Meiosis and Sexual Life Cycles

Meiosis and Sexual Life Cycles
Chapter 13 Meiosis and Sexual Life Cycles

Overview: Variations on a Theme
Living organisms are distinguished by their ability to reproduce their own kind Genetics is the scientific study of heredity and variation Heredity is the transmission of traits from one generation to the next Variation is demonstrated by the differences in appearance that offspring show from parents and siblings Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 13-1 Figure 13.1 What accounts for family resemblance?

It is genes that are actually inherited
Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes In a literal sense, children do not inherit particular physical traits from their parents It is genes that are actually inherited Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Genes are the units of heredity, and are made up of segments of DNA
Inheritance of Genes Genes are the units of heredity, and are made up of segments of DNA Genes are passed to the next generation through reproductive cells called gametes (sperm and eggs) Each gene has a specific location called a locus on a certain chromosome Most DNA is packaged into chromosomes One set of chromosomes is inherited from each parent Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Comparison of Asexual and Sexual Reproduction
In asexual reproduction, one parent produces genetically identical offspring by mitosis A clone is a group of genetically identical individuals from the same parent In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents Video: Hydra Budding Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

0.5 mm Parent Bud (a) Hydra (b) Redwoods Fig. 13-2
Figure 13.2 Asexual reproduction in two multicellular organisms Parent Bud (a) Hydra (b) Redwoods

0.5 mm Parent Bud (a) Hydra Fig. 13-2a
Figure 13.2 Asexual reproduction in two multicellular organisms Parent Bud (a) Hydra

Fig. 13-2b Figure 13.2 Asexual reproduction in two multicellular organisms (b) Redwoods

Concept 13.2: Fertilization and meiosis alternate in sexual life cycles
A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Sets of Chromosomes in Human Cells
Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes A karyotype is an ordered display of the pairs of chromosomes from a cell The two chromosomes in each pair are called homologous chromosomes, or homologs Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited characters Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Figure 13.3 Preparing a karyotype
APPLICATION Figure 13.3 Preparing a karyotype TECHNIQUE 5 µm Pair of homologous replicated chromosomes Centromere Sister chromatids Metaphase chromosome

Fig. 13-3a APPLICATION Figure 13.3 Preparing a karyotype

TECHNIQUE 5 µm Pair of homologous replicated chromosomes Centromere
Fig. 13-3b TECHNIQUE 5 µm Pair of homologous replicated chromosomes Centromere Figure 13.3 Preparing a karyotype Sister chromatids Metaphase chromosome

The sex chromosomes are called X and Y
Human females have a homologous pair of X chromosomes (XX) Human males have one X and one Y chromosome The 22 pairs of chromosomes that do not determine sex are called autosomes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

A diploid cell (2n) has two sets of chromosomes
Each pair of homologous chromosomes includes one chromosome from each parent The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father A diploid cell (2n) has two sets of chromosomes For humans, the diploid number is 46 (2n = 46) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Each replicated chromosome consists of two identical sister chromatids
In a cell in which DNA synthesis has occurred, each chromosome is replicated Each replicated chromosome consists of two identical sister chromatids Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of
Fig. 13-4 Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Figure 13.4 Describing chromosomes Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)

For humans, the haploid number is 23 (n = 23)
A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n) For humans, the haploid number is 23 (n = 23) Each set of 23 consists of 22 autosomes and a single sex chromosome In an unfertilized egg (ovum), the sex chromosome is X In a sperm cell, the sex chromosome may be either X or Y Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Behavior of Chromosome Sets in the Human Life Cycle
Fertilization is the union of gametes (the sperm and the egg) The fertilized egg is called a zygote and has one set of chromosomes from each parent The zygote produces somatic cells by mitosis and develops into an adult Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

At sexual maturity, the ovaries and testes produce haploid gametes
Gametes are the only types of human cells produced by meiosis, rather than mitosis Meiosis results in one set of chromosomes in each gamete Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Multicellular diploid adults (2n = 46)
Fig. 13-5 Key Haploid gametes (n = 23) Haploid (n) Egg (n) Diploid (2n) Sperm (n) MEIOSIS FERTILIZATION Figure 13.5 The human life cycle Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)

