Meiosis and Sexual Reproduction Chapter 12

Meiosis A specialized type of cell reproduction specifically designed to create gametes (ova and sperm or spores) for sexual reproduction Meiosis halves the parental cell’s chromosome number from diploid to haploid Occurs only in sexually reproducing eukaryotic species

Terms and Concepts Asexual reproduction A single parent No genetic variation All offspring are genetically identical to each other and the parent Mechanisms Mitosis Prokaryotic fission Vegetative propagation Pros and Cons No energy expended to find a mate (pro) The phrase “There goes the last female Dodo bird” doesn’t mean extinction (pro) No genetic variation (con)

Fig. 9-1a, p.138

Terms and Concepts Sexual Reproduction Two parents required most of the time—self fertilizing flowers Genetic variation Variation is important for survival and adaptation (and ultimately evolution) Mechanisms Meiosis, gamete formation, and fertilization Pros and Cons Genetic variation (pro) Must expend energy to attract and find a mate (con) The phrase “There goes the last female Dodo bird” does mean extinction (con)

Fig. 9-1b, p.138

Fig. 9-1c, p.138

Terms and Concepts Diploid Haploid Homologs Somatic cells Germ cells Two copies of each type of chromosome Haploid One copy of each type of chromosome Homologs Pair of chromosomes Somatic cells Typical diploid cells of an animal Germ cells Cells committed to go through meiosis to produce gametes Gamete Reproductive cells (Egg/ova, sperm, spores) Zygote The first cell of a new individual (result of fertilization)

Meiosis For sexual reproduction the gametes must have 1/2 the number of chromosomes as the normal cells of an organism For example human cells have 46 chromosomes (23 pairs) If the sperm had 46 and the ovum had 46, then the resulting zygote would have 92 chromosomes! Over time cells would have more and more chromosomes The process of meiosis produces gametes with ½ the number of chromosomes

Fig. 9-12, p.150

Meiosis During meiosis there are two rounds of division Meiosis I Separates homologs Meiosis II Separates sister chromatids

cell pairs with its partner, Each homologue in the cell pairs with its partner, then the partners separate. p.141c

Meiosis The process of meiosis is very similar to mitosis, however there are some key differences that account for producing genetically variable, haploid gametes The following slides will give an overview of meiosis especially pointing out the key differences from mitosis However, the actual processes of each phase will not be presented as they are the same as in mitosis Study figure 12.5

Meiosis Meiosis I Prophase I Homologs pair up along their length The event is called synapsis or pairing of homologous chromosomes The structure of the two chromosomes is called a tetrad There are four strands of DNA, two sets of sister chromatids Crossing over While the homologs are closely associated they can swap segments of DNA This creates novel combinations of gene traits in both strands Figure 12.7

Meiosis Meiosis I Metaphase I Homolog pairs line up at the equator (pushed and pulled by the microtubules of the bipolar spindle, just as in mitosis) Random Alignment (fig 12.7) Homologs can be attached to either spindle pole Each homolog can be packaged into either one of the two new nuclei Increases the number of potential combinations of maternal and paternal alleles in gametes There are over 8 million possible combinations of the maternal and paternal chromosomes which adds to genetic variability of meiosis

combinations possible 1 2 3 combinations possible or or or Fig. 9-7, p.145

Meiosis Meiosis I Anaphase I One of each duplicated chromosome or homolog is pulled towards a spindle pole by the microtubules Its homolog moves to the opposite pole This is the step that creates haploid cells by separating the homologous pairs

p.141c

Meiosis Meiosis I Telophase I One of each type of chromosome has arrived at a spindle pole For most species the cytoplasm will divide For a few, cytoplasmic division occurs after both rounds of meiosis Cells often proceed directly into meiosis II without completely finishing telophase I

Meiosis Meiosis II (both cells created by meiosis I will follow through the steps of meiosis II to divide a second time) Prophase II The centriole pairs (centrosomes) separate and create a new bipolar spindle Microtubules latch onto separate chromatids Chromosomes are Still duplicated (two chromatids each), but only one of each type of chromosome (haploid) Generally still condensed following meiosis I There is no DNA replication between meiosis I and II

Meiosis Meiosis II Metaphase II Anaphase II Telophase II Chromosomes are lined up at the equator Anaphase II Sister chromatids separate and are pulled towards opposite spindle poles Telophase II New nuclear envelope forms around all four new haploid nuclei Cytoplasmic division results in four cells each containing a haploid number of unduplicated chromosomes

p.141d

Meiosis II Prophase II Metaphase II Anaphase II Telophase II

Meiosis Meiosis introduces genetic variations in traits Two parents both contribute genes to their offspring One of each autosomal chromosome and one sex chromosome are passed to offspring Thus offspring get two of each type of chromosome Each pair of chromosomes carries the same genes Genes may not be identical The differences or traits of the same gene are called alleles

Meiosis Meiosis introduces genetic variations (mixes of different alleles) in traits Crossing over (figure 12.6) Prophase I Random alignment (figure 12.7) Metaphase I Remember genetic variation is one of the “pros” of sexual reproduction

Meiosis Results of meiosis 1 diploid parent cell → 4 genetically variable haploid daughter cells

Questions When homologs swap DNA this is called? T or F: Homologs have a predictable pattern they use to line up during metaphase I. What is separated during Anaphase I? What is separated during Anaphase II? What processes of meiosis contribute to genetic variation? How many cells are produced by meiosis?

