, that crossing over and therefore molecular recombination does not occur.
(p) If an organism possesses 36 chromosomes per cell during metaphase II of meiosis, how many DNA double helices does the organism contain per cell during G2 phase of the diploid generation?
(q) During what phase of mitosis or meiosis does a cell contain the most chromosomes? Assume cytokinesis between all occurrences of telophase, telophase I, or telophase II.
(r) Draw a diagram of the human sexual cycle indicating all details relevant to ploidy or cell division.
(s) When karyotyping chromosomes, what three criteria are used to distinguish autosomes?
(t) A typical cell from a given genotypically and phenotypically normal male mammal harbors 2n = 50 chromosomes. How many pairs of homologous autosomes does that imply?
(u) A sexually-reproducing organism that displays an alternation of generations differs from a sexually-reproducing organism that does not in what aspect of its sexual life cycle?
(v) What are the three non-mutational routes to variation in offspring that occur with sexual reproduction?
(w) Crossing over begins and occurs during what phase of meiosis?
(x) Name three ways that chromosomes may be distinguished in a karyotype.
(y) How many autosomes are found in the human haploid?
(z) What kind of nuclear division occurs during cellular reproduction that involves no change in ploidy?
(aa) In terms of relative chromosome number and types of nuclear divisions, how (or why?) does the plant sexual cycle differ from that of humans?
(bb) During which phase of meiosis does chromosome number double?
(cc) Ignoring the effects of crossing over, during what phase of meiosis does independent assortment occur?
(dd) In terms of their DNA sequences, how do the metaphase I (of meiosis) chromatids differ from the interphase I chromatids?
(ee) In a karyotype, chromosomes are distinguished in terms of their size, straining patterns, and _________.
(ff) What is an autosome?
(gg) Are germ line cells diploid or are they haploid?
(hh) Some organisms do not generate their gametes directly via meiosis. What form of nuclear division do these organisms employ (instead) to produce their gametes?
(ii) In plants the multicellular haploid organism is called a(n) _________ to distinguish it from the multicellular diploid organism with which it alternates generations.
(jj) During what phase of meiosis does independent assortment occur?
(kk) Assuming no crossing over, why are the odds of getting chromosomes only from your father into one of your gametes equal to (1/2)23? That is, explain to me what the 1/2 means and what the 23 means in this expression.
(ll) Chiasmata form during what phase of meiosis?
(mm) Which of the following is equivalent to recombination?
(i) Crossing over
(iii) Independent assortment
(iv) Prophage II
(v) Reduction division
(nn) During the formation of a tetrad (i.e., synapsis), prior to the formation of chiasmata, describe what structures are present as well as well as their prefertilization origins.
(oo) During which of the following does DNA duplication not occur:
(i) Interphase I of meiosis
(ii) Interphase II of meiosis
(iii) Interphase of mitosis
(iv) S phase
(v) Sister chromatid pair formation
(pp) During which of the following are tetrads found
(i) Anaphase I of meiosis
(ii) Anaphase II of meiosis
(iii) Interphase II of meiosis
(iv) Prophase I of meiosis
(v) S phase
(qq) Distinguish syngamy from synapsis?
(rr) What is alternation of generations?
(ss) Describe/diagram anaphase I of meiosis during human ovum production.
(tt) Indicate which are haploid (could be more than one):
(i) Gametophyte: __________
(ii) Plant gametes: __________
(iii) Product of meiosis I: __________
(iv) Product of meiosis II: __________
(v) Production of fertilization: __________
(vi) Sporophyte: __________
(uu) Distinguish Soma from Autosome.
(vv) In humans under what normal circumstances does synapsis occur between non-homologous chromosomes?
(ww) A photograph of human chromosomes, arranged in pairs from largest to smallest, is called a(n) __________.
(xx) A single human __________ has a centromere, two telomeres, and is linear.
(yy) What is a locus on a chromosome?
(zz) One-half of a homologous pair of non-sex chromosomes is called a(n) __________.
(aaa) What sort of division gives rise to asexual reproduction by eukaryotic cells?
(bbb) In all cases, Soma is created by what type of eukaryotic cell reproduction?
(ccc) For eukaryotes, the transition from diploidy to haploidy occurs via __________ while the transition from haploidy to diploidy occurs via __________.
