Download Cell Cyles

Cell Cycle and Cell
Division
Prokaryotic Cells - Bacteria
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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
Binary Fission
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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
Escherichia coli Genome
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DNA is circular and encodes ~ 3000 proteins
Bacteria with unusual growth properties have
been identified – contain mutations that
changed the phenotype
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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
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Mixed 2 mutant types together and found that
bacteria can grow on media without either amino acid
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
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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
Integration
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The plasmid can
integrate into the
chromosome and
when bacteria
conjugate they can
pass on chromosomal
genes
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Increases genetic
variation
Transformation
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Bacteria can pick up DNA from surroundings
Take advantage of this in molecular biology labs
to replicate DNA fragments of choice
DNA in the Eukaryotic Cell
A cell’s 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
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With exception of the sex
chromosomes (X and Y) humans have
2 similar copies of each chromosome
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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
Chromosomal Bands
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Set of human
chromosomes is
called a karyotype
The staining
patterns can allow
geneticists to
identify areas of
abnormalities
Types of Chromosomes
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Mitotic chromosomes are those that are
visible during mitosis – condensed
Interphase chromosomes are loose and
string-like - chromatin
Components of Chromosomes
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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
Telomeres
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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
Chromatin vs Chromosome
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Chromatin is a
complex of protein
and DNA – very
dispersed
Chromosome is the
compact form of
DNA so that it is
protected during
mitosis
Nucleosomes
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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
50 nm
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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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
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Nucleosomal histones
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Small and coil DNA into
nucleosomes
H2A, H2B, H3 and H4
H3 and H4 highly conserved
H1 histones
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Larger and less conserved
Histones
Interphase Chromosomes
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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
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
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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
Tortoise Shell and Calico Cats
Cytoskeleton’s Role
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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
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
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New cells need to get
a copy of the entire
genome in the
process – most cells
double their mass as
well
4 Phases of
the Cell Cycle
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Mitosis or M phase
has 2 steps
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Nucleus divides –
mitosis
Cell divides in 2 cytokinesis
4 Phases of the Cell Cycle
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Period between mitosis
is called interphase –
has 3 phases
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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
G1 and G2 Phases
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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
5 Stages of Mitosis
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Continuous event but divided into 5 stages
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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
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
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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
Prometaphas
e
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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
Kinetochore
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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
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Humans bind 20 to 40
microtubules per
kinetochore
3 Classes of Microtubules
Metaphase
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Chromosomes line up
at the metaphase plate
that is halfway between
spindle poles
Chromosomes are
constantly under
tension from both
kinetochore
microtubules
Anaphase
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The connection between chromatids is cut by
proteolytic enzymes – now called daughter
chromosome – pulled to the spindle pole
Process of chromosome segregation
Anaphase A and B
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Anaphase A – separate the chromosomes
to the spindle pole
Anaphase B – the spindle poles separate
further separating the chromosomes
Telophase
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Nuclear envelope reforms around each group
of chromosomes
Nuclear lamina reforms
Chromosomes de-condense – return to
chromatin
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
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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
Contractile Ring
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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
Meiosis
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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
Meiosis
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Germ cells or gamates
(in plants they may be
spores) contain one set
of chromosomes
Egg is large and nonmotile
Sperm is small and
motile
Diploid cells formed by
fertilization
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
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In mitosis all 46 chromosomes line up along the
metaphase plate and 1 copy of each gets taken
to the new daughter cell
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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
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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)
Difference #1
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Mitosis – chromosomes
line up in the metaphase
plate
Meiosis – homologous
pairs line up together
Reassortment
Genetic Recombination
DNA can undergo rearrangements,
caused by genetic recombination
 General recombination – between any
pair of homologous DNA sequences
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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
Holliday Junction
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The Holliday junction is the area of cross-over
that occurs during recombination
Difference #2
⋇
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Mitosis – has only one division
Meiosis – has 2 divisions
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First division is to separate homologous pairs
Second division is to separate sister chromatids
Bivalent Chromosomes
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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
Remainder of Meiosis I
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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
Nondisjunction
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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 Down’s syndrome – they have 3
copies of chromosome 21
Gene
Expression
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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
Gene Regulation
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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