# General Biology/Cells/How Cells Divide

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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

## Bacterial DNA replication

* 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 number

* 1N = number of chromosomes in gamete
* 1N = haploid chromosome number
* 2N = number of chromosomes in somatic cells (cells that are not egg or sperm)
* 2N = diploid
* Deviations from N or 2N are usually lethal in animals
Chromosome numbers

## Eukaryotic chromosomes

* 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)

## Chromosomes

* 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

## Human chromosomes

* Diploid number = 2n = 46 = 23 pairs of homologs
* Haploid number = 23 (gametes)
* Each replicated chromosome contains 2 sister chromatids = 92 chromatids
Cell cycle
* Growth and division cycle of cells
* Precisely controlled by biochemical and gene activity, except in cancer
* Phases
** G1: primary growth phase
** S: DNA replication; chromosome replication
** G2: second growth phase; preparation for mitosis
** M: mitosis; nuclear division
** C: cytoplasmic division

## Mitotic 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.

## Mitosis

* 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
* Momentary alignment of chromosomes in center of cell
Anaphase

## Plant mitosis

* Similar to animal mitosis
* New cell wall formed between cells from membrane partition
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

## Controlling the cell cycle

* 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

## Cancer

* 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

## References

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