Cellular Division Flashcards
Cell division functions in…
growth, repair and reproduction
The Two Types of Cell Divison
mitosis and meiosis
Mitosis
produced two genetically identical daughter cells and conserves the chromosome number (2n)
Meiosis
occurs in sexually reproducing organisms and results in cells with half the chromosome number of the parent cell
Chromosome
highly coiled and condensed strands of DNA; consists of two sister chromatids; the centromere is the region that holds chromatids together
Kinetochore
disc shaped protein on the centromere that attaches the chromatid to the mitotic spindle during cell division
The two factors that limit cell size and promote cell division.
- the ratio of the volume of a cell to the surface area
2. the capacity of the nucleus to control the entire cell
Cell Volume to Surface Area Ratio
the area of the cell membrane increases as the square of the radius while the volume of the cell increases as the cube of the radius; as a cell grow larger the volume increases at a faster rate than the cell membrane
Why does the ratio of the cell volume to surface area matter?
A cell depends on the cell membrane for the exchange of nutrients and waste products.
Capacity of the Nucleus
The nucleus must be able to provide enough information to produce adequate quantities to meet the cell’s needs.
Capacity of the Nucleus: Paramecium
This is a large, sophisticated cell with two nuclei that each control different cell functions.
Capacity of the Nucleus: Human Skeletal Muscle Cells
Giant, multinucleate cells
Five Major Phases of the Cell Cycle
G(1), S and G(2) which make up Interphase. Then, Mitosis and Cytokinesis.
Interphase: G(1) Phase
A period of intense growth and biochemical activity.
Interphase: S Phase
A period of synthesis and replication of DNA.
Interphase: G(2) Phase
A phase when the cell cycle grows and competes preparations for cell division.
Interphase
90% of life is spent here; Chromatin is threadlike and the nucleus contains one or more nucleoli. A single centrosome, consisting of two centrioles can bee seen in the cytoplasm. Plant cells lack centrosomes but have MTOCs.
Four Division of Mitosis
Prophase, Metaphase, Anaphase and Telophase
Prophase
The nuclear membrane begins to disintegrate and the strands of chromosomes condense. The nucleolus disappears. The mitotic spindle forms and begins to extend from one centrosome to the other. Longest phase of mitosis.
Metaphase
Chromosomes line up single file on the metaphase plate. Centrosomes are at opposite poles of the cell. Spindle fibers run from the centrosomes to the kinetochores of the centromeres.
Anaphase
Centromeres of each chromosome separate as spindle fibers split apart the sister chromatids. This is the shortest phase of mitosis.
Telophase
Chromosomes cluster at opposite ends of the cell and the nuclear membrane reforms. The chromosomes uncoil and return to chromatin form. Two individual nuclei form at the end of mitosis.
Cytokinesis: Animal Cells
The dividing of the cytoplasm which begins during anaphase. In animal cells, a cleavage furrow forms as a result of actin and myosin filaments pinching.
Cytokinesis: Plant Cells
A cell plate forms during telophase ad vesicles from the Golgi coalesce down the middle of the cell. Daughter plant cells don’t separate from each other. A sticky middle lamella cements adjacent cells together.
Contact Inhibition / Density-Dependent Inhibition
Normal cells divide and grow until they become too crowded; then they stop dividing and enter G zero.
Anchorage Dependence
Characteristic of normal animal cells; To divide, a cell must be attached or anchored to some surface such as an extracellular membrane.
Cancer Cells
Show neither contact inhibition or anchorage dependence. They divide uncontrollably and do not have to be anchored to any membrane. This is why cancer cells can migrate of metastasize to other regions of the body.
Gametes
sex cells; sperm and ova; haploid chromosome number
Meiosis: Meiosis I
Meiosis I (reduction division) is the process by which homologous chromosome separate.
Meiosis: Meiosis II
Similar to mitosis, but it produces haploid cells.
Meiosis I
Each chromosome pairs up precisely with its homologue into a synaptonemal complex by a process called synapsis and forms a structure know as a tetrad or bivalent.
Why is synapsis important?
- It ensures that each daughter cell will receive one homologue from each parent.
- It makes possible the process of crossing over.
Crossing Over
The process by which homologous chromatids exchange genetic material. A common and highly organized method toe ensure greater genetic variation among gametes.
Meiosis II
Sister chromatids separate in a process similar to mitosis, with prophase, metaphase, anaphase, telophase and cytokinesis. The chromosome number is haploid and are genetically identical to the parent cell.
Meiosis I: Prophase
- Synapsis: the pairing of homologues
- Crossing Over: the exchange of homologous bits of chromosome
- Chiasmata: the visible manifestation of the cross-over events are visible
This is the longest phase.
Meiosis: Metaphase I
Homologous pairs of chromosomes are lined up double file along the metaphase plate. Single fibers from the poles o the cell are attached to the centromere of each pair of homologues.
Meiosis: Anaphase I
Homologous chromosomes are separated as they are pulled by spindle fibers and migrate to opposite poles.
Meiosis: Telophase I
Homologous pairs continue to separate as they reach the poles of the cell. Each pole has the haploid number of chromosomes.
Meiosis: Cytokinesis I
Occurs simultaneously with Telophase I.
Between Meiosis I and Meiosis II
Chromosomes replicate!
Genetic Variation from Meiosis and Fertilization
- Independent Assortment of Chromosomes
- Crossing-Over
- Random Fertilization of an Ovum by a Sperm
Independent Assortment of Chromosomes
Homologous pairs separate depending on the random way in which they line up on the metaphase plate during Metaphase I. There is a 50/50 chance that a particular gamete will receive the maternal chromosome (same for paternal).
Crossover
It produces recombinant chromosomes that combine genes inherited from both parents. 2/3 crossover events occur in a chromosome pair in humans. At metaphase II, recombinant chromosomes line up on the metaphase plate in a random fashion.
Random Fertilization
1 out of 8 million ova and 1 out of 8 million sperm
Cell Cycle Control System
regulates the rate at which cells divide
G(1) Checkpoint
Most important checkpoint in mammals; It’s called the restriction point. If the cell receives the go-ahead it will compete cell division. If not, it will be arrested in the G(0) phase.
Molecules that Determine Timing of Cell Cycle
Cyclins and Cyclin-Dependent Kinases; the first CDK found in humans was M-Phase Promoting (MPF)
Cell Division: Bone Marrow
are always dividing in order to produce a constant supply of red and white blood cells
Cell Division: Cells in the Human Intestine
normally divide twice per day to make up for cells that die during digestion
Cell Division: Nerve & Muscle Cells
arrested in G(0) and do not divide or regenerate at all; this is why there is great danger from spinal cord injuries
Cell Division: Liver Cells
arrested in G(0) but can be induced to divide when the Liver tissue is damaged
