Since organisms start from one cell, yet adults may contain billions of cells, a process for producing new identical cells is required. Likewise, most cells have a relatively short life so they must constantly be replaced.

I. How prokaryotic cells divide

A. Prokaryotic cells lack a nucleus and organelles

B. Prokaryotes have a single circular loop of DNA attached to the inside of the plasma membrane; about 1,000 times the length of the cell

C. Prokaryotes reproduce asexually via binary fission

D. Before cell division takes place, DNA is replicated so two chromosomes are attached inside the plasma membrane

E. Following DNA replication, the two chromosomes separate when cell lengthens and pulls them apart

F. When cell is approximately twice its original length, the plasma membrane grows inward, a new cell wall forms, dividing the cell into two approximately equal daughter cells

II. Eukaryotic chromosomes

A. DNA in the chromosomes of eukaryotic cells is associated with histone proteins

B. When a eukaryotic cell is not undergoing division, DNA within a nucleus is a tangled mass of threads called chromatin

C. At cell division, chromatin becomes highly coiled and condensed, and visible as chromosomes

D. Each species has a characteristic number of chromosomes

1. With the exception of bacteria virtually all organisms have two sets of each chromosome in their non-sex cells, one came from each parent

2. Diploid (2n) number = 2 sets of chromosomes in somatic (body) cells, e.g. humans have 2n = 46

3. Haploid (n) number = one set of chromosomes, usually restricted to sex cells, e.g. human sperm and eggs have n= 23

E.g., Humans have 23 pairs of homologous chromosomes - corresponding chromosomes, one from each parent, which contain the same genes

E. Cell division in eukaryotes involves nuclear division and cytokinesis (division of the cytoplasm)

1. Somatic (body) cells undergo mitosis in the development, growth, and repair of multicellular organisms

a. This nuclear division leaves the chromosome number constant

b. A 2n nucleus divides to provide daughter nuclei that are also 2n

2. A chromosome begins cell division with two sister chromatids

1) Sister chromatids are two parts of a chromosome, at the beginning of cell division, that are attached at a centromere; each consists of a DNA molecule identical to the DNA molecule of the other chromatid

2) Centromere = a region of constriction on a chromosome, where sister chromatids are attached.

III. The cell cycle has three continuous overlapping phases:

A. Interphase - the time between cell divisions

B. Mitosis - division of the nuclear material

C. Cytokinesis - division of the cytoplasm

A. Interphase - the phase between successive mitotic divisions. It is the period of normal metabolic activity

1. Three phases:

a. G1 - period of normal metabolic cellular activities: transcription, translation (protein synthesis) and growth of the cytoplasmic materials including the organelles

b. S - the genetic material (DNA) is duplicated, chromosomes now consist of two chromatids joined at the centromere

c. G2 - metabolic activities in preparation for mitosis

B. Mitosis - the process by which a nucleus gives rise to two identical daughter nuclei, four overlapping phases:

1. Prophase

a. Chromatin condenses and chromosomes become visible

b. Nucleolus and nuclear envelope disappear

2. Metaphase

a. Spindle forms and attaches to kinetochores

Kinetochores are protein containing structures, one on each chromatid, associated with the centromere.

b. chromosomes align on the equatorial plane (metaphase plate)

3. Anaphase

a. centromeres divide

b. daughter chromosomes move towards opposite poles

4. Telophase

a. Spindle disappears

b. Chromosomes decondense and return to chromatin

c. Nuclear envelope reforms and nucleoli reappear.

C. Cytokinesis (cytoplasmic cleavage)

1. In plant cells golgi apparatus produces vesicles that fuse, forming a cell plate

2. In animals a cleavage furrow indents the plasma membrane between the two daughter nuclei at a midpoint; then constriction separates the cytoplasm

IV. Consequences of the cell division

1. Nuclear material was duplicated prior to division

2. Therefore two new genetically identical cells are formed

V. Duration of cell cycle - varies with the tissues and organism involved. Average time for events in plant root tip:

Interphase: 12 - 30 hrs.

Prophase: 1 - 2 hrs.

Metaphase: 5 - 15 min.

Anaphase: 2 - 10 min.

Telophase: 10 - 30 min.

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