From gametes to offspring Sexual reproduction Meiosis → four haploid cells Gamete formation Sperm formation Ova/Egg formation Fertilization Fusion of the haploid nuclei of the ovum and sperm Creates a diploid zygote Adds to variation Which sperm fertilizes which egg is a matter of chance (see next chapter!)

Fig. 9-9, p.147

Fig. 9-10a, p.147

Fig. 9-12, p.150

Sexual Reproduction in Animals Animals use Gametic Meiosis Meiosis produces haploid gametes Gametes do not divide (remain uni-cellular) Fertilization produces a diploid zygote Zygote undergoes mitosis to produce a multicellular diploid body (adult animal)

Sexual Reproduction in Plants Plants use Sporic Meiosis (alternation of generations) Meiosis produces haploid spores Spores undergo mitosis to produce a multicellular haploid body (gametophyte) Fertilization produces a diploid zygote Zygote undergoes mitosis to produce a multicellular diploid body (sporophyte) Alternates between multi-cellular diploid and multi-cellular haploid bodies

multicelled sporophyte mitosis Meiosis Haploid (n) Phase of Cycle spores zygote Diploid (2n) Phase of Cycle Fertilization gametes mitosis multicelled gametophyte mitosis Stepped Art Fig, 21.3, p. 326

Sexual Reproduction in Plants Sporophyte (diploid, multicellular) Produces flowers which contain the germ cells Germ cells are located in the carpel (ova) and stamen (pollen)

Sexual Reproduction in Plants Production of the ova Occurs in flowers within the pistil Made up of 1 or more carpels Stigma Style Ovary

Sexual Reproduction in Plants Production of the ova Occurs in flowers within the pistil The ovary contains 1 or more ovules which produce egg sacs Meiosis of the ovules produces spores Spores undergo mitosis to become the egg sac (gametophyte) Meiosis Spores Egg Sac Mitosis

Diploid Stage Haploid Stage Meiosis an ovule ovary wall megasporocyte integument stalk ovary (cutaway view) Diploid Stage Haploid Stage Double Fertilization Meiosis Fig. 28.6b, p. 454-5

Sexual Reproduction in Plants Production of pollen Occurs in flowers in the stamen Anther Filament

Sexual Reproduction in Plants Production of pollen The anther contains germ cells in pollen sacks Meiosis produces spores Spores undergo mitosis to become the pollen (gametophyte)

pollen sac anther (cutaway view) filament Germ cell Diploid Stage Haploid Stage Meiosis Spores Mitosis Pollen

Sexual Reproduction in Plants Pollination The transfer of pollen from an anther to a stigma (not fertilization) Wind, insects, mammals, etc. Released pollen lands on the stigma Pollen grows a tube through the style to the ovary

Sexual Reproduction in Plants Fertilization Two sperm enter the egg sac from the pollen tube One fuses with the egg forming the diploid zygote Develops into the embryo The other fuses with the central cell to form a triploid endosperm

Sexual Reproduction in Plants Development A seed develops from each mature fertilized ovule Fruit develops from the ovary or other tissue Embryo grows into a new mature sporophyte Nutritive tissue Embryo Seed Ovule Ovary

Fig. 27-5, p.451

Fig. 9-8, p.146

Sexual Reproduction in Fungus Fungus use Zygotic Meiosis Meiosis of the zygote produces haploid spores Spores undergo mitosis to produce a multicellular haploid body (mycelium/fungus) Mycelia of two fungi fuse and develop into a fruiting body (mushroom) Fertilization produces a diploid zygote Zygote remains uni-cellular Zygote is the only diploid cell during the life cycle

a g mycelium Diploid Zygote Diploid Stage nuclear fusion meiosis Haploid Stage Cells with two nuclei (n + n) form on gills g spore (n) at gill margin gill Mitosis cap stalk mycelium cytoplasmic fusion mycelium

Summary Terms and concepts Meiosis I and II Sexual reproduction Pairing of homologs, crossing over, random alignment 4 haploid daughter cells Sexual reproduction Meiosis, gamete formation, fertilization Animal reproduction Plant reproduction Fungus reproduction