Cellular Division - AP Biology

Crossing over, or recombination, is a process that takes place in the earlier stages of meiosis and promotes genetic diversity. During recombination, genetic material is exchanged between two homologous chromosomes. The chromosomes ultimately contain the same amount of genetic material after recombination, and are properly separated during subsequent divisions.

The cleavage of the protein securin is actually what allows the sister chromatids to separate, a process that is essential to maintaining the correct number of chromosomes in each daughter cell.

Nondisjunction is the name given to the phenomenon in which separation of genetic material fails to occur. Either homologous chromosomes fail to separate properly in meiosis I, or sister chromatids fail to separate properly during meiosis II. The result of these nondisjunction events is one cell with an abnormally high number of chromosomes (for example trisomy) and one cell with an abnormally low number of chromosomes (for example monosomy).

Crossing over occurs during prophase of meiosis I (prophase I). This process requires tetrad formation, where the homologous chromosomes (with their sister chromatids) pair with each other. Following tetrad formation, the genetic material from one homologous chromosome can be exchanged with that of the other. This exchange of genetic material leads to genetic recombination and results in production of non-identical gametes. Crossing over occurs only between homologous chromosomes. Sister chromatids are situated to form a single chromosome; crossing over does not include recombination of genetic material within a single chromosome.

Remember that crossing over is not a mutation and is a completely natural process for every sexually reproducing organism.

For this question you have to carefully track the chromosomes through meiosis. A human cell in metaphase I will have formed the tetrads and would have aligned the genetic material along the metaphase plate. The sister chromatids are still attached to one another, so they only count as one chromosome per pair of chromatids. There are a total of 46 chromosomes in metaphase I, each comprised of two sister chromatids. There are 23 homologous pairs, each containing two complete chromosomes.

During telophase II, the cell is in a haploid state. The homologous pairs have been separated during anaphase I, such that each cell contains 23 complete chromosomes. Each chromosome is then broken into its chromatids, such that the total number of chromosomes represented during anaphase II is 46, with each chromatid representing a chromosome. If each of the chromosomes still had its sister chromatid, then the total number of chromosomes would be 23. Telophase II follows anaphase II. The 46 chromatids are sequestered to opposite sides of the cell, but the cell has not yet divided. A cell in telophase II is haploid, containing only one copy of each homologous chromosome, but contains two chromatids for each copy. The total number of chromosomes in a telophase II cell is thus 46. As soon as the cell completes cytokinesis, and two daughter cells are formed, they become haploid cells with 23 chromosomes each.

For this question, remember that a cell containing only one homologue is a haploid cell. Cells containing two homologous chromosomes are considered diploid.

Following the S phase and mitosis, the cells are diploid because they contain pairs of homologous chromosomes.

Following meiosis I, however, the daughter cells are haploid because they contain only one homologue. These homologues still consist of two identical sister chromatids, which will be separated following meiosis II, but the halving of genetic material during meiosis I still generates haploid daughter cells.

In prophase I, homologous chromosomes pair up and facilitate the exchange of genetic information through the process of crossing over.

Remember that the law of independent assortment states that genes on different chromosomes are passed independently of one another to offspring. This phenomenon results from the random alignment of the chromosomes along the metaphase plate. This random alignment allows genes to be segregated independently, and occurs during metaphase I.

Metaphase II involves the alignment of single chromosomes along the metaphase plate for segregation of identical sister chromatids. Remember that independent assortment is only valid for genes on different chromosomes. Genes on the same chromosomes are not passed independently of one another from parent to offspring.

Independent assortment can, thus, only occur during metaphase I, since this phase involves alignment of independent, non-identical chromosomes.

According to the Law of Dominance, each individual has two alleles for each trait and only the dominant allele contributes to the phenotype.

According to the Law of Segregation, during Meiosis, homologous chromosomes segregate into different gametes.

This is the definition of the law of independent assortment; during meiosis, the inheritance of one gene does not influence whether another, separate gene will also be inherited by that gamete.

The laws of inheritance include the laws of segregation (each gamete receives only one copy of each gene from its parent), dominance (in a heterozygote individual, only the dominant allele will influence the phenotype), and independent assortment (inheritance of one gene does not influence inheritance of another gene)

The law of independent assortment states that inheritance of one gene does not influence inheritance of another gene. Thus, inheritance of seed color does not affect the inheritance of seed shape.

According to the Law of Segregation, each gamete receives one allele for each gene from each parent. During Meiosis, each parent’s two copies of each allele are separated from each other, then the gamete receives one copy of each allele from each parent (for a total of two alleles).

Recombination or crossing over happens primarily in Prophase I. In anaphase and telophase, the DNA strands are separated and cannot recombine. In metaphase, homologous chromosomes line up with each other, but do not recombine.

Mitosis and meiosis share many procedural similarities, however, it is important to remember that mitosis makes identical cells while meiosis allows genetic variability between cells.

Meiosis takes place in sex cells, which are capable of creating millions of different variations of offspring. Mitosis takes place in normal cells involved in growth and regeneration within a single, uniform organism.

Meiosis includes two divisions, resulting in two pairs of haploid cells, while mitosis only involves a single division.

Both meiotic and mitotic divisions share the same phases, though there are some differences in the activity of these phases. Mutation can, and does, occur in both mitosis and meiosis. Crossing over, however, is unique to meiosis.

Crossing over ensures genetic variability as it results in daughter cells with different genetic material than their parent cells. This occurs during meiosis I, but is not seen in mitosis or meiosis II.

Both mitosis and meiosis occur in humans. Somatic cells (body cells) divide via mitosis, while gametes (sex cells) divide via meiosis. Because of actions such as crossing over, meiosis results in a higher genetic variability than mitosis.

Prokaryotes, such as bacteria, reproduce asexually, and are incapable of meiosis.

Turner syndrome occurs when a person is missing one sex chromosome, and only has one X-chromosome. As a result, they will be female, and may suffer a variety of symptoms. This is an example of monosomy, in which a person only has one chromosome, when they should have two.

The other three choices are examples of trisomy. Klinefelter is an instance of sex-linked trisomy, with a karyotype of XXY. Down syndrome is cause by trisomy 21, and Edwards syndrome is caused by trisomy 18.

Non-disjunction occurs when sister chromatids fail to separate during meiosis, leading subsequent daughter cells to have an unequal number of chromosomes. This can result in a cell having one extra chromosome (trisomy), or missing one chromosome (monosomy).

The following shows non-disjuction occuring in metaphase II of meiosis. "X" represents a chromosome, "|" represents a cell membrane, and "\" and "/" represent chromatids. When a sperm cell fuses with the egg formed to the left of the membrane (|), it will result in trisomy. When a sperm fuses with the egg to the right of the membrane (|), it will result in monosomy.

Metaphase I: X X

Telophase I: X | X

Metaphase II: / \

Telophase II: / \ |