Chromosomes, the Cell Cycle, and Cell Division

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Chromosomes, the Cell Cycle, and Cell Division
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Chromosomes, the Cell Cycle, and Cell Division

• Systems of Cell Reproduction
• Interphase and the Control of Cell Division
• Eukaryotic Chromosomes
• Mitosis Distributing Exact Copies of Genetic
  Information
• Cytokinesis The Division of the Cytoplasm
• Reproduction Asexual and Sexual
• Meiosis A Pair of Nuclear Divisions
• Meiotic Errors
• Cell Death

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Systems of Cell Reproduction

• Since all living cells are mortal, cell
  reproduction or replacement is universal among
  living organisms.
• This may consist of simple replacement,
  differentiation, or specialization.

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Systems of Cell Reproduction

• Unicellular organisms use cell division primarily
  to reproduce, whereas in multicellular organisms
  cell division also plays important roles in
  growth and in the repair of tissues.
• Two Types of Cell Division
• Mitosis- produces two nuclei with the same number
  of chromosomes as in the original nucleus.
• Meiosis- Gives on half the number of chromosomes
  as in the original nucleus and is associated with
  reproduction.

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Systems of Cell Reproduction

• Four events occur before and during cell
  division
• A signal to reproduce must be received.
• Replication of DNA and vital cell components must
  occur.
• DNA must be distributed to the new cells.
• The cell membrane or cell wall must separate the
  two new cells.

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Systems of Cell Reproduction

• Prokaryotes divide by fission.
• Most prokaryotes have one circular chromosome.
• As DNA replicates, each of the two resulting DNA
  molecules attaches to the plasma membrane.
• As the cell grows, new plasma membrane is added
  between the attachment points, and the DNA
  molecules are moved apart.
• Cytokinesis separates the one cell into two, each
  with a complete chromosome.

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Figure 9.2 Prokaryotic Cell Division
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Systems of Cell Reproduction

• Eukaryotic cells divide by mitosis or meiosis.
• Eukaryotes usually have many chromosomes.
• Eukaryotes have a nucleus, which must replicate
  and, with few exceptions, divide during cell
  division.

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Systems of Cell Reproduction

• The reproduction of eukaryotic cells is typically
  characterized by three steps
• The replication of the DNA within the nucleus
• The packaging and segregation of the replicated
  DNA into two new nuclei (nuclear division)
• The division of the cytoplasm (cytokinesis)

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Systems of Cell Reproduction

• Meiosis is specialized cell division used for
  sexual reproduction. The genetic information in
  the chromosomes is shuffled, and the cells,
  called gametes, typically get one-half of the
  original DNA complement.

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Interphase and the Control of Cell Division

• The cell cycle has two phases mitosis and
  interphase.
• A typical eukaryotic cell will spend most of its
  life in interphase, the period between divisions
  of the cytoplasm.
• Some cells, such as human nerve and muscle cells,
  lose the capacity to divide altogether and stay
  in interphase indefinitely, while other cells
  divide regularly or occasionally.

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Cell Cycle

• There are 4 periods in a cell cycle.
• Mitosis (M)- nucleus and cytoplasm divide and
  form 2 new cells.
• First Gap (G1)- new cells from birth until it
  begins to replicate.
• Synthesis (S)- DNA synthesis, chromosomes are
  replicated.
• Second Gap Period (G2)- end of DNA synthesis
  until cell division or mitosis.

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Interphase and the Control of Cell Division

• Interphase consists of three sub-phases
• G1 is Gap 1, the period just after mitosis and
  before the beginning of DNA synthesis.
• Next is S phase (synthesis), which is the time
  when the cells DNA is replicated.
• G2 is the time after S and prior to mitosis.

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Figure 9.3 The Eukaryotic Cell Cycle
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Interphase and the Control of Cell Division

• Transitions from G1 to S and G2 to M depend on
  activation of a protein called cyclin-dependent
  kinase, or Cdk.
• Several different cyclins exist, which, when
  bound to Cdk, phosphorylate different target
  proteins.

