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Chapter

Chapter 11 General Biology I BSC 2010 Meiosis and Sexual Reproduction credit: modification of work by Frank Wouters Introduction Mitosis create many genetically identical cells. Asexual reproduction in unicellular organisms

Growth, development, repair and regeneration in multicellular organisms Meiosis produces gametes for sexual reproduction resulting in offspring that are genetically different from parent and each other. Introduction Sexual reproduction contributes to genetic variation. Gametes from each parent contain one set of chromosomes (they are haploid). Gametes fuse to produce a single cell, the zygote

(fertilized egg) with two sets of chromosomes (it is diploid). Offspring are genetically different from parents and each other, even before they accumulate mutations. Sperm + Ovum = Zygote Introduction credit a: modification of work by Frank Wouters; credit b: modification of work by

Ken Cole, USGS; credit c: modification of work by Martin Pettitt Each of us, like these other large multicellular organisms, begins life as a fertilized egg. After trillions of cell divisions, we develop into complex, multicellular organisms. 11.1 THE PROCESS OF MEIOSIS In diploid (2n) multicellular organisms, somatic cells (non-gametes) each contain two sets of chromosomes. Homologous chromosomes

For each pair of chromosomes, each parent contributes one homolog (homologues). The two homologs have corresponding, but not identical, genetic information http://staff.jccc.net/PDECELL/celldivision/homolg.gif

11.1 THE PROCESS OF MEIOSIS Download for free at http://cnx.org/content /col11448/latest/ Early in prophase I, homologous chromosomes come tog ether to form a synapse. The chromosomes are already duplicated (having 2 sister chromatids) and are now bound tightly together and in perfect alignment by a protein lattice called a synaptonemal complex, and by cohesin proteins at the centromere.

11.1 THE PROCESS OF MEIOSIS Functions of meiosis: produce gametes reduce chromosome number in half from diploid to haploid (2n to n) ensure each gamete gets one complete set of chromosomes (one of each pair) promote genetic diversity of products 11.1 THE PROCESS OF MEIOSIS

Meiosis consists of two nuclear divisions (Meiosis I and II) DNA is replicated once in Sphase before meiosis chromosome number is reduced by half, from diploid to haploid, ensuring each haploid gamete has a complete chromosome set Products differ from the parent cell, and from each other.

http://www.windows.ucar.edu/earth/Life/images/genetics_meiosis_sm.gif 11.1 THE PROCESS OF MEIOSIS Fertilization is the process when gametes from two parents unite. Gametes (haploid cells) contain one homolog of each pair (n). When haploid gametes fuse in fertilization, they create

diploid (2n) zygote. http://faculty.abe.ufl.edu/~chyn/age2062/lect/lect_08/12_25.GIF 11.1 THE PROCESS OF MEIOSIS The nucleus divides twice (Meiosis I and Meiosis II). In Meiosis I, the homologous chromosomes are separated. The number of chromosomes are reduced in half. In Meiosis II, the sister chromatids are separated. The final products are four haploid gametes (n).

Meiosis I Prophase I homologous pairs, crossing over Metaphase I chromosomes line up in pairs Anaphase I homologous chromosomes move apart Telophase I results in 2 haploid cells (reduction of chromosomes) (chromosomes each have 2 sister chromatids) Meiosis II (similar to mitosis except it starts with two haploid cells) Prophase II only one set of chromosomes

Metaphase II chromosomes line up individually Anaphase II sister chromatids separate Telophase II 4 haploid cells 11.1 THE PROCESS OF MEIOSIS In Prophase I, synapsis occurs: Two homologs join together, held together by a protein complex.

The resulting tetrad consists of two homologous chromosomes, each with two sister chromatids. Download for free at http://cnx.org/content/col11448/latest/ Prophase I

Crossing over occurs between non-sister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes. Download for free at http://cnx.org/content/col11448/latest/

Metaphase I Download for free at http://cnx.org/content/ col11448/latest/ Independent assortment occurs during metaphase. Consider a cell with two sets of 2 chromosomes (n = 2). There are two possible arrangements at the equatorial plane in metaphase I. The total possible number of different gametes is 2n, where n

equals the number of chromosomes in a set. With n = 23 in human cells, there are over 8 million possible combinations of paternal and maternal chromosomes! Anaphase I and II Download for free at http://cnx.org/content/col 11448/latest/ The process of chromosome alignment differs between

meiosis I and meiosis II. In prometaphase I, microtubules attach to fused kinetochores of homologous chromosomes. Homologous chromosomes are at the cell midpoint in metaphase I. Homologous chromosomes are separated in anaphase I. In prometaphase II, microtubules attach to the kinetochores of sister chromatids. Sister chromatids are at the cell midpoint in metaphase II. In anaphase II, sister chromatids are separated. 11.1 THE PROCESS OF MEIOSIS (revisited)

Meiosis I Prophase I homologous pair, crossing over Metaphase I line up in pairs Anaphase I homologous chromosomes separate Telophase I 2 haploid cells (reduction of chromosomes) Meiosis II (similar to mitosis except it starts with two haploid cells) Prophase II Metaphase II line up single file in the middle of the cell Anaphase II sister chromatids separate

Telophase II 4 haploid cells 11.1 THE PROCESS OF MEIOSIS An animal cell with a diploid number of four (2n = 4) proceeds through the stages of meiosis to form 4 haploid daughter cells. Meiosis I Download for free at http://cnx.org/content/col11448/latest/ 11.1 THE PROCESS OF MEIOSIS

Meiosis II Download for free at http://cnx.org/content/col11448/latest/ Meiosis vs. Mitosis Meiosis and mitosis are both preceded by one round of DNA replication; however, meiosis includes two nuclear divisions. The four daughter cells resulting from meiosis are haploid and genetically distinct.

The two daughter cells resulting from mitosis are diploid and identical to the parent cell. Download for free at http://cnx.org/content/col11448/latest/ 11.2 Diploid-Dominant Life Cycle Download for free at http://cnx.org/content/co l11448/latest/

In animals, sexually reproducing adults form haploid gametes from diploid germ cells. Fusion of the gametes gives rise to a fertilized egg cell, or zygote. The zygote will then undergo multiple rounds of mitosis to produce a multicellular offspring. The germ cells (which eventually give rise to gametes) are generated early in the development of the zygote. 11.2 Haploid-Dominant Life Cycle Download for free at http://cnx.org/content/col

11448/latest/ In fungi, such as black bread mold (Rhizopus nigricans), a haploid multi-cellular stage produces specialized haploid spores by mitosis that fuse to form a diploid zygote. A spore gives rise to a multicellular haploid organism by mitosis. The zygote undergoes meiosis to produce haploid spores. 11.2 Alternation of Generations Credit: modification of work by

Fanaberka/Wikime dia Commons Plants alternate between a multicellular haploid organism and a multicellular diploid organism. The diploid plant is called a sporophyte and produces haploid spores by meiosis. The spores develop into multicellular, haploid plants called gametophytes that produce gametes. The gametes of two individuals will fuse to form a diploid zygote that becomes the new sporophyte.

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