Structure, Growth and Division of Plant Cells Chapters 3 and 4

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Structure, Growth and Division of Plant
CellsChapters 3 and 4
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The Plant Cell
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The Plant Cell

• All plant cells have the same basic eukaryotic
  organization
• However, at maturity when they become
  specialized, plant cells may differ greatly from
  one another in their structures and functions
• Even those physically next to each other.
• Even the nucleus can be lost in some plant cells
• Contains many organelles with specific functions
• Enclosed by a membrane which defines their
  boundaries
• Dont Forget the Cell Wall!!!!!!!!!!

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The Plasma Membrane

• Composed of a phospholipid bilayer and proteins.
• The phospholipid sets up the bilayer structure
• Phospholipids have hydrophilic heads and fatty
  acid tails.
• The plasma membrane is fluid--that is proteins
  move in a fluid lipid background

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The Plasma Membrane

• Phospholipids
• Two fatty acids covalently linked to a glycerol,
  which is linked to a phosphate.
• All attached to a head group, such as choline,
  an amino acid.
• Head group POLAR so hydrophilic (loves water)
• Tail is non-polar -hydrophobic
• The tail varies in length from 14 to 28 carbons.

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The Fluid Mosaic Model

• Originally proposed by S. Jonathan Singer and
  Garth Nicolson in 1972.
• Allows for dynamic nature of membrane
• Little transition of lipids can take place
  without specific enzymes to mediate transfer -
  flippase.

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Flippase

• Enzymes located in the membrane responsible for
  aiding the movement of phospholipid molecules
  between the two leaflets that compose a cell's
  membrane
• Two types
• Transverse
• Lateral

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Transverse Diffusion

• Or flip-flop involves the movement of a lipid or
  protein from one membrane surface to the other.
• Is a fairly slow process due to the fact that a
  relatively significant amount of energy is
  required for flip-flopping to occur.

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Transverse Diffusion

• Most large proteins do not flip-flop due to their
  extensive polar regions, which are unfavorable in
  the hydrophobic core of a membrane bilayer.
• This allows the asymmetry of membranes to be
  retained for long periods, which is an important
  aspect of cell regulation.

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Lateral Diffusion

• Refers to the lateral movement of lipids and
  proteins found in the membrane. 
• Membrane lipids and proteins are generally free
  to move laterally if they are not restricted by
  certain interactions.
• Is a fairly quick and spontaneous process

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Flippase

• Potential role of ATP-dependent lipid flippases
  in vesicle formation.
• ATP-dependent lipid translocation might help
  deform the membrane by moving lipid mass towards
  the cytoplasmic leaflet

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Flippase

• This area asymmetry will increase the spontaneous
  curvature of the bilayer, and may thus help
  deform the membrane during vesicle budding.
• Lem3-Cdc50 proteins regulate the localization and
  activity of P4-ATPases.
• P4-ATPases play a pivotal role in the biogenesis
  of intracellular transport vesicles, polarized
  protein transport and protein maturation.

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Flippase

• Interaction of P4-ATPases with peripheral guanine
  nucleotide-exchange factors (GEFs) might cause
  activation of small GTPases.
• GTPases subsequently bind to the membrane and
  facilitate the assembly of coat proteins (if
  required)
• And thus, the endo-membrane system allows gene
  expression, post-translational modification, and
  secretion to occur!

