MEIOSISMEIOSISChapter 10.2

MeiosisMeiosis This kind of cell division

produces gametes containing half the number of chromosomes as a parent’s body cell

Meiosis consists of 2 separate divisions known as Meiosis I and Meiosis II

Diploid cellsDiploid cells Diploid means 2 sets Cell that contains both sets of

homologous chromosomes (one set from each parent)

2n

Haploid cellsHaploid cells Gametes or reproductive

cells Haploid means one set Contain only a single set of

chromosomes n

Homologous chromosomesHomologous chromosomes Paired chromosomes 2 chromosomes of each pair in a

diploid cell Each of a pair of homologous

chromosomes has genes for the same traits

Meiosis IMeiosis I Begins with one diploid (2n) cell During meiosis, a spindle forms and

the cytoplasm divides in the same ways they do during mitosis.

However, what happens to the chromosomes in meiosis is very different

Phases: Prophase I, Metaphase I, Anaphase I, & Telophase I

Prophase IProphase I Chromosomes coil up and spindle

forms Each pair of homologous

chromosomes comes together, matched gene by gene to form a 4 part structure called a tetrad.

Tetrad consists of 2 homologous chromosomes, each made up of 2 sister chromatids

Crossing overCrossing over Tetrads are paired so tightly that

non-sister chromatids from homologous chromosomes sometimes actually exchange genetic material in a process known as crossing over.

Crossing over results in new combinations of genes on a chromosome.

Metaphase IMetaphase I Centromere of each

chromosome becomes attached to a spindle fiber

The spindle fibers pull the tetrads into the middle of the spindle

Homologous chromosomes are lined up side by side as tetrads

Anaphase IAnaphase I Homologous chromosomes,

each with its 2 chromatids, separate and move to opposite ends of the cell.

Centromeres holding sister chromatids together do not split

Telophase ITelophase I Spindle is broken down, the

chromosomes uncoil and cytoplasm divides to yield 2 new cells.

Each cell has only half the genetic information of the original cell

Another cell division is needed because each chromosome is still doubled, consisting of 2 sister chromatids

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Meiosis IIMeiosis II Ends with 4 haploid (n) cells These haploid cells are called sex cells

or gametes Male = sperm Female = egg When a sperm fertilizes an egg, a zygote results

with a diploid number of chromosomes. Zygote can then develop by mitosis into a

multicellular organism. Phases: Prophase II, Metaphase II, Anaphase II,

Telophase II

Newly formed cells in some organisms undergo a short interphase in which the chromosomes do not replicate

In other organisms, however, the cells go from late anaphase of meiosis I directly to metaphase of meiosis II, skipping telophase I, interphase & prophase II.

Prophase IIProphase II A spindle forms in each of

the 2 new cells and the spindle fibers attach to the chromosomes

Prophase II

Metaphase IIMetaphase II The chromosomes, still made

up of sister chromatids, are pulled to the center of the cell and line up randomly at the equator.

Metaphase II

Anaphase IIAnaphase II Centromere of each

chromosome splits, allowing the sister chromatids to separate and move to opposite poles

Anaphase II

Telophase IITelophase II Nuclei reform Spindle breaks down Cytoplasm divides Each haploid cell contains one

chromosome from each homologous pair

These haploid cells will become gametes, transmitting the genes they contain to offspring

Telophase IIhttp://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter11/meiosis_ii.html

Cells that are formed by mitosis are identical to each other and to the parent cell

Meiosis however, provides a mechanism for shuffling the chromosomes and the genetic info they carry.

By shuffling the chromosomes, genetic variation is produced