Chapter 16: The Cell Cycle

Spontaneous Generation

A theory saying that living organisms can arise from non-living matter. Discarded in the mid 18th century.

Rudolph Virchow

In 1855, became the first scientist to publish the conclusion that new cells arise only from the division of other cells. An important part of the argument against spontaneous generation.

Spontaneous Generation

A theory saying that living organisms can arise from non-living matter. Discarded in the mid 18th century.

Rudolph Virchow

In 1855, became the first scientist to publish the conclusion that new cells arise only from the division of other cells. An important part of the argument against spontaneous generation.

Cell Cycle

The life cycle of the cell. A sequence of events from one cell division to the next. How genetic material is duplicated and passed on.

Somatic Cells

Body cells. Their lives vary based on type and environments.

- Blood cells and skin cells are replaced frequently.

- Nerve cells divide infrequently or not at all.

Walther Fleming

A German biologist who, in 1873, stained specimen tissue, revealing the nucleus, which he named chromatin.

Parent Cell

The cell either divides into its daughter cells by meiosis or mitosis.

Daughter Cell

The cells that are produced as a result of mitosis. These cells are identical to each other, and also to the original parent cell.

DNA

Contains the genetic information of a cell. Made of deoxyribonucleic acid, a molecule of nucleic acid that governs processes of heredity in the cells of organisms. Found in each chromosome of a cell.

Each nuclus contains 3m of DNA.

Chromosome

A length of DNA and its associated proteins. In eukaryotic cells, found in the nucleus.

Histones

A highly organized arrangement of proteins. Helps compact DNA into the nucleus.

Eukaryotic

A cell characterized by the presence of a nucleus and other membrane-bound organelles. Eukaryotes can be unicellular (protists) or multicellular (fungi, plants and animals).

Chromatin

A mass of long intertwined cells that make up genetic material for most of a cell's life cycle. "Unpackaged DNA". Only condenses into chromosomes when the cell is about to divide.

Cytokinesis

NOT a stage of mitosis. The division of cytoplasm and organelles to form two identical daughter cells with the same genetic material as the original parent cell. Daughter cells are now in G1 of interphase.

P53

This tumor suppressor gene causes cell cycle arrest in G1, providing time for DNA repair. If repair is successful, cells re-enter the cycle. If unsuccessful, apoptosis.

Micropropagation

The propagation of plants by growing plantlets in tissue culture and then planting them out.

Centromere

The constricted (pinched-in) region in the condensed chromosome.

Homologous Chromosome

Chromosomes with the same gene sequence: one homologue is of maternal origin, the other is of paternal origin. Also known as autosomes.

- Humans have 23 pairs of homologous chromosomes (46 in total).

- 22 pairs of autosomes and 1 pair of sex chromosomes.

Although homologues carry the same set of genes, they may carry different forms of each gene (alleles).

Sex Chromosomes

Determines the sex of and individual. A human female has two X chromosomes and a male has X and Y chromosomes and are still counted as a pair despite not being homologous.

Genes

Areas of DNA that contain specific genetic information at the same location or locus. Homologous chromosomes carry the same genes.

Alleles

Different forms of the same gene. Homologous pairs carry different forms of the same gene.

Ploidy

The number of sets of chromosomes found in a cell or in the cells of an organism.

Diploid

The ploidy of human somatic cells because they contain two sets of chromosomes (2n=46).

Haploid

The ploidy of human gametes because they contain one set of chromosomes. (n=23).

Polyploid

The ploidy of some organisms that contain more than two homologous chromosomes in each cell. Eg. tetraploid, triploid, octoploid.

Karyotype

The particular set of chromosomes an individual possesses. Identified using staining under a light microscope.

Cell Division Cycle

The sequence of events in a cell that lead to duplication of DNA and division of the cytoplasm and organelles to produce two daughter cells. Made up of a growth stage and a division stage.

Interphase

The growth stage of a somatic cell's life when the cell carries out regular metabolic functions and prepares for its next division. There are three phases: G1, S, and G2. Doubling of cytoplasmic components and chromosome duplication.

G1

Early scientists could initially not identify any activities during this phase so they called it "Gap 1". Now, scientists know that important growth processes occur so it is referred to as "Growth 1".

Sister Chromatids

Replicated forms of a chromosome joined together by the centromere and eventually separated during mitosis or meiosis II.

