Deck 25: Sexual Reproduction, Meiosis, and Genetic Recombination

Question 1

The Truth About Sex. For each of the following statements, indicate with an S if it is true of sexual reproduction, with an A if it is true of asexual reproduction, with a B if it is true of both, or with an N if it is true of neither.
(a)Traits from two different parents can be combined in a single offspring.
(b)Each generation of offspring is virtually identical to the previous generation.
(c)Mutations are propagated to the next generation.
(d)Some offspring in every generation will be less suited for survival than the parents, but others may be better suited.
(e)Mitosis is involved in the life cycle.

Answer 1

(a)
The definition of sexual reproduction is the combination of genetic material from two parents.
Therefore, this statement is true of S.
(b)
In perfect asexual reproduction, no mutations will occur and the offspring will be a identical to the parent.
Therefore, this statement is true of A.
(c)
Regardless of whether reproduction is sexual or asexual, errors can occur in the DNA during replication and cell division. As a result, some of the offspring will be mutants for a particular trait.
Therefore, this statement is true of S and A.
(d)
Since mutations can occur in sexual and asexual reproduction, there will be variability between generations. Some traits will be expressed in some offspring while others will not. As a result, each generation will have offspring that have advantageous traits and others that have disadvantageous traits.
Therefore, this statement is true of S and A.
(e)
Gametes are produced via meiosis; however throughout development and life, mitosis is used to divide and create new cells.
Therefore, mitosis is a part of the life cycle for organisms that reproduce sexually and asexually.

Question 2

Ordering the Phases of Meiosis. Drawings of several phases of meiosis in an organism, labeled A through F, are shown in Figure.
FIGURE. Six Phases of Meiosis to Be Ordered and Identified.

(a)What is the diploid chromosome number in this species
(b)Place the six phases in chronological order, and name each one.
(c)Between which two phases do homologous centromeres separate
(d)Between which two phases does recombination occur

Answer 2

(a)
During meiosis, a diploid cell that has two copies of chromosomes divides to form four haploid cells or gametes. Drawing F shows the first stage of meiosis called Prophase.
The number of chromosomes that appear in the given diagram is 4 and the diploid number is represented as 2n.
Since the cell in this initial stage of meiosis is diploid, the diploid chromosome number in this species is 4.
(b)
In meiosis, the thickening of chromosomes and the migration of centrosomes to opposite sides of the diploid cell occurs before the other pictured phases.
Then, the homologous chromosomes join to form bivalents and began to cross over or swap segments of DNA as the nuclear envelope continues to breakdown. In this phase, the centrosomes have completely moved to opposite poles.
Since the bivalents were already formed in the previous phase, the next phase shows the bivalents attached to the spindle fibers.
Next, the haploid cells begin to separate and form separate nuclear envelopes.
In the fifth pictured phase, the chromosomes align along the spindle fibers of the haploid cells.
Finally, the chromatids separate along the spindle fibers in preparation of the final division into four haploid cells.
Although the six phases described do not include all phases of meiosis, the chronological order is Early Prophase I (F), Late Prophase I (E), Metaphase I (D), Telophase I (B), Metaphase II (A), and Anaphase II (C).
(c)
Before the two haploid cells divided into four haploid daughter cells, the sister chromatids separate at the centromeres. These sister chromatids are separated during anaphase and migrated to opposite poles in telophase.
Thus, homologous centromeres separate between Anaphase II and Telophase II.
(d)
Recombination is the process of exchanging DNA for genetic variability. This occurs during crossing over. The process of crossing over occurs in pachytene stage of prophase I of meiosis-I. Prophase occurs in 5 stages namely- Leptotene, zygotene, pachytene, diplotene and diakinesis. The process of crossing over occurs in pachytene stage.
Therefore, recombination occurs between early and late prophase I.

Question 3

Telling Them Apart. Briefly describe how you might distinguish between each of the following pairs of phases in the same organism:
(a)Metaphase of mitosis and metaphase I of meiosis.
(b)Metaphase of mitosis and metaphase II of meiosis.
(c)Metaphase I and metaphase II of meiosis.
(d)Telophase of mitosis and telophase II of meiosis.
(e)Pachytene and diplotene stages of meiotic prophase I.

Answer 3

(a)
The bivalents are aligned along the spindle equator in metaphase of mitosis and metaphase I of meiosis. However, the bivalents in mitosis form a single row along the equator and there are two rows of bivalents in metaphase I.
(b)
Metaphase of mitosis and metaphase II of meiosis look similar because the chromosomes are aligned along the spindle equator in a single row. However, the cell in metaphase of mitosis is diploid and contains two copies of DNA while the cell of metaphase II is haploid.
(c)
The chromosomes are aligned along the spindle equator in metaphase I and II of meiosis. Since no daughter cells have created before metaphase I, the cell is diploid and still has two copies of the chromosomes. Metaphase II occurs after the first round of cell division and cell is haploid.
(d)
The daughter cells forming in telophase of mitosis will be identical copies of the parent cell and contain the same amount of DNA. The daughter cells of telophase II of meiosis are not identical to the parent and will be haploid.
(e)
During the pachytene stage of prophase I, the chromosomes will be shorter and more compact as crossing over occurs. In the diplotene stage, the bivalents will be further apart.