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The Plasma Membrane

• Proteins
• Integral proteins
• Embedded in lipid bylayer serve as ion pumps
• They pump ions across the membrane against their
  concentration gradient
• Peripheral proteins
• Bound to membrane surface by ionic bonds.
• Interact with components of the cytoskeleton
• Anchored proteins
• Bound to surface via lipid molecules

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• Proteins - Add function and structure to membrane
• Extrinsic proteins (peripheral)
• Loosely attached to membrane
• ionic bonds with polar head groups and
  carbohydrates
• hydrophobic bonds with lipid
• proteins have lipids tails

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Integral proteins

• tightly bound to membrane - span both sides
• Protein has both polar and hydrophobic sections
  removed only through disrupting membrane with
  detergents

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Transmembrane proteins

• Has a total molecular weight of about 31,000 and
  is approximately 40 protein and 60
  carbohydrate.
• The primary structure consists of a segment of 19
  hydrophobic amino acid residues with a short
  hydrophilic sequence on one end and a longer
  hydrophilic sequence on the other end.
• The 19-residue sequence is just the right length
  to span the cell membrane if it is coiled in the
  shape of an a-helix.
• The large hydrophilic sequence includes the amino
  terminal residue of the polypeptide chain. 

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Transmembrane proteins

• General Rules of thumb
• takes about 20 aa to cross membrane
• many proteins cross many times
• odd of transmembrane regions,
• -COOH terminal usually cytosolic
• -NH3 terminal extracellular
• can be predicted by amino acid sequence
• high of side chains will be hydrophobic

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The nucleus

• Contains almost all of the genetic material
• What it contains is called the nuclear genome
  this varies greatly between plant species.
• Surrounded by nuclear envelope- double membrane -
  same as the plasma membrane.
• The nuclear pores allow for the passage of
  macromolecules and ribosomal subunits in and out
  of the nucleus.

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The Endoplasmic reticulum

• Connected to the nuclear envelope
• 3D-network of continuous tubules that course
  through the cytoplasm.
• Rough ER Synthesize, process, and sort proteins
  targeted to membranes, vacuoles, or the secretory
  pathway.
• Smooth ER Synthesize lipids and oils.
• Also
• Acts as an anchor points for actin filaments
• Controls cytosolic concentrations of calcium ions

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The Endoplasmic reticulum

• Proteins are made in the Rough ER lumen by an
  attached ribosome.
• Protein detaches from the ribosome
• The ER folds in on itself to form a transport
  vesicle
• This transport vesicle buds off and moves to
  the cytoplasm
• Either
• Fuses with plasma membrane
• Fuses with Golgi Apparatus

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The Golgi Network

• Proteins or lipids made in the ER contained in
  transport vesicles fuse with the Golgi.
• The Golgi modifies proteins and lipids from the
  ER, sorts them and packages them into transport
  vesicles.
• This transport vesicle buds off and moves to
  the cytoplasm.
• Fuse with plasma membrane.

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The Golgi Network
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The Mitochondria

• Contain their own DNA and protein-synthesizing
  machinery
• Ribosomes, transfer RNAs, nucleotides.
• Thought to have evolved from endosymbiotic
  bacteria.
• Divide by fusion
• The DNA is in the form of circular chromosomes,
  like bacteria
• DNA replication is independent from DNA
  replication in the nucleus

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The Mitochondria

• Site of Cellular Respiration
• This process requires oxygen.
• Composed of three stages
• Glycolysis--glucose splitting, occurs in the
  cell. Glucose is converted to Pyruvate.
• Krebs cycle--Electrons are removed--carriers are
  charged and CO2 is produced. This occurs in the
  mitochondrion.
• Electron transport--electrons are transferred to
  oxygen. This produces H2O and ATP. Occurs in the
  mito.

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The Chloroplast

• Contain their own DNA and protein-synthesizing
  machinery
• Ribosomes, transfer RNAs, nucleotides.
• Thought to have evolved from endosymbiotic
  bacteria.
• Divide by fusion
• The DNA is in the form of circular chromosomes,
  like bacteria
• DNA replication is independent from DNA
  replication in the nucleus

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The Chloroplast

• Membranes contain chlophyll and its associated
  proteins
• Site of photosynthesis
• Have inner outer membranes
• 3rd membrane system
• Thylakoids
• Stack of Thylakoids Granum
• Surrounded by Stroma
• Works like mitochondria
• During photosynthesis, ATP from stroma provide
  the energy for the production of sugar molecules

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The Vacuole

• Can be 80 90 of the plant cell
• Contained within a vacuolar membrane (Tonoplast)
• Contains
• Water, inorganic ions, organic acids, sugars,
  enzymes, and secondary metabolites.
• Required for plant cell enlargement
• The turgor pressure generated by vacuoles
  provides the structural rigidity needed to keep
  herbaceous plants upright.

