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8/15/2013 Chapter 13 – Meiosis and Sexual Life Cycles Overview I. Cell Types II. Meiosis I. Meiosis I II. Meiosis II III. Genetic Variation IV. Reproduction Overview: Variations on a Theme Figure 13.1 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 © 2011 Pearson Education, Inc. 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 Inheritance of Genes Genes are the units of heredity, and are made up of segments of DNA Genes are passed to the next generation via 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 © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. 1 8/15/2013 Comparison of Asexual and Sexual Reproduction In asexual reproduction, a single individual passes genes to its offspring without the fusion of gametes Figure 13.2 0.5 mm A clone is a group of genetically identical individuals from the same parent Parent Bud In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents (a) Hydra (b) Redwoods © 2011 Pearson Education, Inc. 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 Reproduction Asexual reproduction: Parent cell divides into two daughter cells (Mitosis). The end result is a two daughter cells identical to parent cell Sexual reproduction: The union of two gametes (sex cells) to form a single zygote Eggs and Sperm are gametes Fertilized egg is zygote Zygote is different from gametes © 2011 Pearson Education, Inc. Fertilization Fertilization is the union between the sperm and the egg. 12 2 8/15/2013 Cell Types Mitosis occurs in all the body’s cells except the cells that are responsible for reproduction Gametes = reproductive cells Sperm and eggs are reproductive cells – gametes Gametes: are the cells that are responsible for reproduction All the rest of the body’s cells are somatic cells The cells that divide to produce gametes undergo meiosis How do gametes overcome this problem? Remember that we have 23 pairs of chromosomes = 46 chromosomes If gametes (sperm and egg) combined with all these chromosomes then the offspring will have 92 chromosomes Before the gametes come together they need to reduce their number of chromosomes in half. So instead of 23 pairs (46 chromosomes) they need to have 23 chromosomes total. The answer to their problem is meiosis – halving their number of chromosomes 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 shape and carry genes controlling the same inherited characters Figure 13.3 APPLICATION TECHNIQUE Pair of homologous duplicated chromosomes 5 m Centromere Sister chromatids Metaphase chromosome © 2011 Pearson Education, Inc. 3 8/15/2013 Sex Chromosomes in Human Cells The sex chromosomes, which determine the sex of the individual, 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 remaining 22 pairs of chromosomes are called autosomes © 2011 Pearson Education, Inc. Sets of Chromosomes in Human Cells 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 Terminology Diploid = Cells that contain two sets of chromosomes. In humans, cells that have 46 chromosomes or 23 pairs; all somatic cells are diploid (2n) Haploid = Cells that have one set of chromosomes. In humans, cells that have 23 chromosomes; gametes are haploid (1n) Polyploidy = three sets of chromosomes; rare in animals, common in plants For humans, the diploid number is 46 (2n = 46) Meiosis is when a diploid cell divides to produce haploid reproductive cells © 2011 Pearson Education, Inc. Meiosis DNA Replication First the chromosomes (DNA) are duplicated during Interphase In a cell in which DNA synthesis has occurred, each chromosome is replicated Then there are two cell divisions Each replicated chromosome consists of two identical sister chromatids Remember that mitosis had chromosome (DNA) duplication followed by one cell division © 2011 Pearson Education, Inc. 4 8/15/2013 Figure 13.4 Remember that there are pairs chromosomes, each chromosome has two chromatids just after DNA replication Key 2n 6 Maternal set of chromosomes (n 3) Paternal set of chromosomes (n 3) Sister chromatids of one duplicated chromosome Centromere Two nonsister chromatids in a homologous pair Figure 13.7-1 Meiosis Pair of homologous chromosomes (one from each set) Interphase Pair of homologous chromosomes in diploid parent cell The DNA has already replicated during interphase – the chromosomes have become duplicated Duplicated pair of homologous chromosomes In Meiosis the chromosome homologous pairs separate and the cell divides = 1st cell division Chromosomes duplicate Sister chromatids Diploid cell with duplicated chromosomes Then the chromatids separate and cell divide = 2 cd cell division The figures are going to show only one pair of chromosomes – but there are 23 pairs at the start Figure 13.7-2 Figure 13.7-3 Interphase Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Sister chromatids Interphase Pair of homologous chromosomes in diploid parent cell Chromosomes duplicate Duplicated pair of homologous chromosomes Sister chromatids Diploid cell with duplicated chromosomes Meiosis I Chromosomes duplicate Diploid cell with duplicated chromosomes Meiosis I 1 Homologous chromosomes separate 1 Homologous chromosomes separate Haploid cells with duplicated chromosomes Haploid cells with duplicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unduplicated chromosomes 5 8/15/2013 Meiosis Overview Remember Meiosis happens to form gametes – the reproductive cells (sperm and eggs) The cells that produce the gametes start out diploid before meiosis, and will end up haploid There are two stages of Meiosis: Meiosis I and II Each Stage of Meiosis has Prophase, Metaphase, Anaphase, and Telophase 1. DNA replicates – chromosomes become duplicated (two chromatids), the cell is diploid (2n). This happens in Interphase. 