CHAPTER 14 Cellular Reproduction Introduction Cells reproduce by

CHAPTER 14 Cellular Reproduction

Introduction • Cells reproduce by the process of cell division. • Mitosis leads to cells that are genetically identical to their parent. • Meiosis leads to production of cells with half of the genetic content of the parent.

14. 1 The Cell Cycle (1) • The cell cycle is the series of stages that a cell goes though. It consists of the M phase and the interphase. – The M phase includes the process of mitosis and cytokinesis. – Mitosis last about an hour or so. – Interphase constitutes the majority of the cell cycle and lasts longer than the M phase; it may extend for days, week, or longer.

An overview of the eukaryotic cell cycle

The Cell Cycle (2) • Interphase includes G 1, S, and G 2 periods. – G 1 takes place between the end of mitosis and the beginning of DNA replication. – DNA replication occurs during the S phase. – G 2 occurs between the end of S and the beginning of mitosis.

The Cell Cycle (3) • Cell cycles in vivo – Three cell types are distinguished based on their capacity to grow and divide. • Specialized cells such as nerve cells have lost the ability to divide. • Cells that do not divide but begin DNA synthesis, such as liver cells. • Cells that divide frequently such as stem cells. – Stem cells have asymmetric cell division in which the daughter cells have different fates.

The Cell Cycle (4) • Control of the Cell Cycle – Cell cycle is focused on initiation of DNA replication and initiation of mitosis. – The cytoplasm contains factors that regulate the state of the nucleus. • The cytoplasm of a replicating cell contains factors that stimulate initiation of DNA synthesis. • The cytoplasm of a cell undergoing mitosis contains factors that trigger chromosomal condensation. • G 1 -S and G 2 -M and both under positive control.

Experimental demonstration that cells contain factors that stimulate entry into mitosis

The Cell Cycle (5) • The Role of Protein Kinases – Entry into the M phase is triggered by activation of a protein kinase called maturation promoting factor (MPF). • MPF consists of two subunits: a kinase and a regulatory subunit, cyclin. • Increased concentration of cyclin activates the kinase. • The cyclin levels fluctuate predictably during the cell cycle.

Fluctuations of cyclin and MPF levels during the cell cycle

The Cell Cycle (6) • The role of protein kinases (continued) – MPF-like, cyclin-dependent kinases (Cdks) occur in yeast cells. • The product of the cdc 2 gene in fission yeast and CDC 28 in budding yeast is a cyclin-dependent kinase responsible for passage through both control points. • For cells to pass through a point of commitment Cdks must be transiently activated by specific cyclins. • Activated Cdk phosphorylates proteins specific for the particular transition.

A model of cell cycle regulation in yeast

The Cell Cycle (7) • Cyclin Binding – Cyclin binds to the catalytic subunit of Cdk. – Different cyclins are transcribed at different points in the cell cycle. – Cyclin-Cdk complexes phosphorylate other proteins.

The Cell Cycle (8) • Cdk Phosphorylation/Dephosphorylation – Cdc-activating kinase (CAK) phosphorylates both a threonine and a tyrosine on the Cdk subunit. – The doubly phosphorylated Cdc-cyclin is inactive. – A phosphatase (cdc 25) removes one phospahte. . – The singly phosphorylated cyclin is active, driving the cell to mitosis. – CAK and cdc 25 are activated by other kinases and phosphatases.

Progression through the yeast cell cycle requires phosphorylation and dephosphoryltion of cdc 2 residues

The Cell Cycle (9) • Cdk Inhibitors – Cdk activity can be blocked by a variety of inhibitors. – A protein called Sic 1 controls cell cycle progression.

The Cell Cycle (10) • Controlled Proteolysis – Occurs via the ubiquitin-proteasome pathway. – Two classes of multisubunit complexes function as ubiquitin ligases. – Destruction of the mitotic cyclins allows a cell to exist mitosis and enter a new cell cycle.

The Cell Cycle (11) • Subcellular Localization – Movement of cyclins between the cytoplasm and the nucleus is another point of control. – If nuclear accumulation of cyclin is blocked, cells fail to initiate mitosis.

Experimental demonstration of subcellular localization during the cell cycle

The Cell Cycle (12) • The pairing between individual cyclins and Cdks is specific, and only some combinations are found. • As in yeast, Cdk 1 is the only Cdk required to drive a mammalian cell through the cell cycle.

