The Cell Cycle: Cell Division and Mitosis How and Why Cells Divide
Dec 28, 2015
The Cell Cycle:Cell Division and
MitosisHow and Why Cells Divide
Cell Division
• Why do Cells Divide?• Surface area to volume ratio decreases as cell
grows. SA can no longer accommodate nutrients and wastes from increased volume that must cross the CM.
• Cells divide to maintain high SA to volume ratio. • DNA can no longer accommodate cell functions
Controls on Cell Division
• Growth and Development• Tissue Repair• Contact with other cells • Internal & External Regulators cyclins
(proteins) respond to events inside & outside of cell
What do you call uncontrolled cell growth?
CANCER (does not respond to cyclins)
*drugs cut off blood supply to cancer cellsdie
The Details:– Chromosomes contain an organism’s genetic
information.– Each new cell gets the same number of
chromosomes and the exact same genes as the original cell.
How Are Chromosomes Replicated ?
• Each Chromosome is a gigantic molecule of DNA
• Starting at one end, DNA molecule UNZIPS• Each Nitrogen base attracts its Complementary
Nucleotide (A with T and G with C)• Chromosomes are duplicated so the parent cell
contains two full identical sets of chromosomes.• Cell then divides resulting in each daughter cell
ending up with one full set of chromosomes.
Chromosomes
• Chromosomes: DNA coiled around proteins called histones.
• DNA & histones form beadlike structure nucleosomes.
• After DNA replication, chromosome structure: 2 identical sister chromatids attached by centromere.
The Cell Cycle
Interphase
G1 cell growth & development
S DNA synthesis
G2 Organelle synthesis
Cell Division
Mitosis & Cytokinesis
Interphase
Interphase is all the time BETWEEN cell divisions• Cells grow in size • Cells synthesize proteins by joining Amino Acids
together in specific sequences• DNA (of each chromosome) unzips and Replicates• This doubles the total number of chromosomes.• Condensing & coiling of chromatin (DNA & proteins)
mesh. • Centrioles replicate
Mitosis Overview
The Steps (phases) of Mitosis
• Prophase
• Metaphase
• Anaphase
• Telophase– Cytokinesis
Prophase (longest phase of Mitosis)
• Chromosomes condense & coil, becoming visible under microscope
• Nuclear membrane disintegrates
• Centrioles begin to migrate to opposite poles
• Microtubules form a structure called a Spindle
Metaphase
• Paired chromatids
lineup in the
middle of the cell,
sometimes called
the equator• Attach to spindle
via centromere
Anaphase
• Centromeres split• Sister Chromatids
separate, moving away from the equator towards opposite Poles
• Microtubules of the Spindle pull them to opposite ends of the cell
Telophase
• Chromosomes uncoil, becoming invisible chromatin mesh
• Spindle disappears• Nuclear membrane
reappears• Cytokinesis (division of
cytoplasm usually follows)– Cytoplasm is divided
evenly into two cells
The Importance of Mitosis
Mitosis is Nature’s 99.9999 percent guarantee
that when ANY Eukaryotic cell divides,
it produces two Genetically Identical Cells.
What kind of cells divide by Mitosis?
Somatic (body) cells
Somatic Cells/Body Cells
• Heart, Muscle, and Nerve cells rarely divide if at all.
• Skin, digestive, and connective tissue cells divide rapidly, replacing worn out, broken down, dead cells.
• Following injury, rapid cell division of these cells slows as healing progresses, due to cyclins.
Different Organisms and Their Chromosome NumbersCat 38
Cow 60
Fruit fly 8
Goldfish 94
Human 46
Onion 16
What do all these chromosome numbers have in common?
How Chromosomes Appear
• Chromosomes are found in homologous pairs. (one from each parent)– Each chromosome has a homolog that
carries traits for the same genes.
• Diploid number (2n)– The total number of chromosomes that
exist in a cell.
Mitosis
in Humans
46
92
ProMAT
DNA Replication
4646
46
92
ProMAT
DNA Replication
4646
2n
4n
ProMAT
DNA Replication
2n2n
MitosisIn Humans In ANY Species