S Phase

About midway through interphase when the cells DNA is copied exactly. The DNA in the chromatin replicates to create a second identical set of DNA (sister chromatids joined at centromere). Because new genetic material is synthesized, it is known as the synthesis phase.

G2 Phase

The last segment of interphase. Because DNA replication consumes a lot of energy, this growth stage lets the cell rebuild its reserves of energy to prepare for division. It also manufactures proteins and other molecules to make structures required for the division of the nucleus and cell.

M Phase

When the division of genetic info (mitosis) and splitting of the cytoplasm (cytokinesis) occur.

Mitosis

The formation of two identical daughter cells from an original (parent cell) is referred to as mitosis.

- Occurs in the somatic cells of all multicellular organisms.

- Daughter cells are genetically identical to parent cells.

- Genetic information is encoded in DNA. It is found within the nucleus of every cell in the body.

- The process of mitosis ensures that DNA is replicated and passed along during each cell division.

- The purpose of mitosis is growth (increased number of cells), development (differentiation of tissues), and repair (of wounds or due to cell loss).

Prophase

"Beginning" Each pair of homologous chromosomes has been duplicated, so the cell contains twice as much DNA as a typical cell.

- Chromatin condenses to form chromosomes.

- Centrioles move to opposite poles of the cell and spindle fibres begin to form.

- Thickened chromosomes are made of two chromatids held together at the centromere.

- Nuclear membrane breaks dissolves.

Centrioles

A pair of cylindrical organelles that move apart to opposite poles of a cell during prophase, forming a spindle network.

Spindle Apparatus

A network of fibres is formed after the centrioles move apart during prophase. Each fibre is made of microtubules, hollow tubes of protein facilitating movement of chromosomes within a cell. Spindle fibre lengthens with the addition of microtubule subunits. Removal of subunits causes spindle fibres to shorten.

Duplication of Chromosomes

Results when a single stranded chromosome is duplicated to form a double stranded chromosome. Both considered ONE chromosome.

Gene Arrangement

A section of DNA on a chromosome that codes for a particular trait.

Plant Cells

Most of these cells do not have centrioles, instead, they divide through microtubule organizing centers (MTOCs) where spindle fibers will form from.

Metaphase

"Middle" Chromosomes composed of sister chromatids line up along the equatorial (metaphase) plate. Spindle fibres attach to the centromere of each chromosome guiding them to the equator of the cell.

- Spindle fibres attach so one sister chromatic faces on pole and the other faces the other pole.

- Each pair of sister chromatids is considered to be a single chromosome as long as the chromatids remain joined at the centromere.

Anaphase

Centromeres divide, spindle fibres shorten and sister chromatids are pulled apart from one another, moving to opposite ends of the cell. If this occurs correctly, two opposing poles are formed with a complete diploid set of chromosomes. At the same time, other microtubules in the spindle apparatus lengthen and force poles of the cell from one another.

Telophase

Chromosomes reach opposite poles of the cell, spindle fibres break down, chromosomes unwind into chromatin, the nuclear membrane forms around each new set of genetic info. The nucleolus also forms.

Telophase and Cytokinesis in Plant Cells

Because plant cells contain a rigid cell wall, cytoplasm will not furrow and pinch off. Instead, a cell plate forms between the two daughter nuclei, eventually developing into a cell wall.

Cancer

A group of diseases associated with uncontrolled, rapid cell division. Moving quickly from one cell division to the next, resulting in a fast-growing mass of non-functional cells called a tumour. Can be caused by mutagens.

Mutagens

Agents which change an organisms genetic material. May be physical, chemical, or biological in origin.

Carcinogens

Mutagens that specifically lead to the formation of cancerous cells.

Cloning

The process of forming identical offspring from a single cell or tissue of a parent organ. A form of asexual reproduction (plants, bacteria, hydra). A form of cloning also results in twins.

Monozygotic Twins

If a fertilized egg undergoes mitosis and splits in two. Leads to identical twins with the same DNA. Fraternal Twins.

Dizygotic Twins

Non-identical twins. Form when two eggs are ovulated at once and fertilized by different sperm. They do not share the same genetic makeup.

Plant Cloning

Cells are extracted from the parent plant (eg. carrots, orchids, tobacco, lettuce). Cells divide by mitosis. Eventually, the newly developing plant can be transferred to the soil: it is genetically identical to the parent plant.

