NOTES FOR BIOLOGY 1001

SECTION 005

Spring 2005

DR. STEVEN POMARICO

CHAPTER 9

CELL DIVISION AND MITOSIS

>>>>>>Cell division functions in reproduction, growth, and repair

---Cell division is the process in which the genetic information of a cell is faithfully passed along to the next generation of cells.

>>>>Cell division consists of nuclear division and cytoplasmic division.

There are 2 types of nuclear division in eukaryotes

                             Mitosis

                             Meiosis

---Mitosis is the division of the nucleus which results in two genetically identical daughter nuclei.

Mitosis is used to increase the number of somatic cells of a multicellular organism.

         -Human somatic cells are diploid and contain 46 chromosomes arranged as pairs of 23 chromosomes.

These pairs are known as homologous chromosome pairs

---Homologous chromosomes (homologues) are a pair of chromosomes that have the same size, centromere position, and staining pattern.

---Diploid is the condition in which cells contain two sets of chromosomes, abbreviated as 2n.

---Meiosis is a two-stage nuclear division in sexually reproducing organisms that results in the production of gametes with a haploid number of chromosomes.

Meiosis takes place in germ cells.

---Haploid is the condition in which cells contain one set of chromosomes; abbreviated as n.

         -the haploid condition is found in the gametes.

---Gametes are haploid reproductive cells

>>>>>The genome of a eukaryotic cell is organized into multiple chromosomes

-The chromosomes of eukaryotes are made up of chromatin

---Chromatin is a complex of DNA and and proteins known as histones, which make up chromosomes in eukaryotic cells. In non-dividing cells, the chromatin appears as a mass of diffuse stained material.

The eukaryotic chromosome has two “arms” which extend from a structure called the centromere (See fig. 9.2)

---Centromere is the centralized region between the arms of a chromosome which joins two sister chromatids.

---Chromatids are a pair of chromosomes which arise by the duplication of a single chromosome

>>>>THE EUKARYOTIC CELL CYCLE

During the cell cycle interphase alternates with mitosis (a.k.a. M phase).

---Interphase is the period of the cell cycle when the cell is not dividing (see fig 9-6)

         -Interphase makes up about 90% of the cell cycle

         -high metabolic activity

         -chromosomes and organelles are duplicated

Interphase consists of three phases:

         1. G₁ phase (gap one)

                                       -first gap in DNA synthesis

         2. S phase (synthesis phase)

                                       -DNA is synthesized

         3. G₂ phase (gap two)

                                       -second gap in DNA synthesis

>>>>Cell division consists of nuclear division and cytoplasmic division.

---Mitosis is the division of the nucleus.

         -During this process the DNA is equally distributed into 2 daughter nuclei.

-The division of the nucleus is usually followed quickly by cytokinesis

---Cytokinesis is the division of the cytoplasm that forms 2 separate daughter cells each containing a single nucleus.

The mitosis phase of the cell cycle is divided into four phases

         (See figs 9.5 and 9.7):

      1. Prophase

                   -chromatin condenses

                   -spindle apparatus forms

                             -spindle consists of microtubules between two microtubule                                        organizing centers containing pairs of centrioles

                   -nuclear envelope disappears

                   -kinetochore microtubules attached kinetochore of chromosomes                              to the mitotic spindle

                   -polar microtubules radiate toward the cell’s equator

      2. Metaphase

                   -centrosomes positioned at opposite ends (poles) of the cell

                   -chromosomes move to and align on the equator of the cell.

      3. Anaphase

                   -sister chromatids divide into separate daughter chromosomes and                              move toward opposite poles.

                   -movement of the chromosome is accomplished by depolymerization                              of the microtubules at the kinetochore

                   -cell begins to elongate because the polar microtubules slide past each                                other and push the poles apart.

      4. Telophase

                   -daughter chromosomes arrive at poles

                   -kinetochore microtubules disappear

                   -polar microtubules elongate (pushing out on the ends of the cells) helps                                                   causes the formation of the cleavage furrow.

                   -daughter nuclear envelopes form

                   -chromatin begins de-condensing.

By the end of telophase replication of the nucleus is complete and two genetically identical daughter nuclei are present.

>>>>>>Cytokinesis divides the cytoplasm

      Cytokinesis

                   -the cytoplasm divides

The mechanism of division of the cytoplasm differs for plants and animals.

       In plants: (See fig. 9.8)

                   -cell plate formation

                   -cell wall deposition

      In animals: (See fig. 9.9)

                   -cleavage furrow (shallow grove) forms near the site of the metaphase                                          plate (cell equator)

                   -a contractile ring forms inside the plasma membrane at the cleavage                                             furrow and pinches the parent cell into two daughter cells.