(ddd) There exist three general variations on the sexual cycle. In animals only the diploid stage (typically) reproduces mitotically. In plants there is an alternation of generations. What is the third variation, e.g., as displayed by sexually reproducing fungi?
(eee) True or False, the Sporophyte stage of an alternation of generations is formed mitotically (i.e., via multiple rounds of mitosis), starting with a single spore.
(fff) What occurs during prophase I of meiosis that does not occur during prophase (or prometaphase) of mitosis?
(ggg) What are the three sources of variation in the sexual cycle that are not realized given asexual reproduction?
(hhh) What is a locus? (no, I am not looking for a description of a type of insect)
(iii) What are the three ways that chromosomes may be distinguish within a karyotype?
(jjj) What is an autosome?
(kkk) Which of the following is not a characteristic of asexual reproduction?
(i) In humans involves X and Y chromosomes
(ii) Limited to diploid cells only
(iii) Little genetic variation between parent and offspring
(iv) No change in ploidy
(v) Only mitotic division (in eukaryotes)
(lll) In humans, meiosis occurs only among __________ cells.
(mmm) In various fungi, protozoa, and algae gametes are generated by __________ rather than by __________.
(nnn) What is a sporophyte?
(ooo) During what phase of meiosis do tetrads line up in an imaginary plane?
(ppp) What is a tetrad?
(qqq) What are chiasmata?
(48) Practice question answers [index]
(b) both haploid and diploid plant cells undergo mitosis whereas only diploid human cells do so (i.e., haploid cells don't)
(d) Prophase I and anaphase I (crossing over and independent assortment, respectively)
(e) Approximately 1 in 8 million. That is, this is one of the 8 million possibilities we discussed in class which may be generated by independent assortment. The important thing to keep in mind is that there is only one way that this gamete may be generated (and we have described that way). Alternatively you can calculate the probability, i.e., (1/2)23 = 1 / 8,388,608.
(f) There is no synthesis phase.
(g) fertilization à diploidy à mitosis à multicellularity à meiosis à haploidy à gametes à fertilization.
(h) Drawing should include centrosomes at opposite poles, asters, spindle fibers, chromosomes (6 chromatids, 6 chromosomes), the chromosomes being dragged by their kinetochore, non-kinetochore microtubules, etc. Note that in addition one could indicate that, or how the chromatids are products of recombination.
(i) One-half of a homologous pair of chromosomes
(k) (vi), gametes don't divide
(l) a generation (i.e., within which mitosis occurs) that is haploid and which gives rise to gametes
(m) anaphase II, prophase I, metaphase I, prophase I
(n) independent assortment
(o) (0.5)4 = 1/16. That is, there are four chromosomes in the gamete and each has a one-half chance of having come from mom
(p) 72, this is past the reduction division so it is a haploid cell, but prior to the separation of the chromosomes into two
(q) anaphase of mitosis, or telophase before cytokinesis
(r) diploid cell in germ line à meiosis à haploid gamete formation à fertilization à zygote formation (diploid) à mitosis (numerous)
(s) size, banding pattern, location of centromere
(t) 24 (i.e., X and Y are not homologous pairs nor autosomes)
(u) the organism that does not display an alternation of generations has either a haploid or a diploid stage that is not capable of dividing mitotically
(v) crossing over, independent assortment, random mating
(w) prophase I
(x) Size, banding pattern, location of centromere
(y) 22 (46/2 - 1 or 46-2/2… either way it is the number of chromosomes in the diploid divided by two less the number sex chromosomes)
(aa) Plants display an alternation of generations; that is, their haploid cells are capable of undergoing mitosis (not just no division followed by fertilization as is seen in humans)
(bb) Anaphase II (this is when the sister chromatids are finally separated)
(cc) Independent assortment occurs during Anaphase I, i.e., this is when the tetrads are separated and chromosomes that originally came from mom versus dad are separated (sorted) randomly to the now-haploid progeny cells
(dd) The metaphase I chromosomes are a mixture of the chromosomes from mom and dad; thus, the sequence of a given chromosome at interphase I is whatever sequence the chromosome had in dad's sperm or mom's ova; after prophase I, however, the sequence is some combination of the sequence of two homologous chromosomes, the one that came from mom and the one that came from dad
