Chapter 13

__ -- eggs and sperm-- are made by a special sort of cell division called __, which occurs only in reproductive organs. In __, a __ cell undergoes two consecutive divisions to produce four __ cells, which have only half as many chromosomes as the parent cell.

A haploid cell, such as an egg or sperm. Gametes unite during sexual reproduction to produce a diploid zygote.

A two-stage type of cell division in sexually reproducing organisms that results in cells with half the chromosome number of the original cell.

A cell containing two sets of chromosomes (2n), one set inherited from each parent.

A cell containing only one set of chromosomes (n).

Like mitosis, __ is preceded by an __, during which the chromosomes replicate. The __ also duplicates in preparation for cell division.

__ begins with condensation of the chromosomes.

__, each made up of two sister __, come together in pairs. This pairing is called __. Each chromosome pair is called a __, a complex of four chromatids.

Chromosome pairs of the same length, centromere position, and staining pattern that possess genes for the same characters at corresponding loci. One homologous chromosome is inherited from the organism’s father, the other from the mother.

The pairing of replicated homologous chromosomes during prophase I of meiosis.

__ of __ cross over each other and exchange parts at __ . Meanwhile, other cellular components prepare for the division of the nucleus. The __ move away from each other, and spindle microtubules form between them. The nuclear envelope and nucleoli disperse. Finally, spindle microtubules capture the __ that form on the chromosomes, and the chromosomes begin moving to the __.

The X-shaped, microscopically visible region representing homologous chromatids that have exchanged genetic material through crossing over during meiosis.

At __ , the chromosome __ are aligned on the __. For each __, __microtubules from one pole of the cell are attached to one __, while __microtubules from the other pole of the cell are attached to the other chromosome of the pair. Thus, the __are poised to move to opposite poles of the cell.

During __, each pair of chromosomes is pulled apart and the __ move toward opposite poles. Note that sister chromatids remain attached at their __and move as a unit toward the same pole.

The chromosomes finish their journey during __ , and __ occurs, producing two __daughter cells. Note that each chromosome still consists of two sister chromatids. __ isn't over yet; remember that it consists of two consecutive divisions. During __, the sister chromatids will be separated.

The second division of meiosis-- __-- is essentially the same as mitosis. The important thing to remember is that __ begins with a __ cell. During __, a spindle forms in each cell and the chromosomes move toward the middle of each cell.

During __, the chromosomes align on the __.

Sister chromatids are pulled apart in __.

In __, nuclei form at opposite poles of each dividing cell, and __splits the cells apart. Meiosis has produced four __cells, each with one set of chromosomes.

The duplicated homologous chromosomes pair up, and crossing-over occurs. At this point each homologous chromosome pair is visible as a tetrad, a tight grouping of four chromatids, two of them sisters and two of them non-sisters. The sites of crossing-over are seen as crisscrossed chromatids and are called chiasmata (singular: chiasma).

While chromosome duplication took place prior to meiosis I, no new chromosome replication occurs before meiosis II. In this, then, the chromosomes are seen as pairs of sister chromatids attached by their centromeres.

The homologous pairs of chromosomes (tetrads) become aligned in the middle of the cell and are attached to the now fully formed meiotic spindle. Each homologue consists of two sister chromatids, so there are, in total, four chromosomes.

Each chromosome becomes aligned in the middle of the cell, much as the chromosomes do in mitosis, and is attached to the now fully formed spindle. Each chromosome consists of one pair of sister chromatids, so there are, in total, two chromosomes.

The two chromosomes of each tetrad separate and start to move toward opposite poles of the cell. The sister chromatids remain attached at their centromeres and move together toward the poles.

The centomeres separate, and the two chromatids of each chromosome--now called daughter chromosomes--move separately to opposite poles on the spindle.

The homologous chromosome pairs complete their migration to the two opposite poles. Now a set of chromosomes is at each pole, with each chromosome still having two chromatids

The two chromatids of each chromosome have separated during anaphase II to produce individual chromosomes, each of a single chromatid, and that is what is seen in each nuclear area of it.

In sexually reproducing organisms, three processes lead to most genetic variation:

The union of haploid gametes to produce a diploid zygote.

Each pair of __ consists of one chromosome inherited from the father and one from the mother. Each pair of chromosomes lines up independently of the other pairs in __ of meiosis.

There are two different ways that each chromosome pair can line up. That means that in the organism shown here, with a diploid number of four, independent orientation of chromosomes at __ can produce __with four different combinations of maternal and paternal chromosomes. In a human being, with 46 chromosomes, more than eight million combinations are possible.

The reciprocal exchange of genetic material between nonsister chromatids during prophase I of meiosis.

Now let's look at how __ creates even more genetic variability. During __ of meiosis, __ pair up very closely, and corresponding parts of two nonsister chromatids may trade places. This process of crossing over creates variation by producing chromosomes that combine the genes inherited from two parents.

In humans, __ happen an average of two or three times per chromosome pair, greatly increasing the variation among eggs and sperm. Note that crossing over produces some __ with chromosomes like those of the parents, and some recombinant gametes with a mixture of genes from both sets of chromosomes.

__and __ occur simultaneously during meiosis, multiplying the number of genetic variations among gametes.

Because each pair of chromosomes lines up independently, and crossovers can occur almost anywhere along each pair of chromosomes, it is possible for a human being to do what??

A sperm fertilizes an egg, producing a __.

The random nature of __ adds to the variation arising from meiosis.

Each parent is capable of producing a huge variety of genetically different __. The number of possible combinations among their offspring is staggering.