MEIOSIS BABY AND SOURCES OF GENETIC VARIATION MEIOSIS
MEIOSIS (BABY)! AND SOURCES OF GENETIC VARIATION
MEIOSIS • Halves # of chromosomes • is preceded by the replication of chromosomes • two sets of cell divisions, called meiosis I and meiosis II • Results in 4 unique daughter cells – each with one set of unduplicated chromosomes Each daughter cell has only half as many chromosomes as the parent cell © 2011 Pearson Education, Inc.
FIGURE 13. UN 04
FIGURE 13. 7 -1 Interphase Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Sister chromatids Chromosomes duplicate Diploid cell with duplicated chromosomes
FIGURE 13. 7 -2 Interphase Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Sister chromatids Chromosomes duplicate Diploid cell with duplicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with duplicated chromosomes
FIGURE 13. 7 -3 Interphase Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Sister chromatids Chromosomes duplicate Diploid cell with duplicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with duplicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unduplicated chromosomes
THE ORDER OF EVENTS… Interphase (2 n, chromosomes replicate) àMeiosis I (2 daughter cells) Meiosis II (1 n, 4 daughter cells) Bio. Flix: Meiosis http: //www. youtube. com/watch? v=k. VMb 4 Js 99 t. A © 2011 Pearson Education, Inc.
FIGURE 13. 8 MEIOSIS I: Separates sister chromatids MEIOSIS I: Separates homologous chromosomes Prophase I Metaphase I Centrosome (with centriole pair) Sister chromatids Chiasmata Fragments of nuclear envelope Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2 n 6 in this example. Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids remain attached Centromere (with kinetochore) Spindle Homologous chromosomes Telophase I and Cytokinesis Anaphase I Metaphase plate Homologous chromosomes separate Microtubule attached to kinetochore Chromosomes line up by homologous pairs. Cleavage furrow Each pair of homologous chromosomes separates. During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Sister chromatids separate Two haploid cells form; each chromosome still consists of two sister chromatids. Haploid daughter cells forming
SO, IN BRIEF… - Meiosis I separates Homologous Chromosomes - Meiosis II separates Sister Chromatids
FIGURE 13. 8 A Prophase I Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Telophase I and Cytokinesis Anaphase I Metaphase I Sister chromatids remain attached Centromere (with kinetochore) Metaphase plate Fragments Homologous chromosomes of nuclear envelope Homologous chromosomes separate Microtubule attached to kinetochore Cleavage furrow Each pair of homologous chromosomes separates. Chromosomes line up Duplicated homologous chromosomes (red and blue) by homologous pairs. pair and exchange segments; 2 n 6 in this example. Two haploid cells form; each chromosome still consists of two sister chromatids.
FIGURE 13. 8 B Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Sister chromatids separate Haploid daughter cells forming
And at the end of meiosis… Each daughter cell is genetically distinct from the others and from the parent cell © 2011 Pearson Education, Inc.
A COMPARISON OF MITOSIS AND MEIOSIS Mitosis : cells 2 n (diploid diploid) • daughter cells identical to parent • Growth and repair • 1 division Meiosis: 2 n 1 n (diploid haploid) - daughter cells unique from parents and each other • - Synapsis and Crossing over occur • - 2 divisions © 2011 Pearson Education, Inc.
FIGURE 13. 9 A MEIOSIS MITOSIS Parent cell MEIOSIS I Chiasma Prophase I Duplicated chromosome Chromosome duplication 2 n 6 Chromosome duplication Homologous chromosome pair Metaphase I Anaphase Telophase Anaphase I Telophase I Daughter cells of meiosis I 2 n Daughter cells of mitosis 2 n Haploid n 3 MEIOSIS II n n Daughter cells of meiosis II
FIGURE 13. 9 B SUMMARY Property Mitosis Meiosis DNA replication Occurs during interphase before mitosis begins Occurs during interphase before meiosis I begins Number of divisions One, including prophase, metaphase, and telophase Two, each including prophase, metaphase, and telophase Synapsis of homologous chromosomes Does not occur Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Number of daughter cells and genetic composition Two, each diploid (2 n) and genetically identical to the parent cell Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Role in the animal body Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes
ORIGINS OF GENETIC VARIATION AMONG OFFSPRING • Three mechanisms contribute to genetic variation §Independent assortment of chromosomes §Crossing over §Random fertilization © 2011 Pearson Education, Inc.