Stem Cells

Cells that have not yet specialized.

Totipotent Cell

Capable of dividing and developing to form a complete, mature organism such as a zygote. It is unethical to obtain stem cells from this source.

Pluripotent Stem Cell

Capable of developing into many different cell types. Obtained from adult tissues.

Telomeres

Caps found at the end of each chromosome that shortens every time a cell divides. Eventually, they will become so short that a cell will no longer be able to undergo mitosis and cell death occurs. Serves as a "molecular clock" for cellular aging.

Meiosis

The process that produces haploid gametes from diploid cells in the ovaries and testes. Has two outcomes, reduction division (form of cell division producing daughter cells with fewer chromosomes than the parent cells), and recombination (products of meiosis have different combinations of genes. Genetic recombination gives rise to offspring that are genetically distinct from one another and their parents.)

Produces 4 daughter cells rather than two.

Germ Cells

Gamete producing cells, proceed through the growth and synthesis phases from interphase before dividing. Contains duplicated chromosomes at the start of meiosis made up of a pair of identical sister chromatids held together at the centromere.

Prophase I

Chromatin condenses to form chromosomes, spindle fibres form, the nuclear membrane dissolves, synapsis and crossing over occur leading to increased genetic diversity.

Synapsis

The aligning of homologous chromosomes side by side leading them to pair up making four chromatids (tetrad).

Tetrad

Made up of. a pair of sister chromatids synapsed with another pair of sister chromatids. In the middle, two homologous but non identical chromatids (non sister chromatids) lie side by side. This plays an important role in genetic recombination.

Crossing Over

The exchange of genetic material between two homologous chromosomes during meiosis.

Metaphase I

Spindle fibres attach to the centromere of each chromosome, chromosomes line up as homologous pairs (tetrads) along the equatorial plate. The orientation of each pair is independent of the orientation of other pairs (independent assortment).

Independent Assortment

The random distribution of the pairs of genes on different chromosomes to the gametes. Increases genetic diversity due to the number of potential chromosome combinations. For a homologous pair, the maternal chromosome is orientated to one pole and the paternal is orientated to the other.

Anaphase I

The spindle fibres shorten. This causes the homologous chromosomes to separate from one another. The homologues move to opposite poles of the cell. Because the sister chromatids are still held together, the centromeres do not split as they do in mitosis. The result is that a single chromosome (made up of two sister chromatids) from each homologous

pair moves to each pole of the cell. This stage is the reduction division of chromosome number.

Telophase I

Homologous chromosomes begin to uncoil, spindle fibres disappear. Cytoplasm is divided and a nuclear membrane forms around each new set of chromosomes. Each new cell is haploid containing either the maternal or paternal chromosome from each tetrad. These daughter cells are similar, but not identical.

Meiosis II

Cells undergo prophase II (chromosomes condense), metaphase II (chromosomes line up along equatorial plate), anaphase II (sister chromatids separate and move to opposite poles), telophase II (chromosomes gather into nuclei), and cytokinesis. The result is four haploid daughter cells with a single copy of each chromosome. Each haploid cell formed after telophase II will proceed through cytokinesis to form gametes.

Spermatagonium

A diploid germ cell that starts spermatogenesis. At the beginning of puberty, they are stimulated to divide by mitosis to form two daughter cells. One cell replenishes the spermatogonia cell population and the other develops into a primary spermatocyte.

Primary Spermatocyte

Formed from the daughter cell of spermatogonia. Undergoes meiosis I to form two secondary spermatocytes.

Secondary Spermatocyte

Formed from the primary spermatocyte (2 formed) then undergoes meiosis II to form four spermatids.

Spermatid

Formed from secondary spermatocytes after they undergo meiosis II. Four formed. Go through a final set of developmental stages in order to develop into mature sperm. The nucleus and certain enzymes are organized into a "head" region. The midsection holds many mitochondria, which serve as an energy resource for the cell. Finally, a long tail-like flagellum develops for locomotion.

Oogonium

A diploid germ cell that starts oogenesis. Each undergoes mitosis to form two primary oocytes.

Primary Oocyte

Made from the mitosis of oogonium. Remain in an arrested prophase I until puberty where every month, one primary oocyte undergoes meiosis with and unequal division of cytoplasm.

Secondary Oocyte

The cell that receives most of the cytoplasm and later undergoes meiosis II.

