Cell Division

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Cell Division

• Chapter 8

http//www.youtube.com/watch?vQ6ucKWIIFmg
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Cell Division

• Doubling organelles and proteins
• DNA replication
• Nuclear division
• Cytoplasmic division

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Cell Increase and Decrease

• Maintain homeostasis
• Cell numbers kept in check by this mechanism
• Through cell division of somatic cells and cell
  death
• Cell division interphase, mitosis and
  cytokinesis
• Cell death apoptosis

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Cell Increase and Decrease

• Somatic cells
• Asexual reproduction
• increase in number of somatic cells
• Increase in number unicellular organisms
• Germ cells
• Sexual reproduction
• requires the production of eggs and sperm

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Important terms

• DNA
• Chromosomes
• Chromatin
• Chromatid

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Mitosis verse Meiosis??

• Mitosis
• Cell division mechanism that occurs in
  nonreproductive cells
• somatic cell nuclei
• Meiosis
• Cell division mechanism that occurs in cells that
  participate in sexual reproduction
• gamete nuclei

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Cell Cycle Somatic cells

• Set of stages that involves cell growth and
  nuclear division
• Consists of
• Interphase
• G1
• S
• G2
• Meiotic stage
• Mitosis and Cytokinesis

http//www.cellsalive.com/cell_cycle.htm
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Interphase

• When the cell carries on its usual functions
• Main stages
• G1
• Gap before DNA synthesis begins
• S
• Time when DNA duplicated
• G2
• Gap between time DNA duplication ends and mitosis
  begins

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Interphase in Meiosis?

• Meiosis is a "one-way" process
• Cannot be said to engage in a cell cycle as
  mitosis does
• Preparatory steps that lead up to meiosis are
  identical in pattern and name to the interphase
  of the mitotic cell cycle

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Cytokinesis in Animal Cells

• Cytoplasmic cleavage
• Accompanies mitosis
• Separate process
• Cleavage furrow forms between daughter nuclei
• Contractile ring contracts deepening the furrow
• Continues until separation is complete

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Human DNA in somatic cells

• 22-23 pairs of homologous chromosomes
• Difference?
• Autosomes (1-22)
• Sex chromosomes (23)
• Somatic cells have 46 chromosomes
• Diploid
• 2n

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Human DNA in gametes

• Due to reductional division
• Halves the diploid number (2n) to a haploid
  number (n)
• 23 total chromosomes

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Division of the Nucleus

• Nucleus must be divided
• Parent cells DNA into 2 nuclei
• 2 ways nucleus can divide
• Mitosis
• Meiosis

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Mitosis

• Maintaining the Chromosome Number

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DNA ReplicationSomatic cells

• Duplicated chromosome
• Composed of 2 sister chromatids
• held together by a centromere
• Sister chromatids
• Genetically identical
• When separate, each daughter nucleus gets a
  chromosome

DNA copied
DNA divided
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Chromosomes

• Chromosomes are paired in somatic cells
• homologous chromosomes, homologues
• contain information about the same traits but the
  information may vary
• Cells that have two of each type of chromosome
  are called diploid cells
• one chromosome of each pair is inherited from the
  mother and the other is inherited from the father

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The difference between homologous chromosomes and
sister chromatids
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Mitosis

• Mitosis
• 4 main stages
• Prophase
• Metaphase
• Anaphase
• Telophase

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How the cell cycle works
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Mitosis

• 1. Prophase Mitosis begins!
• Threadlike form
• Spindle fibers appear
• DNA start to condense
• Aster formed
• Nuclear envelope starts to break apart
• Centrioles move to opposite sides of the cell

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Mitosis

• 2. Metaphase
• Duplicated chromosomes aligned midway between the
  poles
• Associated with spindle fibers

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Mitosis

• 3. Anaphase
• Sister chromatids separate from each other and
  move to opposite poles
• Become daughter chromosomes

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Mitosis

• 4. Telophase
• Return to threadlike form as in prophase
• New nuclear envelope separates each chromosome
  cluster
• 2 new nuclei!!!!!

http//www.cellsalive.com/mitosis.htm
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Meiosis

• Reducing the Chromosome Number

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Meiosis

• Mechanism for dividing the nucleus of germ cells
• Oogonia and spermatogonia
• 2n
• Meiosis must take place prior to formation of
  gametes
• Sperm and eggs
• n
• First stage in sexual reproduction

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Meiosis Overview

• Occurs in the life cycle of sexually reproducing
  organisms
• Reduces the chromosome number
• 2 divisions, 4 daughter cells
• Cells are diploid at beginning of meiosis
• TWO consecutive divisions
• Result is 4 haploid nuclei
• Divided into
• Meiosis I
• Meiosis II

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Reducing the Chromosome Number

• Genetic Recombination
• Promotes genetic variability
• Happens by
• Crossing Over
• Independent Assortment of paired chromosomes
• Random Fertilization

