Chapter 12

• Mitotic Cell division results in genetically identical daughter cells
• The mitotic phase alternates with interphase in the cell cycle

I.
Cell Division Overview
II.
The Cell Cycle
III. Mitosis – nuclear division/cytokinesis
IV. Stages of Mitosis

• Life depends on the ability of cells to reproduce/copy
• Cell Division = 1 cell divides into 2
• Unicellular organisms cell division = reproduction
– Prokaryotes, Protists
– Bacteria = binary fission

• Multicellular organisms cell division = growth and development, repair and replace.
– Protists, fungi, plants and animals
• 2 types of multicellular organism cells:
1. Somatic Cells: Body cells of an organism that do all the daily function of the organism
2. Germ Cells: reproductive cells (eggs and sperm)

• G
0
: the cell spends its life working and growing (i.e. breaking down sugars, making
ATP, enzymes)
• Sometimes a cell will prepare to divide
– 1 mother cell  2 daughter cells
• Interphase
– G
1
(Gap1): Growth, development, organelle production, etc. Cell must enter reproduction (S phase) or not (G
0
)
– S (Synthesis): DNA replication
– G
2
(Gap2): continued growth and preparation for division (centrosomes/centrioles duplicate)

• Mitosis (M): nuclear division
• Cytokinesis: division of cytoplasm and cell membrane/cell wall

• Chromatin: spread out DNAprotein complex. In Eukaryotes the proteins are histones. Human cells have 3 billion base pairs.
• Chromosomes: condensed strands of DNA distinct in number and length for each organism.
One chromosome is made up of
30 to 150 million base pairs.

• Humans have 46 chromosomes (23 pairs).
• Each chromosome contains a few thousand genes that code for cellular proteins

• Diploid indicates that our somatic
(body) cells have 23 pairs of chromosomes (2n)
• Haploid indicates that our gametes
(sex cells) have 23 chromosomes (1n or n)

• Problem: distribution of one exact copy of each of their chromosomes to each new cell.
• Each chromosome contains thousands of genes, each necessary to the proper functioning of the organism.
• Humans have ~120,000 genes spread over 46 chromosomes in each somatic cell.

• During Mitosis all access to the DNA stops as chromatin coils up and condenses into visible chromosomes
• Each replicated chromosome is composed of 2 identical parts (sister chromatids) held together by a centromere.

• Sister chromatids are pulled apart during mitosis, and partitioned into 2 daughter cells
• Result of mitosis & cytokenesis: 2 daughter cells genetically identical to the parent cell.

• Prophase: Chromosomes condense, centrosomes are pushed apart by growing spindle, nuclear envelope disintegrates

• Prometaphase: the 2 centromeres of each chromosome attach to one kinetochore spindle fiber; centrosomes move to opposite poles
Kinetochore spindle fiber
Non Kinetochore spindle fiber

• Metaphase: Chromosomes line up at the metaphase plate (equator); centrosomes are at opposite poles

• Anaphase: Chromosomes separate due to shortening of kinetochore spindle fibers; cell elongates due to lengthening of non-kinetochore spindle fibers
Non Kinetochore spindle fiber
Kinetochore spindle fiber

• Telophase: Chromosomes immediately uncoil and resume transcription activities; spindle proteins disassemble; nuclear envelope reassembles

• http://vcell.ndsu.nodak.edu/animations
/mitosis/movie-flash.htm

• Spindle fibers are microtubules made of tubulin proteins that are always present in the cytoplasm
• Enzymes assemble and disassemble spindle fibers

1. Centromere protein clamp, holding the 2 sister chromatids together; deactivated during anaphase
2. Kinetochore motor protein; 1 on each sister chromatid

• Several microtubules attach to each kinetochore motor protein
• Motor protein starts moving (walks along the spindle fibers); kinetochore microtubules shorten, pulling chromsomes (at the metaphase plate) apart
– ATP hydrolysis powers the motion

• Microtubules lengthen, pushing the two poles apart (cell expands)
Non Kinetochore spindle fiber
Kinetochore spindle fiber

Animal:
• Cleavage furrow
– Made by microfilaments (actin fibers)
– A drawstring around the middle of the cell
• Cell pinches off into 2 daughter cells

• Plants have centrosomes w/out centrioles
• No cleavage furrow
– plant cells cannot separate due to cell wall
• Cell plate grows through divided cell

• Prokaryotic Cells – 20 minutes
• No DNA/Histone complex + fast DNA replication (500 NT/sec)
• No spindle fibers, no mitosis
• 2 DNA rings are attached to 2 spots of the plasma membrane which grows apart
• 1 plasmid

• Eukaryotic Cells – 12-48 hrs
• Longest phase of the cell cycle is interphase
• Longest phase of mitosis is prophase

• Depends on cell type:
Cell type Life span/frequency
Esophagus, epidermis 2-3 days
Small intestine
Large intestine
1-2 days
6 days
Red blood cells
White blood cells
Nerve cells
3 months
Up to 10 years
Lifetime (G
0
)