CH 14 Mastering Genetics

M The M phase (mitosis and cytokinesis) follows G2, the last stage of interphase.

sister chromatids Sister chromatids are duplicated copies of a chromosome that remain attached at the centromere.

Cohesin Cohesin localizes between the sister chromatids and holds them together to resist the pull of the kinetochore microtubules.

actin The contractile ring that forms the cleavage furrow is composed of actin microfilaments.

kinases Cyclin-dependent kinases (Cdks) form complexes with specific cyclin proteins. The kinase is activated only when it is part of the cyclin-Cdk complex. The activated cyclin-Cdk complex phosphorylates numerous targets to regulate progress through the cell division cycle.

a gene that normally blocks progression through the cell cycle A tumor suppressor gene is a gene whose normal function, when properly expressed, is to block progression through the cell division cycle.

. . . G1 - S - G2 - M - G1 . . . The S phase is both preceded and followed by a period of growth (G1 and G2, respectively). After the M phase, the cell re-enters the G1 phase.

False Both haploid and diploid cells can undergo mitosis.

A A) polar microtubules contract, pulling attached chromosomes toward the poles B) chromosomes condense C) microtubules assemble between centrioles D) nuclear membrane breaks down This describes the role of kinetochore microtubules during anaphase. Polar microtubules lengthen during anaphase, causing the cell to become elliptical.

bacteria and archaea Bacteria and archaea lack sexual cycles and therefore reproduce exclusively by cell division.

synaptonemal complex The synaptonemal complex is a tri-layer protein structure that holds homologs together and facilitates recombination.

sites where DNA strand exchange between non-sister chromatids has occurred Chiasmata are the points where non-sister chromatids cross over and recombination occurs.

anaphase I of meiosis

Males are hemizygous for X-linked genes. Males carry only one copy of the X chromosome, so they are termed hemizygous (half zygous).

some of their male progeny may have white eyes If the female is heterozygous, approximately half of the male progeny will have white eyes.

true Males inherit only one X chromosome. That chromosome is contributed by the female parent

His mother had at least one white allele. Because this male had white eyes, he must have inherited a white allele from his mother.

c) Only males will display the recessive trait. This statement is false. Homozygous recessive females will also display the recessive trait.

One X chromosome is randomly inactivated in female somatic cells. One X chromosome is randomly inactivated early in embryogenesis. This leaves female somatic cells with one copy of the X chromosome that is expressed, which is equivalent to what is found in male cells, which carry a single X chromosome

anaphase I

a) no According to law of segregation, the allele pairs segregate during gamete formation and randomly fuse during fertilization. Thus, the cross over between one pair of homologs does not affect the expected proportion of gamete genotypes. b) no According to law of independent assortment, each pair of alleles segregates independently of the other pair during gamete formation and randomly fuses during fertilization. Thus, the cross over between one pair of homologs does not affect the expected proportion of gamete genotypes.

law of segregation The law of segregation states that the two alleles for a gene separate during gamete formation, and end up in different gametes. In the case of the heterozygous green-pod plant (Gg), one gamete will receive the dominant allele (G), and the other gamete will receive the recessive allele (g). The law of segregation accounts for the prediction that 50% of the offspring of the test cross will have green pods and 50% will have yellow pods.

meiosis I, anaphase Alleles separate from one another during anaphase of meiosis I, when the homologous pairs of chromosomes separate.

Cross the green-pod plant with a yellow-pod plant A cross between a plant of unknown genotype and one that is known to be homozygous recessive is called a test cross because the recessive homozygote tests whether there are any recessive alleles in the unknown. Because the recessive homozygote will contribute an allele for the recessive characteristic to each offspring, the second allele (from the unknown genotype) will determine the offspring's phenotype.

why a collection of meiotic products that includes gametes of genotype Ab will also include gametes of genotype AB in roughly the same proportion Because alignment of the chromosomes is random, the alignment that produces Ab is just as likely as the alignment that produces AB. Therefore, the two genotypes should both occur with equal frequency.

false Even though there would be no genotypic differences in the products of such meioses, random alignment of chromosomes would still have occurred.

AaBC This gamete contains two copies of gene "A". Proper segregation would have separated A from a and allowed only one copy per gamete.

Pachynema Crossing over occurs during pachynema when bivalents are closely paired.

true Chromosomes are duplicated during interphase; at synapsis of prophase I, one chromosome (with two chromatids) in a tetrad is paternally inherited while the other is maternally inherited.

Anaphase II Sister chromatids from each dyad separate during anaphase II.

at anaphase in mitosis and anaphase II in meiosis

true

reciprocal exchange of DNA between homologs during prophase I

Knowing the terms and relationships shown in this concept map will help you understand the role that meiosis plays in heredity, sexual reproduction, and genetic variability. To understand the process of meiosis, it is essential that you can differentiate between sister chromatids, nonsister chromatids, homologous chromosomes, and non-homologous chromosomes. Meiosis creates gametes (eggs and sperm) with only a single chromosome set (haploid or n) from parental cells with two chromosome sets (diploid or 2n). During fertilization, the haploid sperm (n) and egg (n) fuse, producing a diploid zygote (2n). The cells of the zygote then divide by mitosis (which does not change the ploidy level) to produce an adult organism (still 2n) of the next generation. In sexual life cycles, meiosis and fertilization keep the number of chromosomes constant from generation to generation.

Meiosis involves two sequential cellular divisions. In meiosis I, homologous chromosomes pair and then separate. Thus, although the parent cell is diploid (containing two chromosome sets, one maternal and one paternal), each of the two daughter cells is haploid (containing only a single chromosome set). In meiosis II, the sister chromatids separate. The four daughter cells that result are haploid.