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The Vacuole

• In general, the functions of the vacuole include
• Isolating materials that might be harmful or a
  threat to the cell
• Containing waste products
• Containing water in plant cells
• Maintaining internal hydrostatic pressure
  or turgor within the cell
• Maintaining an acidic internal pH
• Containing small molecules
• Exporting unwanted substances from the cell
• Allows plants to support structures such as
  leaves and flowers due to the pressure of the
  central vacuole
• In seeds, stored proteins needed for germination
  are kept in 'protein bodies', which are modified
  vacuole

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The cytoskeleton

• Three main components
• Microtubules are a and b proteins that create
  scaffolding in a cell. MTs are formed from the
  protein tubulin. 13 rows of tubulin 1
  microtubule
• Microfilaments solid (7 nm) made from G-actin
  protein. Consists of 2 chains of actin subunits
  that intertwine in a helical fashion

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The cytoskeleton

• Intermediate filaments a diverse group of
  helically wound linear proteins.
• Dimers line up parallel to each other
• These form anti-parallel Tetramers
• These join together to form a filament

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The cytoskeleton

• All these elements can assemble and disassemble
• Involved in plant cell division
• During mitosis
• Process of division that produces two daughter
  cells with identical chromosomal content of
  parent cell

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Plamodesmarta

• Each contains a tube called a Desmotubule, which
  is part of the ER.
• This is what connects adjacent cell and allow
  chemical communication and transport of material
  throughout the whole plant.
• The restriction acts to control the size of the
  molecules which pass through.

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The Plant Cell wall

• Cell walls are held together by the middle
  Lamella.
• Made up of
• Cellulose
• Xyloglucan
• Pectin
• Proteins
• Ca ions
• Lignin
• other ions
• Water

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The Plant Cell
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Replication of DNA
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• Composed of 4 nucleotide bases, 5 carbon sugar
  and phosphate.
• Base pair rungs of a ladder.
• Edges sugar-phosphate backbone.
• Double Helix
• Anti-Parallel

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The bases

• Chargaffs Rules
• AT
• GC
• led to suggestion of a double helix structure for
  DNA

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The Bases

• Adenine (A) always base pairs with thymine (T)
• Guanine (G) always base pairs with Cytosine (C)

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The Bases

• The CT pairing on the left suffers from carbonyl
  dipole repulsion, as well as steric crowding of
  the oxygens. The GA pairing on the right is also
  destabilized by steric crowding (circled
  hydrogens).

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DNA Replication

• Adenine (A) always base pairs with thymine (T)
• Guanine (G) always base pairs with Cytosine (C)
• ALL Down to HYDROGEN Bonding
• Requires steps
• H bonds break as enzymes unwind molecule
• New nucleotides (always in nucleus) fit into
  place beside old strand in a process called
  Complementary Base Pairing.
• New nucleotides joined together by enzyme called
  DNA Polymerase

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DNA Replication

• Each new double helix is composed of an old
  (parental) strand and a new (daughter) strand.
• As each strand acts as a template, process is
  called Semi-conservative Replication.
• Replication errors can occur. Cell has repair
  enzymes that usually fix problem. An error that
  persists is a mutation.
• This is permanent, and alters the phenotype.