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Figure 9.4 Cyclin-Dependent Kinases and Cyclins
Trigger Transisions in the Cell Cycle
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Interphase and the Control of Cell Division

• Cyclin-Cdk complexes act as checkpoints. When
  functioning properly, they allow or prevent the
  passage to the next cell cycle stage, depending
  on the extra- and intracellular conditions.
• In cancer cells, these cyclin-Cdk controls are
  often disrupted.

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Interphase and the Control of Cell Division

• Some cells which no longer go through the cell
  cycle may respond to growth factors provided by
  other cells.
• Examples include platelet-derived growth factor,
  interleukins, and erythropoietin.
• Growth factors act by binding to target cells,
  and triggering events within the target cell that
  initiate the cell cycle.
• Cancer cells cycle inappropriately because they
  either make their own growth factors or no longer
  require them to start cycling.

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Eukaryotic Chromosomes

• The basic unit of the eukaryotic chromosome is a
  gigantic, linear, double-stranded molecule of DNA
  complexed with many proteins to form a dense
  material called chromatin.
• After the DNA of a chromosome replicates during S
  phase, each chromosome consists of two joined
  chromatids.

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Figure 9.5 Chromosomes, Chromatids, and Chromatin
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Eukaryotic Chromosomes

• Interphase chromosomes are wrapped around
  proteins called histones.
• These wraps of DNA and histone proteins are
  called nucleosomes and resemble beads on a
  string.
• During mitosis and meiosis, the chromatin becomes
  even more coiled and condensed.

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Figure 9.6 DNA Packs into a Mitotic Chromosome
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Mitosis Distributing Exact Copies of Genetic
Information

• When the cell enters S phase and DNA is
  replicated, the centrosome replicates to form two
  centrosomes.
• During G2-to-M transition, the two centrosomes
  separate from each other and move to opposite
  ends of the nuclear envelope.
• The orientation of the centrosomes determines the
  cells plane of division.
• Centrosomes are regions where microtubules form.

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Phases of Mitosis

• 5 Phases
• Prophase
• Metaphase
• Anaphase
• Telophase
• Cytokinesis

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Mitosis Distributing Exact Copies of Genetic
Information

• Prophase marks the beginning of mitosis.
• Chromosomes compact and coil, becoming more dense
  and visible, form a set of sister chromatids.
• Polar microtubules form between the two
  centrosomes and make up the developing spindle.
• The mitotic spindle serves as a railroad track
  along which chromosomes will move later in
  mitosis.
• Mitotic spindle begins to push the ends or poles
  apart.
• A cell will not begin to divide until it has
  about doubled in size.

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Figure 9.8 Mitosis (Part 1)
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Mitosis Distributing Exact Copies of Genetic
Information

• During metaphase
• Chromosomes are fully condensed and have
  distinguishable shapes.
• Cohesins break down.
• Mitotic spindle starts to tug and lines all the
  chromatids up at the equator of the spindle.

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Figure 9.8 Mitosis (Part 2)
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Mitosis Distributing Exact Copies of Genetic
Information

• Anaphase
• Each of the sister chromatids separate, thereby
  becoming independent chromosomes.
• They are pulled to opposite poles of the spindle.
• Forms a complete set of chromosomes which is the
  basis for a new nucleus.
• The cell is pushed farther apart so it becomes
  much longer.

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Mitosis Distributing Exact Copies of Genetic
Information

• Telophase begins when the chromosomes finish
  moving.
• 2 nuclei are organized.
• The chromosomes at each end unwind into masses of
  chromatin.

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Cytokinesis

• Cytokinesis- The Division of the Cytoplasm
• Usually begins in early anaphase.
• A ring of microfilaments form around the cells
  equator.
• The filaments constrict the cell to form a
  cleavage furrow and eventually pinches the
  cytoplasm in two.

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Cytokinesis The Division of the Cytoplasm

• Plants have cell walls and the cytoplasm divides
  differently.
• After the spindle breaks down, vesicles from the
  Golgi apparatus appear in the equatorial region.
• The vesicles fuse to form a new plasma membrane,
  and the contents of the vesicles combine to form
  the cell plate, which is the beginning of the new
  cell wall.