1. CROSSING OVER • Prophase I • nonsister chromatids exchange DNA segments • Creates recombinant chromosomes © 2011 Pearson Education, Inc.
CROSSING OVER • Each pair of chromosomes lines up gene by gene and forms a tetrad (group of 4 chromatids) • Each tetrad has one or more chiasmata (X-shaped regions where crossing over occurred) • Homologous portions of two nonsister chromatids trade places © 2011 Pearson Education, Inc.
In crossing over, homologous portions of two nonsister chromatids trade places Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome © 2011 Pearson Education, Inc.
FIGURE 13. 11 -1 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis
FIGURE 13. 11 -2 Prophase I of meiosis Pair of homologs Chiasma Centromere TEM Nonsister chromatids held together during synapsis
FIGURE 13. 11 -3 Prophase I of meiosis Pair of homologs Chiasma Centromere TEM Anaphase I Nonsister chromatids held together during synapsis
FIGURE 13. 11 -4 Prophase I of meiosis Pair of homologs Chiasma Centromere TEM Anaphase II Nonsister chromatids held together during synapsis
FIGURE 13. 11 -5 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere TEM Anaphase II Daughter cells Recombinant chromosomes
FIGURE 13. 11 A Chiasma Centromere TEM
2. INDEPENDENT ASSORTMENT OF CHROMOSOMES Homologous pairs of chromosomes orient randomly at metaphase I of meiosis Each pair of chromosomes are sorted independently of the other pairs (Law of Segregation states that allele pairs separate during meiosis and then recombine during fertilization) © 2011 Pearson Education, Inc.
The number of combinations possible when chromosomes assort independently into gametes is 2 n, where n is the haploid number For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes © 2011 Pearson Education, Inc.
FIGURE 13. 10 -1 Possibility 2 Possibility 1 Two equally probable arrangements of chromosomes at metaphase I
FIGURE 13. 10 -2 Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Metaphase II
FIGURE 13. 10 -3 Possibility 2 Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4
3. RANDOM FERTILIZATION • any sperm can fuse with any ovum (unfertilized egg) • fusion of two gametes (each with 8. 4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations © 2011 Pearson Education, Inc.
Crossing over adds even more variation Each zygote has a unique genetic identity Animation: Genetic Variation © 2011 Pearson Education, Inc.
GAMETOGENESIS HAPPENS DIFFERENTLY IN MALES AND FEMALES Germ cell = cell that will undergo meiosis and lead to gametes - a germ line mutation is one that will be passed down to future generations Oogenesis – making eggs Spermatogenesis – making sperm
SPERMATOGENESIS • Makes 4 functional sperm cells It takes about 72 days for a sperm to fully mature
OOGENESIS • Makes one functional egg and 3 polar bodies
THE GOOD OLE’ EVOLUTION CONNECTION… (Genetic variation produced in sexual life cycles contributes to evolution) • Mutations (changes in an organism’s DNA) are the original source of genetic diversity • Mutations create different versions of genes called alleles • Reshuffling of alleles during sexual reproduction produces genetic variation © 2011 Pearson Education, Inc.
THE EVOLUTIONARY SIGNIFICANCE OF GENETIC VARIATION WITHIN POPULATIONS • Natural selection results in the accumulation of genetic variations favored by the environment Therefore…. Sexual reproduction contributes to the genetic variation in a population, which originates from mutations © 2011 Pearson Education, Inc.
FIGURE 13. 12 200 m
FIGURE 13. UN 01 Prophase I: Each homologous pair undergoes synapsis and crossing over between nonsister chromatids with the subsequent appearance of chiasmata. Metaphase I: Chromosomes line up as homologous pairs on the metaphase plate. Anaphase I: Homologs separate from each other; sister chromatids remain joined at the centromere.
FIGURE 13. UN 02 F H
FIGURE 13. UN 03
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