General Biology/Cells/How Cells Divide

How cells divide

Prokaryote cell division

Binary fission
- Doubling of cell contents, including DNA
- Fission to divide contents
Segregation of replicated genomes by growth of membrane between attachment points
Partitioning of cytoplasmic components
Escherichia coli
- Capable of cell division every 20 minutes under optimal conditions (DNA in continuous state of replication)
- Model organism of bacterial cell division

Replication follows rules of base pairing, with each polynucleotide chain serving as template for synthesis of its complement.
Genetic evidence showed that the bacterial chromosome is circular long before there was corroborating physical evidence.

Eukaryotic chromosomes

Discovered by Walther Fleming in 1882 in dividing cells of salamander larvae, following improvements in microscopes and staining technology
- He called division mitosis (mitos = “thread”)
Chromosome number is constant in a species
- Ranges from 2 to >500 (46 in human somatic cells)
- Homologous pairs, one contributed by each parent
- Change in number is cause and consequence of speciation
Chromosome constancy and their precise division in mitosis and meiosis led biologists to postulate that they were carriers of hereditary information

Chromosome numbers

Consist of chromatin
- DNA and associated proteins, mainly histones
- Nucleosomal organization
- Euchromatin: unwound chromatin, in basic nucleosomal configuration; genes available for expression
- Heterochromatin: highly condensed except during replication
Karyotype: array of chromosomes an individual possesses
- Clinical importance (Down syndrome; cancer)
- Evolutionary importance (speciation)

Homologous pairs
- Inherited one from each parent
- Identical in length and position of centromere
- Contain identical or similar genes
- Homologous pair = homologs
Morphology
- After replication, consist of two sister chromatids attached to a centromere

Cell cycle

Cells exiting the cell cycle are said to be in G0
Cell cycle time varies with stages of life cycle and development, with G1 the most variable
DNA replication occurs during S phase of the cell cycle following G1.

 - at this point the chromosomes are composed of two sister chromatids connected by a common centromere.

Nuclear division
- equational division of replicated chromosomes
- chromatids move to opposite poles
Continuous process
- prophase
- metaphase
- anaphase
- telophase
Driven by motors and microtubules
No change in chromosome number
- N → N by mitosis
- 2N → 2N by mitosis
May be accompanied by cytokinesis

Kinetochore Microtubules attach to kinetochores. Metaphase

Anaphase

Cell cycle control

Cell cycle events are regulated by protein complexes and checkpoints
Discovered by microinjection of proteins in to eggs, by mutational analysis and by techniques of molecular biology

Molecular control of cell cycle: Cdk and cyclin

Cyclin dependent protein kinase (Cdk)
- Phosphorylate serine/threonine of target regulatory proteins
- Function only when bound to cyclin
Cyclin: short-lived proteins that bind to cdks

External signals initiate cell division in multicellular organisms
Growth factors: extracellular regulatory signals
- Usually soluble; bind to cell surface receptor
- Sometimes membrane bound, requiring cell-cell contact with receptor
- E.g., upon wound, platelets release PDGF which stimulates fibroblasts to enter cell cycle (exit G0), to heal wound

Unregulated cell proliferation
Cancer cells have numerous abnormalities
- >46 chromosomes
- Mutations in proto-oncogenes
Encode proteins stimulating the cell cycle
May be regulated by phosphorylation
Often over expressed in cancer cells
- Mutations in tumor-suppressor genes
Encode proteins inhibiting the cell cycle
- Often bind to products of proto-oncogenes
May be regulated by phosphorylation

This text is based on notes very generously donated by Paul Doerder, Ph.D., of the Cleveland State University.