2. La nuova biologia.blu Le cellule e i viventi
David Sadava, David M. Hillis,
H. Craig Heller, May R. Berenbaum
La nuova biologia.blu
Le cellule e i viventi

3. The Cell Cycle and Cell Division

4. How Do Prokaryotic and Eukaryotic Cells Divide?
The life cycle of an organism is closely linked to cell division.
Cell division is important in growth and repair of tissues in multicellular organisms, and in the reproduction of all organisms.

5. How Do Prokaryotic and Eukaryotic Cells Divide?
Four events must occur for cell division:
A reproductive signal initiates cell division
Replication of DNA
Segregation: distribution of DNA into two new cells
Cytokinesis: separation of cellular material into the two new cells

6. How Do Prokaryotic and Eukaryotic Cells Divide?
In prokaryotes, binary fission results in two new single-celled organisms.

7. How Is Eukaryotic Cell Division Controlled?
Cell cycle: period from one cell division to the next; divided into mitosis/cytokinesis and interphase.
Interphase: nucleus is visible and cell functions, including DNA replication, occur; begins after cytokinesis, ends when mitosis starts (M phase).
Interphase has three sub- phases: G1, S, and G2.
Duration of interphase is highly variable.

8. How Is Eukaryotic Cell Division Controlled?
The signals act through cyclin-dependent kinases (Cdk’s).
Protein kinases catalyze transfer of a phosphate group from ATP to a protein (phosphorylation). The shape and function of the protein changes.
Cdk’s play important roles in the cell cycle.

9. Cyclin-Dependent Kinases Regulate Progress through the Cell Cycle
Cyclin–Cdk’s act at cell cycle checkpoints to regulate progress.
Figure Cyclin-Dependent Kinases Regulate Progress through the Cell Cycle By acting at checkpoints (red lines), different cyclin–Cdk complexes regulate the orderly sequence of events in the cell cycle.

11. What Happens during Mitosis?
Eukaryotic DNA molecules are extensively “packed” and organized by histones—proteins with positive charges that attract the negative phosphate groups of DNA.
Interactions result in the formation of beadlike units, or nucleosomes.
DNA molecules are complexed with proteins to form chromatin.

12. DNA is Packed into a Mitotic Chromosome
After replication, the sister chromatids are held together during G2 by proteins called cohesins.
Figure DNA is Packed into a Mitotic Chromosome The nucleosome, formed by DNA and histones, is the essential building block in this highly compacted structure.
At mitosis the cohesin is removed, except at the centromere region.

13. What Happens during Mitosis?
Mitosis (M phase) ensures accurate segregation of chromosomes to daughter cells.

14. The Phases of Mitosis
Figure The Phases of Mitosis Mitosis results in two new nuclei that are genetically identical to each other and to the nucleus from which they were formed. In the micrographs, the green dye stains microtubules (and thus the spindle); the red dye stains the chromosomes. The chromosomes in the diagrams are stylized to emphasize the fates of the individual chromatids.

15. What Happens during Mitosis?
The spindle apparatus (or mitotic spindle) moves sister chromatids apart.

16. Cytokinesis Differs in Animal and Plant Cells
Cytokinesis: division of the cytoplasm.
In animal cells the plasma membrane pinches in between the nuclei.
In plant cells, vesicles from the Golgi apparatus appear along the plane of cell division. These fuse to form a new plasma membrane and a new cell wall.
Figure Cytokinesis Differs in Animal and Plant Cells (A) A sea urchin zygote (fertilized egg) that has just completed cytokinesis at the end of the first cell division of its development into an embryo. (B) A dividing plant cell in late telophase. Plant cells divide differently from animal cells because plant cells have cell walls.

18. What Role Does Cell Division Play in a Sexual Life Cycle?
Asexual reproduction is based on mitotic divisions.
Unicellular organisms can reproduce by fission
Multicellular organisms can also reproduce asexually, producing clones.
Aspen trees have shoots that sprout from the root system. All the trees in a stand may be clones of a single parent.

