Cell Cycle and Cell Division

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Cell Cycle and Cell Division
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Prokaryotic Cells - Bacteria

• E. coli is a bacteria that resides in our colon
  and can be beneficial to us our major source of
  vitamin K that is necessary for proper
  coagulation
• Have only 1 copy of their DNA, considered to be
  haploid, any change in the sequence may be
  readily seen in the phenotype observable
  characteristics

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Binary Fission

• E coli can grow and duplicate every 20 minutes
• DNA is attached to the cell membrane and after it
  duplicates, it separates as the cell enlarges
• When cell is double in size, the cell divides by
  adding a plasma membrane and cell wall between
  the 2 cells

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Escherichia coli Genome

• DNA is circular and encodes 3000 proteins
• Bacteria with unusual growth properties have been
  identified contain mutations that changed the
  phenotype
• Rifampicin growth because of mutation in the RNA
  polymerase that allows for mRNA synthesis while
  in presence of the drug
• Bacteria can pass DNA back and forth between one
  another
• Mixed 2 mutant types together and found that
  bacteria can grow on media without either amino
  acid

5
Gene Transfer

• Bacterial mating or conjugation can be achieved
  in bacteria that contain plasmids small,
  circular dsDNA that is separate from rest of
  chromosome
• Plasmids can replicate independently from the
  bacterial chromosome
• F plasmid or fertility plasmid allows for mating
  and gene transfer

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Bacterial Mating

• Bacteria with F plasmid (donor) will encounter a
  bacteria without the F plasmid (recipient) and
  establish a cytoplasmic bridge to pass on the F
  plasmid
• Now both are donor bacteria
• Method of passing on antibiotic resistance

7
Integration

• The plasmid can integrate into the chromosome and
  when bacteria conjugate they can pass on
  chromosomal genes
• Increases genetic variation

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Transformation

• Bacteria can pick up DNA from surroundings
• Take advantage of this in molecular biology labs
  to replicate DNA fragments of choice

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DNA in the Eukaryotic Cell

• A cells DNA is divided into a set of chromosomes
  each being a long linear DNA associated with
  proteins to help fold into chromatin
• Also associated with proteins involved in gene
  expression, DNA replication and DNA repair

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Chromosomes

• With exception of the sex chromosomes (X and Y)
  humans have 2 similar copies of each chromosome
• one from our mother (?)
• one from our father (?)
• Called homologous chromosomes
• One instance of non-homologous chromosomes are
  males because of their X and Y
• Chromosomes are similar but not identical

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Chromosomal Bands

• Set of human chromosomes is called a karyotype
• The staining patterns can allow geneticists to
  identify areas of abnormalities

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Types of Chromosomes

• Mitotic chromosomes are those that are visible
  during mitosis condensed
• Interphase chromosomes are loose and string-like
  - chromatin

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Components of Chromosomes

• 3 important DNA sequences needed for accurate DNA
  copying, complete genome to new daughter cells
• DNA replication origin a place to start DNA
  replication, multiple sites
• Centromere a place to attach the mitotic
  spindle for chromosome separation
• Telomere at end of linear DNA to prevent the
  chromosome from getting shorter in every round of
  replication

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Important Parts During Cell Cycle
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Telomeres

• Telomerase is the enzyme responsible for adding
  DNA sequence to the end of the chromosome to act
  as a template to allow complete DNA replication
• Also protects the DNA from degradation

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Chromatin vs Chromosome

• Chromatin is a complex of protein and DNA very
  dispersed
• Chromosome is the compact form of DNA so that it
  is protected during mitosis

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Nucleosomes

• Nucleosome bead-on-a-string look to DNA
• The string is the DNA and the bead is the
  nucleosome core particle that the DNA is wrapped
  around the protein is histone

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Histone Octmer

• Nucleosome is the histone proteins and the
  adjacent linker DNA threads the histones
  together
• Histone is core around which the DNA coils
• Disc shaped with 2 copies of H2A, H2B, H3 and H4

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Histones

• Proteins with a large proportion of positively
  charged amino acids (Lys and Arg)
• Helps to bind the negatively charged DNA
• 5 types in 2 main groups
• Nucleosomal histones
• Small and coil DNA into nucleosomes
• H2A, H2B, H3 and H4
• H3 and H4 highly conserved
• H1 histones
• Larger and less conserved