Question 4

Your Centromere Is Showing. Suppose you have a diploid organism in which all the chromosomes contributed by the sperm have cytological markers on their centromeres that allow you to distinguish them visually from the chromosomes contributed by the egg.
(a)Would you expect all the somatic cells (cells other than gametes)to have equal numbers of maternal and paternal centromeres in this organism Explain.
(b)Would you expect equal numbers of maternal and paternal centromeres in each gamete produced by that individual Explain.

Question 5

How Much DNA Let X be the amount of DNA present in the gamete of an organism that has a diploid chromosome number of 4. Assuming all chromosomes to be of approximately the same size, how much DNA ( X , 2 X, 1/2 X , and so on)would you expect in each of the following
(a)A zygote immediately after fertilization
(b)A single sister chromatid
(c)A daughter cell following mitosis
(d)A single chromosome following mitosis
(e)A nucleus in mitotic prophase
(f)The cell during metaphase II of meiosis
(g)One bivalent

Question 6

Meiotic Mistakes. Infants born with Patau syndrome have an extra copy of chromosome 13, which leads to developmental abnormalities such as cleft lip and palate, small eyes, and extra fingers and toes. Another type of genetic disorder, called Turner syndrome, results from the presence of only one sex chromosome-an X chromosome. Individuals born with one X chromosome are females exhibiting few noticeable defects until puberty, when they fail to develop normal breasts and internal sexual organs. Describe the meiotic events that could lead to the birth of an individual with either Patau syndrome or Turner syndrome.

Question 7

Down Syndrome Revisited. About 5% of individuals with Down syndrome (see Human Connections, page 756)have a chromosomal translocation in which a thud copy of chromosome 21 is attached to chromosome 14. Such individuals receive this chromosome from one of their parents due to a malformation in a gonadal cell. What is the ploidy of such individuals with Down syndrome Why does receiving the modified chromosome from one of their parents lead to Down syndrome in such children

Question 8

QUANTITATIVE Punnett Squares as Genetic Tools. The genetic characters of seed color (where Y is the allele for yellow seeds and y for green seeds)and seed shape (where R is the allele for round seeds and r for wrinkled seeds)were used by Mendel in some of his crosses.
(a)Mendel performed a one-factor cross between parent plants that were both heterozygous for seed color Yy × Yy. Using a Punnett square, explain the 3:1 phenotypic ratio Mendel observed for the offspring of such a cross.
(b)Mendel went on to perform a two-factor cross between plants heterozygous for both seed color ( Yy )and seed shape (Rr). How does the Punnett square of Figure 25-26 reflect Mendel's law of independent assortment
(c)Complete the Punnett square of Figure 25-26 by writing in each of the possible progeny genotypes. How many different genotypes will be found in the progeny In what ratios
(d)For the case of Figure 25-26, how many different phenotypes will be found in the progeny In what ratios

Question 9

Genetic Mapping. The following table provides data concerning the frequency with which four genes ( w , x, y, and z)located on the same chromosome recombine with each other.
(a)Construct a genetic map indicating the order in which these four genes occur and the number of map units that separate the genes from each other.
(b)In constructing this map, you may have noticed that the map distances are not exactly additive. Can you provide an explanation for this apparent discrepancy

Question 10

Homologous Recombination. Bacterial cells use at least three different pathways for carrying out genetic recombination. All three pathways require the RecA protein, but in each case, a different set of steps precedes the action of RecA in catalyzing strand invasion. One of these three pathways utilizes an enzyme complex called RecBCD, which binds to double-strand breaks in DNA and exhibits both helicase and single-strand nuclease activities.
(a)Briefly describe a model showing how the RecBCD enzyme complex might set the stage for genetic recombination.
(b)When bacterial cells are co-infected with two different strains of bacteriophage 1, genes located near certain regions of the phage DNA, called CHI sites, recombine at much higher frequencies than other genes do. However, in mutant bacteria lacking the RecBCD protein, genes located near CHI sites do not recombine any more frequently than do other genes. How can you modify your model to accommodate this additional information

Question 11

Gene Cloning and Recombination. Not only are the plasmid vectors used in molecular biology engineered, but the strains of E. coli used in cloning are as well.
(a)Nearly all strains of E. coli used in DNA cloning carry mutations in the recA gene that result in loss of RecA activity. Why would a recA mutation make an E. coli cell a better host for propagating recombinant plasmid DNA
(b)Recall that restriction endonucleases are normally made by bacteria such as E. coli. E. coli used in molecular biology also carry mutations in restriction endonucleases. Why do you think these mutations would be useful