CELL REPRODUCTION THE CELL CYCLE AND MITOSIS CELL

CELL REPRODUCTION THE CELL CYCLE AND MITOSIS

CELL REPRODUCTION • Cell Division: process by which a cell divides to form two new cells (daughter cells) • Three types of cell division, or cell reproduction – Prokaryotes (bacteria) • Binary fission divides forming two new identical cells – Eukaryotes • Mitosis – Cell or organism growth – Replacement or repair of damaged cells • Meiosis – formation of sex cells, or gametes

Why do cells divide? 1: DNA Overload – If cells grow without limit, an “information crisis” would develop – DNA cannot serve the needs of the increasing size of cell 2: Exchange of materials – Food and oxygen have to cross membrane very quickly – Waste must get out – If cell is too large, this occurs too slowly and cell will die

PROKARYOTIC CELL DIVISION • Binary fission – 3 main steps: 1: DNA Replication—DNA is copied, resulting in 2 identical chromosomes 2: Chromosome Segregation— 2 chromosomes separate, move towards ends (poles) of cell 3: Cytokinesis—cytoplasm divides, forming 2 cells – Each new daughter cell is genetically identical to parent cell

I. Bacterial Cell Division • • Cell Growth and Binary Fission Fts Proteins and Cell Division Mre. B and Determinants of Cell Morphology Peptidoglycan Synthesis and Cell Division

Cell Growth and Binary Fission • Binary fission: cell division following enlargement of a cell to twice its minimum size • Generation time: time required for microbial cells to double in number • During cell division, each daughter cell receives a chromosome and sufficient copies of all other cell constituents to exist as an independent cell

One generation Cell elongation Septum formation Completion of septum; formation of walls; cell separation

Fts Proteins and Cell Division • Fts (filamentous temperature-sensitive) Proteins – Essential for cell division in all prokaryotes – Interact to form the divisome (cell division apparatus) • Fts. Z: forms ring around center of cell; related to tubulin • Zip. A: anchor that connects Fts. Z ring to cytoplasmic membrane • Fts. A: helps connect Fts. Z ring to membrane and also recruits other divisome proteins – Related to actin

Fts Proteins and Cell Division • DNA replicates before the Fts. Z ring forms • Location of Fts. Z ring is facilitated by Min proteins – Min. C, Min. D, Min. E • Fts. K protein mediates separation of chromosomes to daughter cells

Outer membrane Peptidoglycan Cytoplasmic membrane Divisome complex Cytoplasmic membrane Fts. Z ring

Mre. B and Determinants of Cell Morphology • Prokaryotes contain a cell cytoskeleton that is dynamic and multifaceted • Mre. B: major shape-determining factor in prokaryotes – Forms simple cytoskeleton in Bacteria and probably Archaea – Forms spiral-shaped bands around the inside of the cell, underneath the cytoplasmic membrane – Not found in coccus-shaped bacteria

Mre. B and Determinants of Cell Morphology • Mre. B (cont’d) – Localizes synthesis of new peptidoglycan and other cell wall components to specific locations along the cylinder of a rod-shaped cell during growth

Figure 5. 4 a Fts. Z Cell wall Cytoplasmic membrane Mre. B Sites of cell wall synthesis

Figure 5. 4 b

Mre. B and Determinants of Cell Morphology • Most archaeal genomes contain Fts. Z and Mre. Blike proteins, thus cell morphology is similar to that seen in Bacteria

Peptidoglycan Synthesis and Cell Division • Production of new cell wall material is a major feature of cell division – In cocci, cell walls grow in opposite directions outward from the Fts. Z ring – In rod-shaped cells, growth occurs at several points along length of the cell

Peptidoglycan Synthesis and Cell Division • Preexisting peptidoglycan needs to be severed to allow newly synthesized peptidoglycan to form – Beginning at the Fts. Z ring, small openings in the wall are created by autolysins – New cell wall material is added across the openings – Wall band: junction between new and old peptidoglycan

Figure 5. 5 Fts. Z ring Wall bands Septum © 2012 Pearson Education, Inc. Growth zone

Figure 5. 4 a Fts. Z Cell wall Cytoplasmic membrane Mre. B Sites of cell wall synthesis © 2012 Pearson Education, Inc.

Peptidoglycan Synthesis and Cell Division • Bactoprenol: carrier molecule that plays major role in insertion of peptidoglycan precursors – C 55 alcohol – Bonds to N-acetylglucosamine/ N-acetylmuramic acid/pentapeptide peptidoglycan precursor © 2012 Pearson Education, Inc.

Peptidoglycan Synthesis and Cell Division • Glycolases: enzymes that interact with bactoprenol – Insert cell wall precursors into growing points of cell wall – Catalyze glycosidic bond formation © 2012 Pearson Education, Inc.

Figure 5. 7 a Peptidoglycan Transglycosylase activity Cytoplasmic membrane Out Growing point of cell wall Autolysin activity In Pentapeptide Bactoprenol © 2012 Pearson Education, Inc.

Peptidoglycan Synthesis and Cell Division • Transpeptidation: final step in cell wall synthesis – Forms the peptide cross-links between muramic acid residues in adjacent glycan chains – Inhibited by the antibiotic penicillin © 2012 Pearson Education, Inc.

Figure 5. 7 b Transpeptidation © 2012 Pearson Education, Inc.