Cell division involves the distribution of identical genetic material, DNA, to two daughters cells. What is most remarkable is the fidelity with which the DNA is passed along, without dilution or error, from one generation to the next.

In order to better understand the concept of cell division and genetics, some basic definitions are in order:

• gene - basic unit of heredity; codes for a specific trait
• locus - the specific location of a gene on a chromosome (locus - plural loci)
• genome - the total hereditary endowment of DNA of a cell or organism
• somatic cell - all body cells except reproductive cells
• gamete - reproductive cells (i.e. sperm & eggs)
• chromosome - elongate cellular structure composed of DNA and protein - they are the vehicles which carry DNA in cells
• diploid (2n) - cellular condition where each chromosome type is represented by two homologous chromosomes
• haploid (n) - cellular condition where each chromosome type is represented by only one chromosome
• homologous chromosome - chromosome of the same size and shape which carry the same type of genes
• chromatid - one of two duplicated chromosomes connected at the centromere
• centromere - region of chromosome where microtubules attach during mitosis and meiosis

composed of DNA and protein (histones) all tightly wrapped up in one package duplicated chromosomes are connected by a centromere

Example - an organism is 2n = 4.  Chromosomes 1 & 2 are homologous chromosomes Chromosomes 3 & 4 are homologous chromosomes Chromosomes 1 & 3 came from the mother Chromosomes 2 & 4 came from the father

G1 - first gap S - DNA synthesis (replication) G2 - second gap M - mitosis

• mitosis - nuclear/chemical events resulting in two daughter nuclei which have identical genetic material to each other and to the mother cell
• cytokinesis - division of the cytoplasm. This usually occurs with mitosis, but in some organisms this is not so

• The stages of the cell cycle can be broken down into six stages:
• Interphase, Prophase, Metaphase, Anaphase, Telophase

Prophase - the first stage of mitosis. The chromosomes condense and become visible The centrioles form and move toward opposite ends of the cell ("the poles") The nuclear membrane dissolves Compare Prophase to the Prophase I and to the Prophase II stages of mitosis.

Metaphase Microtubules grow from the centrioles and attach to the centromere The chromosomes line up in the middle of the cell ("the equator") Compare Metaphase to the Metaphase I and to the Metaphase II stages of mitosis.

Anaphase The centromeres break and the sister chromosomes separate The chromosomes migrate toward opposite poles Cytokinesis begins Compare Anaphase to the Anaphase I and to the Anaphase II stages of mitosis.

Telophase The chromosomes decondense The nuclear envelope forms Cytokinesis reaches completion, creating two daughter cells Compare Telophase to the Telophase I and to the Telophase II stages of mitosis.

Cytokinesis Divides the Cytoplasm

• First, a cleavage furrow appears
• cleavage furrow = shallow groove near the location of the old metaphase plate
• A contractile ring of actin microfilaments in association with myosin, a protein
• Actin and myosin are also involved in muscle contraction and other movement functions
• The contraction of a the dividing cell's ring of microfilaments is like the pulling of drawstrings
• The cell is pinched in two

• Cytokinesis in plant cells is different because plant cells have cell walls.
• There is no cleavage furrow
• During telophase, vesicles from the Golgi apparatus move along microtubules to the middle of the cell (where the cell plate was) and coalesce, producing the cell plate
• Cell-wall construction materials are carried in the vesicles and are continually deposited until a complete cell wall forms between the two daughter cells

Regulation of the Cell Cycle

• The cell-cycle control system is driven by a built-in clock that can be adjusted by external stimuli (chemical messages)
• Checkpoint - a critical control point in the cell cycle where stop and go-ahead signals can regulate the cell cycle
• Animal cells have built-in stop signals that halt the cell cycles and checkpoints until overridden by go-ahead signals.
• Three Major checkpoints are found in the G1, G2, and M phases of the cell cycle
• The G1 checkpoint - the Restriction Point
• If a cell receives a go-ahead signal at the G1 checkpoint, it will usually continue with the cell cycle
• If the cell does not receive the go-ahead signal, it will exit the cell cycle and switch to a non-dividing state called G0
• Actually, most cells in the human body are in the G0 phase

Rhythmic fluctuations in the abundance and activity of cell-cycle control molecules pace the events of the cell cycle.

• Kinase - a protein which activates or deactivates another protein by phosphorylating them.
• Kinases give the go-ahead signals at the G1 and G2 checkpoints
• The kinases that drive these checkpoints must themselves be activated
• The activating molecule is a cyclin, a protein that derives its name from its cyclically fluctuating concentration in the cell
• Because of this requirement, these kinases are called cyclin-dependent kinases, or Cdk's

• Cyclins accumulate during the G1, S, and G2 phases of the cell cycle
• By the G2 checkpoint (the red bar in the figure), enough cyclin is available to form MPF complexes (aggregations of Cdk and cyclin) which initiate mitosis
• MPF apparently functions by phosphorylating key proteins in the mitotic sequence
• Later in mitosis, MPF switches itself off by initiating a process which leads to the destruction of cyclin
• Cdk, the non-cyclin part of MPF, persists in the cell as an inactive form until it associates with new cyclin molecules synthesized during interphase of the next round of the cell cycle

PDGF is required for the division of fibroblasts which are essential in wound healing