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Figure 9.10 Cytokinesis Differs in Animal and
Plant Cells
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Reproduction Asexual and Sexual

• Mitosis by repeated cell cycles can give rise to
  vast numbers of genetically identical cells.
• Meiosis results in just four progeny, which
  usually do not further duplicate. The cells can
  be genetically different.

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Reproduction Asexual and Sexual

• Asexual reproduction involves the generation of a
  new individual that is essentially genetically
  identical to the parent. It involves a cell or
  cells that were generated by mitosis.
• Variation of cells is likely due to mutations or
  environmental effects.

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Reproduction Asexual and Sexual

• Sexual reproduction involves meiosis.
• Two parents each contribute a set of chromosomes
  in a sex cell or gamete.
• Gametes fuse to produce a single cell, the
  zygote, or fertilized egg. This creates variety
  among the offspring beyond that attributed to
  mutations or the environment.

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Reproduction Asexual and Sexual

• In multicellular organisms, somatic cells (cells
  that are not specialized for reproduction) each
  contain two sets of chromosomes.
• In each recognizable pair of chromosomes, one
  comes from each of the two parents.
• The members of the pair are called homologous
  chromosomes and have corresponding but generally
  not identical genetic information.
• In humans there are 46 homologous chromosomes in
  most body cells.

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Reproduction Asexual and Sexual

• Haploid cells contain just one homolog of each
  pair. The number of chromosomes in a single set
  is denoted by 1n.
• When haploid gametes fuse in fertilization, they
  create the zygote, which is 2n, or diploid.
• Tetraploid- 4 homologous chromosomes of each type
  4n.

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Reproduction Asexual and Sexual

• Haplontic organisms have a predominant life cycle
  in a 1n (haploid) state.
• Some organisms have an alternation of generations
  that includes both a 1n vegetative life stage and
  a 2n vegetative life stage.
• In diplontic organisms, which include animals,
  the organism is usually diploid.

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Figure 9.12 Fertilization and Meiosis Alternate
in Sexual Reproduction (Part 1)
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Figure 9.12 Fertilization and Meiosis Alternate
in Sexual Reproduction (Part 2)
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Reproduction Asexual and Sexual

• Cells in metaphase can be killed and prepared in
  a way that spreads the chromosomes around a
  region on a glass slide.
• A photograph of the slide can be taken, and
  images of each chromosome can be organized based
  on size, number, and shape. This spread is called
  a karyotype.

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Figure 9.13 Human Cells Have 46 Chromosomes
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Phases of Meiosis

• Prophase I- Finds homolog and line up together.
• Metaphase I- Tetrads line up at spindle equator.
• Anaphase I- They begin to separate.
• Telophase I- Organize into a new nucleus.
• No DNA replication between meiosis I and II.
• Prophase II- new spindle forms.
• Metaphase II- sister chromatids line up at
  spindle.
• Anaphase II- sister chromatids separate.
• Telophase II- organized into new haploid nuclei.

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Meiosis A Pair of Nuclear Divisions

• Meiosis consists of two nuclear divisions that
  reduce the number of chromosomes to the haploid
  number. The DNA is replicated only once.
• The functions of meiosis are
• To reduce the chromosome number from diploid to
  haploid.
• To ensure each gamete gets a complete set of
  chromosomes.
• To promote genetic diversity among products.

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Meiosis A Pair of Nuclear Divisions

• Meiosis
• It takes 2 divisions during meiosis to make each
  nucleus haploid.
• The 2 divisions are split up into meiosis I and
  II.
• Both divisions use a spindle and move
  chromosomes, so it looks similar to mitosis.
• Meiosis can take days to complete, where mitosis
  usually takes only hours or minutes.

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Figure 9.14 Meiosis (Part 1)
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Meiosis A Pair of Nuclear Divisions

• The homologous chromosomes separate in anaphase
  I.
• The individual chromosomes are pulled to the
  poles, with one homolog of a pair going to one
  pole and the other homolog going to the opposite
  pole.

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Figure 9.14 Meiosis (Part 2)
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Figure 9.14 Meiosis (Part 3)
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Meiosis A Pair of Nuclear Divisions

• The second meiotic division separates the
  chromatids.
• Meiosis II is similar to mitosis but one
  difference is that DNA does not replicate before
  meiosis II.
• The number of chromosomes in the resulting cells
  is therefore half that found in diploid mitotic
  cells.
• In meiosis II, sister chromatids are not
  identical and there is no crossing-over.

