Cell Division

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Cell Division

• Mitosis, meiosis and cytokinesis

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Cell Division

• Results in the formation of two identical
  daughter cells
• Each daughter cell will be roughly ½ the size of
  the parent cell
• Before the cell divides, the DNA of the cell must
  be copied and separated (mitosis)
• Actual cell division is called cytokinesis.

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Why do cells have to divide

• Limits to size!
• Surface area/volume ratio
• Trouble exchanging materials with its outside
  environment
• Too much demand placed on DNA
• Larger the cell, more proteins need to be made!
• Only one copy of each DNA molecule in a cell!

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Cell Division

• Mitosis occurs exclusively in eukaryotic cells.
• In multicellular organisms, the somatic (body
  cells) cells undergo mitosis, while germ cells
  (cells destined to become sperm in males or ova
  in females) divide by a related process called
  meiosis.
• Prokaryotic cells, which lack a nucleus, divide
  by a process called binary fission.

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Forms of DNA

• DNA Proteins Chromosomes
• Most of the time, DNA is unspooled into loose
  strands called chromatin
• can be used to provide instructions in this form.
• Before a cell divides, the DNA winds around
  histone proteins and becomes visible as
  chromosomes
• The chromosomes can be counted in this form.

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Structure of DNA
Nucleosome
Chromosome
DNA double helix
Coils
Supercoils
Histones
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Chromosome number

• Eukaryotic organisms have a specific number of
  different types of chromosomes.
• They have two of each type so chromosomes come
  in pairs.
• Cells with chromosomes present in pairs are said
  to be diploid (2N)
• one of each is haploid

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Chromosome identity

• Each chromosome type can be identified by shape
  and size.
• If stained, characteristic patterns of bands can
  be seen as well.
• A karyotype is a display of all of an
  individuals chromosomes arranged by type.
• Can be used to identify major genetic disorders.

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Chromosome number

• Each organism has a different number of
  chromosomes
• Camel 70 Chicken 78
• Opossum 22 Housefly 22
• Bat 44 Corn 24
• Lentil 14 Rice 24
• Goat 60 Barley 14
• Apple 34 Lettuce 12
• HUMANS HAVE 46! 23 different pairs

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DNA ? By the Numbers

• In a diploid (2N) eukaryotic cell, one set of
  chromosomes comes from the mother and another
  from the father.
• Each human gets 23 from each parent (46 total)
• 44 are autosomes (general body characteristics)
  and two are sex chromosomes (determine sex and
  carry general characteristics)

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• The two corresponding chromosomes are called
  homologous chromosomes. Homologous chromosomes
  need not be genetically identical.
• For example, a gene for eye color at one locus
  (location) on the father chromosome may code for
  green eyes, while the same locus on the mother
  chromosome may code for brown.

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• Regardless of the number, before a cell can
  divide, it must generate an exact copy of each
  chromosome.
• When the cell divides, each daughter cell will
  get one copy of each of the total number of
  chromosomes.

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Before Cell Division, each chromosome must
replicate!

• Individual chromosomes replicate and form sister
  chromatids.
• Each sister chromatid is destined for one of the
  two resulting daughter cells

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• The copies are called sister chromatids, and
  together they are considered one chromosome.
• After separation, however, each sister chromatid
  is considered a full-fledged chromosome by
  itself. The two copies of the original chromosome
  are then called sister chromosomes

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After Replication ? Chromosomes
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The Cell Cycle

• Sequence of events a cell goes through as it
  grows and divides
• (G1 Phase) Cell grows and synthesizes proteins
  and new organelles
• (S Phase) Chromosomes replicate
• (G2 Phase) Organelles and molecule used in cell
  division are produced
• (M Phase) mitosis and cytokinesis, the actual
  division of the cell into two daughter cells.

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Cell Division M-Phase

• Consists of TWO steps (Mitosis and Cytokinesis)
• Mitosis ? process by which a cell separates its
  duplicated genome into two identical halves.
  Mitosis only separates the newly replicated
  chromosomes DNA replication does not occur
  during mitosis.
• broken down into five phases (PMAT)
• Prophase, Metaphase, Anaphase, Telophase.
• Cytokinesis which divides the cytoplasm and cell
  membrane.

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Mitosis ? Prophase

• Longest phase of mitosis
• Chromosomes condense (become visible)
• Centrioles (in cytoplasm) separate and move to
  opposite sides of cell
• Nuclear membrane breaks-down
• Microtubule structure called the spindle develops
  (attaches from centrioles to chromosomes

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Chromosome Structure

• Prior to separation, the two sister chromatids
  are attached together in a specialized region of
  the chromosome known as the centromere.

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Mitosis ? Metaphase

• Chromosomes line-up along center of cell
  (metaphase plate)
• Each chromosome is connected to its centromere by
  a spindle fiber

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Mitosis ? Anaphase

• Sister chromatids separate into separate
  chromosomes
• Separated chromosomes pulled to opposite sides

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Mitosis ? Telophase

• Chromosomes move together at opposite ends of the
  cell and become less condensed
• Spindle breaks apart
• Two new nuclear membrane form
• Result is one cell with 2 nuclei!

