Meiosis Genetic Variation. Homologous Chromosomes homologous chromosomes- a pair of chromosomes that...
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Transcript of Meiosis Genetic Variation. Homologous Chromosomes homologous chromosomes- a pair of chromosomes that...
Homologous Chromosomes• homologous chromosomes- a pair of
chromosomes that code for the same trait (one from mom one form dad)
• 23 pairs in humans
• 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)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Meiosis
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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