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Interphase Chromosomes

• While not as condensed as mitotic chromosomes,
  there are areas more tightly packed than others
• Depends on the genes that are being expressed
• Most highly condensed chromatin is called
  heterochromatin
• The more extended chromatin is called euchromatin
  and is either being transcribed or easily
  available for transcription

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X Chromosome

• Females have 2 X chromosomes and males have 1 X
  and 1 Y chromosome
• One X becomes inactive and highly condensed
  Barr body
• Some cells have the paternal X off and some have
  the maternal X off
• This happens during early development and once
  off, all cells that arise from that cell will
  have it off
• This is then passed down (inherited) in all the
  cells that arise from that cell

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Barr Body Formation
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Tortoise Shell and Calico Cats
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Cytoskeletons Role

• Mitotic spindle microtubules used to separate
  the chromosomes formation starts in late G2
• Contractile ring myosin and actin filaments
  used to separate the 2 daughter cells formation
  starts in M phase

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Organelles Fragment

• Chromosomes divide as discussed in Chapter 6
• Mitochondria and chloroplasts will divide and
  double their numbers
• Membrane bound organelles fragment to increase
  the likelihood to be divided evenly between 2
  daughter cells

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Cell Cycle

• The orderly sequence of events which a cell
  duplicates its contents (DNA and organelles) and
  then divides in 2
• Fundamental task is to copy and pass on its
  genetic information to daughter cells

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Cell Cycle

• New cells need to get a copy of the entire genome
  in the process most cells double their mass as
  well

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4 Phases of the Cell Cycle

• Mitosis or M phase has 2 steps
• Nucleus divides mitosis
• Cell divides in 2 - cytokinesis

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4 Phases of the Cell Cycle

• Period between mitosis is called interphase has
  3 phases
• Synthesis (S) phase is when DNA is replicated
• Gap1 (G1) phase is phase between M and S phase
• Gap2 (G2) phase is phase between S and M phase

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G1 and G2 Phases

• Portion of the cell cycle that allows for the
  synthesis of proteins, lipids and other molecules
  needed for cell division
• Duplicates the organelles and keeps the cells
  from getting smaller on each division
• At end of G2, the DNA condenses into chromosomes
  from its normal chromatin structure

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5 Stages of Mitosis

• Continuous event but divided into 5 stages
• Prophase chromosomes condenses and mitotic
  spindle forms outside the nucleus
• Prometaphase nuclear membrane breaks down,
  attach mitotic spindle to chromosomes
• Metaphase mitotic spindle lines up chromosomes
  along the equator of the cell
• Anaphase sister chromatids separate and move to
  opposite ends
• Telophase nuclear envelope reassembles and cell
  is ready for cytokinesis

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Prophase

• Centrosome is duplicated in late S phase and as
  prophase starts they separate and move to
  opposite poles of the cell using motor proteins
  and ATP hydrolysis
• Each centrosome organizes its microtubules which
  then interact to form the mitotic spindle which
  undergoes dynamic instability

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Spindle Poles

• During prophase some of the microtubules become
  stabilized and form the mitotic spindle
• Some microtubules interact and are called polar
  microtubules and the centrosome is called the
  spindle pole

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Prometaphase

• Starts with the break up of the nuclear envelope
  because of the disassembly of the intermediate
  filaments in the nuclear lamina
• Spindle microtubules will bind to the chromosomes
  at a structure called the kinetochore which form
  on chromosomes during late prophase on the
  centromere

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Kinetochore

• Kinetochore protein assembles on centromere
• Microtubules probe into the area and when
  encounters a kinetochore will attach now called
  the kinetochore microtubule
• A microtubule from each pole will bind to the
  chromosome
• Humans bind 20 to 40 microtubules per kinetochore

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3 Classes of Microtubules
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Metaphase

• Chromosomes line up at the metaphase plate that
  is halfway between spindle poles
• Chromosomes are constantly under tension from
  both kinetochore microtubules

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Anaphase

• The connection between chromatids is cut by
  proteolytic enzymes now called daughter
  chromosome pulled to the spindle pole
• Process of chromosome segregation

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Anaphase A and B

• Anaphase A separate the chromosomes to the
  spindle pole
• Anaphase B the spindle poles separate further
  separating the chromosomes

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Telophase

• Nuclear envelope reforms around each group of
  chromosomes
• Nuclear lamina reforms
• Chromosomes de-condense return to chromatin

