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

Bacterial DNA replication
Eukaryotic chromosomes
 * 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.
 * 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
Chromosome numbers
 * 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

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
Cell cycle
 * Diploid number = 2n = 46 = 23 pairs of homologs
 * Haploid number = 23 (gametes)
 * Each replicated chromosome contains 2 sister chromatids = 92 chromatids
 * 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
- at this point the chromosomes are composed of two sister chromatids connected by a common centromere.
 * 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
Kinetochore Microtubules attach to kinetochores. Metaphase Anaphase
 * 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
 * Momentary alignment of chromosomes in center of cell

Plant mitosis
Cell cycle control Molecular control of cell cycle: Cdk and cyclin
 * Similar to animal mitosis
 * New cell wall formed between cells from membrane partition
 * 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
 * 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