Cell Division and Mitosis
- Dividing Cells: The Bridge Between Generations
- Overview of Division Mechanisms
- Before cells are able to reproduce, there must be a division of the nucleus and its DNA.
- Mitosis and meiosis are eukaryotic nuclear division mechanisms that lead to the
distribution of DNA to new nuclei in forthcoming daughter cells.
- Mitosis is used by multicelled organisms for growth by repeated divisions of somatic
- Meiosis occurs only in germ cells that divide to form gametes.
- Some Key Points About Chromosomes
- Each chromosomes is a molecule of DNA complexed with proteins.
- Prior to division, each threadlike chromosome is duplicated to form two sister
chromatids held together by a centromere.
- The centromere is also the region where the duplicated chromosome will attach to the
microtubules of the spindle during nuclear division.
- Mitosis and the Chromosome Number
- Each species has a characteristic chromosome number (for example: human somatic cells
contain 46 chromosomes).
- Chromosomes exist as pairs: one member of each pair from each parent.
- Somatic cells are diploid; that is, they have two of each type of chromosome.
- Germ cells (egg and sperm) are haploidonly one chromosome of each type.
- Mitosis maintains the chromosome number of the species through all the divisions of
development, growth, and repair.
- The Cell Cycle
- The cell cycle is a recurring sequence of events that extends from the time of a
cells formation until each division is completed.
- Most of a cells existence (about 90 percent) is spent in interphase; mitosis
occupies only a small part.
- During interphase the cells mass increases, the cytoplasmic components
approximately double in number, and the DNA is duplicated (S).
- Some cells are arrested in interphase and never divide again (example: brain cells).
- Stages of MitosisAn Overview
- The four sequential stages of mitosis are: prophase, metaphase, anaphase, and telophase.
- The spindle apparatus moves the chromosomes.
- The spindle apparatus is composed of two sets of microtubules.
- Microtubules (components of the cytoskeleton) extend from the two "poles" of
the cell and overlap at the cell equator.
- Prophase: Mitosis Begins
- Chromosomes become visible as rodlike units, each consisting of two sister chromatids.
- In the cytoplasm, the microtubules of the cytoskeleton break apart and begin
reassembling near the nucleus.
- Microtubules are composed of numerous subunits called tubulins.
- Some microtubules extend from the centromeres to one of the two poles; others extend
from the poles, overlap in the middle of the cell, but do not contact the chromosomes.
- The nuclear envelope begins to disintegrate.
- The centrioles, which have duplicated by the time prophase is underway, are now moved by
the microtubules to the opposite poles of the cell
- Transition to Metaphase
- The nuclear membrane now breaks up completely in the transition between pro- and
- Sister chromatids, each attached to microtubules, become oriented toward opposite poles.
- When all the chromosomes are aligned at the cells equator, halfway between the
poles, we call the stage metaphase.
- From Anaphase Through Telophase
- Sister chromatids separate and move toward opposite poles.
- Microtubules attached to the centromeres shorten and pull the chromosomes toward
- Other microtubules at the spindle poles ratchet past each other to push the two
spindle poles apart.
- Once separated, each chromatid is now an independent chromosome.
- Telophase begins when the two daughter chromosomes of each original chromatid pair
arrive at opposite poles.
- Chromosomes return to the threadlike form typical of interphase.
- The nuclear envelope reforms from the fusion of small vesicles
- Each daughter cell has the same number of chromosomes as the parent cell; mitosis is
- A Closer Look at the Cell Cycle
- The Wonder of Interphase
- During G1, most of the carbohydrates, lipids, and proteins for a cells own use and
for export are assembled.
- During the S phase, the cell copies its DNA and synthesizes proteins used in organizing
the condensed chromosomes.
- During G2, the proteins that will drive mitosis to completion are produced.
- Chromosomes, Microtubules, and the Precision of Mitosis.
- Metaphase chromosomes are highly organized.
- Proteins called histones tightly bind to DNA and cause spooling into a structural unit
called a nucleosome.
- By late prophase, each chromatid has a constricted region called the centromere, on the
surface of which is the kinetochorea docking site for the spindle microtubules.
- To prevent tangling of the DNA molecules, an enzyme called DNA topoisomerase recognizes
possible tangling and corrects it.
- The microtubules of the spindle are continually assembling and disassembling.
- At anaphase, the kinetochores slide over the microtubules on their way to the poles; the
microtubules shorten behind them by disassembling themselves.
- Two proteins, dynein and kinesin, drive the sliding motion.
- Division of the Cytoplasm
- Cell Plate Formation in Plants
- Because of the rather rigid cell wall, the cytoplasm of plant cells cannot just be
pinched in two.
- Instead vesicles containing remnants of the microtubular spindle form a disklike
structure during cell plate formation.
- Cleavage of Animal Cells
- The flexible plasma membrane of animal cells can be squeezed in the middle to separate
the two daughter cellsa process called cleavage.
- Parallel arrays of contractile microfilaments slide past one another at the cleavage
furrow, pulling the plasma membrane inward.