MITOSIS
CHAPTER 8
PROKARYOTES HAVE A SIMPLE CELL
CYCLE
• Cell division in
prokaryotes takes
place in two stages,
which together make
up a simple cell
cycle.
• 1. Copy the DNA
• This process is called
replication.
• 2. Split the cell in two to
form daughter cells
• This process is called
binary fission.
PROKARYOTES HAVE A SIMPLE CELL
CYCLE
• The hereditary information in a prokaryote is
stored in DNA.
• The prokaryotic chromosome is a single circle of
DNA.
• DNA replication begins with the unzipping of the
double-stranded.
PROKARYOTES HAVE A SIMPLE CELL
CYCLE
• A new double helix is
formed by adding
complementary
nucleotides to the
exposed DNA strands
that have been
unzipped.
• The end result of
replication is that the
cell possess two
complete copies of
the hereditary
information.
PROKARYOTES HAVE A SIMPLE CELL
CYCLE
• After replication, the
cell grows in order to
partition the
replicated DNA
molecules.
Origin of
replication
Prokaryotic
cell
Prokaryotic
chromosome:
Double-stranded DNA
Replication
of DNA
(a)
• When the cell reaches
an appropriate size,
the cell splits into two
equal halves.
• New plasma
membrane and cell
wall are added at a
point between the
partitioned DNA.
• Eventually the cell constricts in two to
form two daughter cells.
• Each daughter cell is a complete,
living cell with its own DNA.
Elongation of cell
Cell pinches in two
(b)
Daughter cells
EUKARYOTES HAVE A COMPLEX CELL
CYCLE
• Eukaryotic cells contain more DNA than
prokaryotic cells and the DNA is also
packaged differently.
• Cell division in eukaryotic cells is more
complex.
• DNA in eukaryotic cells is linear and
packaged into a compact chromosome.
• There is more than one chromosome in a
eukaryotic cell.
EUKARYOTES HAVE A COMPLEX CELL
CYCLE
• Eukaryotic cells undergo two different
mechanisms to divide up the DNA.
• Mitosis is a cell division mechanism that occurs in
nonreproductive cells.
• These cells are called somatic cells.
• Meiosis is a cell division mechanism that occurs in
cells that participate in sexual reproduction.
• These cells are called germ cells.
EUKARYOTES HAVE A COMPLEX CELL
CYCLE
• The eukaryotic cell
cycle is divided into
distinct phases:
• Interphase (G1,S, and
G2 phases)
• Mitosis (M phase)
• Cytokinesis (C
phase)
CELL CYCLE
• Interphase
• The first phase of the cycle.
• It is sometimes considered a resting phase but is
actually a period of activity.
• It is comprised of three phases.
CELL CYCLE
• G1 phase
• The primary growth phase of the cell after
division.
• Most cells spend majority of their time in this
phase.
• S phase
• DNA replication occurs in preparation for cell
division.
• G2 phase
• Further preparation for cell division, including
replication of mitochondria and synthesis of
microtubules.
EUKARYOTES HAVE A COMPLEX CELL
CYCLE
• Mitosis (M phase)
• A microtubular apparatus binds to the
chromosomes and moves them apart.
• Cytokinesis (C phase)
• The cytoplasm divides, creating two daughter
cells.
KEY BIOLOGICAL PROCESS: THE CELL
CYCLE
Interphase. The chromosomes
are extended and in use during the
G1, S, and G2, Phases.
Prophase. The chromosomes
condense, the nuclear envelope
down, and the spindle forms.
2
1
Growth (G1,S, G2 Phases)
Cytokinesis (C Phase)
6
Cytokinesis. The cytoplasm of
the cell is cleaved in half.
Metaphase. The chromosomes
line upon the central plane of
the cell.
3
Mitosis (M phase)
5
Telophase. The chromosomes
uncoil, and a new nuclear envelope
forms. The spindle fibers disappear.
4
Anaphase. The centromeres
divide, and the chromatids move
toward opposite poles.
CHROMOSOMES
• Chromosome
number varies
among organisms.
• Most eukaryotes
have between 10
and 50 chromosomes
in their somatic cells.
CHROMOSOMES
• Chromosomes are paired in somatic cells.
• These pairs are called homologous chromosomes,
or homologues.
• Homologues contain information about the same
traits but the information may vary.
• Cells that have two of each type of chromosome
are called diploid cells.
• One chromosome of each pair is inherited from
the mother and the other is inherited from the
father.
CHROMOSOMES
Homologous
chromosomes
• Prior to cell
division, each of
the homologous Centromere
chromosomes
replicates,
forming two
identical copies
called sister
chromatids.
