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Biology II – Chapter 11: The Continuity of Life: Cellular Reproduction
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Cell division is the process by which cells reproduce.
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Unicellular organisms divide and form two new individuals.
Multicellular organisms result from divisions of the zygote – a single cell.
o Even a zygote is produced by the combining of two other cells – usually a sperm cell and an egg
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Information in the cell is contained in genes.
o Genes code for the proteins that carry out cellular functions.
When a cell divides – genes are separated into daughter cells in an orderly, predictable process
o This process involves the formation of chromosomes.
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Chromosomes
o Chromatin – uncoiled strands of chromosomes – consist of DNA and proteins
o When cell division begins, DNA replicates.
o These replicated strands begin to coil into tightly wrapped, rod-shaped structures called
chromosomes.
o The only difference between chromatin and chromosomes is the physical arrangement
o Each chromosome contains two identical parts – each called a chromatid
o Two chromatids are often called sister chromatids.
o The point where each pair of chromatids is attached is called the centromere.
o Every species has a characteristic number of chromosomes in each cell – the number varies
among species.
 Fruit fly = 8 chromosomes
 Alligator = 32 chromosomes
 Cat = 32 chromosomes
 Humans = 46 chromosomes
 Apes = 48 chromosomes
 Ameba = 50 chromosomes
 Horse = 64 chromosomes
 Goldfish = 96 chromosomes
o The number of chromosomes does not indicate the complexity of the species.
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Mitosis: The division of the cell nucleus in which the chromosomes in the parent cell divide into two
identical sets is called mitosis.
o Each daughter cell contains the same number of chromosomes as the parent cell.
o Involved in asexual reproduction
o In body cells – also called somatic cells – mitosis is the process where:
 The number of cells is increased without changing the information contained in the DNA
 The amount of DNA in those cells is not changed.
o Unicellular organisms – mitosis is the cycle of reproduction
o Multicellular organisms – mitosis is the cycle of growth and repair
o Cell Cycle: The sequence of events that occurs in a cell from mitosis to mitosis.
 A cell typically passes through 5 events of the cell cycle.
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The five events consist of three phases:
 Interphase – 1st – 3rd events – a period of cell growth and development when the
cell is not dividing
o G1 phase – cell growth
o S phase – DNA replication occurs
o G2 phase – rapid growth and preparation for mitosis
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Mitosis – 4th event – the division of the cell nucleus
o Prophase
o Metaphse
o Anaphase
o Telophase
Cytokinesis – 5th event – the division of the cytoplasm of a parent cell and its
contents into two daughter cells
Interphase
 A continuous process – contains 3 distinct periods:
o G1 phase – 1st period – cell doubles in size, enzymes and organelles also
double in number
o S phase – 2nd period – DNA replication takes place
o G2 phase – 3rd period – cell undergoes rapid growth and synthesizes
necessary enzymes and structures that prepares it for mitosis
o The Stages of Mitosis
 The 4th event of the cell cycle and is divided into 4 phases
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Prophase
 1st stage of mitosis
 Chromatin form into chromosomes
 The nuclear envelope begins to break down
 The 2 pair of centrioles begin to move to opposite ends of the cell
 Spindle fibers and asters begin to appear – made of microtubules
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Metaphase
 2nd stage of mitosis
 Chromosomes begin to line up at the middle (the equator) of the cell on
specialized spindle fibers called kinetochore fibers
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Anaphase
 3rd phase of mitosis
 The chromatids divide at the centromeres and move to opposite sides (poles) of
the cell with the aid of specialized spindle fibers called polar fibers and
kinetochore fibers
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Telophase
 The final phase of mitosis
 Centrioles and spindle fibers disappear
 Chromosomes become chromatin again
 Nuclear envelopes begin to form around the new developing daughter cells
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Cytokinesis
 Immediately follows mitosis (sometimes included in telophase)
 Occurs when the cytoplasm from the parent cell divides equally between the 2
new daughter cells
o All organelles and structures are also divided equally during this process
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Animal cells
o Cell membrane pinches in and divides the cell at its center – called a
cleavage furrow
o Cell membrane eventually divides in two, forming 2 complete daughter
cells – each with its own cell membrane
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Plant cells
o Vesicles formed by the golgi apparatus fuse at the equator of the parent
cell and form a cell plate
 Cell plate – a membrane across the middle of the cell
o A new cell wall forms at the cell plate eventually separating into two new
plant daughter cells.
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Meiosis
o The process of nuclear division that reduces the number of chromosomes by half
o Involves the same 4 phases as mitosis
o Produces gametes – or sex cells – egg and sperm
o Involved in sexual reproduction in both animals and plants
 In animals – results in a haploid egg cell and a haploid sperm cell
 In plants – results in haploid spores that lead to the production of egg and sperm cells
o Meiosis keeps the chromosome number constant from one generation to another
o Also ensures that each generation has a different genetic makeup than the previous one
o Meiosis takes place in 2 cell divisions instead of one:
 Meiosis I – 1st division – homologous chromosomes are separated into 2 separate cells
 Meiosis II – 2nd division – the chromatids of each chromosome are separated into 4
daughter cells
 Each contains only half the number of chromosomes as the parent cell
o Meiosis I
 Prophase I
 Homologous chromosomes pair up by
twisting together to form a tetrad –
exchanging portions of their chromatids called
crossing-over
 This results in an exchange of
genes
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Metaphase I
 The tetrads move to the middle (equator) of
the cell
 Homologous chromosomes remain together
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Anaphase I
 The homologous chromosomes separate
 One chromosome from each pair is pulled to
opposite poles of the cell
o Each chromosome is still composed of
two chromatids joined by a centromere
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Telophase I
 The final stage of meiosis I
 The cytoplasm divides – forming 2 daughter cells
o Meiosis II
 Prophase II
 The nuclear envelope begins to fragment, the nucleolus begins to disappear
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Metaphase II
 The chromosomes line up at the equator of the cell
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Anaphase II
 The centromeres divide – freeing each sister chromatid from the other – each pole
receives the same number of chromosomes
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Telophase II
 The nuclear envelopes form around the chromosomes of the daughter cells
 The cell membrane furrows
 4 daughter cells are the final result – each containing half the number of
chromosomes of the parent cell
o Formation of Egg and Sperm
 The daughter cells produced by meiosis form gametes – sexual reproductive cells
 In animals – males result in sperm cells and females result in egg cells
o Meiosis I & II in males – results in 4 cells that become sperm
o Meiosis I & II in females – results in 1 cell that becomes an egg and three
polar bodies
o Asexual and Sexual Reproduction
 Asexual Reproduction – the production of offspring from one parent, without the union
of gametes
 Offspring are genetically identical to the parent
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Unicellular organisms
o New cells are created by mitosis
Multicellular organisms
o Occurs in body (somatic) cells for growth and repair by mitosis
o New cells are formed by various means – such as cloning or budding
Sexual Reproduction – the production of offspring through meiosis and the fusion of
gametes
 Offspring are different from both parents
o Genes are combined in new ways through genetic recombination
o Genetic recombination introduces variation between parents and offspring
 This allows a better chance of survival in a changing environment
 Gives organisms that reproduce sexually a survival advantage over
asexual organisms.