The Variety of Sexual Life Cycles
The alternation of meiosis and fertilization is common to all organisms that reproduce sexually The three main types of sexual life cycles differ in the timing of meiosis and fertilization Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Gametes are the only haploid cells in animals
In animals, meiosis produces gametes, which undergo no further cell division before fertilization Gametes are the only haploid cells in animals Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Figure 13.6 Three types of sexual life cycles
Key Haploid (n) Haploid unicellular or multicellular organism Diploid (2n) Haploid multicellular organism (gametophyte) n Gametes n n Mitosis n Mitosis Mitosis n Mitosis n n n n n MEIOSIS FERTILIZATION Spores n n Figure 13.6 Three types of sexual life cycles Gametes Gametes n MEIOSIS FERTILIZATION Zygote MEIOSIS FERTILIZATION 2n 2n 2n 2n Diploid multicellular organism Zygote Diploid multicellular organism (sporophyte) 2n Mitosis Mitosis Zygote (a) Animals (b) Plants and some algae (c) Most fungi and some protists

Key Haploid (n) Diploid (2n) Gametes n n n MEIOSIS FERTILIZATION
Fig. 13-6a Key Haploid (n) Diploid (2n) Gametes n n n MEIOSIS FERTILIZATION Figure 13.6a Three types of sexual life cycles—animals Zygote 2n 2n Diploid multicellular organism Mitosis (a) Animals

Plants and some algae exhibit an alternation of generations
This life cycle includes both a diploid and haploid multicellular stage The diploid organism, called the sporophyte, makes haploid spores by meiosis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

A gametophyte makes haploid gametes by mitosis
Each spore grows by mitosis into a haploid organism called a gametophyte A gametophyte makes haploid gametes by mitosis Fertilization of gametes results in a diploid sporophyte Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

(b) Plants and some algae
Fig. 13-6b Key Haploid (n) Haploid multicellular organism (gametophyte) Diploid (2n) Mitosis Mitosis n n n n n Spores Gametes Figure 13.6b Three types of sexual life cycles—plants and some algae MEIOSIS FERTILIZATION 2n 2n Zygote Diploid multicellular organism (sporophyte) Mitosis (b) Plants and some algae

The zygote produces haploid cells by meiosis
In most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multicellular diploid stage The zygote produces haploid cells by meiosis Each haploid cell grows by mitosis into a haploid multicellular organism The haploid adult produces gametes by mitosis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Haploid unicellular or multicellular organism Diploid (2n)
Fig. 13-6c Key Haploid (n) Haploid unicellular or multicellular organism Diploid (2n) Mitosis Mitosis n n n n Figure 13.6c Three types of sexual life cycles—most fungi and some protists Gametes n MEIOSIS FERTILIZATION 2n Zygote (c) Most fungi and some protists

However, only diploid cells can undergo meiosis
Depending on the type of life cycle, either haploid or diploid cells can divide by mitosis However, only diploid cells can undergo meiosis In all three life cycles, the halving and doubling of chromosomes contributes to genetic variation in offspring Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Like mitosis, meiosis is preceded by the replication of chromosomes
Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid Like mitosis, meiosis is preceded by the replication of chromosomes Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis Each daughter cell has only half as many chromosomes as the parent cell Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

In the second cell division (meiosis II), sister chromatids separate
The Stages of Meiosis In the first cell division (meiosis I), homologous chromosomes separate Meiosis I results in two haploid daughter cells with replicated chromosomes; it is called the reductional division In the second cell division (meiosis II), sister chromatids separate Meiosis II results in four haploid daughter cells with unreplicated chromosomes; it is called the equational division Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number
Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number

Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes

Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unreplicated chromosomes

The single centrosome replicates, forming two centrosomes
Meiosis I is preceded by interphase, in which chromosomes are replicated to form sister chromatids The sister chromatids are genetically identical and joined at the centromere The single centrosome replicates, forming two centrosomes For the Cell Biology Video Meiosis I in Sperm Formation, go to Animation and Video Files. BioFlix: Meiosis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Figure 13.8 The meiotic division of an animal cell
Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Centrosome (with centriole pair) Sister chromatids remain attached Centromere (with kinetochore) Sister chromatids Chiasmata Spindle Metaphase plate Figure 13.8 The meiotic division of an animal cell Sister chromatids separate Haploid daughter cells forming Homologous chromosomes Homologous chromosomes separate Cleavage furrow Fragments of nuclear envelope Microtubule attached to kinetochore