2. Meiosis 1: homologous chromosomes separate and the cell divides resulting in two haploid cells (1n) 3. Meiosis 2: The chromatids separate and then the cell divides resulting in four haploid cells (1n) Interphase Meiosis I is preceded by interphase, when the chromosomes are duplicated to form sister chromatids The sister chromatids are genetically identical and joined at the centromere The single centrosome replicates, forming two centrosomes BioFlix: Meiosis © 2011 Pearson Education, Inc. Homologous Chromosomes in Prophase I During Prophase I the homologous chromosomes are attracted to each other and become associated with each other forming a tetrad. The process of homologous chromosomes pairing up during prophase I is called synapsis. A tetrad contains two chromosomes, both are duplicated so there are four chromatids. Crossing Over Prophase I: Duplicated homologous chromosomes condense and intertwine – this produces genetic variation Crossing over: genetic material is exchanged between the homologous chromosomes The sites of crossing over are called chiasmata (singular, chiasma) 6 8/15/2013 Crossing Over Prophase I 37 Metaphase I 38 Independent Assortment During Metaphase I homologous pairs of chromosomes line up the at the center of the cell (the equator) The tetrads arrange themselves randomly – this also gives genetic variation = independent assortment (alignment) 39 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 © 2011 Pearson Education, Inc. 7 8/15/2013 Independent Assortment Anaphase I 44 Telophase I Anaphase I 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 46 © 2011 Pearson Education, Inc. End of Telophase I - Cytokinesis 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 We now have two haploid cells (1n) which means there are 23 chromosomes total in each cell The chromosomes are still in the duplicated form – two chromatids Note: not all species have cytokinesis after telophase I © 2011 Pearson Education, Inc. 8 8/15/2013 Interphase Meiosis II Interphase between Meiosis I and II is brief. The S phase does not take place Preparation for Meiosis II: centrosome replicates Prophase II: The 23 chromosomes are already condensed. The Nuclear membrane dissolves. Metaphase II: Chromosomes line up at the equator Anaphase II: Chromatids separate Telophase II and cytokinesis: Cells separate Now there are four haploid cells: each has 23 chromosomes (not in the duplicated state) Figure 13.8b Prophase II Metaphase II Prophase 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 52 Metaphase II Anaphase II 53 54 9 8/15/2013 Figure 13.8 Telophase II 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; 2n 6 in this example. Metaphase II Prophase II Telophase II and Cytokinesis Anaphase II 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. During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Cleavage furrow Each pair of homologous chromosomes separates. Sister chromatids separate Haploid daughter cells forming Two haploid cells form; each chromosome still consists of two sister chromatids. 55 Figure 13.8a 1. 2. 3. 4. Metaphase plate At the end of Meiosis I are these cells haploid or diploid? 50% 50% At the end of Meiosis I, how many chromosomes are there in each cell? 50% 1. 23 chromosomes 2. 46 chromosomes 50% ch ro m 23 oi d ip l D H ap lo id os o m es 1. Haploid 2. Diploid ur Two haploid cells form; each chromosome still consists of two sister chromatids. Fo Chromosomes line up by homologous pairs. 25% m es Each pair of homologous chromosomes separates. 25% re e Microtubule attached to kinetochore Cleavage furrow O Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n 6 in this example. Homologous chromosomes separate 25% Th Fragments of nuclear envelope 25% ne Homologous chromosomes One Two Three Four os o Spindle Sister chromatids remain attached Centromere (with kinetochore) ch ro m Chiasmata At the end of Meiosis I how many cells are there? 46 Centrosome (with centriole pair) Sister chromatids Telophase I and Cytokinesis Anaphase I o Metaphase I Tw Prophase I 10 8/15/2013 ur At the end of Meiosis II, how many chromosomes are there in each cell? 50% 1. 23 chromosomes 2. 46 chromosomes 50% os o ch ro m 46 23 ip l oi d ch ro m os o m es 50% D lo id ap H 25% Fo O 25% re e o 25% m es 50% 25% Th One Two Three Four N Ye At the end of Meiosis II are these cells haploid or diploid? 1. Haploid 2. Diploid 1. 2. 3. 4. o s 50% Tw 50% 1. Yes 2. No At the end of Meiosis II how many cells are there? ne At the end of Meiosis I, are the chomosomes in the duplicated state? At the end of Meiosis II, are the chromosomes in the duplicated state? 