The life cycle of a multicellular organism includes How to Make a Sea Star The life cycle of a multicellular organism includes development reproduction This sea star embryo (morula) shows one stage in the development of a fertilized egg (fusion of egg and sperm) The cluster of cells will continue to divide as development proceeds by a series of cell division

Some organisms can also reproduce asexually This sea star is regenerating a lost arm Regeneration results from repeated cell divisions

CONNECTIONS BETWEEN CELL DIVISION AND REPRODUCTION Cell division is at the heart of the reproduction of cells and organisms Organisms can reproduce sexually or asexually Asexual reproduction- no variety Sexula reproduction Variety

8.1 Like begets like, more or less Some organisms make exact or identical copies of themselves, asexual reproduction Figure 8.1A

Other organisms make similar but not identical copies of themselves in a more complex process, sexual reproduction Figure 8.1B

8.2 Cells arise only from preexisting cells All cells come from cells Cellular reproduction is called cell division Cell division allows an embryo to develop into an adult It also ensures the continuity of life from one generation to the next

8.3 Prokaryotes reproduce by binary fission Prokaryotic cells divide asexually These cells possess a single chromosome (packaged DNA during cell division) containing genes The chromosome is replicated The cell then divides into two cells, a process called binary fission Prokaryotic chromosomes Figure 8.3B

Binary fission of a prokaryotic cell Plasma membrane Prokaryotic chromosome Cell wall Duplication of chromosome and separation of copies Continued growth of the cell and movement of copies Division into two cells Figure 8.3A

THE EUKARYOTIC CELL CYCLE AND MITOSIS 8.4 The large, complex chromosomes of eukaryotes duplicate with each cell division A eukaryotic cell has many more genes than a prokaryotic cell The genes are grouped into multiple chromosomes, found in the nucleus The chromosomes of this plant cell are stained dark purple Figure 8.4A

Chromosomes contain a very long DNA molecule with thousands of genes Individual chromosomes are only visible during cell division

Before a cell starts dividing, the chromosomes are duplicated Sister chromatids This process produces sister chromatids Centromere Figure 8.4B

Chromosome distribution to daughter cells When the cell divides, the sister chromatids separate Chromosome duplication Two daughter cells are produced Each has a complete and identical set of chromosomes Sister chromatids Centromere Chromosome distribution to daughter cells Figure 8.4C

8.5 The cell cycle multiplies cells The cell cycle consists of two major phases: Interphase, where chromosomes duplicate and cell parts are made The mitotic phase, when cell division occurs Figure 8.5

8.6 Cell division is a continuum of dynamic changes Eukaryotic cell division consists of two stages: Mitosis Cytokinesis

In mitosis, the duplicated chromosomes are distributed into two daughter nuclei After the chromosomes coil up, a mitotic spindle (microtubules) moves them to the middle of the cell

Centrosomes (with centriole pairs) Early mitotic spindle Centrosome INTERPHASE PROPHASE Centrosomes (with centriole pairs) Early mitotic spindle Centrosome Fragments of nuclear envelope Kinetochore Chromatin Centrosome Spindle microtubules Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Figure 8.6

The sister chromatids then separate and move to opposite poles of the cell The process of cytokinesis divides the cell into two genetically identical cells

TELOPHASE AND CYTOKINESIS METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS Cleavage furrow Nucleolus forming Metaphase plate Nuclear envelope forming Spindle Daughter chromosomes Figure 8.6 (continued)

8.7 Cytokinesis differs for plant and animal cells In animals, cytokinesis occurs by cleavage This process pinches the cell apart Cleavage furrow Cleavage furrow Contracting ring of microfilaments Figure 8.7A Daughter cells

In plants, a membranous cell plate splits the cell in two Cell plate forming Wall of parent cell Daughter nucleus In plants, a membranous cell plate splits the cell in two Cell wall New cell wall Vesicles containing cell wall material Cell plate Daughter cells Figure 8.7B

Most animal cells divide only when stimulated, and others not at all 8.8 Anchorage, cell density, and chemical growth factors affect cell division Most animal cells divide only when stimulated, and others not at all In laboratory cultures, most normal cells divide only when attached to a surface They are anchorage dependent

Cells continue dividing until they touch one another This is called density-dependent inhibition Cells anchor to dish surface and divide. When cells have formed a complete single layer, they stop dividing (density-dependent inhibition). If some cells are scraped away, the remaining cells divide to fill the dish with a single layer and then stop (density-dependent inhibition). Figure 8.8A

Cell Cycle control The cell cycle has critical checkpoint which determine whether the cycle will continue, pause or stop. Signals affecting critical checkpoints determine whether the cell will go through a complete cycle and divide These checkpoints are controlled by specific proteins inside cells and growth factor signaling from outside the cell. G1 checkpoint Control system M checkpoint G2 checkpoint

Cell cycle control system Growth factors are proteins secreted by cells that stimulate other cells to divide They act by binding to the cell surface receptor and activate signalling mechanisms inside the cell to regulate proteins affecting the cell cycle control checkpoints Growth factor Plasma membrane Relay proteins Receptor protein G1 checkpoint Signal transduction pathway Cell cycle control system Figure 8.8B Figure 8.8B

Cancer cells have abnormal cell cycles 8.10 Connection: Growing out of control, cancer cells produce malignant tumors Cancer cells have abnormal cell cycles They divide excessively and can form abnormal masses called tumors Radiation and chemotherapy are effective as cancer treatments because they interfere with cell division

8.11 Review of the functions of mitosis: Growth, cell replacement, and asexual reproduction When the cell cycle operates normally, mitotic cell division functions in: Growth (seen here in an onion root) Figure 8.11A

Cell replacement (seen here in skin) Dead cells Epidermis, the outer layer of the skin Dividing cells Dermis Figure 8.11B

Asexual reproduction (seen here in a hydra) Figure 8.11C