Cellular reproduction (cont.2) Biology 102 Cellular reproduction (cont.2) Chapter 1

Lecture outline Sexual vs. asexual reproduction Advantages and Disadvantages Steps and details of meiosis Errors in meiosis (briefly) Chapter 1

Sexual vs. asexual reproduction Advantages of asexual reproduction: Can reproduce on own. Why an advantage? ALL of your genes carried on to offspring. Allows for rapid production of offspring. Relatively "cheap": each replication produces new individuals. Disadvantages of asexual reproduction: No genetic recombination No chance to create offspring that might deal better with new situations Genes dispersed in exactly same "package“ Chapter 1

Sexual vs. asexual reproduction Advantages of sexual reproduction Genetic recombination Potential for offspring to deal better with novel situations than parent type Your siblings aren't clones of each other. One genetic combinations might do markedly better Disadvantages of sexual reproduction Must find compatible partner (difficult in some habitats) Each offspring carries only 50% of your genes, not 100% New combination could be less effective than what you have! Must produce many gametes… Chapter 1

Biology: Life on Earth (Audesirk) Meiosis I Homologous chromosomes exchange DNA & align on equator Homologous chromosomes pair and cross over Figure: 11.14a-d Title: The details of meiotic cell division Caption: In meiotic cell division (meiosis and cytokinesis), the homologous chromosomes of a diploid cell are separated, producing four haploid daughter cells. Each daughter cell contains one member of each pair of parental homologous chromosomes. In these diagrams, two pairs of homologous chromosomes are shown, large and small. The yellow chromosomes are from one parent (for example, the father), and the violet chromosomes are from the other parent. (a) Prophase I. Duplicated chromosomes condense. Homologous chromosomes pair up and chiasmata occur as chromatids of homologues exchange parts. The nuclear envelope disintegrates, and spindle microtubules form. (b) Metaphase I. Paired homologous chromosomes line up along the equator of the cell. One homologue of each pair faces each pole of the cell and attaches to spindle microtubules via its kinetochore (red). (c) Anaphase I. Homologues separate, one member of each pair going to each pole of the cell. Sister chromatids do not separate. (d) Telophase I. Spindle microtubules disappear. Two clusters of chromosomes have formed, each containing one member of each pair of homologues. The daughter nuclei are therefore haploid. Cytokinesis commonly occurs at this stage. There is little or no interphase between meiosis I and meiosis II. Prophase I Metaphase I Chapter 1 Chapter 1

Biology: Life on Earth (Audesirk) Crossing Over Review steps What is accomplished? Figure: 11.15 Title: The mechanism of crossing over Caption: 1) Homologous chromosomes pair up side by side. 2) One end of each chromosome binds to the nuclear envelope. Protein strands “zip” homologous chromosomes together. 3) Homologous chromosomes are fully joined by protein strands. 4) Recombination enzymes bind to the chromosomes. Recombination enzymes snip chromatids apart and reattach the chromatids. Chiasmata are formed when one end of a chromatid of a paternal chromosome (yellow) is attached to the other end of a chromatid of a maternal chromosome (violet). 5) The protein strands and recombination enzymes leave as the chromosomes condense. The chiasmata remain as locations where homologous chromosomes are twisted around each other, helping to hold homologues together. Chapter 1 Chapter 1

Biology: Life on Earth (Audesirk) Meiosis I Homologous chromosomes exchange DNA & align on equator Homologous chromosomes pair and cross over Homologous chromosomes move to opposite poles Figure: 11.14a-d Title: The details of meiotic cell division Caption: In meiotic cell division (meiosis and cytokinesis), the homologous chromosomes of a diploid cell are separated, producing four haploid daughter cells. Each daughter cell contains one member of each pair of parental homologous chromosomes. In these diagrams, two pairs of homologous chromosomes are shown, large and small. The yellow chromosomes are from one parent (for example, the father), and the violet chromosomes are from the other parent. (a) Prophase I. Duplicated chromosomes condense. Homologous chromosomes pair up and chiasmata occur as chromatids of homologues exchange parts. The nuclear envelope disintegrates, and spindle microtubules form. (b) Metaphase I. Paired homologous chromosomes line up along the equator of the cell. One homologue of each pair faces each pole of the cell and attaches to spindle microtubules via its kinetochore (red). (c) Anaphase I. Homologues separate, one member of each pair going to each pole of the cell. Sister chromatids do not separate. (d) Telophase I. Spindle microtubules disappear. Two clusters of chromosomes have formed, each containing one member of each pair of homologues. The daughter nuclei are therefore haploid. Cytokinesis commonly occurs at this stage. There is little or no interphase between meiosis I and meiosis II. Prophase I Metaphase I Anaphase I Telophase I Chapter 1 Chapter 1