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Cytokinesis

• Remember, NOT part of mitosis
• Animals
• Cell membrane pinches off cytoplasm into two
  equal parts at a region called the cleavage
  furrow
• Plants
• Cell Plate develops between two new nuclei which
  grows into a separating membrane and ultimately a
  separating cell wall

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Mitosis Animation

• Cell Cycle Animation
• http//www.cellsalive.com/mitosis.htm

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Limits to Division?

• The big problem with eukaryotes is that they have
  to replicate linear chromosomes. The polymerase
  enzyme cant work all the way to the end, so the
  chromosome gets shorter with each round of
  replication.
• Solution use special ends called telomeres that
  get recreated with a special enzyme called
  telomerase. This may play a crucial role in
  human aging if the chromosome ends fail to
  replicate properly, the chromosomes gradually
  lose parts of their end sequence.

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Regulation of Cell Cycle

• Not all cells move through cell cycle at same
  rate
• Bone marrow cells/skin cells ? continuous
  division
• Nerve and muscle cells ? seldom or never

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Cycle Regulators

• The cell cycle is regulated by special proteins
  called cyclins and cyclin-dependent kinases.
• High concentrations of cyclin influences a cell
  to divide.
• Internal Regulators ? proteins that respond to
  internal stimuli cell cycle checkpoints!
• Ex. Cell will not enter mitosis until all
  chromosomes are replicated
• External Regulators ? proteins that respond to
  external stimuli
• Ex. Cell will begin to divide rapidly after
  injury
• Ex. When dividing cells come in contact with
  adjacent cells, division will slow

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WHEN CELLS GO BAD!

• When a bodys cell lose the ability to control
  growth, cancer is the result.
• Cancer cells do not respond to chemical signals
  that tell them to stop growing.
• Form masses of cells called tumors that damage
  surrounding tissues.

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Meiosis

• We know that regular somatic (body) cells contain
  TWO sets of chromosomes (diploid/ 2N)
• When a sexually reproducing organism produces
  gametes (sex cells) they must somehow separate
  these pairs of chromosomes so gametes only get
  one set.
• WHY?

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Ex. Humans

• Normal Diploid (2N) somatic cell contains _____
  chromosomes (_____ pairs)
• Gametes (sperm and egg cells) need to contain
  _________ chromosomes.
• We generate these HAPLOID (N) cells through the
  process of meiosis!

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Steps of Meiosis

• Divided into two distinct stages
• Meiosis I
• Meiosis II
• Starts with one diploid cell and ends with 4
  haploid daughter cells
• Before meiosis begins, DNA undergoes replication
  just like in mitosis!

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Meiosis I Prophase I

• Appearance of the chromosomes, the development of
  the spindle, and the breakdown of the nuclear
  membrane (envelope).
• Each replicated chromosome pairs up with its
  corresponding homologous chromosome
• Paired chromosomes (4 chromatids) form a tetrad

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Tetrads and crossing over

• It is during this alignment that chromatid arms
  may overlap and temporarily fuse (chiasmata, or
  synapsis), resulting in crossovers
• Segments of homologous chromosomes may switch
  places where overlap occurs.

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What is Crossing Over?

• Paired-up homologous chromosomes, may exchange
  portions of their chromatids
• Advantage?

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Meiosis I Metaphase I

• Here is where the critical difference occurs
  between Metaphase I in meiosis and metaphase in
  mitosis. In the latter, all the chromosomes line
  up on the metaphase plate in no particular order.
  In Metaphase I, the chromosome pairs are aligned
  on either side of the metaphase plate.

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Meiosis I Anaphase I

• During Anaphase I the homologous pairs separate
  from each other and move along the spindle fibers
  to each pole of the cell.

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End of Meiosis I

• At the end, each daughter cell has a single set
  of chromosomes, half the total number in the
  original cell where the chromosomes were present
  in pairs.
• While the original cell was diploid, the daughter
  cells are now haploid. This is why Meiosis I is
  often called reduction division.

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Meiosis II

• Meiosis II is quite simple in that it is simply a
  mitotic division of each of the haploid cells
  produced in Meiosis I.
• There is no Interphase between Meiosis I and
  Meiosis II

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Meiosis II Prophase II

• A new set of spindle fibers forms and the
  chromosomes begin to move toward the equator of
  the cell.

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Meiosis II Metaphase II

• All the chromosomes in the two cells align with
  the metaphase plate.

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Meiosis II Anaphase II

• Sister chromatids separate as they are pulled by
  spindle fibers

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Meiosis II Telophase II

• A cleavage furrow develops, followed by
  cytokinesis and the formation of the nuclear
  membrane (envelope).
• When Meiosis II is complete, there will be a
  total of four daughter cells, each with half the
  total number of chromosomes as the original cell.

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Meiosis in Males and Females

• In male animals, meiosis results in the formation
  of 4 ___________ cells
• In female animals, meiosis results in the
  formation of one _______ cell and three small
  polar bodies which die.

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Advantages/Disadvantages of sexual reproduction?

• Recombination of maternal and paternal
  chromosomes in the gamete results in genetic
  variation among the offspring. In an environment
  which changes, this allows the process of natural
  selection to occur.