Mastering Biology

A normal number of chromosomes in all gametes is not a possible outcome as the result of meiotic non-disjunction. If both homologs in meiosis I move to the same pole of the parent cell, all of the products of meiosis will be abnormal. If both sister chromatids in meiosis II to the same pole of the parent cell, half of the gametes will be abnormal.

A zygote with a genetic profile of 2n-1 is called a monosomy. Cells that have too many or too few chromosomes of a particular type are said to be aneuploid ("without-form"). Trisomy ("three-bodies") occurs when a cell has three copies of the chromosome.

Which of these cells is (are) haploid?

After Meiosis I and Meiosis II (C &D)

When during meiosis do homologous chromosomes separate?

Anaphase 1

Homologous chromosomes migrate to opposite poles during _____.

Anaphase I

During _____ sister chromatids separate.

Anaphase II

When are sister chromatids separated during meiosis?

Anaphase II

During _____ chromosomes align single file along the equator of a haploid cell.

Metaphase II

Crossing Over

Pairing of homologous chromosomes is followed by crossing over, the exchange of genetic material between nonsister chromatids of homologous chromosomes.

When does the spindle apparatus first form in meiosis?

Prophase

When during meiosis do homologous chromosomes pair up?

Prophase 1

Synapsis occurs during _____.

Prophase I

A normal diploid cell in G1 has 2 pg of DNA. At the end of meiosis II, one daughter cell __________.

is haploid and has 1 pg of DNA

A normal diploid cell in G1 has 2 pg of DNA. At the end of meiosis I, one daughter cell __________.

is haploid and has 2 pg of DNA

At the end of _____ and cytokinesis there are four haploid cells.

telophase II, telophase II

Meiosis I produces _____ cells, each of which is _____.

two, haploid

Human gametes are produced by _____.

Meiosis produces haploid gametes from a diploid parental cell.

A diploid organism whose somatic (nonsex) cells each contain 32 chromosomes produces gametes containing _____ chromosomes.

16 is half of 32.

Normal human gametes carry _____ chromosomes.

23

A cell at the end of S phase (DNA synthesis phase of interphase) contains 100 picograms of nuclear DNA. If this cell undergoes meiosis, how much nuclear DNA will be in each daughter cell?

25 picograms

If the kidney cells of an animal have 12 chromosomes, the sperm cells would have how many chromosomes?

6

Recombinant Chromosomes

A chromosome created when crossing over combines the DNA from two parents into a single chromosome. Each of the chromosomes in gametes B and C are composed of material derived from both parents.

Which of the following is not a possible outcome as the result of meiotic non-disjunction?

A normal number of chromosomes in all gametes

metaphase I

At metaphase I, the chromosome tetrads are aligned on the metaphase plate. For each tetrad, kinetochore microtubules from one pole of the cell are attached to one homologous chromosome, while kinetochore microtubules from the other pole of the cell are attached to the other chromosome of the pair. Thus, the homologous chromosomes are poised to move to opposite poles of the cell. During metaphase I tetrads align along the metaphase plate.

At the end of meiosis I, one daughter cell will have four copies of chromosome 22, whereas the other will have zero copies of chromosome 22.

At the end of meiosis II, the former will divide into two sperm cells that have two copies of chromosome 22 (for a total of 24 chromosomes), whereas the latter will result in sperm cells that have zero copies of chromosome 22 (for a total of 22 chromosomes).

The region of a chromosome where microtubules connect to a chromosome pair is called the centromere. Microtubules of the spindle apparatus attach to kinetochores located at the centromere of each chromosome. The centromere is a region on the chromosome; kinetochores are structures in that region.

Centrosomes are a microtubule organizing center located near the nucleus that contain two bundles of microtubules called centrioles. The two centrioles inside a centrosome consist of microtubules as triplets arranged in a circle.

Which statement correctly describes how cellular DNA content and ploidy levels change during meiosis I and meiosis II?

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.

anaphase I

During anaphase I, each pair of chromosomes is pulled apart and the homologous chromosomes move toward opposite poles. Note that sister chromatids remain attached at their centromeres and move as a unit toward the same pole. During anaphase I homologous chromosomes, consisting of sister chromatids, migrate to opposite poles.

