Chapter 2: Chromosomes and cellular reproduction

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Chapter 2: Chromosomes and cellular reproduction I.

usually 1 circular molecule Small genome; DNA not

membrane-bound organelles Nucleus present Larger

molecules Larger genome; DNA complexed with histones

1 Chapter 2: Chromosomes and cellular reproduction I. Contrast between prokaryotes and eukaryotes. See Figure 2.1 Nucleus absent Small diameter 1 to 10 µm Genome usually 1 circular molecule Small genome; DNA not complexed with histones No cytoskeleton or membrane-bound organelles Nucleus present Larger diameter 10 to 100 µm Genome multiple linear molecules Larger genome; DNA complexed with histones Cytoskeleton and membrane-bound organelles Eukaryotic DNA is complexed with histones (Fig. 2.2) Prokaryotic DNA Eukaryotic DNA (Fig. 2.3) 1

Viruses consist of nucleic acid and protein (Fig. 2.

Prokaryotic cells replicate by simple division.

5 Diploid eukaryotic cells have two sets of

6) Homologous pairs Diploid somatic cells Haploid

2 Viruses consist of nucleic acid and protein (Fig. 2.4) II. Prokaryotic cells replicate by simple division. See Figure 2.5 II. Prokaryotic cells replicate by simple division. See Figure 2.5 Diploid eukaryotic cells have two sets of chromosomes. (Fig. 2.6) Homologous pairs Diploid somatic cells Haploid germline cells Cell division in eukaryotes is more complicated, so we need a language to describe the chromosomes. 2

Anatomy of a eukaryotic chromosome (Fig. 2.

The cell cycle consists of interphase a period of cell growth M phase a period of nuclear

1 G 0 : stable, nondividing period of variable length S: Synthesis of DNA Fig. 2.

3 Anatomy of a eukaryotic chromosome (Fig. 2.7) Chromosome type based on centromere position (Fig. 2.8) III. The cell cycle consists of interphase a period of cell growth M phase a period of nuclear and cell division Fig. 2.9 and Table 2.1 G 0 : stable, nondividing period of variable length S: Synthesis of DNA Fig Prophase: centrioles migrate to opposite ends of the cell; spindle fibers are organized nuclear membrane breaks down; nucleolus disintegrates chromosomes condense chromosome two sister chromatids connected at the centromere 3

Fig. 2.10 Metaphase: chromosomes attached to spindle fibers at the centromeres centromeres line up on the metaphase plate.

4 Fig Metaphase: chromosomes attached to spindle fibers at the centromeres centromeres line up on the metaphase plate. Anaphase: centomeric regions divide sister chromatids pull themselves to opposite ends of the cell. The spindle fibers do not pull the chromosomes to the poles of the cell Instead, the kinetochore disassembles the tubulin subunits, and the chromosome pulls itself toward the centrosome. Fig

Telophase: Chromosome number and amount of DNA in

produces two genetically identical cells, with no

5 Telophase: Chromosome number and amount of DNA in mitosis. (Fig. 2.12) nuclear membrane reforms cytoplasm partitioned (cytokinesis). produces two genetically identical cells, with no net change in chromosome number. IV. consists of two cell divisions I a reductional division II an equational division Fig

6 Prophase I: chromosomes condense telomeres are attached to nuclear membrane homologous chromosomes synapse synaptonemal complex forms crossing over occurs chiasmata move to ends of chromosomes nucleolus and nuclear membrane break down centromeres are attached to spindle fibers Metaphase I: centromeres do not divide tetrads move to the metaphase plate Anaphase I: one chromosome (pair of sister chromatids) moves to each pole. Telophase I: variable nuclei haploid, but with two chromatids for each chromosome Prophase II: very brief 6

7 Metaphase II: centromeres line up on metaphase plate Anaphase II: centromeres divide each sister chromatid pulls itself to opposite poles of the cell Telophase II: nuclear membrane forms 4 products of meiosis 7

different alleles than those that gave rise to the

8 Fig V. Crossing over (recombination) occurs during prophase I and produces genetic variation. Alleles are shuffled Chromosomes can be produced that carry different alleles than those that gave rise to the individual. V. Genetic variation is produce by the random assortment of chromosomes during meiosis. 3 homologous pairs gives rise to 8 different gametes 8

9 V. Genetic variation is produce by the random assortment of chromosomes during meiosis. Ignoring recombination, if n is the number of homologous pairs of chromosomes (the dipliod number), then the number of different gametes that can be produce is 2 n In humans n = 23 2 n = 8,388,608 Crossing over shuffles alleles on the same chromosome, random assortment shuffles alleles on different chromosomes. Both processes increase the genetic variation among gametes produced during meiosis. Contrast and gametogenesis Spermatogenesis has ongoing meiosis from progenitor cells in each generation; oogenesis has single meiosis from each progenitor cells in each generation. Spermatogenesis produces 4 gametes per meiosis; oogenesis produces 1 gamete per meiosis. produces four haploid products that are genetically variable. 9

Contrast of mitosis and miosis (Fig. 2.19): 1. One cell division, resulting in two daughter cells. Two cell divisions, resulting in four products of meiosis. 2. Chromosome number per nucleus maintained.

10 Contrast of mitosis and miosis (Fig. 2.19): 1. One cell division, resulting in two daughter cells. Two cell divisions, resulting in four products of meiosis. 2. Chromosome number per nucleus maintained. Chromosome number halved in products of meiosis. 3. One premitotic S phase per cell division. One premeiotic S phase for both cell divisions. 4. Normally, no pairing of homologs. Full synapsis of homologs in prophase I. 5. Normally, no crossing over. Normally, at least one crossover per chromosome arm. 10

11 6. Centromeres divide at anaphase. Centromeres do not divide at anaphase I, but do divide at anaphase II. 7. Conservative process: daughter cell genotypes identical to parental cell's genotype. Promotes variation among the products of meiosis. 8. Cell undergoing mitosis can be diploid or haploid. Haploid cell cannot undergo meiosis. 11

Typical Life Cycle of Algae and Fungi. 5 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Typical Life Cycle of Algae and Fungi. 5 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Module 3B Meiosis and Sexual Life Cycles In this module, we will examine a second type of cell division used by eukaryotic cells called meiosis. In addition, we will see how the 2 types of eukaryotic cell