Metaphase I Alignment Scenarios Biology: Life on Earth (Audesirk) Metaphase I Alignment Scenarios Note side-by-side alignment Note independent assortment of chromosomes Figure: 11.UN16 Title: Shuffling of homologues in anaphase I and genetic variability Caption: Three pairs of homologous chromosomes will produce eight possible sets of chromosomes in anaphase I. Chapter 1 Chapter 1

Biology: Life on Earth (Audesirk) Meiosis II Similar to Mitosis Figure: 11.14e-I Title: The details of meiotic cell division Caption: In meiotic cell division (meiosis and cytokinesis), the homologous chromosomes of a diploid cell are separated, producing four haploid daughter cells. Each daughter cell contains one member of each pair of parental homologous chromosomes. In these diagrams, two pairs of homologous chromosomes are shown, large and small. The yellow chromosomes are from one parent (for example, the father), and the violet chromosomes are from the other parent. (e) Prophase II. If chromosomes have relaxed after telophase I, they recondense. Spindle microtubules re-form and attach to the sister chromatids. (f) Metaphase II. Chromosomes line up along the equator, with sister chromatids of each chromosome attached to spindle microtubules that lead to opposite poles. (g) Anaphase II. Chromatids separate into independent daughter chromosomes, one former chromatid moving toward each pole. (h) Telophase II. Chromosomes finish moving to opposite poles. Nuclear envelopes re-form, and the chromosomes become extended again (not shown here). (i) Four haploid cells. Cytokinesis results in four haploid cells, each containing one member of each pair of homologous chromosomes (shown here in condensed state). Four Haploid Cells Prophase II Metaphase II Anaphase II Telophase II Chapter 1 Chapter 1

Errors in meiosis Down Syndrome XXY or XYY Chromosome pair 21 doesn’t split up (Metaphase 1) Egg has three homogous chromosomes Why more prevalent in older women? Meiosis stops in Prophase 1 until ovulation… XXY or XYY Similar mechanism, but with sex chromosomes Effects Chapter 1

Meiosis vs. Mitosis: Comparison of Stages Biology: Life on Earth (Audesirk) Meiosis vs. Mitosis: Comparison of Stages Figure: 1.1 Title: A comparison of mitotic and meiotic cell divisions in animal cells Caption: In these diagrams, comparable phases are aligned. In both mitosis and meiosis, chromosomes are replicated during interphase. Meiosis I, with the pairing of homologous chromosomes, formation of chiasmata, exchange of chromosome parts, and separation of homologues to form haploid daughter nuclei, has no counterpart in mitosis. Meiosis II, however, is similar to mitosis. Chapter 1 Chapter 1

Meiosis vs. Mitosis: Comparison of Spindles Biology: Life on Earth (Audesirk) Meiosis vs. Mitosis: Comparison of Spindles Meiosis: Duplicated chromosomes with one kinetochore; Paired homologues go to opposite poles. Figure: 11.16 Title: A comparison of the spindles formed during mitosis and meiosis I Caption: (a) In mitosis, homologous chromosomes are not paired. The kinetochores of sister chromatids are attached to kinetochore microtubules that lead to opposite poles. When the sister chromatids separate during anaphase, the newly independent daughter chromosomes move to opposite poles of the cell. (b) In meiosis I, homologous chromosomes are paired. Both kinetochores of the sister chromatids of a single chromosome are attached to kinetochore microtubules that lead to the same pole. During anaphase I, sister chromatids of each chromosome remain together, moving to the same pole, but homologous chromosomes separate and move to opposite poles. Mitosis: Duplicated chromosomes with two kinetochores; Unpaired homologs split between sister chromatids, which go to opposite poles. Chapter 1 Chapter 1