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CELL REPRODUCTION
Humans replace 25 million new cells each second.
Cells must be large enough to support the cell, but as size increases, the volume
increases faster than the surface area increases. This can present a problem for
nutrition. If a cell doubles, it needs eight times more nutrients and will have
eight times more waste, but the surface area only increases fourfold to handle
these new diffusion needs. The cell would die or starve, so they must divide
before this happens.
CELL CYCLE
Has two phases – Interphase and Division
Interphase – busiest part of a cell’s life
• Carries on routine cell activities – ATP synthesis, repair, eliminate wastes,
etc.
• Grows – makes proteins and new organelles
• Metabolism of the cell continues
• Chromosomes and DNA are duplicated
Division – Two types – Mitosis and Meiosis
• Mitosis is for regular body cells of 46 chromosomes. These cells are also
called autosomes. This is dividing to copy. A cell divides to make two
new cells with 46 chromosomes each. Since these cells have 23 pairs of
chromosomes, they are called diploid, represented by the symbol 2n.
• Meiosis is the division of sex cells, also called gametes. This is dividing to
split. A cell divides to make two four cells with 23 chromosomes each.
Since these cells have only 23 chromosomes with no pairs, they are called
haploid, represented by the symbol n. This way sperm and egg “add
together” to create a baby with 46 chromosomes in each cell.
46 is the number of chromosomes for humans. Different species will have
different numbers of chromosomes.
Mitosis
Meiosis
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MITOSIS
PROPHASE
• Longest phase of Mitosis
• Sister chromatids form 2 halves, exact copies, held together by a
centromere
• Nuclear envelope and nucleus dissolve until they are completely absent at
the end of prophase.
• Centrioles move to opposite poles (centrioles are found only in animal
cells)
• Spindle fibers, made of microtubules, form between the centrioles.
METAPHASE
• Shortest phase of mitosis
• Sister chromatids attach to spindles by centromere
• Spindles line up pole-to-pole to ensure a complete set of chromosomes for
each new cell.
ANAPHASE
• Sister chromatids separate
• Centromeres split apart
• Separation is believed to occur by shortening of spindle fibers
TELOPHASE
• Daughter chromosomes reach opposite poles
• Prophase reverses to form independent cells
• Chromosomes unwind, spindles break down
• Nucleus reappears; new nuclear envelope forms
• Cytoplasm divides, plasma membranes separate
• Cytokinesis – Actual division of the cell
• Plant cells form a new cell plate that becomes a new cell wall to complete
Cytokinesis.
MEIOSIS
Prophase I
• Chromosomes coil up and new spindle fibers form
• Homologous chromosomes join to form a 4-part structure called a tetrad.
Tetrads pair so tightly that they can cross over to provide different
combinations of alleles. An allele is a gene form for each variation of a
trait, like blue for eye color.
Metaphase I
• Tetrads line up along the equator attached by centromeres
Anaphase I
• Centrioles DO NOT SPLIT like they do in mitosis. This makes sure that
each new cell receives only one chromosome from each homologous pair.
• Homologous chromosomes (homologues) do separate to opposite poles.
Homologous chromosomes are genes for the same trait, like genes for eye
color.
Telophase I
• Reverse of Prophase I
• Spindles break down; chromosomes uncoil
• Cytoplasm divides.
PMAT II – the same as mitosis.
Meiosis provides mechanisms for shuffling of chromosomes to provide variation
of traits. Genetic recombination allows the reassortment of chromosomes and
their genetic info either by crossing over in Prophase I or by Independent
Segregation.
CANCER
Enzyme production monitors cell progress. Some trigger the cell cycle while
others inhibit progression of the cycle. Cancer results from cell death, or
uncontrolled division of cells.
• Can result from failure to produce certain enzymes, which may be genetic
• Damaged genes are forced into action by environmental triggers
• Cancer cell deprive normal cells of nutrients
• Cancer cells can also enter the bloodstream and spread through the body
to start new tumors.
• Second leading cause of death in the US. Heart disease is first
• Can affect any tissue. Main areas are lung, colon, breast and prostate
• Rates differ by geography