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Figure 9.14 Meiosis (Part 4)
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Figure 9.14 Meiosis (Part 5)
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Figure 9.14 Meiosis (Part 6)
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Meiosis A Pair of Nuclear Divisions

• During meiosis I, the chromosomes appear to repel
  each other except at the centromere and at points
  of attachments, called chiasmata, which appear
  x-shaped.
• These chiasmata reflect the exchange of genetic
  material between homologous chromosomes, a
  phenomenon called crossing-over.
• This crossing-over increases genetic variation by
  reshuffling the genes on the homologs.

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Figure 9.16 Crossing Over Forms Genetically
Diverse Chromosomes
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Meiosis A Pair of Nuclear Divisions

• Meiosis leads to genetic diversity.
• Synapses and crossing-over during prophase I mix
  genetic material of the maternal with that of the
  paternal homologous chromosomes.
• Which member of a homologous pair segregates or
  goes to which daughter cell at anaphase I is a
  matter of chance. This phenomenon is called
  independent assortment.

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Meiotic Errors

• Nondisjunction occurs when homologous chromosomes
  fail to separate during anaphase I, or sister
  chromatids fail to separate during anaphase II.
• The result is a condition called aneuploidy.

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Figure 9.18 Nondisjunction Leads to Aneuploidy
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Meiotic Errors

• One reason for aneuploidy may be a lack of
  cohesions.
• Failure of chromosome 21 to separate in humans
  results in trisomy 21Down syndrome.
• Translocation, a process in which part of a
  chromosome attaches to another, can also cause
  abnormality.

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Meiotic Errors

• Polyploids have extra whole sets of chromosomes,
  and this abnormality in itself does not prevent
  mitosis.
• Triploids are 3n tetraploids are 4n.
• Although mitosis usually is unimpaired, meiosis
  is problematic, especially for odd numbers of
  sets, as in triploidy.

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Cell Death

• Cells die in one of two ways necrosis and
  apoptosis.
• Necrosis occurs when cells either are damaged by
  poisons or are starved of essential nutrients.
  These cells swell and burst.

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Cell Death

• Genetically programmed cell death is called
  apoptosis
• The cell may no longer be needed, e.g., cells of
  the weblike tissue between the fingers of a
  developing human fetus.
• Cells that are old or damaged may need to be
  replaced.
• The cell death cycle is controlled by signals.
• The cell becomes isolated, chops up its own
  chromatin, and gets ingested by surrounding
  living cells.

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Gamete Formation in Animals

• Pgs. 820-824
• Gametes- term used to define sperm or egg.
• Sperm or Spermatozoa- male gametes.
• Sperm are small with little cytoplasm and have
  flagellum so they can move.
• Sperm are produced in the testes by the process
  of spermatogenesis.
• Germ cells are cells that reproduce to form
  diploid cells, in the male these cells are called
  spermatogonia cells.
• Cells that undergo meiosis are called
  spermatocytes.

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Gamete Formation in Animals

• These divide by meiosis into 4 spermatids.
• To become mature sperm the spermatids must
  undergo further differentiation.
• The mature sperm consists of a tail and head.
• Oogenesis- is the formation of the female
  gametes called eggs or ova (sing.) ovum (plur.)
• These cells are the largest cells in the female
  body where as sperm are of the smallest in the
  male body.
• Division in this process is unequal. An original
  cell in the female only produces one ovum and 2
  polar bodies (small cells).

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Gamete Formation in Animals

• In the ovary the germ cells are called oogonia,
  which divide by mitosis.
• They eventually stop dividing and become oocytes.
• In mammals all this occurs while the female is
  still in embryo. This process is stopped at the
  point of prophase I in meiosis until the female
  reaches sexual maturity.
• In meiosis I the chromosomes separate as usual
  but the cytoplasm is divided unequally, which
  forms the first polar body.
• In meiosis II the same thing happens which
  produces the 2nd polar body and one large ovum.
• The polar bodies are just a means of shedding
  excess chromosomes from the developing egg and
  they soon disintegrate.