Meiosis Genetic Variation •Describe how chromosome assortment during meiosis contributes to genetic variation. •Explain how crossing over contributes to genetic variation. •Compare and contrast mitosis and meiosis.

Genetic Variety

• As you already know, offspring that result from sexual reproduction (2 parents) are genetically different from their parents and from one another.

• This genetic variety in offspring is the raw material for natural selection, which will be explored later.

• This section explores how genetic variety arises through meiosis and fertilization.

Tetrads• Tetrads form during metaphase I.• A tetrad (or bivalent) is 2 duplicated,

homologous chromosomes (dyads) joined together

• The process of dyads pairing up across the equator of a cell is called SYNAPSIS.

• A tetrad has 4 chromatids.

SYNAPSIS

Assortments of Chromosomes• Here is one way that meiosis contributes to genetic variety. The example is an organism with

a diploid chromosome number of four (2n = 4). How the chromosomes in each homologous pair (tetrads) line up and separate at metaphase I is a matter of chance, like the flip of a coin. So, the assortment of chromosomes that end up in the resulting cells occurs randomly. In this example, four combinations are possible.

• In a diploid cell with four chromosomes (two homologous pairs), there are two equally possible ways for the chromosomes inherited from the two parents to be arranged during metaphase I. This variation in the orientation of chromosomes leads to gametes with four equally possible combinations of chromosomes.

Independent Assortment• If you know the haploid number for an organism, you

can calculate the number of possible combinations in the gametes. The possible combinations are equal to 2n, where n is the haploid number.

• For this organism n = 3, so the number of chromosome combinations is 23, or 8.

Law of Independent Assortment

CLICK for an excellent explanation of the Law of Independent AssortmentHomologous

Chromosomes

Homologous Chromosomes

•For a human, n = 23, so there are 223, or about 8 million, possible chromosome combinations!•Simply put, this means that if we only consider the independent assortment of chromosomes during metaphase I, it is possible for human males to produce about 8 million genetically different sperm cells!

Independent Assortment of ChromosomesIncreases genetic variability

Crossing Over• The number of different chromosome

combinations in gametes is just one factor that contributes to genetic variation. A 2nd factor is crossing over—the exchange of genetic material between homologous chromosomes.

• This exchange occurs during prophase I of meiosis.

• When crossing over begins, homologous chromosomes are closely paired as tetrads from the process of synapsis. There is a precise gene-by-gene alignment between adjacent chromatids of the two homologous chromosomes.

• Segments of the two chromatids can be exchanged at one or more sites.

• Early in prophase I, a chromatid from one chromosome exchanges a segment with the corresponding segment from the other chromosome. These altered chromosomes give rise to what are known as "recombinant chromosomes" in the gametes.

Two homologous chromosomes (dyads) paired up to form a

tetrad.

S

phaseMetaphase I

• So, on top of all the possible chromosome combinations resulting from independent assortment, crossing over adds another source of variation.

• Crossing over can produce a single chromosome that contains a new combination of genetic information from different parents, a result called genetic recombination.

• Because chromosomes may contain hundreds of genes, a single crossover event can affect many genes.

Independent assortment & crossing over genetic variability.

Review: Comparison of Mitosis and Meiosis

You have now learned about 2 versions of cell reproduction in eukaryotic organisms.

1. Mitosis, which provides for growth, repair, and asexual reproduction, produces daughter cells that are genetically identical to the parent cell.

2. Meiosis, which takes place in a subset of specialized cells in sexually reproducing organisms, yields haploid daughter cells with only one set of homologous chromosomes. This set consists of one member of each homologous pair.

In both mitosis and meiosis, the chromosomes duplicate only once, in the preceding interphase. Mitosis involves one division of the genetic material in the nucleus, and it is usually accompanied by cytokinesis, producing two diploid cells. Meiosis involves two nuclear divisions, yielding four haploid cells.

•In prophase I, the duplicated homologous chromosomes form tetrads, and crossing over occurs. Then, during metaphase I, the tetrads (rather than individual doubled chromosomes) are aligned at the center of the cell.

•In anaphase I, sister chromatids stay together and go to the same pole when the homologous chromosomes separate.

•At the end of meiosis I, the chromosome number in each of the two daughter cells is haploid, but each chromosome still consists of two sister chromatids.

•Meiosis II is basically identical to mitosis. The sister chromatids separate, and each cell divides in two. Because these cells are already haploid, the cells they produce are haploid, too.

The key events that distinguish meiosis from mitosis occur during the stages of meiosis I.

•Meiosis II is basically identical to mitosis. The sister chromatids separate, and each cell divides in two. •Because these cells are already haploid, the cells they produce are haploid, too.

Mitosis & Meiosis

• Mitosis and meiosis both make it possible for cells to inherit genetic information in the form of chromosome copies.

• Both mitosis and meiosis begin after the chromosomes have been duplicated during interphase. Though similar, the results of the two processes differ in the number of cells produced and in the number of chromosomes the cells contain.

9.6 Online Review

• Independent assortment of chromosomes animation• Independent assortment tutorial & quiz• Meiosis overview & crossing over tutorial • Crossing over and genetics tutorial (Advanced)• Meiosis/Mitosis Tutorial• Meiosis Quiz 1, Quiz 2, Quiz 3, Quiz 4, Quiz 5

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