B02 - The Cell Cycle - pdf PDF

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THE CELL CYCLE AND SEXUAL LIFE CYCLES Reading, Campbell Biology, 2nd Canadian edition  Chapter 12, pp. 243-253  Chapter 13 A. THE CONTINUITY OF LIFE 1. During the history of biological science, the continuity of life was first understood to consist of the continuity between generations of individual organisms, then of cells, then of chromosomes, and finally of the base sequence of DNA molecules.

2. Life also has generational continuity in terms of things other than DNA sequences – for example, DNA base modifications and biological membranes.

3. To refer to genes and DNA as a “blueprint” is a misleading metaphor.

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B. THE CELL CYCLE 1. The cell cycle is a controlled sequence of events that comprises cell growth and division.

Fig. 12.6 What activities does the cell engage in during the G1 part of interphase? How much DNA is there in the cell’s nucleus during G2 compared to G1? from Campbell Biology , 2nd Canadian edition, by Reece et al. 2018

2. Cells that progress through the cell cycle growing unicellular organisms embryonic cells in young animals stem cells in mature animals cells in the meristems (growing points) of plants cells at the growing points of algae and fungi cells that divide to repair an injury in animals and plants

3. Cells that do not normally progress through the cell cycle differentiated cells performing specialized functions

dormant or resting stages of unicellular or multicellular organisms

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4. The cell cycle is subject to complex control. Cell growth, division of the cell's contents, and division of the cell's genetic information must all be coordinated.

C. MITOSIS 1. Mitosis is a division of the nucleus that involves an equational division of the chromosomes. What is a chromosome?

After mitosis is finished, each daughter nucleus will be genetically identical to

2. Before mitosis, the chromosomes are duplicated in S phase to give two sister chromatids.

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Fig. 12.5 Can light microscopy resolve individual chromosomes at all stages of the cell cycle? from Campbell Biology , 2nd Canadian edition, by Reece et al. 2018

As a result of chromosome duplication, each allele (version of a gene) is copied. 3. Stages of mitosis prophase

prometaphase

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Fig. 12.7 How many chromosomes are shown in the diagram of prometaphase? How many chromatids? from Campbell Biology , 2nd Canadian edition, by Reece et al. 2018

metaphase

anaphase

telophase

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4. Mitosis is usually (but not always) followed by cytokinesis, division of the cell’s contents into two separate cells. animal cells –

plant cells –

A coenocyte is a cell in which one or more rounds of mitosis has occurred without cytokinesis.

D. MEIOSIS 1. Meiosis is a division of the nucleus that involves a reductional division of the chromosomes. This is necessary for organisms with sexual life cycles.

2. Each daughter nucleus will get half the chromosomes of the mother nucleus. Each daughter will be genetically distinct from the mother nucleus and the other daughter nuclei.

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3. Every species has its characteristic chromosome numbers. chromosome number in a nucleus before meiosis: diploid, or 2n

chromosome number after meiosis: haploid, or 1n

4. In a diploid cell there are two similar, but not identical, versions of each kind of chromosome. These two versions are called homologous chromosomes.

For each pair of homologous chromosomes in each of your cells, you received one homologue from your mother and one from your father at fertilization.

5. Before meiosis the chromosomes must be duplicated, as they are before mitosis.

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Fig. 13.7 It is important that you understand in what respects homologous chromosomes are similar, and in what respects they are different. Two members of a homologous pair will have genes governing the same characters in the same positions; but for many genes, the two homologues might differ in the traits specified by their specific versions of the gene (alleles). You must also understand the distinction between homologous chromosomes and sister chromatids. from Campbell Biology , 2nd Canadian edition, by Reece et al. 2018

6. Stages of meiosis Meiosis I prophase I: homologous chromosomes find each other (homologue pairing) and might exchange arms. The exchange is called crossing over, and is usually reciprocal down to the base pair.

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metaphase I: paired homologues, still associated, line up on the metaphase plate

anaphase I: homologues separate

Fig. 13.8 The crossover events depicted here imply that a given chromosome that you inherited from your mother was a mixture of her mother's and her father's DNA. Crossing over recombined your grandparents’ DNA in your mother’s meiocyte (meiotic cell). The same thing happened during the meiosis in your father’s meiocyte that led to the sperm cell. from Campbell Biology , 2nd Canadian edition, by Reece et al. 2018

Meiosis II Sister chromatids line up in metaphase II, and then separate in anaphase II.