19. What Role Does Cell Division Play in a Sexual Life Cycle?
Sexual reproduction: offspring are not identical to the parents.
Requires gametes created by meiosis; two parents each contribute one gamete to an offspring.
Somatic cells — body cells not specialized for reproduction.

20. What Role Does Cell Division Play in a Sexual Life Cycle?
Gametes contain only one set of chromosomes — one homolog of each pair. Chromosome number is haploid (n).
Fertilization: two haploid gametes (female egg and male sperm) fuse to form a diploid zygote; chromosome number = 2n.
Evolution has generated many different versions of the sexual life cycle.

21. What Role Does Cell Division Play in a Sexual Life Cycle?

22. What Happens during Meiosis?
Meiosis consists of two nuclear divisions (Meiosis I and Meiosis II), but DNA is replicated only once.
Reduces chromosome number from diploid to haploid.
Ensures that each haploid product has a complete set of chromosomes
Generates genetic diversity among the products

23. Meiosis: Generating Haploid Cells
Figure Meiosis: Generating Haploid Cells In meiosis, four daughter nuclei are produced, each of which has half as many chromosomes as the original cell. Four haploid cells are the result of two successive nuclear divisions. The micrographs show meiosis in the male reproductive organ of a lily; the diagrams show the corresponding phases in an animal cell. (For instructional purposes, the chromosomes from one parent are colored blue and those from the other parent are red.)

24. What Happens during Meiosis?
During prophase I and metaphase I, the homologous chromosomes pair by adhering along their lengths: synapsis.
The four chromatids of each homologous pair form a tetrad. At some point, the homologs seem to repel each other but are held together by cohesins at regions called chiasmata that form between non- sister chromatids.

25. Crossing Over Forms Genetically Diverse Chromosomes
Crossing over: exchange of genetic material occurs between non-sister chromatids at the chiasmata.
Crossing over results in recombinant chromatids and increases genetic variability of the products.
In anaphase I, independent assortment also allows for chance combinations.

26. What Happens during Meiosis?
Meiosis II:
Not preceded by DNA replication
Sister chromatids are separated
Chance assortment of the chromatids contributes further to the genetic diversity.
Final products are four haploid daughter cells (n).

27. Meiosis: Generating Haploid Cells
Figure Meiosis: Generating Haploid Cells In meiosis, four daughter nuclei are produced, each of which has half as many chromosomes as the original cell. Four haploid cells are the result of two successive nuclear divisions. The micrographs show meiosis in the male reproductive organ of a lily; the diagrams show the corresponding phases in an animal cell. (For instructional purposes, the chromosomes from one parent are colored blue and those from the other parent are red.)

28. Nondisjunction Leads to Aneuploidy (Part 1)
There can be errors in meiosis. Nondisjunction which results in aneuploidy — chromosomes are lacking or present in excess.
In humans, Down syndrome results from a gamete with two copies of chromosome 21. After fertilization, there are three copies (trisomic).
A fertilized egg that did not receive a copy of chromosome 21 will be monosomic, which is lethal.

29. Nondisjunction Leads to Aneuploidy (Part 2)

30. Mitosis and Meiosis: A Comparison
Cells resulting from mitosis are genetically identical to the parent cell.
Meiosis is nuclear division in cells involved in sexual reproduction.
The cells resulting from meiosis are not identical to the parent cells.
Figure Mitosis and Meiosis: A Comparison Meiosis differs from mitosis chiefly by the pairing of homologs and by the failure of the centromeres to separate at the end of metaphase I.

31. What Happens during Meiosis?
When cells are in metaphase of mitosis, it is possible to count and characterize the chromosomes.
The karyotype is the number, shapes, and sizes of all the chromosomes of a cell.
Karyotypes can be used to diagnose abnormalities such as trisomies by a branch of medicine called cytogenetics.

32. Inc. All rights reserved
Adapted from
Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014
Inc. All rights reserved