MEIOSIS AND SEXUAL REPRODUCTION SEXUAL REPRODUCTION Benefits Creates
MEIOSIS AND SEXUAL REPRODUCTION
SEXUAL REPRODUCTION Benefits? Creates variation and new gene combinations Joining of 2 cells called gametes (n, sperm + egg) to make zygote (2 n) Gametes are made by the process of meiosis (NOT mitosis!) Egg 23 fertiliz Sperm 23 fertilization Zygote 46
MEIOSIS Makes haploid gametes (cuts chromosome number in half) Involves two divisions (no replication inbetween) resulting in 4 cells with half the number of chromosomes Homologous chromosomes pair up and cross over during genetic recombination Homologous chromosomes are then divided up into different daughter cells.
REMEMBER… Homologous chromosomes are pairs of chromosomes that have information for the same traits. One set of information came from Mom. One set of information came from Dad.
Meiosis sexual reproduction haploid, diploid, gamete homologous chromosome two stages of meiosis independent assortment crossing-over FIGURE 13. 4 DESCRIBING CHROMOSOMES IN A DIPLOID CELL AT METAPHASE - IN A EUKARYOTE Key Maternal set of chromosomes (n = 3) 2 n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)
STEPS OF MEIOSIS Meiosis I Prophase I Metaphase I Anaphase I Telophase I Reduction Division: separates homologous pairs Meiosis II Prophase II Metaphase II Anaphase II Telophase II 2 nd division separates sister chromatids (identical process to mitosis)
PROPHASE I Homologous chromosomes find each other and pair up Crossing over occurs at sites of contact Chromosomes actually break and swap DNA Results in new combinations of traits in the next generation
METAPHASE I Homologous pairs line up on metaphase plate. Independent assortment: random orientation of chromosomes pairs on metaphase plate
ANAPHASE I Homologous pairs are pulled apart from one another A haploid set of chromosomes ends up at each pole Note: each chromosome still has the sister chromatid attached
INTO 2 DAUGHTER CELLS THAT ARE HAPLOID
Meiosis two stages of meiosis FIGURE 13. 8 THE MEIOTIC DIVISION OF AN ANIMAL CELL MEIOSIS I: Separates homologous chromosomes INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Sister chromatids remain attached Centromere (with kinetochore) Centrosomes (with centriole pairs) Sister chromatids Nuclear envelope Chiasmata Spindle Metaphase plate Homologous Microtubule chromosomes attached to Chromatin separate kinetochore Tetrads line up Chromosomes duplicate Homologous chromosomes (red and blue) pair and exchange segments; 2 n = 6 in this example Tetrad Pairs of homologous chromosomes split up
MEIOSIS II Involves 2 cells at once Exact same process as mitosis, except each cell only has 23 chromosomes now. Sister chromatids are separated Results in 4 cells, with 23 chromosomes each (haploid).
Meiosis sexual reproduction haploid, diploid, gamete homologous chromosome two stages of meiosis independent assortment crossing-over Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes FIGURE 13. 7 OVERVIEW OF MEIOSIS: HOW MEIOSIS REDUCES CHROMOSOME NUMBER Sister chromatids Diploid cell with replicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unreplicated chromosomes
FIGURE 13. 9 A COMPARISON OF MITOSIS AND MEIOSIS Meiosis sexual reproduction haploid, diploid, gamete homologous chromosome two stages of meiosis independent assortment crossing-over MITOSIS MEIOSIS Chiasma (site of crossing over) Parent cell (before chromosome replication) MEIOSIS I Prophase Chromosome replication Duplicated chromosome (two sister chromatids) Chromosome replication Tetrad formed by synapsis of homologous chromosomes 2 n = 6 Chromosomes positioned at the metaphase plate Metaphase Sister chromatids separate during anaphase Anaphase Telophase 2 n Tetrads positioned at the metaphase plate Homologues separate during anaphase I; sister chromatids remain together Metaphase I Anaphase I Telophase I Haploid n=3 Daughter cells of meiosis I 2 n MEIOSIS II Daughter cells of mitosis n n n Daughter cells of meiosis II Sister chromatids separate during anaphase II n
Meiosis FIGURE 13. 10 THE INDEPENDENT sexual reproduction ASSORTMENT OF HOMOLOGOUS haploid, diploid, gamete homologous chromosome CHROMOSOMES IN MEIOSIS two stages of meiosis independent assortment Key crossing-over Maternal set of chromosomes Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4
Meiosis sexual reproduction haploid, diploid, gamete homologous chromosome two stages of meiosis independent assortment crossing-over Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase II FIGURE 13. 11 THE RESULTS OF CROSSING OVER DURING MEIOSIS Daughter cells Recombinant chromosomes
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