Sexually reproducing organisms create new offspring with the fusion of two parental cells.  If two somatic cells, with a full complement of chromosomes, were allowed to unite, each new cell would have twice the number of chromosomes its parents had.  To prevent this doubling from occurring in sexual reproduction, a special division process (meiosis) is needed.  The process of meiosis produces gametes (sex cells) that contain only half the diploid number.  This is called the haploid number.  This ensures that when fertilization occurs during sexual reproduction, two haploid gametes join to make a single diploid cell called the zygote.

Chromosome Number and Structure:

Human somatic cells:

◦diploid number of 46 and is designed 2n
◦chromosomes are arranged in 23 pairs of homologous chromosomes
◦one chromosome of each pair came from the individuals mother
◦one chromosome of each pair came from the individuals father
◦23 chromosomes from the mother are called the maternal set
◦23 chromosomes from the father are called the paternal set

Homologous

◦carry the same genes (hereditary traits) as its partner

Chromosomes:

◦carry genes for the same trait at the same locus (location)
◦the genes for a specific trait are not necessarily identical
◦the different forms of the same gene are called alleles
◦are the same size and shape and have the same centromere location

Important Meiosis Details:

Prophase I:

◦homologous chromosomes come together (synapsis) forming a tetrad (4 chromatids)
◦during synapsis crossing over occurs, breaking and reuniting of  chromsomes; allows the exchange of genetic material (genetic variation)

Metaphase I:

◦tetrads move to the metaphase plate
◦independent assortment occurs

Anaphase I:

◦homologous move apart, one chromosome of each pair going to each pole
◦the number of chromosomes has just been halved: from 2n to n
◦this is called reduction division

Telophase I:

◦two haploid daughter cells with exactly half the number of chromosomes as the parent are produced

Meiosis II:

◦no duplication of genetic material between meiotic divisions
◦similar to mitosis, but begins with half the genetic material
◦the four cells produced contain a haploid number of chromosomes
◦four cells go on to form sperm (males) or eggs (females)

Independent Assortment:

When the homologous pairs line up at the metaphase plate, there is no necessity for chromosomes that came from the mother to stay together.  In other words, homologous pairs line up at the metaphase plate independently of each other.  Some gametes may receive all maternal chromosomes and some may receive all maternal chromosomes.  Others may receive a mixture of maternal and paternal chromosomes.

The number of different possible kinds of gametes produced by random assortment of chromosomes is 2n where n is the haploid number of chromosomes in the organism.

Independent assortment is the second way that meiosis produces genetic variability.

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