• When injury occurs, platelets (blood cells important in blood clotting) release PDGF
• Fibroblasts are a connective tissue cells which possess PDGF receptors on their plasma membranes
• The binding of PDGF activates a signal-transduction pathway that leads to a proliferation of fibroblasts and a healing of the wound

• Cells grown in culture will rapidly divide until a single layer of cells is spread over the area of the petri dish, after which they will stop dividing
• If cells are removed, those bordering the open space will begin dividing again and continue to do so until the gap is filled - this is known as contact inhibition
• Apparently, when a cell population reaches a certain density, the amount of required growth factors and nutrients available to each cell becomes insufficient to allow continued cell growth

• For most animal cells to divide, they must be attached to a substratum, such as the extracellular matrix of a tissue or the inside of the culture jar
• Anchorage is signaled to the cell-cycle control system via pathways involving membrane proteins and the cytoskeleton

• Cancer cells do not respond normally to the body's control mechanism.
• They divide excessively and invade other tissues
• If left unchecked, they can kill the organism
• Cancer cells do not exhibit contact inhibition
• If cultured, they continue to grow on top of each other when the total area of the petri dish has been covered
• They may produce required external growth factor (or override factors) themselves or possess abnormal signal transduction sequences which falsely convey growth signals thereby bypassing normal growth checks
• Cancer cells exhibit irregular growth sequences
• If growth of cancer cells does cease, it does so at random points of the cell cycle
• Cancer cells can go on dividing indefinitely if they are given a continual supply of nutrients
• Normal mammalian cells growing in culture only divide 20-50 times before they stop dividing

More definitions:

• Allele - alternate forms of the same gene

• Homozygous - having two identical alleles for a given gene

• Heterozygous - having two different alleles for a given gene

• Genotype - genetic makeup of an organism

• Phenotype - the expressed traits of an organism

• In an organism with a heterozygous gene, often one allele will be expressed as a physical attribute while the other does not (it is masked).
• The expressed gene is the dominant gene while the masked one is known as the recessive gene.
• In the above example of Hair Color, the dark hair allele is dominant to the light hair allele because the heterozygous individual is possesses dark hair
• Of course, there are exceptions.

• Meiosis - a two-stage cell division resulting in gametes with half the chromosome number found in the original cell (each chromosome type is represented by one chromosome).
• Meiosis is only seen in the gametes (i.e. the sperm and eggs) of sexually reproducing organisms.
• Why is Meiosis important in sexually reproducing organisms???

• A cell undergoing meiosis first passes though Interphase; therefore, the DNA will be duplicated during the S stage of Interphase.

The stages of meiosis can be broken down into two main stages, Meiosis I and Meiosis II

• Meiosis I can be broken down into four substages: Prophase I, Metaphase I, Anaphase I and Telophase I

• Meiosis II can be broken down into four substages: Prophase II, Metaphase II, Anaphase II and Telophase II

Prophase I - most of the significant processes of Meiosis occur during Prophase I The chromosomes condense and become visible The centrioles form and move toward the poles The nuclear membrane begins to dissolve The homologs pair up, forming a tetrad Each tetrad is comprised of four chromotids - the two homologs, each with their sister chromatid Homologous chromosomes will swap genetic material in a process known as crossing over (abbreviated as XO) Crossing over serves to increase genetic diversity by creating four unique chromatids Compare Prophase I to Prophase II and to the Prophase stage of mitosis.

Genetic material from the homologous chromosomes is randomly swapped This creates four unique chromatids Since each chromatid is unique, the overall genetic diversity of the gametes is greatly increased

Metaphase I Microtubules grow from the centrioles and attach to the centromeres The tetrads line up along the cell equator Compare Metaphase I to Metaphase II and to the Metaphase stage of mitosis.

Anaphase I The centromeres break and homologous chromosomes separate (note that the sister chromatids are still attached) Cytokinesis begins Compare Anaphase I to Anaphase II and to the Anaphase stage of mitosis.

Telophase I The chromosomes may decondense (depends on species) Cytokinesis reaches completion, creating two haploid daughter cells Compare Telophase I to Telophase II and to the Telophase stage of mitosis.

Prophase II Centrioles form and move toward the poles The nuclear membrane dissolves Compare Prophase II to Prophase I and to the Prophase stage of mitosis.

Metaphase II Microtubules grow from the centrioles and attach to the centromeres The sister chromatids line up along the cell equator Compare Metaphase II to Metaphase I and to the Metaphase stage of mitosis.

Anaphase II The centromeres break and sister chromatids separate  Cytokinesis begins Compare Anaphase II to Anaphase I and to the Anaphase stage of mitosis.

Telophase II The chromosomes may decondense (depends on species) Cytokinesis reaches completion, creating four haploid daughter cells Compare Telophase II to Telophase I and to the Telophase stage of mitosis.

Some questions to ponder

• How does the number of daughter cells produced from mitosis and meiosis differ?
• How does the ploidy of the daughter cells produced from mitosis and meiosis differ?
• Do the daughter cells produced from mitosis contain identical genetic complements?
• Do any of the daughter cells produced from meiosis contain identical genetic complements?
• When do the homologous chromosomes separate during mitosis?
• When do the homologous chromosomes separate during meiosis?
• When do sister chromatids separate during mitosis?
• When do sister chromatids separate during meiosis?
• Click the cockroach below to view the answers to these questions.