Combinations between various cyclins and Cdks at different stages in the mammalian cell cycle

The Cell Cycle (13) • Checkpoints, Kinase Inhibitors, and Cellular Responses – Progress through the cell cycle can be arrested at a checkpoint by: • Sensors that detect chromosomal abnormalities. • Transmitters that signal the information. • Effectors that inhibit cell cycle machinery.

Model for the mechanism of action of two DNAdamage checkpoints

The Cell Cycle (14) • Checkpoints (continued) – Pathways that stop the cell cycle progress: • Chk 1 inhibits Cdc 25 when DNA is damaged. • The protein p 21 inhibits the G 1 Cdk when DNA is damaged. – Cells may synthesize other proteins that inhibit progression through the cell cycle.

p 27: a Cdk inhibitor that arrests cell cycle progression

14. 2 M Phase: Mitosis and Cytokinesis (1) • Mitosis is a process of nuclear division in which two nuclei with identical genetic content are produced. – It is usually accompanied by cytokinesis. – Mitosis maintains the chromosome number. – Mitosis can occur in either haploid or diploid cells. – The phases of mitosis are: prophase, metaphase, and telophase.

The stages of mitosis in animal and plants cells

M Phase: Mitosis and Cytokinesis (2) • Prophase – In prophase, duplicated chromosomes are prepared for segregation and the mitotic machinery is assembled. – Formation of the Mitotic Chromosome • Chromosome compaction occurs in early phophase. • Condensin protein is also responsible for compaction.

Chromosomes in early prophase

M Phase: Mitosis and Cytokinesis (3) • Prophase (continued) – Compact chromosomes appear as rod-like structures • Each mitotic chromosome consists of two chromatids. • Chromosome compaction requires topoisomerase II. • Prior to replication, the DNA of each chromosome is associated with cohesin, which forms a ring to encircle the two sister DNA molecules.

The mitotic chromosome

Model for the role of condensin and cohesin in the formation of mitotic chromosomes

Each mitotic chromosome is comprised of a pair of sister chromatids connected by cohesin

M Phase: Mitosis and Cytokinesis (4) • Centromeres and Kinetochores – Centromeres occur at a primary constriction on chromosomes and serve as the binding site for proteins. – Kinetochores are on the outer surface of centromeres. – Kinetochores are the sites where chromosomes attach to the microtubules of the mitotic spindle.

The Kinetochore

M Phase: Mitosis and Cytokinesis (5) • Formation of the Mitotic Spindle – The centrosomeis a microtubule-organizing structure. – The mitotic spindle is made of microtubules. – The centrosome cycle, in which centrioles are duplicated, progresses along with the cell cycle.

The centrosome cycle of an animal cell

The centrosome cycle of an animal cell

M Phase: Mitosis and Cytokinesis (6) • Formation of the mitotic spindle (continued) – In the mitotic spindles of animal cells, microtubules are arranged in an aster around each centrosome. • Certain types of animal cells and most plants lack centrosomes.

Formation of the mitotic spindle

M Phase: Mitosis and Cytokinesis (7) • Prometaphase – During prometaphase the definitive mitotic spindle is formed and chromosomes are moved by microtubules into the center of the cell. • The microtubules grow into the region around a chromosome. • A single kinetochore is attached to microtubules form both spindle poles.

Prometaphase

Microtubule behavior during formation of the metaphase plate

M Phase: Mitosis and Cytokinesis (8) • Prometaphase (continued) – Chromosomes arrive at the center of the cell by differential growth of microtubules at opposite poles. – Eventually each chromosome moves into the center of the spindle.

The engagement of a chromosome during prometaphase and its movement to the metaphase plate

M Phase: Mitosis and Cytokinesis (9) • Metaphase – In metaphase, chromosomes are aligned at the spindle equator on the metaphase plate. – Microtubules in a mitotic spindle are highly organized. • Astral microtubules radiate from the centrosome to the region outside the body of the spindle.

The mitotic spindle of an animal cell

M Phase: Mitosis and Cytokinesis (10) • Metaphase (continued) – Chromosomal microtubules move chromosomes to the poles. – Polar microtubules maintain the integrity of the spindle. – Microtubule flux in the metaphase spindle involves tubulin treadmilling toward the poles.

Tubulin flux through the microtubules of the mitotic spindle at anaphase

M Phase: Mitosis and Cytokinesis (11) • Anaphase – Anaphase begins when sister chromatids split and move apart. – The Role of Proteolysis in Progression Through Mitosis • The anaphase promoting complex (APC) is activated at the metaphase/anaphase transition. • An APC subunit marks an anaphase inhibitor (securin) for destruction.

SCF and APC activities during the cell cycle