Anaphase II

During anaphase II sister chromatids separate and migrate to opposite poles.

A normal diploid cell in G1 has 2 pg of DNA. At the end of meiosis I, one daughter cell is haploid and has 2 pg of DNA.

During meiosis I, the diploid (2n) parent cell produces two haploid (n) daughter cells. Notice, however, that each chromosome still consists of two sister chromatids, meaning that chromosomes are still replicated at the end of meiosis I. Remember that G1 comes before S phase, when DNA is replicated. As the cell goes into G2, it has 4 pg of DNA.

metaphase II

During metaphase II, the chromosomes align on the metaphase plate. Metaphase II is essentially the same as mitotic metaphase except that the cells are haploid.

What is the most common result of gamete trisomy, when that gamete is used to form a zygote?

Failure of the zygote to develop into an embryo

A cell at the end of S phase (DNA synthesis phase of interphase) contains 100 picograms of nuclear DNA. If this cell undergoes meiosis, there will be 25 picograms of nuclear DNA in each daughter cell.

G1 comes before S phase, when DNA is replicated. As the cell goes into G2, it has 100 pg of DNA, so it had to have 50 pg of DNA in G1. Each of the two daughter cells will have 50 pg of DNA at the end of meiosis I, and each of the four daughter cells will have 25 pg of DNA at the end of meiosis II.

What would happen to genetic variation if crossing over were eliminated while independent assortment was maintained?

Genetic variation would decrease.

What would happen to genetic variation if maternal chromosomes always lined up together on one side of the metaphase plate during meiosis I and paternal chromosomes always lined up on the other side?

Genetic variation would decrease.

Organisms that have a single copy of each gene are called __________.

Haploid -having one of each type of chromosome (n) Bacteria and archaea are haploid, as are many plant and animal gametes are haploid.

A pair of chromosomes made of members from two parents is called __________.

Homologous Chromosomes -is a set of one maternal chromosome and one paternal chromosome that pair up with each other inside a cell during mitosis.

A normal diploid cell in G1 has 2 pg of DNA. At the end of meiosis II, one daughter cell is haploid and has 1 pg of DNA.

In meiosis, one diploid cell with replicated chromosomes gives rise to four haploid cells with unreplicated chromosomes. If the parental cell has 2 pg of DNA in G1, it has 4 pg of DNA in G2. At the end of meiosis II, each gamete has 1 pg of DNA and is haploid.

telophase II and cytokinesis

In telophase II, nuclei form at opposite poles of each dividing cell, and cytokinesis splits the cells apart. Meiosis has produced four haploid cells, each with one set of chromosomes. Four haploid cells are present at the end of telophase II and cytokinesis.

interphase

Like mitosis, meiosis is preceded by an interphase, during which the chromosomes replicate. The centrosome also duplicates in preparation for cell division. During interphase the centrosome duplicates and the chromosomes are not condensed.

prophase I

Prophase I begins with condensation of the chromosomes. Homologous chromosomes, each made up of two sister chromatids, come together in pairs. This pairing is called synapsis. Each chromosome pair is called a tetrad, a complex of four chromatids. Chromatids of homologous chromosomes cross over each other and exchange parts at chiasmata (singular, chiasma). Meanwhile, other cellular components prepare for the division of the nucleus. The centrosomes move away from each other, and spindle microtubules form between them. The nuclear envelope and nucleoli disperse. Finally, spindle microtubules capture the kinetochores that form on the chromosomes, and the chromosomes begin moving to the metaphase plate. Homologous chromosomes pair during prophase I..

During _____ a spindle forms in a haploid cell.

Prophase II

The cells that produce sperm in humans contain 46 chromosomes. If one of these cells undergoes meiosis to form sperm cells, and chromosomal nondisjunction occurs in chromosome 22 during meiosis I, the chromosome number in each of the resulting sperm is 24, 24, 22, and 22.

Remember that n = 23 in humans.

