Meiosis

Where are genes? The chromosomes contain the genes Remember that chromosomes are made up of DNA and protein Genes are segments of DNA that code for a specific protein the unit by which hereditary characteristics are transmitted

How are genes passed down? We need to look at specialized cells that pass genetic information to offspring Reproductive cells called gametes Each organism contains two copies of every gene ex: a gene for eye color may have the mothers blue eye form and also the father’s brown eye form

Therefore… When a mother passes genetic info to her offspring through reproduction, she only passes one half or one copy. The father does the same The genetic information gets passed in the gamete or reproductive cell Sperm and egg

Sperm and Egg

Gametes (sperm and egg) Contain half the number of chromosomes found in other cells in the organism Human cells contain 46 chromosomes Gametes: sperm and egg contain 23 chromosomes haploid (n) nonfunctional When the sperm and egg unite, the 23 c-somes from the mother and father unite to create an offspring with 46 c-somes This process is called fertilization and produces a zygote. The resulting cell is diploid (2n) and functional

Meiosis referred to as the reduction division number of chromosomes in a diploid cell is reduced by half produces haploid cells called gametes (sex cells) from 1 diploid cell you end up with 4 haploid cells (sex cells) that are genetically different there are 2 stages: Meiosis I and Meiosis II 8

Telophase I and Cytokinesis Phases of Meiosis I Meiosis I Meiosis I Interphase I During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell. Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis

Meiosis I I) Interphase I most common resting stage preparation to divide 10

Meiosis I II) Prophase I chromosomes become visible homologous chromosomes pair up (tetrad) centrioles descend from equator to poles nuclear membrane disappears 11

Prophase I When homologous chromosomes form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing over or synapsis. Crossing-over produces new combinations of alleles.

Meiosis I III) Metaphase I homologous chromosomes line up in pairs along the equator spindle fibers attach to the centromere 13

Anaphase I IV) Anaphase I spindle fibers shorten pulling homologous chromosome pairs apart 14

Telophase I V) Telophase I 1) cell divides (cytokinesis) 2) nuclear membrane reappears 3) centrioles ascend back to the equator 4) end up with 2 distinct daughter cells (diploid) 15

Meiosis 16

Meiosis II

Meiosis II I) Interphase II 1) skipped 2) chromosomes don’t replicate 18

Meiosis II II) Prophase II (reduction phase) 1) chromosomes become visible 2) centrioles descend from equator to poles 3) nuclear membrane disappears 19

Meiosis II III) Metaphase II 1) chromosomes line up along the equator 2) spindle fibers attach to the centromere 20

Meiosis II IV) Anaphase II 1) spindle fibers shorten pulling sister chromatids apart 21

Meiosis II V) Telophase II 1) cell divides 2) nuclear membrane reappears 3) centrioles ascend back to the equator 4) end up with 4 haploid daughter cells Meiosis 22

Meiosis II 23

Meiosis Overview 24

Gamete Formation Gamete Formation In male animals, meiosis results in four equal-sized gametes called sperm. Meiosis produces four genetically different haploid cells. In males, meiosis results in four equal-sized gametes called sperm.

Gamete Formation In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction. Meiosis produces four genetically different haploid cells. In females, only one large egg cell results from meiosis. The other three cells, called polar bodies, usually are not involved in reproduction.

Comparing Mitosis and Meiosis Cells produced by mitosis have the same number of chromosomes and alleles as the original cell. Mitosis allows an organism to grow and replace cells. Some organisms reproduce asexually by mitosis.

Comparing Mitosis and Meiosis Cells produced by meiosis have half the number of chromosomes as the parent cell. These cells are genetically different from the diploid cell and from each other. Meiosis is how sexually-reproducing organisms produce gametes.

What type of cell does it occur with?   Mitosis Meiosis What type of cell does it occur with? Number of daughter cells produced? Are the cells identical or different? How many divisions? Type of reproduction (sexual or asexual) Crossing over (synapsis) Homologous chromosomes separate (name the phase) Sister chromatids separate (name the phase)

11-4 If the body cells of humans contain 46 chromosomes, a single sperm cell should have 46 chromosomes. 23 chromosomes. 92 chromosomes. between 23 and 46 chromosomes.

11-4 During meiosis, the number of chromosomes per cell is cut in half through the separation of daughter cells. homologous chromosomes. gametes. chromatids.

11-4 The formation of a tetrad occurs during anaphase I. metaphase II. prophase I. prophase II.

11-4 In many female animals, meiosis results in the production of only 1 egg. 1 egg and 3 polar bodies. 4 eggs. 1 egg and 2 polar bodies.

11-4 Compared to egg cells formed during meiosis, daughter cells formed during mitosis are genetically different, while eggs are genetically identical. genetically different, just as egg cells are. genetically identical, just as egg cells are. genetically identical, while egg cells are genetically different.