BIO102 Ch12
Cell Replication
- Cells replicate through:
- Copying DNA.
- Separating the copies.
- Dividing the cytoplasm to create two complete cells.
Definitions
- DNA: Encodes the cell’s genetic information.
- Gene: A segment of DNA that contains hereditary information.
- Chromosome: A single long double helix of DNA wrapped around proteins.
- Ploidy: Number of copies of each type of chromosome.
- Haploid (N): One copy of each type of chromosome.
- Diploid (2N): Two copies of each type of chromosome.
- Chromatin: DNA and the proteins associated with it.
- Genome: All the genetic material in an organism.
Eukaryotic Chromosome Morphology
- Unreplicated Chromosome: Consists of a single, long DNA double helix wrapped around proteins.
- Replicated Chromosome: Consists of two copies of the same DNA double helix.
- Condensed Replicated Chromosome: DNA condensed around its associated proteins, resulting in a compact chromosome.
- Sister Chromatids: Two copies of a replicated chromosome, connected at the centromere.
- Centromere: The region where sister chromatids are most closely joined.
Eukaryotic Cell Cycle
- New cells arise from the division of pre-existing cells.
- Cell Cycle:
- Interphase (G1, S, G2 phases)
- M-Phase (Mitosis or Meiosis)
Overview of Cell Cycle
- G1 Phase: Cell growth and preparation for DNA replication.
- S Phase: DNA replication occurs, resulting in sister chromatids.
- G2 Phase: Further growth and preparation for mitosis.
- M Phase: Mitosis (nuclear division) and cytokinesis (cytoplasmic division).
M Phase (Nuclear Division)
- Dividing phase; chromosomes are highly condensed.
- Mitosis and Meiosis are examples of nuclear divisions (karyokinesis).
- Mitosis:
- Occurs in both haploid and diploid cells.
- Produces daughter cells that are genetically identical to each other and the parent cell (same number of chromosomes).
- Usually accompanied by cytokinesis.
Stages of Mitosis
- Prophase
- Metaphase
- Anaphase
- Telophase
Prophase
- Chromosomes condense.
- The mitotic spindle begins to form, composed of microtubules.
- The nuclear envelope starts to break down.
- Chromosomes attach to the spindle via kinetochores.
Kinetochore
- Structure that attaches microtubules to chromosomes.
- Kinetochore fibers connect the kinetochore to the spindle microtubules.
Metaphase
- The nuclear envelope is completely gone.
- Formation of the mitotic spindle is completed.
- Chromosomes line up along the metaphase plate.
Anaphase
- Centromeres separate.
- Sister chromatids separate and migrate towards opposite poles of the cells; after separation, they are called daughter chromosomes.
- Two forces pull chromosomes apart:
- Kinetochore microtubules shrink (depolymerization).
- Motor proteins of the polar microtubules push the two poles of the cell away from each other.
Telophase
- Chromosome migration is complete.
- Each pole receives a complete set of chromosomes.
- Chromosomes decondense.
- A new nuclear envelope forms around each set of chromosomes.
- The spindle breaks down.
- Mitosis is complete when two independent nuclei have formed.
Mitosis Summary
- Interphase: After chromosome replication, each chromosome is composed of two sister chromatids; centrosomes have replicated.
- Prophase: Chromosomes condense, and the spindle apparatus begins to form; the nuclear envelope breaks down; microtubules contact chromosomes at kinetochores.
- Metaphase: Chromosomes complete migration to the middle of the cell.
- Anaphase: Sister chromatids separate into daughter chromosomes, which are pulled to opposite poles of the spindle apparatus.
- Telophase: The nuclear envelope re-forms, and chromosomes de-condense; cell division begins as an actin–myosin ring causes plasma membrane to begin pinching in. Cell division is complete at the completion of telophase; two daughter cells form.
Structures Involved in Mitosis
| Structure | Definition |
|---|---|
| Chromosome | A structure containing genetic information in the form of genes. |
| Chromatin | The material that makes up eukaryotic chromosomes; consists of a DNA molecule complexed with histone proteins. |
| Chromatid | One double-stranded DNA copy of a replicated chromosome with its associated proteins. |
| Sister chromatids | The two attached, double-stranded DNA copies of a replicated chromosome. When chromosomes are replicated, they consist of two sister chromatids. The genetic material in sister chromatids is identical. When sister chromatids separate during mitosis, they become independent chromosomes. |
| Centromere | A specialized region of a chromosome where sister chromatids are most closely joined to each other. |
| Kinetochores | The structures on sister chromatids where microtubules attach. |
| Microtubule-organizing center | Any structure that organizes microtubules. |
| Centrosome | The microtubule-organizing center in animals and certain plants and fungi. |
| Centrioles | Cylindrical structures consisting of microtubule triplets, located inside animal centrosomes. |
Binary Fission in Prokaryotes (Bacterial Cell Division)
- DNA is copied, and protein filaments attach.
- DNA copies are separated; a ring of protein forms.
- The ring of protein draws in the membrane.
- Fission is complete.
Cell-Cycle Checkpoints
- G1 Checkpoint: Determines whether the cell will divide, delay division, or enter G0.
- G2 Checkpoint: Checks for DNA damage and chromosome replication completeness after S phase.
- M-phase Checkpoints: Ensure that sister chromatids separate correctly.
- Mature cells that do not pass the G1 checkpoint enter the G0 state
Cancer: Out-of-Control Cell Division
- Involves uncontrolled cell division.
- Examples of cancer types include prostate, breast, lung, colon and rectal, melanoma, bladder, non-Hodgkin lymphoma, renal cell, thyroid, endometrial, leukemia, and pancreatic.