If the chromosomes in each homologous pair don't separate from each other during the first meiotic division, it will lead to trisomy. The chromosomes in each homologous pair must separate from each other during the first meiotic division, so that only one homolog ends up in each daughter cell

Sister chromatids must separate from each other and move to opposite poles of the dividing cell during meiosis II. If both homologs in meiosis I or both sister chromatids in meiosis II move to the same pole of the parent cell, the products of meiosis will be abnormal. The replication of DNA is required for the cell cycle to continue into mitosis or meiosis. It will not affect the occurrence of trisomy.

A key difference between the two processes is that the homologous chromosomes are paired in meiosis, but not mitosis. Chromosomes are replicated in the S stage of interphase, which occurs prior to any cell division (mitosis or meiosis).

Sister chromatids result from DNA replication, which occurs prior to both mitosis and meiosis. Mitosis and meiosis are not defined by the ploidy state of the parent cell.

The random separation of maternal and paternal homologous chromosomes during meiosis I leads to independent assortment. Independent assortment occurs when pairs of homologous chromosomes line up during meiosis I and the homologs separate. This leads to a variety of combinations of maternal and paternal chromosomes. Each daughter cell gets a random assortment of maternal and paternal chromosomes.

Sister chromatids separate in anaphase II. Crossing over produces new combinations of alleles within a chromosome—combinations that did not exist in either parent. This phenomenon is known as recombination.

What is the difference between synapsis and a chiasma?

Synapsis is the physical pairing of two homologous chromosomes during prophase I of meiosis; a chiasma is the x-shaped structure formed during meiosis by crossing over between the non-sister chromatids of homologous chromosomes.

The cells that produce sperm in humans contain 46 chromosomes. If one of these cells undergoes meiosis to form sperm cells, and chromosomal nondisjunction occurs in chromosome 22 during meiosis I, what is the chromosome number in each of the resulting sperm?

THE CORRECT ANSWER 24, 24, 22, and 22

At the end of _____ and cytokinesis, haploid cells contain chromosomes that each consist of two sister chromatids.

Telophase I

The region of a chromosome where microtubules connect to a chromosome pair is called the __________.

centromere

telophase I and cytokinesis

The chromosomes finish their journey during telophase I, and cytokinesis occurs, producing two haploid daughter cells. Note that each chromosome still consists of two sister chromatids. Meiosis isn't over yet; remember that it consists of two consecutive divisions. During meiosis II, the sister chromatids will be separated. At the end of telophase I and cytokinesis there are two haploid cells.

Which of the following will lead to trisomy?

The chromosomes in each homologous pair don't separate from each other during the first meiotic division.

Prophase II

The first phase of meiosis II. Prophase II is identical to mitotic prophase, except that the number of chromosomes was reduced by half during meiosis I. The events of prophase II are essentially the same as those of mitotic prophase except that prophase II cells are haploid.

What is a key difference between meiosis and mitosis?

The homologous chromosomes are paired in meiosis, but not mitosis.

What process leads to independent assortment?

The random separation of maternal and paternal homologous chromosomes during meiosis I.

How do microtubules move chromosomes in anaphase?

They are degraded at the kinetochore. Chromosome movement is driven by fraying of the ends of microtubules at each kinetochore, just as it is in mitosis (shown below for reference).

A karyotype reveals that a developing rat fetus, which died before birth, carried an extra copy of chromosome 1. Genetically, this individual would best be described as showing __________.

Trisomy

Failure of the zygote to develop into an embryo is the most common result of gamete trisomy. The consequences of meiotic mistakes are almost always severe. Aneuploidy (including trisomy) tends to be lethal to embryos if it involves chromosomes that contain a large number of genes. In many cases, one can assume the zygote was not able to even develop into a viable embryo due to the cells only having one copy of vital genes.

Trisomy-21 is common most likely because it involves a small chromosome with a correspondingly small number of genes. With the exception of trisomy-21, most trisomies and monosomies observed in humans involve the sex chromosomes. In all these cases, the result of trisomy was still viable and does not represent all instances of trisomy in the gametes.

Meiosis II typically produces _____ cells, each of which is _____.

four, haploid