Download Mitosis

Why do cells divide?
1. To heal/repair tissue
2. For multicellular organisms to grow in size
3. To keep cell sizes small for increased efficiency
4. For the reproduction of unicellular organisms (like
bacteria)
The Cell Cycle
The Cell Cycle is the
preparation and division of
the nucleus of a cell to
form two new cells
Interphase and Mitosis
make up the Cell Cycle
Stages of the cell cycle
• INTERPHASE
– G1 stage = "GAP 1”
– S stage = "Synthesis”
• This is the stage when DNA
replication occurs
– G2 stage = "GAP 2”
• MITOSIS
– M stage = mitosis & cytokinesis
Cell Cycle
What is Mitosis?
• Nuclear division plus division of
the cell itself (cytokinesis)
• Produces two identical daughter
cells.
Regulation of the Cell Cycle
• Very complex
• Errors can lead to cancer.
–In cancer, regulation of the cell cycle
goes awry
–Normal cell growth & behavior is lost.
Regulatory Enzymes
• Cdk (cyclin dependent kinase)
• Major control switch, along with
other cyclins
–causes cell to move from G1 to S
or G2 to M.
• MPF (Maturation Promoting Factor)
–triggers progression through cell
cycle.
Cancer Promotors
• p53 = protein that blocks the cell
cycle if DNA is damaged. Cell can
die.
• p53 mutation is the most frequent
mutation leading to cancer.
• p27 = protein that blocks entry into
S phase
–Low levels may mean eventual
breast cancer
Chromosome Characteristics -
Ploidy
• Number of sets of chromosomes in
a cell
• Haploid (n)-- one set chromosomes
• Diploid (2n)-- two sets
chromosomes
• Most plant and animal adults are
diploid (2n)
• Eggs and sperm are haploid (n)
Chromosome Characteristics
• Diploid set for humans; 2n = 46
(n=23)sperm + (n=23)egg = (2n=46)zygote
• Autosomes = body chromosomes
• Female-sex chromosomes =
homologous (XX)
• Male-sex chromosomes =
non-homologous (XY)
Interphase - G1 S G2
• The cell is metabolizing, growing,
replicating DNA and preparing for
mitosis
Interphase – G1 S G2
• Chromosomes are not clearly seen
• Nucleolus may be visible
• In animal cells ONLY, a pair of
centrioles may start moving into
position.
Interphase summary
Three Phases:
• The cell grows &
develops (Gap 1)
• DNA is replicated (S)
• Necessary materials
are made (Gap 2)
ANIMAL
PLANT
How do you know these cells are in
Most ofinterphase?
these cells are
Intact Nucleus
in Interphase
Mitosis (M Phase)
The M Phase consists of:
• Mitosis and Cytokinesis
Mitosis: the division of the nucleus
Cytokinesis: the division of the cytoplasm
Four Phases of Mitosis:
•
•
•
•
Prophase
Metaphase
Anaphase
Telophase
Preparing
for
Division
Late Interphase:
Prophase
Centromere
G
Early
Spindle Apparatus
2
Nuclear
membrane
disappears
Chromosome consisting of two sister chromatids
becomes visible
Prophase
• Chromatin begins to condense and
becomes visible.
• Nucleolus disappears.
• Centrioles begin
moving to opposite
ends of the cell
How is DNA packaged into
chromosomes?
DNA coils around set of 4
histone proteins, creating
a “bead on a string” called
a nucleosome
The nucleosome coils
into structures called
chromatin
Chromatin supercoils =
chromosome!
• The chromosome is
duplicated (during S)
• Each duplicated
chromosome consists
of two sister
.
chromatids
• These are connected
by a centromere .
Late Prophase
• Nuclear membrane dissolves
• Proteins attach to centromeres
creating the kinetochores.
• Microtubules attach at
centromeres/kinetochores
and chromosomes
begin moving.
Metaphase
Chromosomes line up at an
imaginary line between the
centrosomes at opposite ends
of the cell.
The spindle attaches to
the centromeres.
Metaphase
• Spindle fibers align the
chromosomes along metaphase
plate (middle).
• This helps to ensure
each new nucleus will
receive one copy
of each chromosome.
Anaphase
Spindles pull the
“daughter”
chromosomes apart
Anaphase
• Paired chromosomes separate at
kinetochores and move to opposite
sides
Anaphase
Remember: the
DNA in these
chromosomes
is identical on
each end!
IDENTICAL
DNA
Telophase
• Chromatids arrive at opposite
poles
• Membranes re-form around
daughter nuclei.
• Chromosomes & spindles
disperse and are no
longer visible.
• Cytokinesis may begin
Telophase
Two nuclei become visible
In animal cells, a Cleavage
Furrow is visible
In plant cells, a cell plate is
now visible
Cytokinesis
The cytoplasm and
organelles divide.
Telophase &
Cytokinesis
Cytokinesis: Animals
• Fiber ring of protein called actin around
center of cell contracts and pinches cell into
two identical daughter cells
Plants
• In plant cells,
the rigid wall
requires that
a cell plate be
synthesized
between the
two daughter
cells.
The END RESULT
of the CELL CYCLE
Two identical cells as the
original, ready to go through
interphase and mitosis,
repeating the process
Mitosis & Cytokinesis
animal cell division
What is the purpose of
the Cell Cycle?
To replace somatic cells
(skin, bone, lining, etc.)
Somatic cells are body cells
Some cells like your nerve, muscle and red blood
cells do not undergo division. (One reason
stem cell research is so important.)
Asexual Reproduction
• Uses only mitosis
• Cells duplicate to form
2 exact copies (clones)
of each other
• Most primitive form of reproduction
–NO GENETIC VARIATION
• Used by higher organisms when
conditions are not favorable for
sexual reproduction
Chromosome # during Mitosis
Diploid cell
=2n (46 in
humans)
2(2n) = 92
due to DNA replication
chromosomes
in humans
one cell division occurs
Diploid cell
=2n (46 in
humans)
TWO
IDENTICAL
CELLS
Diploid cell
=2n (46 in
humans)