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, the units of heredity, are found on chromosomes.
Inheritance of Genes Genes, the units of heredity, are found on chromosomes. A gene’s specific location on a chromosome is called it’s locus In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Comparison of Asexual and Sexual Reproduction
One set of chromosomes are inherited from each parent, (specifically, each gamete). Therefore, a fertilized egg (a zygote) contains 2 sets of chromosomes. A diploid cell (2n) has two sets of chromosomes. For humans, the somatic cells are the diploids, and the number is 46. 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
The two chromosomes of each pair (maternal and paternal) are homologous Chromosomes in a homologous pair are the same length, as they carry the same number of similar genes controlling the same inherited traits. Similar genes on a homologous pair are known as alleles. A karyotype is an ordered display of these pairs of chromosomes from a single cell 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

Karyotype: 5 µm Pair of homologous chromosomes Sister chromatids
Fig. 13-3b Karyotype: 5 µm Pair of homologous chromosomes Figure 13.3 Preparing a karyotype Sister chromatids

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

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

Chromosomal number in somatic cells of various species:
Humans: 46 Chimps: 48 African Wild Dog: 78 Fruit fly: 8 Hedgehog: 90 Broccoli: 18 Yeast: 32 Adder’s Tongue fern: (highest known of any species)

Will pair up only in meiosis
Fig. 13-4 DIPLOID CELL Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome. These are identical. HOW MANY CHROMATIDS ARE HERE? HOW MANY IDENTICAL CELLS SUCH AS THIS ONE CAN WE MAKE? HOW MANY SPERM CELLS CAN WE MAKE? Centromere Pair of homologous chromosomes (one from each set) Will pair up only in meiosis

At sexual maturity, the ovaries and testes produce haploid gametes
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)

Behavior of Chromosome Sets in the Human Life Cycle
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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 Unlike mitosis, meiosis takes place in two sets of cell divisions, meiosis I and meiosis II The two consecutive cell divisions, then, result in four daughter cells, rather than the two daughter cells in mitosis The 4 cells each have half as many chromosomes as the parent cell, and each cell is unique, not a clone Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The Stages of Meiosis: REVIEW
The primary difference of meiosis from mitosis is that during meiosis: Homologous chromosomes join together briefly, exchange genes, and then randomly separate forever…..like two ships that pass in the night. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Meiosis I Meiosis II Fig. 13-7-3 Homologous pair of chromosomes
Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Diploid cell with replicated chromosomes Homologous chromosomes join here and exchange genes Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number Homologous chromosomes separate Meiosis I 1 Haploid cells with replicated chromosomes Sister chromatids separate Meiosis II 2 Haploid cells with unreplicated 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

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

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

In Prophase I, synapsis occurs
In Prophase I, synapsis occurs. Synapsis is the process where homologous chromosomes form a TETRAD. During synapsis, chromosomes in tetrad formation undergo crossing over, where paired homologous chromosomes exchange alleles. The lovers entwine… Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Pairs of lovers gather…
Metaphase I In metaphase I, all tetrads line up at the metaphase plate, with one homologous chromosome from each pair facing opposite poles, kinetichore microtubules attached. Pairs of lovers gather… Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

to kinetochore attached to centromere
Fig. 13-8b Prophase I Metaphase I Sister chromatids Crossing over Figure 13.8 The meiotic division of an animal cell Tetrad Microtubule attached to kinetochore attached to centromere

The lovers say goodbye…
Anaphase I In anaphase I, homologous chromosomes separate from their tetrad, and move randomly to either end of the cell. This is called independent assortment. It is 50/50 whether a chromosome will wind up at one pole or the other. (However, chromatids remain attached at the centromere and move as one unit toward the pole) The lovers say goodbye… 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

NOTE: 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

Meiosis II is basically mitosis

Metaphase II In metaphase II, the chromosomes, with sister chromatids, are arranged at the metaphase plate Remember, due to crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical 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

At the end of meiosis, there are four daughter cells, each with a haploid set of chromosomes
Each daughter cell is genetically distinct from the others and from the parent cell In spermatogenesis, the four cells grow tails. In oogenesis, one of the four cells gets all the cytoplasm and becomes the egg. The other three become “polar bodies”. 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

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

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

Replicated chromosome
Fig. 13-9a MITOSIS MEIOSIS MEIOSIS I Parent cell Crossing Over 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

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

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

RESULTS 100 80 60 Spore cases (%) 40 20 Shugoshin+ Shugoshin–
Fig b RESULTS 100 80 Figure What prevents the separation of sister chromatids at anaphase I of meiosis? 60 Spore cases (%) 40 20 Shugoshin+ Shugoshin–

(ex: blue eyes / brown eyes) But variation also exists HERE
Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution Mutation (incorrect or missing nucleotides, accidental copying or deletion of genes) is a key source of genetic diversity. Mutations are very rarely beneficial to a species, but when they are, they tend to stick around. Mutations may create different versions of genes called alleles. (ex: blue eyes / brown eyes) But variation also exists HERE Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Independent Assortment of Chromosomes
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

Independent Assortment of Chromosomes
The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number Each chromosome has a 50% chance of moving to one pole of the cell or the other. For humans (n = 23), there are more than million (223) possible combinations of chromosomes. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Crossing Over Crossing over produces recombinant chromosomes, which combine genes inherited from each parent The total possible combinations of genes crossing over on a pair of homologs are…very large indeed. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Random Fertilization Finally, random fertilization adds to genetic variation because any sperm may fuse with any ovum. The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a human zygote with any of about 70 trillion diploid combinations. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Animation: Genetic Variation
Each zygote has a unique genetic identity. You are one in 70 trillion! And THAT is just among two individuals!!! 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

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

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

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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