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Protein synthesis in Plants
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Central Dogma of Molecular Biology

• DNA holds the code
• DNA makes RNA
• RNA makes Protein
• DNA to DNA is called REPLICATION
• DNA to RNA is called TRANSCRIPTION
• RNA to Protein is called TRANSLATION

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Central Dogma of Molecular Biology
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Gene Structure in Eukaryotes - contains
Exons and Introns - Exons contains coding
info - Introns does not contain coding info
introns are intervening sequence that
is transcribed but then must be removed
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Summary of protein synthesis

• Proteins
• Chains of Amino Acids
• Three nucleotide base pairs code for one amino
  acid.
• Proteins are formed from RNA
• The nucleotide code must be translated into an
  amino acid code.

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Occurs in the cytoplasm or on Rough ER
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RNA

• Formed from 4 nucleotides, 5 carbon sugar,
  phosphate.
• Uracil is used in RNA.
• It replaces Thymine
• The 5 carbon sugar has an extra oxygen.
• RNA is single stranded.

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Translation

• Translation requires
• Amino acids
• Transfer RNA (tRNA) Appropriate to its time,
  transfers AAs to ribosomes. The AAs join in
  cytoplasm to form proteins. 20 types. Loop
  structure
• Ribosomal RNA (rRNA) Joins with proteins made in
  cytoplasm to form the subunits of ribosomes.
  Linear molecule.
• Messenger RNA (mRNA) Carries genetic material
  from DNA to ribosomes in cytoplasm. Linear
  molecule.

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Translation

• Initiation
• mRNA binds to smaller of ribosome subunits, then,
  small subunit binds to big subunit.
• AUG start codon--complex assembles
• Elongation
• add AAs one at a time to form chain.
• Incoming tRNA receives AAs from outgoing tRNA.
  Ribosome moves to allow this to continue
• Termintion Stop codon--complex falls apart

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Translation

• With respect to the mRNA, the three sites are
  oriented 5to 3 E-P-A, because ribosomes moves
  toward the 3' end of mRNA. So, for elongation to
  occur, the following happens
• The A site binds the incoming tRNA with the
  complementary codon on the mRNA. It should be
  remembered that each tRNA contains an Anticodon.
  
• The P site holds the tRNA with the growing
  polypeptide chain.

Used with permission from http//education-portal.
com
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Translation

• The E site holds the tRNA without its amino acid.
  
• When a tRNA initially binds to its corresponding
  codon on the mRNA, it is in the A site.
• Then, a peptide bond forms between the amino acid
  of the tRNA in the A site and the amino acid of
  the charged tRNA in the P site.

Used with permission from http//education-portal.
com
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Translation

• The growing polypeptide chain is transferred to
  the tRNA in the A site.
• Translocation occurs, moving the tRNA in the P
  site, now without an amino acid, to the E site
  the tRNA that was in the A site, now charged with
  the polypeptide chain, is moved to the P site.
• Finally, The tRNA in the E site leaves and
  another tRNA enters the A site to repeat the
  process.

Used with permission from http//education-portal.
com
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Cell Division in Plants
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Most plant cells divide by Mitosis

• Mitosis Process of division that produces two
  daughter cells with identical chromosomal content
  of parent cell.
• Mitosis is one stage of the cell cycle.
• Cell cycle--cycle of stages a cell goes through
  in order to grow and divide.

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Most plant cells divide by Mitosis

• G0 phase
• The term "post-mitotic" is sometimes used to
  refer to both quiescent and senescent cells.
  Nonproliferative cells in multicellular
  eukaryotes generally enter the quiescent G0 state
  from G1 and may remain dormant for long periods
  of time, possibly indefinitely.
• This is very common for cells that are fully
  differentiated.
• Cellular senescence occurs in response to DNA
  damage or degradation that would make a cell's
  progeny nonviable

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Most plant cells divide by Mitosis

• G1 phase
• It is also called the growth phase.
• During this phase the biosynthetic activities of
  the cell, which had been considerably slowed down
  during M phase, resume at a high rate.
• This phase is marked by the use of 20 amino acids
  to form millions of proteins and later on enzymes
  that are required in S phase, mainly those needed
  for DNA replication.
• Duration of G1 is highly variable, even among
  different cells of the same species.

