Download The Cell Cycle

The Cell Cycle
Cell Division in Prokaryotes:
Binary Fission
 Binary fission: The division of a prokaryotic cell into two offspring cells.
 1. The chromosome (single, circular, attached to the inside of the cell
membrane) which is attached to the inside of the membrane makes a copy of
itself. The copies stay attached to the cell membrane.
 2. The cell continues to grow until it is about two times the cells original cell.
 3. A cell wall forms between the two chromosomes and the cell splits into two
new cells.
 4. Each cell has one of the identical chromosomes that resulted from the copying
of the original cells chromosome.
Organization of Genetic
Material I
 All the DNA in a cell constitutes the cell’s genome
 A genome can consist of a single DNA molecule
(common in prokaryotic cells) or a number of DNA
molecules (common in eukaryotic cells)
 DNA molecules and proteins in a cell are packaged into
chromosomes
 Every eukaryotic species has a characteristic number of
chromosomes in each cell nucleus
 Somatic cells (non-reproductive cells) have two sets of
chromosomes
 Gametes (reproductive cells: sperm and eggs) have half as
many chromosomes as somatic cells
 Eukaryotic chromosomes consist of chromatin, a
combination of DNA and protein that condenses during
cell division
Organization of Genetic
Material II
 DNA coiled into chromosomes
 DNA is associated with proteins called histones that help to maintain the shape
of the chromosome and help it pack.
 There are also non-histone proteins (not shown) that are involved in controlling
the activity in specific regions of DNA.
 In a non-dividing cell, DNA and protein is in a loose form that is called chromatin
Distribution of Chromosomes
 In preparation for cell division, DNA is
replicated (DNA synthesis) and the
chromosomes condense (prophase)
 Each duplicated chromosome has
two sister chromatids, which
separate during cell division
 The centromere is the narrow “waist”
of the duplicated chromosome,
where the two chromatids are most
closely attached
 Eukaryotic cell division consists of:
 Mitosis, the division of the nucleus
 Cytokinesis, the division of the
cytoplasm
Chromosome Number and
Karyotypes
 Animals chromosomes are
catergorized as autosomes or sex
chromosomes
 Humans have 46 chromosomes.
 44 are autosomes
 2 are sex chromosomes
 Males are XY
 Females are XX
 Each individual gets two copies of
each type of chromosome. The two
copies are called homologous
chromosomes or homologues.
 Homologous chromosomes are
the same size and shape.
 They carry the genes for the same
trait.
 If one chromosome in a pair of
homologous chromosomes
carries the trait for nose shape,
so does the other chromosome
in the pair.
 Karyotypes: a picture of the
chromosomes in a dividing cell
found in a normal human.
 Karyotypes help us determine the
sex of an individual as well as if
there are any chromosome
number abnormalities.
Diploid v. Haploid Cells
 Diploid: Cells that have both chromosomes for each
homologous pair and 2 sex chromosomes.
 All normal cells are homologous.
 Abbreviated 2n where n = # of homologous pairs
 In humans: 2n = 46 where n = 22 homologous pairs + 2 sex
chromosomes
 Haploid: Have only one chromosome for each
homologous pair and one sex chromosome.
 Abbreviated 1n or n.
 Sperm and egg cells are haploid.
 Diploid condition is restored when sperm and egg combine to
form a new organism:
 Haploid sperm (n = 23 in humans) + haploid egg (n) = zygote
(first cell that forms from fertilization = 2n = 46 in humans)
 n + n = 2n (23 + 23 = 46)
Stages of the Cell Cycles
 The cell cycle consists of
 Interphase (cell growth and
copying of chromosomes in
preparation for cell division)
 About 90% of the cell cycle
 G1 phase (“first gap”)
 S phase (“synthesis”)
 G2 phase (“second gap”)
 The cell grows during all
three phases, but
chromosomes are
duplicated only during
the S phase
 Mitotic (M) phase (mitosis and
cytokinesis)
G1 – Growth 1of Interphase
 After a cell 1st divides, it is about ½ of the size it
should be. (awww… baby cell!)
 In G1 (G = growth or gap) the cell grows to a
mature size
 Under the right conditions, they will then proceed
to the S phase of interphase.
 Cells can also EXIT the cell cycle at this point and go
into a phase called G0
 In Go cells are mature, operating normally, but in
a non-dividing state.
 For example, all your nerve cells are in a G0 state.
S – Synthesis Phase of
Interphase
 This the phase in which DNA is copies (Section
10.1 and power point)
 After the DNA is copied, the two copies will STAY
ATTACHED for awhile near the middle of each
strand. This will be important for the rest of cell
division.
 Remember key ideas:
 Enyzmes: helicase, DNA polymerase, ligase
 Replication bubbles and forks
 Leading v lagging strand
 Base pairing
 End result
G2 – Growth 2 of Interphase
 DNA is replicated, now the rest of the cell must
divide.
 G2 is the time when a cell prepares for cell
division.
Mitosis: The division of the
nucleus.
 Mitosis is conventionally divided into four
phases:
 Prophase
 Metaphase
 Anaphase
 Telophase
 Cytokinesis is well underway by late telophase
Mitosis: Prophase
 The chromatid condenses into chromosomes.
 The two copies of the chromosomes (called
chromatins) are attached together at the
centromere
 The nuclear envelope starts to break down.
 The nucleolus disappears.
 Centrosomes, two dark spots, composed of two
centrioles appear next to the disappearing nucleus.
 The centrosomes start moving to opposite sides
(poles) of the cell forming spindle fibers. The
spindle fibers are microtubules that will help the
chromatids separate from each other.
 The spindle fibers arrays is called a mitotic
spindle
 There are two types of spindle fibers
 Kinetochore fibers: attached to a disk
shaped protein in the centromere called
a kinetochore
 Polar fibers: extend across the dividing cell
from centrosome to centrosome
Mitosis: Metaphase
 Chromosomes are fully
condensed.
 The kinetochores at the
centromere (the “waist” of
the chromosome) is attached
to the kinetochore fibers.
 Chromosomes line up on an
imaginary line in the middle of
the cell called a “metaphase
plate”
 The centromeres line up ON
the metaphase plate – this
is important for when we
talk about meiosis.
Mitosis: Anaphase
 Chromatids of each
chromosome separate at
the centromere
 They are pulled to opposite
sides of the cell.
 When they separate, they
are considered to be full
chromosomes.
Mitosis: Telophase
 After the chromosomes reach
the opposite sides of the cell,
prophase is undone.
 The spindle fibers disassemble.
 The chromomoses uncoil and
return to chromatin
 The nuclueolus in each
daughter cell forms.
 Cytokinesis, the division of the
cytoplasm, is usually
happening at the same time.
Cytokinesis: The division of the cytoplasm.
Each cell formed by cytokinesis has ½ the
cytoplasm and ½ of the organelles of the
original cell.
Animal Cells
 The cell
membrane
pinches in ½ way
between the
dividing cells poles.
 This area is called
a cleavage furrow
 The cleavage
furrow eventually
separates the
cell into two
identical
daughter cells.
Plant Cells
 There is a cell wall
plants have to
deal with.
 Vesicles formed by
the Golgi
apparatus fuse in
the middle of the
cell to form a “cell
plate” (aww…
baby cell wall!)
 The cell plate will
grow until it
eventually divides
the cell into two
daughter cells.
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