Division in meiosis I occurs in four phases:
– Prophase I – Metaphase I – Anaphase I – Telophase I and cytokinesis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Prophase I Metaphase I Anaphase I Centrosome (with centriole pair)
Fig. 13-8a Telophase I and Cytokinesis Prophase I Metaphase I Anaphase I Centrosome (with centriole pair) Sister chromatids remain attached Centromere (with kinetochore) Sister chromatids Chiasmata Spindle Metaphase plate Figure 13.8 The meiotic division of an animal cell Cleavage furrow Homologous chromosomes Homologous chromosomes separate Fragments of nuclear envelope Microtubule attached to kinetochore

Chromosomes begin to condense
Prophase I Prophase I typically occupies more than 90% of the time required for meiosis Chromosomes begin to condense In synapsis, homologous chromosomes loosely pair up, aligned gene by gene Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

In crossing over, nonsister chromatids exchange DNA segments
Each pair of chromosomes forms a tetrad, a group of four chromatids Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Metaphase I In metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad Microtubules from the other pole are attached to the kinetochore of the other chromosome Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Prophase I Metaphase I Centrosome (with centriole pair) Centromere
Fig. 13-8b Prophase I Metaphase I Centrosome (with centriole pair) Centromere (with kinetochore) Sister chromatids Chiasmata Spindle Metaphase plate Figure 13.8 The meiotic division of an animal cell Homologous chromosomes Fragments of nuclear envelope Microtubule attached to kinetochore

In anaphase I, pairs of homologous chromosomes separate
One chromosome moves toward each pole, guided by the spindle apparatus 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

Telophase I and Cytokinesis
In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids Cytokinesis usually occurs simultaneously, forming two haploid daughter cells Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms
No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Telophase I and Cytokinesis
Fig. 13-8c Telophase I and Cytokinesis Anaphase I Sister chromatids remain attached Figure 13.8 The meiotic division of an animal cell Homologous chromosomes separate Cleavage furrow

Division in meiosis II also occurs in four phases:
– Prophase II – Metaphase II – Anaphase II – Telophase II and cytokinesis Meiosis II is very similar to mitosis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Telophase II and Cytokinesis
Fig. 13-8d Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II Figure 13.8 The meiotic division of an animal cell Sister chromatids separate Haploid daughter cells forming

In prophase II, a spindle apparatus forms
In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Metaphase II In metaphase II, the sister chromatids are arranged at the metaphase plate Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical The kinetochores of sister chromatids attach to microtubules extending from opposite poles Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Prophase II Metaphase II Fig. 13-8e
Figure 13.8 The meiotic division of an animal cell

In anaphase II, the sister chromatids separate
The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Telophase II and Cytokinesis
In telophase II, the chromosomes arrive at opposite poles Nuclei form, and the chromosomes begin decondensing Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Cytokinesis separates the cytoplasm
At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes Each daughter cell is genetically distinct from the others and from the parent cell Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Telephase II and Cytokinesis
Fig. 13-8f Telephase II and Cytokinesis Anaphase II Figure 13.8 The meiotic division of an animal cell Sister chromatids separate Haploid daughter cells forming

A Comparison of Mitosis and Meiosis
Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Figure 13.9 A comparison of mitosis and meiosis in diploid cells
Parent cell Chiasma Chromosome replication Chromosome replication Prophase Prophase I Homologous chromosome pair Replicated chromosome 2n = 6 Metaphase Metaphase I Anaphase Anaphase I Telophase Telophase I Haploid Figure 13.9 A comparison of mitosis and meiosis in diploid cells n = 3 Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II 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, anahase, and telophase Two, each including prophase, metaphase, anaphase, 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 (2n) 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 amoung the gametes