(ee) Position of centromere
(ff) An autosome is a non-sex chromosome, typically found in homologous pairs within the cells of diploid organisms
(gg) Germ line cells are diploid; they give rise to the haploid gametes via the reduction division of meiosis
(hh) Organisms such as the plant gametophytes that do not generate their gametes by meiosis typically do so employing only mitosis
(jj) Independent assortment occurs during anaphase I of meiosis; this is when the sister chromatid pairs coming from Mom versus Dad are independently sorted into different recipient cytoplasms
(kk) The 1/2 refers to the one-half probability of your Dad's chromosome finding its way into one (but not the other) recipient cytoplasm during anaphase I (independent assortment) while the 23 refers to the total number of chromosomes that came from Dad, i.e., the haploid set of human chromosomes; in other words, 23 times a decision must be made to place Dad's chromosome into only one of two possible recipients during independent assortment
(ll) Prophase I of meiosis
(mm) (i) Crossing over
(nn) Two sister chromatid pairs come together during synapsis to form a tetrad; one pair is derived from the parent ova (mom) while the other ultimately came from the parental sperm (dad)
(oo) (ii) Interphase II of meiosis
(pp) Prophase I of meiosis
(qq) Syngamy is the synonymous with fertilization, the fusion of haploid gametes to form a diploid zygote; synapsis is the coming together of homologous chromosomes during prophase I of meiosis
(rr) Alternation of generations refers to the life cycle of certain organisms (e.g., plants) that involves mitotic division of both haploid and diploid cells, in alternation
(ss) Separation of sister chromatid pairs, with one sister chromatid pair going to one daughter cell and the other sister chromatid pair going to the other
(tt) (i) Haploid, (ii) Haploid, (iii) Haploid, (iv) Haploid, (v) Diploid, (v) Diploid
(uu) Soma are the non-germ line cells in your body; an autosome is a non-sex chromosome
(vv) Sperm production, X and Y for a tetrad of sorts
(yy) A locus is the physical location of a gene on a chromosome
(aaa) Mitotic division
(ccc) Meiosis, Fertilization (syngamy)
(ddd) Mitotic division of the haploid stage, only; syngamy gives rise to a diploid cell (or, simply, nucleus) which goes on immediately (i.e., without intervening mitosis) to divide meiotically
(eee) False (spores are haploid, the sporophyte is diploid and gives rise to spores meiotically)
(fff) Synapisis, tetrad formation, chiasmata formation, crossing over, recombination
(ggg) Independent assortment, crossing over, random fertilization
(hhh) A locus is a specific location on a chromosome, e.g., the location of a gene
(iii) Size, banding patterns, and location of the centromere
(jjj) An autosome is a non-sex chromosome
(kkk) (ii) Limited to diploid cells only
(mmm) Mitosis, meiosis
(nnn) A sporophyte is a plant generation that is diploid and which gives rise to spores meiotically
(ooo) Metaphase I
(ppp) A tetrad is two metaphase chromosomes lined up together
(qqq) Chiasmata are the sites of crossing over/recombination that occur during Prophase I of meiosis
Chapter 13, Bio 113 questions:
(#) What is a karyotype?
A: A karyotype is is an arrangement of chromosomes, in order from largest to smallest, on a photographic plate
(#) What are two ways in which chromosomes can be distinguished in a karyotype?
A: Size, banding patterns, location of centromere
(#) Two chromosomes of the same type found within a cell's cytoplasm, one from mom, one from dad, are said to be:
(iv) Sex chromosomes
A: (ii) Homologous
(#) What is an autosome?
A: An autosome is a non-sex chromosome
(#) Sexual reproduction involves changes in __________ going from parent to offspring, whereas asexual reproduction does not.
(#) True or False, in Fungi (like animals) meiosis directly gives rise to gametes.
(#) The conversion of the haploid stage to the diploid stage is generally known as __________.
A: fertilization; syngamy
(#) Name this overall sexual cycle:
A: Alternation of generations
(#) What occurs during prophase I of meiosis that does not occur during prophase of mitosis?
A: synapsis, chiasmata formation, tetrad formation
(#) Describe the product of anaphase I in terms of ploidy.
(#) Independent assortment physically occurs during what phase(s) of meiosis.
A: Metaphase I and/or Anaphase I of meiosis