Meiosis

Genetic Variation

Homologous Chromosomes• homologous chromosomes- a pair of

chromosomes that code for the same trait (one from mom one form dad)

• 23 pairs in humans

Karyotype• A karyotype - is an ordered display of the pairs

of homologous chromosomes from a cell

• sex chromosomes - are called X and Y• Human females (XX) homologous pair• Human males (XY)homologous pair• autosomes -The 22 pairs of chromosomes that

do not determine sex (determine everything else)

Chromosomes Types

XXXY

• Meiosis – creates four haploid (n) gametes (egg or sperm) (two divisions)

• Egg and sperm unite to form a diploid (2n) zygote. (develops into an embryo)

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Meiosis

Fig. 13-4 Interphase• The same as mitosis

Prophase I• Longest phase (two main things happen in

prohase one)• Synapsis • Crossing over

Synapsis• Synapsis – homologous chromosomes pair up

• Sister chromatids connnect to other homologous sister chromatids to form a tetrad.

(Synapsis)

Tetrad

Crossing Over• Crossing over – homologous chromosomes

exchange DNA while connected as tetrads.

• Chiasma -the point at which they connect to exchange DNA

• *

• In metaphase I, tetrads line up at the metaphase plate• Independent assortment – homologous chromosomes

line up randomly on any side (ex. All of mom’s chromosomes don’t line up on one side)

Metaphase I

• In anaphase I, tetrads separate (homologous chromosomes are separated)

• Sister chromatids remain attached at the centromere and move as one unit toward the pole

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Anaphase I

• The cell is divided into two halves with sister chromatids in each half. They are no longer identical because of crossing over.

• Cytokinesis usually occurs simultaneously, forming two daughter cells (not identical). 2 cells are now present.

Telephase I

Cleavage Furrow

• Meiosis II is very similar to mitosis: • Prophase II – spindle fibers and kinetichores

form• Metaphase II – chromatids line up• Anaphase II – chromatids are separated into

individual chromosomes• Telophase II and cytokinesis – each cell splits,

and nuclei reform (4 unidentical haploid cells are now present)

• *

Meiosis II (repeat)

Fig. 13-8d

Prophase II Metaphase II Anaphase II Telophase II andCytokinesis

Sister chromatidsseparate Haploid daughter cells

forming

Mitosis vs. Meiosis

Creating Variation• The behavior of chromosomes during

meiosis and fertilization is responsible for most of the variation that arises in each generation

• Three mechanisms contribute to genetic variation:– Independent assortment of chromosomes– Crossing over– Random fertilization

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Independent Assortment• The number of combinations possible when

chromosomes assort independently into gametes is 2n, where n is the haploid number

Crossing Over• Crossing overproduces recombinant chromosomes(new combinations of chromosomes), which combines genes inherited from each parent

Random Fertilization• Random fertilization adds to genetic variation

because any sperm can fuse with any egg• An egg and sperm can make about 70 trillion

diploid combinations• *

You should now be able to:

1. Distinguish between the following terms: somatic cell and gamete; autosome and sex chromosomes; haploid and diploid

2. Describe the events that characterize each phase of meiosis

3. Describe three events that occur during meiosis I but not mitosis

4. Name and explain the three events that contribute to genetic variation in sexually reproducing organisms

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings