Bio 178 Lecture 21The Cell Cycle (Cntd.) and Sexual Reproduction

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Reading

• Chapters 11 & 12

Quiz Material

• Questions on P 226 & 239

• Chapters 11 & 12 Quizzes on Text Website (www.mhhe.com/raven7)

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. 2002. Molecular Biology of the cell.

Prokaryotic Cell Division

Prokaryotic Cell Division(Binary Fission)

• Duplication of Genetic MaterialDNA starts replicating at the origin of replication.

Replication proceeds bidirectionally until it reaches the termination site.

• Cell Elongation and DNA PartitioningCell grows and DNA is partitioned in the cell (by attachment to membrane).

• Cell DivisionA 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)

http://www.umass.edu/microbio/chime/pipe/ftsz/present/septum.htm

Control of the Cell Cycle

Control of the Cell CycleCells 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:

• G1/SIs the cell ready to divide?

Important factors - Cell size, DNA damage, nutritional state, growth factors.

• G2/MHas DNA replicated successfully?• Spindle Are all the chromosomes attached to the spindle fibers?

Molecular Mechanisms of Cell Cycle Control• Role of PhosphorylationKinases - Add phosphates

Phosphatases - Remove phosphates

• Cell Cycle specific Kinases

Cyclin dependent kinases (Cdk) phosphorylate proteins involved in the cell cycle.

• CyclinProteins 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 Activation1. Cyclin binding.

2. Phosphorylation at one site activates it (and phosphorylation at another site inactivates it).

• G1/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)

• G2/M Checkpoint

A cdk (M phase promoting factor (MPF)) complexed with its cyclin, is activated.

• Spindle CheckpointMPF 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

Control of the Cell Cycle

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellCycle.html

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.