First Polar Body

Formed after meiosis I, not functional and will soon degenerate. The same as the second polar body.

Asexual Reproduction

The reproductive process in which a parent organism produces genetically identical offspring. The key mechanism is mitosis.

Sexual Reproduction

Involves the production of gametes by meiosis, followed by fertilization between genetically distinct parental gametes to

produce genetically distinct offspring.

Binary Fission

When single celled organisms split into two identical daughter cells leading to rapid growth of identical populations. Used by bacteria. Bacteria cannot undergo mitosis because it has a single, circular chromosome and no nucleus.

Exponential Growth

The sequence of repeat doubling of cells. Allows bacteria to produce huge populations in a fairly short time. However, these populations are genetically identical. If one cell is vulnerable to a particular toxin or virus, then every cell in the colony will also be vulnerable to the same toxin or virus.

Conjugation

Involves the transfer of genetic material from one cell to another by cell to cell contact through a bridging structure called a pilus. This creates cells with new genetic combinations. Used by bacteria.

Budding

A form of asexual reproduction in which a complete but miniature version of the parent grows out from the parent's body. The new organism then separates to become an independent organism. Eg. Hydra, Coral, Sponge.

Vegetative Reproduction

A form of budding specific to plants. Takes place in strawberry plants that spread by extending thin, creeping stems that take root. Once the plant has taken root, the stem disintegrates separating the new plant from its parent.

Fragmentation

Another form of asexual reproduction involving the creation of new plants from a fragment (portion) of a parent plant. Done in the cultivation of potatoes.

Parthenogenesis

A form of asexual reproduction in which an unfertilized egg develops into an adult. Eg. Queen bee lays fertilized and unfertilized eggs. Fertilized = female worker bees, unfertilized = male drones.

Spore

A structure that contains genetic material and cytoplasm surrounding by a protective sheath/wall. The wall protects the contents until conditions are favourable at which the spore wall opens and the organism develops. Readily dispersed in water and wind. Spores may be haploid or diploid and not all are products of asexual reproduction. Many fungi.

Autosome

Any chromosome that is not a sex chromosome.

Sporophyte

A diploid generation of plants. Through meiosis, produces multiple haploid spores, which develop without fertilization.

Gametophyte

A haploid generation of plants. Produce male and female gametes that develop into sporophytes upon fertilization.

Alternation of Generations

The life cycle of plants consisting of two generations: a haploid generation and a diploid generation that alternate. One generation or the other is characteristically dominant in different plant groups.

Mosses

- Leafy green mat = gametophyte.

- Stalk from mat = sporophyte.

- Spores cast from the cap and develop into the gametophyte on the ground, special structures makes sperm and egg = fertilization.

- Because sperm must swim to egg, mosses can only grow in moist environments.

Conifers

- The tree itself is the diploid sporophyte.

- The haploid gametophytes are microscopic structures with male and female cones.

- Single celled female gametophyte develops from a spore produced by a specialized structure at the top of each scale of the female cone (larger cone).

- The female gametocyte remains inside spore producing structure.

- Male gametophyte produced by structure on male cone released in pollen that reaches the female cone.

Cnidaria

Include jellyfish, sea anemones, and corals. They alternate between reproductive phase. Two distinct adult forms: a non-motile polyp and a free swimming medusa. In jellyfish, the medusa stage is dominant. In anemones, the polyp stage is dominant. Obelia alternate.

Nondisjunction

Occurs when homologous chromosomes or sister chromatids do not properly separate during meiosis. In either case, gametes with either too few or too many chromosomes result. In humans, this results in sperm or eggs with 24 (one too many) or 22 (one too few) chromosomes.

If gametes produced by nondisjunction are fertilized, it may result in disease.

Kleinfelter Syndrome

When a person receives the chromosomes XXY. Appears male at birth but produces high levels of female sex hormones at puberty, resulting in sterility.

Monosomy

A condition in which one chromosome (usually x) lacks its homologous partner. Appear female but do not develop sexually. Turner syndrome = most fetuses are miscarried by the 20th week of pregnancy.

Trisomy

Occurs when an extra chromosome above the typical number (46) is present. Results in down syndrome (cognitive delay and common physical traits).

Karyotyping

The process of sorting chromosomes into their matched pairs. An important technique for investigating chromosomal disorders.