Homologous
chromosomes
Replication
Sister
Sister
chromatids chromatids
• The sister chromatids are joined together by a
structure called a centromere.
• Humans have 23 pairs of homologous
chromosomes.
CHROMOSOMES
Homologous
pair
• A karyotype is an
arrangement of
chromosomes.
• Chromosomes can
be compared based
on size, shape, and
centromere location.
• The karyotype at right
shows the 23 pairs of
human
chromosomes.
© Andrew S. Bajer
CHROMOSOMES
• Chromosomes are comprised of chromatin,
a complex of DNA and protein.
• There is also some RNA associated with
chromosomes.
• The DNA in a chromosome is one very long
double-stranded fiber that extends unbroken
for the length of the chromosome.
• The DNA is coiled in order to allow it to fit into
a small space despite being very long.
CHROMOSOMES
• DNA is coiled around proteins called
histones.
• The histones have positive charges to counteract
the negative charges associated with the
phosphate groups of the DNA.
Scaffold protein
Chromatin loop
Scaffold protein
Solenoid
DNA
DNA
Central
histone
Rosettes of chromatin loops
Nucleosome
CELL DIVISION
•
Interphase sets the stage for cell division.
•
•
Chromosomes are first duplicated
Although not visible, chromosomes begin to
wind up tightly in a process called
condensation.
CELL DIVISION
•
The cell division that follows interphase is a
division of the nuclear contents, known as
mitosis.
•
Mitosis is a continuous process but it is divided,
for ease of study, into four distinct stages:
1. prophase
2. metaphase
3. anaphase
4. telophase
CELL DIVISION
• Prophase
• The beginning of mitosis and
the point where the
condensed chromosomes
first become visible.
• The nuclear envelope begins
to disintegrate.
• Centrioles (if present)
separate in the cell center
and migrate to opposite
“poles” of the cell.
• The centrioles form a network of protein cables
called the spindle, made of microtubules.
• Some of the microtubules extend toward the
centromere of the chromosomes.
• These microtubules will grow from each pole
until attached to a centromere at a disc of
protein called a kinetochore.
CELL DIVISION
• Metaphase
• The chromosomes
attached to
microtubules of the
spindle are aligned in
the center of the cell.
• The centromeres are
aligned along an
imaginary plane that
divides the cell in
half, known as the
equatorial plane.
CELL DIVISION
• Anaphase
• Sister chromatids
separate.
• The microtubules of
the spindle are
dismantled starting at
the poles.
• This pulls the
chromatids toward
the poles.
CELL DIVISION
• Telophase
• The spindle is
dismantled.
• A nuclear envelope
forms around the set
of chromosomes at
each pole.
• The chromosomes
begin to
uncondense.
CELL DIVISION
CleavageCell wall
furrow
Nuclei
• Cytokinesis
• Occurs at the end of mitosis
and is a division of the
cytoplasm into roughly
equal halves.
• In animals, cytokinesis occurs (a)
by actin filaments
contracting and pinching
the cell in two.
• This action is evident as a
(b)
Vesicles containing membrane
cleavage furrow that
components fusing to form
appears between the
cell plate
daughter cells.
• In plants, a new cell wall is laid down to divide the two
daughter cells.
• The cell wall grows at right angles to the mitotic spindle
and is called the cell plate.
CELL DEATH
• During fetal development, many cells are
programmed to die.
• This results in the formation of fingers and toes
from paddlelike hands and feet.
AGING AND THE CELL CYCLE
• In 1978, Elizabeth Blackburn suggested an
explanation for the “Hayflick limit”.
• In body cells, a portion of the protective telomere
cap was lost by a chromosome during each
cycle of DNA replication.
• So each time a cell divides, its chromosomes
get a little shorter.
• Eventually, after some 50 replication cycles,
the protective telomeric cap is used up.
• The cell line then enters senescence, no
longer able to proliferate.
WHAT IS CANCER?
• Cancer is a growth
disorder of cells.
• Begins when
apparently normal
cells grow
uncontrollably and
spread to other body
parts.
• The result is a
growing cluster of
cells called a
tumor.
WHAT IS CANCER?
• Benign tumors are surrounded by a healthy
layer of cells (also known as encapsulated)
and do not spread to other areas.
• Malignant tumors are not encapsulated and
are invasive.
• Cells from malignant tumors leave and spread to
different areas of the body to form new tumors.
• These cells are called metastases.
WHAT IS CANCER?
• Cancer is caused by a gene disorder in
somatic tissue in which damaged genes fail
to properly control the cell cycle.
• Mutations cause damage to genes.
• May result from chemical or environmental
exposure, such as UV rays.
• Viral exposure may also alter DNA.