Replicated chromosome
Fig. 13-9a MITOSIS MEIOSIS MEIOSIS I Parent cell Chiasma Chromosome replication Chromosome replication Prophase Prophase I Homologous chromosome pair 2n = 6 Replicated chromosome Metaphase Metaphase I Figure 13.9 A comparison of mitosis and meiosis in diploid cells Anaphase Telophase Anaphase I Telophase I Haploid n = 3 Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II

Figure 13.9 A comparison of mitosis and meiosis in diploid cells
Fig. 13-9b 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, anaphase, and telophase Two, each including prophase, metaphase, anaphase, and telophase Synapsis of homologous chromosomes Figure 13.9 A comparison of mitosis and meiosis in diploid cells 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 (2n) 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

Three events are unique to meiosis, and all three occur in meiosis l:
– Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information – At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes – At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Protein complexes called cohesins are responsible for this cohesion
Sister chromatid cohesion allows sister chromatids of a single chromosome to stay together through meiosis I Protein complexes called cohesins are responsible for this cohesion In mitosis, cohesins are cleaved at the end of metaphase In meiosis, cohesins are cleaved along the chromosome arms in anaphase I (separation of homologs) and at the centromeres in anaphase II (separation of sister chromatids) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Two of three possible arrangements of labeled chromosomes
Fig EXPERIMENT Shugoshin+ (normal)+ Shugoshin– Spore case Fluorescent label Metaphase I Anaphase I Metaphase II OR Anaphase II ? ? ? ? Figure What prevents the separation of sister chromatids at anaphase I of meiosis? Mature spores ? ? ? ? Spore Two of three possible arrangements of labeled chromosomes RESULTS 100 80 Spore cases (%) 60 40 20 Shugoshin+ Shugoshin–

Two of three possible arrangements of labeled chromosomes
Fig a EXPERIMENT Shugoshin+ (normal) Shugoshin– Spore case Fluorescent label Metaphase I Anaphase I Figure What prevents the separation of sister chromatids at anaphase I of meiosis? Metaphase II OR Anaphase II ? ? ? ? Mature spores ? ? ? ? Spore Two of three possible arrangements of labeled chromosomes

Mutations create different versions of genes called alleles
Concept 13.4: 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Origins of Genetic Variation Among Offspring
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 of Chromosomes
Homologous pairs of chromosomes orient randomly at metaphase I of meiosis In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number For humans (n = 23), there are more than million (223) possible combinations of chromosomes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Possibility 2 Possibility 1 Two equally probable arrangements of
Fig Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Figure The independent assortment of homologous chromosomes in meiosis

Fig Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Figure The independent assortment of homologous chromosomes in meiosis Metaphase II

Fig Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Figure The independent assortment of homologous chromosomes in meiosis Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4

Crossing Over Crossing over produces recombinant chromosomes, which combine genes inherited from each parent Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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

Prophase I Nonsister of meiosis chromatids held together
Fig Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Figure The results of crossing over during meiosis

Fig Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Figure The results of crossing over during meiosis

Fig Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Figure The results of crossing over during meiosis Anaphase I

Fig Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Figure The results of crossing over during meiosis Anaphase I Anaphase II

Recombinant chromosomes
Fig Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Figure The results of crossing over during meiosis Anaphase I Anaphase II Daughter cells Recombinant chromosomes

Random Fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) The 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Animation: Genetic Variation
Crossing over adds even more variation Each zygote has a unique genetic identity Animation: Genetic Variation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The Evolutionary Significance of Genetic Variation Within Populations
Natural selection results in the accumulation of genetic variations favored by the environment Sexual reproduction contributes to the genetic variation in a population, which originates from mutations Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Prophase I: Each homologous pair undergoes
Fig. 13-UN1 Prophase I: Each homologous pair undergoes synapsis and crossing over between nonsister chromatids. 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.

Fig. 13-UN2 F H

Fig. 13-UN3

Fig. 13-UN4

You should now be able to:
Distinguish between the following terms: somatic cell and gamete; autosome and sex chromosomes; haploid and diploid Describe the events that characterize each phase of meiosis Describe three events that occur during meiosis I but not mitosis 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

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