Chapter IV Part 2 Reproduction of EUKARYOTIC cell By Pn. Aslizah Binti Mohd Aris 06-4832168

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

Figure 13.1 What accounts for family resemblance?

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 0.5 mm Parent Bud Figure 13.2 Asexual reproduction in two multicellular organisms (a) Hydra (b) Redwoods

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 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

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 Centromere Figure 13.4 Describing chromosomes 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 Ovary Testis Diploid zygote (2n = 46) Figure 13.5 The human life cycle 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 multi- cellular organism (gametophyte) n Gametes n n Mitosis n Mitosis Mitosis n Mitosis n n n n n MEIOSIS FERTILIZATION Spores n Gametes n Gametes n MEIOSIS FERTILIZATION Zygote MEIOSIS FERTILIZATION 2n 2n 2n 2n Diploid multicellular organism Zygote Diploid multicellular organism (sporophyte) 2n Mitosis Mitosis Zygote Figure 13.6 Three types of sexual life cycles (a) Animals (b) Plants and some algae (c) Most fungi and some protists

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

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 Meiosis I 1 Homologous chromosomes separate Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unreplicated chromosomes

Meiosis I 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 Sister chromatids separate Haploid daughter cells forming Homologous chromosomes Homologous chromosomes separate Cleavage furrow Fragments of nuclear envelope Microtubule attached to kinetochore Figure 13.8 The meiotic division of an animal cell

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 Cleavage furrow Homologous chromosomes Homologous chromosomes separate Figure 13.8 The meiotic division of an animal cell Fragments of nuclear envelope Microtubule attached to kinetochore

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

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 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

Meiosis II 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 Sister chromatids separate Haploid daughter cells forming Figure 13.8 The meiotic division of an animal cell

Prophase II 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 Prophase II Metaphase II Figure 13.8 The meiotic division of an animal cell

Anaphase II 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 Sister chromatids separate Haploid daughter cells forming Figure 13.8 The meiotic division of an animal cell

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 Chiasma MEIOSIS I Parent cell Chromosome replication Chromosome replication Prophase Prophase I Homologous chromosome pair Replicated chromosome 2n = 6 Metaphase Metaphase I Anaphase Anaphase I Telophase 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 SUMMARY Figure 13.9 A comparison of mitosis and meiosis in diploid cells 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

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

Spermatogenesis is production of mature sperm The timing and pattern of meiosis in mammals differ for males and females Gametogenesis, the production of gametes by meiosis, differs in females and males Sperm are small and motile and are produced throughout the life of a sexually mature male Spermatogenesis is production of mature sperm

Spermatogenesis and oogenesis in an animal cell

Spermatogenesis In diploid animals, the only haploid cells are gametes produced by meiosis and used in sexual reproduction. Gametes are produced by specialized cells. In males, spermatogenesis produces spermatozoa within the testes. Primordial germ cells (primary spermatogonia) undergo mitosis to produce secondary spermatogonia. Secondary spermatogonia transform into primary spermatocytes (meiocytes), which undergo meiosis I, giving rise to two secondary spermatocytes. Each secondary spermatocyte undergoes meiosis II, producing haploid spermatids that differentiate into spermatozoa.

Figure 46.12 Human gametogenesis Fig. 46-12a Epididymis Seminiferous tubule Testis Cross section of seminiferous tubule Primordial germ cell in embryo Mitotic divisions Sertoli cell nucleus Spermatogonial stem cell 2n Mitotic divisions Spermatogonium 2n Mitotic divisions Primary spermatocyte 2n Meiosis I Lumen of seminiferous tubule Secondary spermatocyte n n Figure 46.12 Human gametogenesis For the Cell Biology Video Motion of Isolated Flagellum, go to Animation and Video Files. For the Cell Biology Video Flagellum Movement in Swimming Sperm, go to Animation and Video Files. Meiosis II Neck Spermatids (at two stages of differentiation) Early spermatid n n n n Tail Midpiece Head Plasma membrane Differentiation (Sertoli cells provide nutrients) Mitochondria Sperm n n n n Nucleus Acrosome

Primordial germ cell in embryo Fig. 46-12c Primordial germ cell in embryo Mitotic divisions Spermatogonial stem cell 2n Mitotic divisions Spermatogonium 2n Mitotic divisions Primary spermatocyte 2n Meiosis I Secondary spermatocyte n n Meiosis II Early spermatid n n n n Figure 46.12 Human gametogenesis Differentiation (Sertoli cells provide nutrients) Sperm n n n n

Oogenesis In females, oogenesis produces eggs (oocytes) in the ovary. Primordial germ cells (primary oogonia) undergo mitosis to produce secondary oogonia. Secondary oogonia transform into primary oocytes, which grow until the end of oogenesis. Primary oocytes undergo meiosis I and unequal cytokinesis, producing a large secondary oocyte, and a small cell called the first polar body. The secondary oocyte produces two haploid cells in meiosis II. One is a very small cell, the second polar body, and the other rapidly matures into an ovum. (5) The first polar body may or may not divide during meiosis I. Polar bodies have no function in most species and degenerate, so that a round of meiosis produces only one viable gamete, the ovum. Human oocytes form in the fetus, completing meiosis only after fertilization.

Oogenesis Eggs contain stored nutrients and are much larger Oogenesis is development of mature oocytes (eggs) and can take many years

Figure 46.12 Human gametogenesis Fig. 46-12e Ovary Primary oocyte within follicle In embryo Growing follicle Primordial germ cell Mitotic divisions 2n Oogonium Mitotic divisions Primary oocyte (present at birth), arrested in prophase of meiosis I 2n Mature follicle Ruptured follicle Completion of meiosis I and onset of meiosis II First polar body n n Secondary oocyte, arrested at metaphase of meiosis II Figure 46.12 Human gametogenesis Ovulated secondary oocyte Ovulation, sperm entry Completion of meiosis II Second polar body Corpus luteum n n Fertilized egg Degenerating corpus luteum

Figure 46.12 Human gametogenesis Fig. 46-12g In embryo Primordial germ cell Mitotic divisions 2n Oogonium Mitotic divisions Primary oocyte (present at birth), arrested in prophase of meiosis I 2n Completion of meiosis I and onset of meiosis II First polar body n n Secondary oocyte, arrested at metaphase of meiosis II Ovulation, sperm entry Figure 46.12 Human gametogenesis Completion of meiosis II Second polar body n Fertilized egg n

Spermatogenesis differs from oogenesis: In oogenesis, one egg forms from each cycle of meiosis; in spermatogenesis four sperm form from each cycle of meiosis Oogenesis ceases later in life in females; spermatogenesis continues throughout the adult life of males Oogenesis has long interruptions; spermatogenesis produces sperm from precursor cells in a continuous sequence