Crossing over occurs during early prophase I when homologous chromosomes loosely pair up along their lengths. Crossing over occurs only between nonsister chromatids within a homologous pair of chromosomes, not between the sister chromatids of a replicated chromosome. Crossing over is reduced near the centromere; segments that are not adjacent to the centromere (for example, segments near the ends of chromosomes) are more likely to cross over.

DNA content is halved in both meiosis I and meiosis II. Ploidy level changes from diploid to haploid in meiosis I, and remains haploid in meiosis II. During anaphase of both meiosis I and meiosis II, the DNA content (number of copies of chromosomes) in a cell is halved. However, the ploidy level changes only when the number of unique chromosome sets in the cell changes. This occurs only in meiosis I (where separation of homologous chromosomes decreases the ploidy level from 2n to n and produces daughter cells with a single chromosome set).

all are in both box (DNA relocation, Crossing over, fertilization, metaphase I to anaphase I, metaphase II to anaphase II) In organisms that reproduce sexually, the processes of DNA replication, the precise pairing of homologs during crossing over, chromosome alignment and separation in meiosis I and II, and fertilization ensure that traits pass from one generation to the next. Unlike with asexual reproduction, offspring of sexual reproduction are genetically different from each other and from both of their parents. Mechanisms that contribute to genetic variation include errors (mutations) that occur during DNA replication the production of recombinant chromosomes due to crossing over the independent assortment of homologous chromosomes in meiosis I the separation of sister chromatids (which are no longer identical due to crossing over) in meiosis II the random fusion of male and female gametes during fertilization

One aspect of meiosis that generates genetic variation is the random orientation of homologous pairs of chromosomes at metaphase I. Each pair can orient with either its maternal or paternal homolog closer to a given pole; as a result, each pair sorts into daughter cells independently of every other pair. Due to independent assortment alone, a diploid cell with 2n chromosomes can produce 2 n possible combinations of maternal and paternal chromosomes in its daughter cells. For the cell in this problem (n = 3), there are 23, or 8, possible combinations; for humans (n = 23), there are 223, or 8.4 million, possible combinations. Note that when crossing over occurs, the number of possible combinations is even greater.

There would be less genetic variation among gametes. Crossing over contributes significantly to the genetic variation seen in gametes. This is because the exchange of maternal and paternal genes between the nonsister chromatids of a homologous chromosome pair creates recombinant chromosomes with unique combinations of alleles. However, crossing over is not the only process that introduces genetic variation in meiosis I. The independent assortment of homologous chromosomes (which are never identical) in meiosis I produces daughter cells that differ from each other. The effect of crossing over on genetic variation is shown below. Without crossing over, sister chromatids remain identical and thus, pairs of daughter cells would be identical. With crossing over, however, all four daughter cells are genetically unique.

Homologous chromosomes are a pair of chromosomes, one maternal and one paternal, that come together during fertilization. They have the same centromere position, and the same genetic loci (genes); however the DNA sequence is not identical. Crossing over between homologous chromosomes during prophase I of meiosis generates genetic diversity in the offspring. Homologous chromosomes pair up along the midline during metaphase I of meiosis, and move apart during anaphase I. Sister chromatids are the result of the replication of a single chromosome. They are identical in DNA sequence (apart from mutation or crossing over with a chromatid from a homologous chromosome). During metaphase of mitosis, chromosomes line up down the midline and sister chromatids separate during anaphase. In meiosis, sister chromatids separate during meiosis II.

There are a number of important differences between the production of male and female gametes, even though the events that occur during meiosis are similar in all cells. Spermatogenesis, which takes place in the testes, produces four sperm cells with the haploid number of chromosomes and equal amounts of cytoplasm from one undifferentiated diploid germ cell called a spermatogonium. Because humans can reproduce year-round, spermatogenesis occurs throughout the life of the mature male. Oogenesis, which takes place in the ovaries, differs in several important ways from spermatogenesis. Oogenesis results in the production of one egg cell and other haploid products called polar bodies from a diploid germ cell called an oogonium. The egg cell contains the majority of cytoplasm which contributes to zygote development during fertilization, while the polar bodies contain less cytoplasm. Unlike spermatogenesis, much of oogenesis is completed before birth, with oocytes remaining arrested in prophase I until many years later, when meiosis is continued just prior to ovulation. Additionally, while females produce one gamete per month during their mature reproductive years, males produce millions of sperm every day.

During ovum production, primary oocytes are arrested for years in meiosis I, increasing the likelihood of components involved in chromosome segregation to break down. During spermatogenesis, germ cells are produced daily, while during oogenesis, germ cells are produced prenatally. The difference in the "age" of the gametes at the time of fertilization may contribute to the observed effect. Although scientists do not know with certainty why nondisjunction increases with advanced maternal age, both the age of the ovum at the time of fertilization and the fact that oocytes are arrested in meiosis I for years are thought to contribute to this effect. Oogenesis begins during fetal development and oocytes are arrested in prophase I by birth. During puberty, ovulation begins and meiosis is reinitiated in one egg during each ovulatory cycle. As a result, each ovum that is released has been arrested in meiosis I for one month longer than the one released during the preceding cycle. Therefore, women in their 30's and 40's are producing ova that are much older than those which were ovulated when puberty began, which may contribute to nondisjunction.

a) End of mitotic telophase. 48 chromosomes: 46 autosomes, 2 sex chromosomes,each chromosome will be composed of a single chromatid. b) Meiotic metaphase I. 48 chromosomes, 96 chromatids. c) End of meiotic anaphase II. 48 chromosomes d) Early mitotic prophase. 48 chromosomes, 96 chromatids. e) Mitotic metaphase. 48 chromosomes, 96 chromatids. f) Early prophase I. 48 chromosomes, 96 chromatids.