Bio 178 Lecture 21 The Cell Cycle (Cntd.) and Sexual Reproduction © Biodidac. 7F&File_type=GIF

Reading Chapters 11 & 12 Quiz Material Questions on P 226 & 239 Chapters 11 & 12 Quizzes on Text Website (

Outline Cell Division  Eukaryotic Cell Cycle (Cntd.)  Prokaryotic Cell Division  Control of the Cell Cycle in Eukaryotes Sexual Reproduction

Cytokinesis in Plant Cells

Cytokinesis (Cntd.) Fungi and Protists Nuclear envelope does not dissolve during mitosis. Once mitosis is complete nuclear division occurs.

Mitosis in Plant Cells Video : Alberts et al Molecular Biology of the cell.

Prokaryotic Cell Division

(Binary Fission) Duplication of Genetic Material DNA starts replicating at the origin of replication. Replication proceeds bidirectionally until it reaches the termination site. Cell Elongation and DNA Partitioning Cell grows and DNA is partitioned in the cell (by attachment to membrane). Cell Division A new membrane (septum) forms at the cell midpoint. This begins with a ring of FtsZ protein. Cell pinches into 2 daughter cells.

Bidirectional DNA Replication McGraw-Hill Video

Bacterial Septum (in Bacillus)

Control of the Cell Cycle

Cells use checkpoints to ensure that proper conditions have been achieved before allowing the cell to proceed to the following stage of the cell cycle. These checkpoints are at: G 1 /S Is the cell ready to divide? Important factors - Cell size, DNA damage, nutritional state, growth factors. G 2 /M Has DNA replicated successfully? Spindle Are all the chromosomes attached to the spindle fibers?

Molecular Mechanisms of Cell Cycle Control Role of Phosphorylation Kinases - Add phosphates Phosphatases - Remove phosphates Cell Cycle specific Kinases Cyclin dependent kinases (Cdk) phosphorylate proteins involved in the cell cycle. Cyclin Proteins that are synthesized and degraded at specific stages of the cell cycle. Cdks are only active when complexed with cyclin.

Cdk-Cyclin Complex

Molecular Mechanisms of Cell Cycle Control (Cntd) Cdk Activation 1. Cyclin binding. 2. Phosphorylation at one site activates it (and phosphorylation at another site inactivates it). G 1 /S Checkpoint A cyclin specific to this stage complexes with Cdk (eg. cdc2). The cdk phosphorylates proteins required for S phase.

Molecular Mechanisms of Cell Cycle Control (Cntd) G 2 /M Checkpoint A cdk (M phase promoting factor (MPF)) complexed with its cyclin, is activated. Spindle Checkpoint MPF activates APC (anaphase-promoting complex). If all the chromosomes are correctly aligned on the metaphase plate APC activates a protein that degrades the cohesin complex. This initiates the events of prophase.

Control of the Cell Cycle

Sexual Reproduction Involves 2 major processes which maintain chromosome number from generation to generation: Meoisis (Reduction division) Halves chromosome number  Gametes (in animals). Fertilization (Syngamy) Restores chromosome number  Zygote.