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Fig. 13.8 You should inspect the chromosomes in each of the daughter nuclei to confirm that each nucleus is haploid, and is genetically distinct from the other nuclei. from Campbell Biology , 2nd Canadian edition, by Reece et al. 2018 7. Sources of genetic variability among products of a meiotic division uncorrected errors (mutations) resulting from DNA replication

crossing over of homologue arms during prophase I independent assortment of homologous chromosomes at metaphase I: maternal and paternal homologues of different homologous pairs line up independently at metaphase I (and therefore segregate independently at anaphase I)

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8. Fate of the products of meiosis in animals

in plants

E. CHROMOSOME SETS 1. one set of chromosomes one version (homologue) for each chromosome in the genome one version (allele) of each gene

2. haploid nucleus (n or 1n) one chromosome set one homologue per chromosome one allele per gene

3. diploid nucleus (2n) two chromosome sets two homologues per chromosome two alleles per gene

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F. SEXUAL LIFE CYCLES 1. Sexually reproducing organisms have both 1n and 2n phases of their life cycle.

2. Life cycles of sexual organisms can be represented in terms of chromosome numbers.

Fig. 13.6 The sexual life cycles shown here are sometimes given these names: a) diploid life cycle b) alternation of generations life cycle c) haploid life cycle from Campbell Biology , 2nd Canadian edition, by Reece et al. 2018

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STUDY QUESTIONS – THE CELL CYCLE AND SEXUAL LIFE CYCLES Answers are given on the next page. 1. What is the result, genetically, of mitosis? Meiosis? 2. What would happen if a cell underwent mitosis but no cytokinesis? 3. If there are 16 chromosomes in an animal cell in the G1 stage of the cell cycle, what is the diploid number of chromosomes for this species? The haploid number? 4. How much DNA is present in a G2 nucleus, compared to a G1 nucleus? 5. A nucleus containing 88 chromatids at the start of mitosis would produce nuclei containing how many chromosomes after mitosis was over? If the same nucleus were at the beginning of meiosis, how many chromosomes would each of the four daughter nuclei have at the end? 6. What are the two phases of meiosis that are the source of genetic variation in the four resulting daughter cells, and why? 7. What are some of the activities a cell engages in during the G1 part of interphase? 8. Draw a picture of a diploid cell with eight chromosomes (four pairs of homologues) at metaphase I in meiosis. Your diagram should include different shading for maternal and paternal homologues, different centromere positions and chromosome sizes among the homologous pairs, and crossing over in two of the four pairs of homologues. Next, draw four products of this meiosis.

Answers to Study Questions 1. Mitosis: the result is two cells, each with the same chromosome number as and genetically identical to the mother cell. Meiosis: the result is four cells, each with half the chromosomes as the mother cell and each genetically distinct from the others. 2. A cell with two or more nuclei would result. This is a reasonably common occurrence, by the way. There are green algae with millions of nuclei in a common cytoplasm. Individuals in a fungal phylum, the Zygomycota, also have many nuclei in one cytoplasmic compartment; coconut milk is a third example. The term referring to this multinucleate condition is coenocytic.

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3. 16; 8 4. Since G2 follows S phase, when the chromosomes are duplicated, a G2 nucleus would contain twice as much DNA as a G1 nucleus. 5. 44 chromosomes in each nucleus; 22 chromosomes in each nucleus 6. Prophase I (because of crossing over in this phase) and metaphase I (because of the independent assortment of maternal and paternal homologues in this phase). 7. gene expression, acquisition of nutrients, growth

Questions from your text, Campbell Biology, second Canadian edition 

Scientific Skills Exercise, p. 259



pp. 261-262, Questions 1, 5 – 9, 11



p.280, Questions 1-8, 11

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All figures used with permission from Campbell Biology, 2 Canadian edition, by Jane B. Reece et al., Pearson, 2018. Fig. 12.5, p. 245; Fig. 12.6, p. 246; Fig. 12.7, p. 247; Fig. 13.6, p. 270; Fig. 13.7, p. 271; Fig. 13.8, p. 272-3.

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