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Cytokinesis

• Separation of the new nuclei and cytoplasmic
  organelles between 2 new daughter cells
• Starts in anaphase but is not complete until
  after 2 new nuclei are formed
• Caused by the contractile ring of actin and myosin

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Cleavage Furrow

• Evidence of cytokinesis is by the presence of the
  cleavage furrow
• Dependent on the location of the mitotic spindle
• Usually forms around the equator of the cell -
  exception is during early development

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Contractile Ring

• Array of actin and myosin filaments that
  assembles during anaphase
• Force to divide cells comes from the actin moving
  over the myosin getting continuously smaller
• Disassembles when cell is divided into 2

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Meiosis

• Process of getting a cell with half the number of
  chromosomes cell would be haploid
• Normal cells would have a full set of
  chromosomes from the mom and the dad and are
  called diploid
• Fertilization will return the haploid sex cell to
  a diploid cell by uniting sperm and egg

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Meiosis

• Germ cells or gamates (in plants they may be
  spores) contain one set of chromosomes
• Egg is large and non-motile
• Sperm is small and motile
• Diploid cells formed by fertilization

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Meiosis

• Involves the duplication of the chromosomes and
  then 2 successive cell divisions to yield the
  haploid cells
• Can get genetic recombination between the
  chromosomes prior to the first cell division
  since they are in such close proximity allows
  for new phenotypes

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Mitosis vs Meiosis

• In mitosis all 46 chromosomes line up along the
  metaphase plate and 1 copy of each gets taken to
  the new daughter cell
• Each cell gets both ? and ? (46 chromosomes)
• In meiosis the chromosomes find their homologous
  pair and these line up at the metaphase plate
  get parental chromosome shuffling during the
  division
• First division the cell gets either the ? or ?
  chromosome but not both (23 sister chromatids)
• Second division the cell gets a single copy of
  the chromosome (23 chromosomes)

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Difference 1

• Mitosis chromosomes line up in the metaphase
  plate
• Meiosis homologous pairs line up together

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Reassortment
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Genetic Recombination

• DNA can undergo rearrangements, caused by genetic
  recombination
• General recombination between any pair of
  homologous DNA sequences
• 2 copies of same chromosome
• Crossing over of chromosomes during meiosis
• No nucleotides get altered no gain or loss of
  nucleotides at the cross over point

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Genetic Recombination
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Holliday Junction

• The Holliday junction is the area of cross-over
  that occurs during recombination

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Difference 2
?

• Mitosis has only one division
• Meiosis has 2 divisions
• First division is to separate homologous pairs
• Second division is to separate sister chromatids

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Bivalent Chromosomes

• Homologous pairs form a structure called bivalent
  consists of 4 chromatids
• Genetic recombination can occur at this time
  mixing of the ? and ? genes by crossing over
  one arm or portion of arm can be exchanged due to
  close proximity
• Allows for offspring having a novel assortment of
  genes

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Remainder of Meiosis I

• Very similar to mitosis from this point on
• Nuclear membrane disassembles
• Mitotic spindle attaches to the bivalent
• Line up on metaphase plate
• Homologs separate
• Nuclear membrane reforms and cells divide

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Daughter Cells from Meiosis I

• Differ from diploid cells
• They have 46 chromosomes but both copies come
  from the same parent with the exception of
  whether there was any crossing over
• Inherited as if a single chromosome

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Meiosis II

• No DNA replication in this step and no
  significant interphase
• Follows like mitosis but the new daughter cell
  have only 1 copy of each chromosome total of 23

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Meiosis
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(No Transcript)
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Nondisjunction

• Occasionally homologs do NOT separate during
  meiosis
• This causes one of the haploid cells to have an
  extra copy of the chromosome and the other cell
  to have no copy
• Upon fertilization, one embryo will have 3 copies
  and the other would have 1 copy (from the
  normal parent)
• Cause of Downs syndrome they have 3 copies of
  chromosome 21

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Gene Expression

• Hard to believe that a lymphocyte and a neuron
  contain the same DNA
• All cells contain the entire genome
• Differences in cells arises from what genes are
  expressed where

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Gene Regulation

• Differentiation occurs because various cell types
  have different genes being expressed
• In the frog, you can take the nucleus from an
  adult cell and put it in an egg and still get a
  tadpole