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Meiosis comparison

• Meiosis I

• Meiosis II

• Crossing over
• Homologous chromosomes line up
• Homologous chromosomes split
• Two haploid cells formed
• Cytokinesis occurs

• Essentially the same as Meiosis I
• Starts with a haploid cell that has NOT undergone
  chromosome duplication

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Comparisons between males and females

• Spermatogenesis
• Begins at puberty and continues throughout life
• Oogenesis
• Begins in the fetus
• Primary oocytes are arrested in prophase I
• At puberty, one primary oocyte continues the
  process of meiosis during each menstrual cycle

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Comparisons

• Mitosis
• DNA replication occurs only once
• Requires only one division
• Produces two daughter cells
• Diploid daughter cells
• 2n
• Genetically identical cells produced
• Occurs all the time

• Meiosis
• DNA replication occurs only once
• Requires two divisions
• Produces four daughter cells
• Haploid daughter cells
• n
• Genetically variable cells produced
• Occurs only at certain times

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MEIOSIS
MITOSIS
Parent cell (before chromosome duplication)
Site of crossing over
MEIOSIS I
Prophase I
Prophase
Tetrad formed by synapsis of homologous chromosome
s
Duplicated chromosome (two sister chromatids)
Chromosome duplication
Chromosome duplication
2n 4
Tetrads align at the metaphase plate
Chromosomes align at the metaphase plate
Metaphase I
Metaphase
Anaphase I Telophase I
Anaphase Telophase
Sister chromatids separate during anaphase
Homologous chromosomes separate (anaphase
I) sister chroma- tids remain together
Haploid n 2
Daughter cells of meiosis I
No further chromosomal duplication sister chromat
ids separate (anaphase II)
2n
2n
MEIOSIS II
Daughter cells of mitosis
n
n
n
n
Daughter cells of meiosis II
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Comparisons
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Overview of the Life Cycle of Humans
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Cell division in other organisms
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How Plant Cells Divide

• Occurs in meristematic tissues
• Same phases as animal cells
• Plant cells do not have centrioles or asters

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Plant Cells
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Cytokinesis in Plant Cells

• Flattened, small disk appears between daughter
  cells
• Golgi apparatus produces vesicles which move to
  disk
• Release molecules which build new cell walls
• Vesicle membranes complete plasma membranes

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Prokaryotes Have a Simple Cell Cycle

• Cell division in prokaryotes takes place in two
  stages (simple cell cycle)
• copy the DNA
• this process is called replication
• split the cell in two to form daughter cells
• this process is called binary fission

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Cell Division in Prokaryotes

• Binary Fission
• Prokaryotes have a single chromosome
• Chromosomal replication occurs before division
• Cell elongates to twice its length
• Cell membrane grows inward until division is
  complete

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Alterations of chromosome number and structure
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Extra copy of chromosome 21 causes Down syndrome
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• Trisomy 21 involves the inheritance of three
  copies of chromosome 21
• Trisomy 21 is the most common human chromosome
  abnormality
• Imbalance in chromosome number causes Down
  syndrome, which is characterized by
• Characteristic facial features
• Susceptibility to disease
• Shortened life span
• Mental retardation
• Variation in characteristics
• The incidence increases with the age of the mother

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0
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90
80
70
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Infants with Down syndrome (per 1,000 births)
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40
30
20
10
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40
45
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35
30
25
50
Age of mother
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Accidents during meiosis can alter chromosome
number
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• Nondisjunction is the failure of chromosomes or
  chromatids to separate during meiosis
• Fertilization after nondisjunction yields zygotes
  with altered numbers of chromosomes

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Abnormal numbers of sexchromosomes
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• Sex chromosome abnormalities tend to be less
  severe as a result of
• Small size of the Y chromosome
• X-chromosome inactivation
• In each cell of a human female, one of the two X
  chromosomes becomes tightly coiled and inactive
• random process that inactivates either the
  maternal or paternal chromosome
• Inactivation promotes a balance between the
  number of X chromosomes and autosomes

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New species can arise from errors in cell division
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• Polyploid species have more than two chromosome
  sets
• Observed in many plant species
• Seen less frequently in animals
• Example
• Diploid gametes are produced by failures in
  meiosis
• Diploid gamete Diploid gamete ? Tetraploid
  offspring
• The tetraploid offspring have four chromosome sets

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Alterations of chromosome structure can cause
birth defects and cancer
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• Structure changes result from breakage and
  rejoining of chromosome segments
• Deletion is the loss of a chromosome segment
• Duplication is the repeat of a chromosome segment
  
• Inversion is the reversal of a chromosome segment
• Translocation is the attachment of a segment to a
  nonhomologous chromosome can be reciprocal
• Altered chromosomes carried by gametes cause
  birth defects
• Chromosomal alterations in somatic cells can
  cause cancer

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Deletion
Duplication
Homologous chromosomes
Inversion
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Chromosome 9
Reciprocal translocation
Chromosome 22
Philadelphia chromosome
Activated cancer-causing gene