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Most plant cells divide by Mitosis

• S phase
• Starts when DNA replication commences when it is
  complete, all of the chromosomes have been
  replicated, i.e., each chromosome has two
  (sister) chromatids.
• Thus, during this phase, the amount of DNA in the
  cell has effectively doubled, though the ploidy
  of the cell remains the same.
• During this phase, synthesis is completed as
  quickly as possible due to the exposed base pairs
  being sensitive to external factors
• ie - pesticides

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Most plant cells divide by Mitosis

• G2 phase
• During the gap between DNA synthesis and mitosis,
  the cell will continue to grow.
• The G2 checkpoint control mechanism ensures that
  everything is ready to enter the M (mitosis/
  Mieosis) phase and divides

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Stages of Division

• Prophase--nuclear envelope breakdown, chromosome
  condensation, spindle formation.
• Metaphase--chromosomes are lined up precisely on
  the metaphase plate, or middle of the cell.
• Anaphase--spindle pulls sister chromatids apart.
  
• Telophase--chromatids begin to decondense and
  become chromatin. Spindle disappears.
• Cytokinesis--divide cell and organelles. Actin
  ring, or cleavage furrow splits cell.

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• Prophase--nuclear envelope breakdown, chromosome
  condensation, spindle formation.
• Metaphase--chromosomes are lined up precisely on
  the metaphase plate, or middle of the cell.

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• Anaphase--spindle pulls sister chromatids apart.
  
• Telophase--chromatids begin to decondense and
  become chromatin. Spindle disappears.
• NEW CELL WALL IS FORMED
• Cytokinesis--divide cell and organelles. Actin
  ring, or cleavage furrow splits cell.

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Remember the cytoskeleton?

• Changes in microtubule arrangements (yellow)
  during different stages of the cell cycle of
  wheat root cells. DNA is shown in blue.

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Gamete Production -Meiosis

• In order to reproduce plants must produce
  gametes.
• Meiosis blends DNA from parental plant
  contributions to produce a mixed up half or
  haploid, set of DNA.
• Crossing over is critical for producing haploid
  DNA with genetic diversity.

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The Process of Meiosis
Interphase

• Haploid gametes are produced in diploid organisms
• Two consecutive divisions occur, meiosis I and
  meiosis II, preceded by interphase

Centrosomes (with centriole pairs)
Nuclear envelope
Chromatin
Chromosomes duplicate
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Prophase -I
Replicated pairs of chromosomes line up side by
side. These pairs are called Homologous--both
have same gene order (gene for eye color, hair
color, etc). Sister chromatid from one pair
interact with a Sister chromatid from another
pair. One sister is from father, one sister
from mother, but they have same gene order.
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Prophase -I

• This interaction is called Synapsis.
• Synapsis results in the formation of a Tetrad
  (4 sisters together).
• Crossing over swaps sections of homologous genes.

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Figure 2.9 (1)
Meiosis - I
Prophase I
Metaphase I
Anaphase I
Telophase I
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Figure 2.9 (2)
Meiosis - II
Prophase II
Metaphase II
Anaphase II
Telophase II
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• Meiosis I

Meiosis I Homologous chromosomes separate
Telophase I and Cytokinesis
Anaphase I
Prophase I
Metaphase I
Microtubules attached to Chromosomes
Sister chromatids remain attached
Cleavage furrow
Sites of crossing over
Spindle
Sister chromatids
Tetrad
Centromere
Tetrads line up
Homologous chromosomes pair and exchange segments
Two haploid cells form chromosomes are still
double
Pairs of homologous chromosomes split up
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• Meiosis II

Meiosis II Sister chromatids separate
Telophase II and Cytokinesis
Prophase II
Anaphase II
Metaphase II
Sister chromatids separate
Haploid daughter cells forming
During another round of cell division, the sister
chromatids finally separate four haploid
daughter cells result, containing single
chromosomes
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ANY QUESTIONS?