50% Three events are unique to meiosis, and all three occur in meiosis l o N s 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 Ye 1. Yes 2. No 50% © 2011 Pearson Education, Inc. 11 8/15/2013 Genetic diversity through meiosis Independent Assortment There are three places in this process that contribute to the genetic diversity of the offspring. Prophase I: The pairs of chromosomes crossing over. Metaphase I: The way the chromosomes line up on the equator is random = independent assortment Random fertilization 68 Figure 13.10-1 Figure 13.10-2 Possibility 2 Possibility 1 Possibility 2 Possibility 1 Two equally probable arrangements of chromosomes at metaphase I 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 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 8 million (223) possible combinations of chromosomes Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 © 2011 Pearson Education, Inc. 12 8/15/2013 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 Nonsister chromatids held together during synapsis Chiasma Centromere TEM Figure 13.11-3 Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Figure 13.11-4 Prophase I of meiosis Pair of homologs Chiasma Nonsister chromatids held together during synapsis Chiasma Centromere Centromere TEM TEM Anaphase I Anaphase I Anaphase II Figure 13.11a Random Fertilization Chiasma Centromere TEM 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 © 2011 Pearson Education, Inc. 13 8/15/2013 Meoisis and Gender The gametes now contain 23 chromosomes, haploid, and are not in the duplicated form One of these chromosomes will be a sex chromosome Eggs will contain a X chromosome Sperms will contain either a X or a Y chromosome X and Y are non-homologous chromosomes Animation: Genetic Variation Rightclick slide / select “Play” © 2011 Pearson Education, Inc. 14 8/15/2013 Spermatogenesis In the male testes sperm are produced. One cell produces 4 sperm. Each sperm has 23 chromosomes, they are not in the duplicated form The sperm can have either an X or a Y sex chromosome The sperm have a small head and a long tail = flagellum for locomotion The sperm need to contain the genetic material and deliver it to the egg. The heads contain the chromosomes and lots of mitochondria to power the flagella About 400 million sperm are produced each day Egg Formation cont All of the cells that produce the eggs are made before the female mother is even born. So when a girl is born, her ovaries contain all the cells that produce her eggs Each month one of these cells will leave the ovary and go on to mature – and produce the egg and polar bodies Egg Formation The ovaries in females produce eggs One cell will produce one egg and three non-functioning “polar bodies” The one egg gets most of the cytoplasm, leaving the other three cell not able to survive The one egg has 23 chromosomes, with a X sex chromosome The one egg is large enough to support the embryo Fertilization Fertilization is the union between the sperm and the egg. Results in a diploid zygote. 15 8/15/2013 Reproduction in Plants Mosses and Ferns – use spores and egg and sperm, have alternation of generation Angiosperms – flowing plants use sperm and egg Reproduction using spores Fig. 11.2c Gametophytes produce gametes (eggs and sperm) Sporophyte: Produce spores Are dependent on the gametophytes (they grow out of the gametophytes) Sexual Reproduction in Angiosperms Sperm – in pollen produced in anther of stamen Egg – in ovary of carpel Both these cells are haploid Fruit – mature ovary Pollination – transfer of pollen to stigma of female carpel Fertilization – when the pollen grain fuses with the egg – producing the diploid zygote Embryo develops inside seed Seed germinates into plant 16 8/15/2013 Review of Mitosis vs Meiosis Mitosis Mitosis and Meiosis both start with a diploid cell (46 chromosomes, 23 pairs) Before both Mitosis and Meiosis the DNA replicates during interphase, forming duplicated chromosomes, each containing two chromatids Mitosis occurs in somatic cells (cells other than those that produce the gametes), Meiosis produces gametes During Mitosis: The chromatids are separated to produce two cells, each with 46 chromosomes, 23 pairs of non duplicated chromosomes These cells are diploid (2n) cells There is no exchanging of genetic material Meiosis – Two stages Meiosis I: the pairs of chromosomes line up and the chromosomes are separated, resulting in 2 cells, each with 23 chromosomes, in the duplicated state = haploid cells Meiosis II: The chromatids are separated producing two haploid cells that contain 23 non duplicated chromosomes. One original cell produces four haploid cells Important concepts Know all the vocabulary presented in the lecture Know which cells undergo mitosis vs meiosis How is genetic diversity introduced into meiosis? What events contribute to genetic diversity and when (what stage of meiosis) do these events take place How is the gender of the offspring determined. 101 17 8/15/2013 Important Concepts How many functioning sperm are produced from one spermatocyte. What sex chromosomes can a sperm have? How many functioning eggs are produced from one oocyte? What sex chromosomes do eggs have? Know what pollen and eggs are in plants, know the reproductive parts of plants, know what pollination and fertilization are in plants, what is the seed Important Concepts for Lab Exam For Meiosis: Know each stage, the order of the stages, and what happens in each stage. Know what the end result is of meiosis I and II Know what state the cell and the chromosomes are in at the beginning and end of mitosis, meiosis I and at the end of meiosis II. For example: Are the cells haploid or diploid? Are the chromosomes duplicated, or not duplicated? How many chromosomes are there in the cell? Are they in pairs? 18