How Do Cells Divide?
CELL CYCLE
What you will learn…
 1. Why Do Cells Divide?
 2. Chromosome structure
 3. Cell Division in Prokaryotes
 4. Cell Cycle
 5. Mitosis
 6. Cytokinesis
 7. Control of Cell Division and Cancer
 8. Meiosis
 9.Why do cells need two types of cell division?
 10. Gamete Formation
1. Why Do Cells Divide?
 Virchow: Cells can only come from preexisting cells
 In unicellular organisms, can reproduce an entire
organism
 Allows multicellular organisms to reproduce asexually
 Basis of sexual reproduction sperm and egg
 Allows fertilized egg, or zygote, to develop into an
adult organism
 Replaces worn-out or damaged cells
 Enables multicellular organism to grow to adult size
http://www.pbs.org/wgbh/nova/miracle/progr
am.html
Cell size
 Cells are small, because they have to be
able to carry materials from one side of the
cell to the next in a short period of time.
 Cells must have a large enough surface
area to be able to take in nutrients and
oxygen and release waste quickly.
2. Chromosome Structure
DNA can be in the form of Chromatin:
Diffuse mass of long, thin fibers, not seen under
the microscope, less tightly coiled
Combination of DNA and protein
DNA must be tightly packaged before cell
division, so it can be evenly divided between
the two new cells.
DNA will now be in the form of Chromosome!
http://www.dnalc.org/resources/3d/07-how-dna-is-packaged-basic.html
2. Chromosome Structure
Chromosomes
 Rod-shaped structure
 Coiled up, compact forms of chromatin
 Contains one long DNA molecule bearing
hundreds or thousands of genes.
 DNA is attached to protein molecules called
histones
DNA wraps with protein like wrapping paper on a
present giving it the X-shape
Only found in eukaryotic cells (prokaryotes have
naked, circular shaped chromosomes)
2. Chromosome Structure
2. Chromosome Structure
 Sister chromatids
 Each duplicated chromosome contains two
identical copies.
 Centromere
 The point by which two chromatids are joined.
 Chromatin
 Diffuse mass of long, thin fibers, not seen under
the microscope, less tightly coiled
 Combination of DNA and protein
2. Chromosome Structure
3. Cell Division in
Prokaryotes
 Binary fission
 Process by prokaryotes reproduce by cell division.
 Steps:
 Duplication of chromosomes and separation of
copies.
 Cell elongates
 Divides into two daughter cells
3. Cell Division in
Prokaryotes
4. Cell Cycle
 In your own body, millions of cells must divide
every second to maintain the total number of
about 100 trillion cells.
 Some cells divide once a day, and some do
not at all (mature muscle cells, brain cells)
4. Cell Cycle
 Starts out with Interphase
 Occurs when the cell is between cell division
 Interphase stages:
 G1: Cells grow to mature size
 S: DNA is copied
 G2: Cell prepares for division
Cells exit the cell cycle via…
G0: Cells do not copy DNA or prepare for mitosis, but are still alive
(e.g. nervous system)
5. Mitosis
 The last stage of the cell cycle when the
nucleus of a cell divides to produce two new
daughter cells (after cytokinesis) each with
the same amount and type of chromosomes
as the parent cells.
 Mitosis is divided into four phases:




A.Prophase
B. Metaphase
C. Anaphase
D. Telophase
5. Mitosis
 A.Prophase:
 What does the cell look like?
 Centrioles and spindle fibers appear
 Nuclear envelope disappears, and chromosomes are
visible
 What happens to the DNA and nucleus?
 Chromosomes form when chromatin tightens and coils
 Nuclear membrane breaks down and disappears
 What two things appear near where the nucleus was?
 Centrioles and spindle fibers
5. Mitosis
 A. Prophase
5. Mitosis
 B. Metaphase
 What does the cell look like?
 Chromosomes move to the middle
 Where are the chromosomes during metaphase?
 Middle of the cell
5. Mitosis
B. Metaphase
5. Mitosis
 C. Anaphase:
 What does the cell look like?
 Chromosomes move to the end of cell
 What happens to the chromosomes?
 Chromosome splits at centromere into 2 chromatids
and moves to end of cell
5. Mitosis
 C. Anaphase
5. Mitosis
 D. Telophase
 What does the cell look like?
 Cell starts to pinch in
 Nucleus starts to reform
 Chromosomes are at opposite ends
 What happens to the chromosomes and nucleus?
 Nucleus forms back around single chromatids
5. Mitosis
 D. Telophase
6. Cytokinesis
 What is cytokinesis?
 Cytoplasm and contents (other organelles) divide
 What’s special about cytokinesis in plants?
 Cell wall also divides with new cell plate in middle
 What’s special about cytokinesis in animals?
 Takes place when the cell membrane pinches in
until the cytoplasm is pinched into two equal halfs
7. Control of Cell Division and
Cancer
 Cell division is a complex process that needs
to be regulated.
 These regulators determine when and how
the cell should divide.
 External Regulators
 Internal Regulators
7. Control of Cell Division
and Cancer
 External regulators:
 If the cell touches other cells, then cell division slows
down.
 If enough space between cells and nutrients are
available, cells divide or speed up their cell cycle.
7. Control of Cell Division
and Cancer
 Internal regulators:
 1.Cyclins –
 proteins that regulate the timing of the cell cycle in
eukaryotic cells.
 2.Check points
 Proteins that make sure that certain things happen in the
cell before the cell moves to the next phase of the cell
cycle
 3 major checkpoints in the cell cycle.
 3.The age of the cell.
7. Control of Cell Division
and Cancer
 Cancer cells
 lack normal checkpoints and continue to grow
without inhibition
 do not respond to normal signals
within the cell
 are not inhibited by other cells
 will divide indefinitely
7. Control of Cell Division
and Cancer
 Mutations in the genes of these checkpoint
proteins may lead to cancer:
 The uncontrolled growth of cells.
 Tumor: an abnormally growing mass of body
cells
 Benign tumor
 If abnormal cells remain at original site
 Can be problematic if disrupt certain organs, but
usually easily removed by surgery
 Malignant tumor
 If abnormal cells spread into other tissues and body
parts, interrupting organ function
7. Control of Cell Division
and Cancer
Tumor Progression
1. Tumor growth
2. Blood vessels
feed tumor
3. Tumor cells
enter blood and
lymph vessels
4. Secondary
tumors form in
other parts of
the body
Movie clips on cancer, its nature and experiments to treat it (Parts 2 and 6)
http://www.pbs.org/wgbh/nova/cancer/program.html
http://www.youtube.com/watch?v=HonoQ6mE6dY&feature=related
7. Control of Cell Division
and Cancer
 Treatment of Cancer:
 1.Surgical removal of tumor – Most effective when tumor is
in a defined area
 2.Chemotherapy – Medicines that disrupt the process of
mitosis in rapidly growing cells
 3.Radiation Therapy - High energy gamma radiation is
aimed at the growing tumour. This damages the DNA in
rapidly dividing cells and helps to destroy the tumor.
8. Meiosis
 Many of the stages of meiosis closely resemble
corresponding stages in mitosis.
 Type of cell division that produces haploid gametes in
diploid organisms.
8. Meiosis
8. Meiosis
 Like mitosis, is preceded by the replication of
chromosomes.
 However, this single replication is followed by two
consecutive cell divisions, called Meiosis I and
Meiosis II.
 These divisions result in four daughter cells, with
only half as many chromosomes as the parent cell
 Each new cell is known as haploid
8. Meiosis
 Starts with diploid cells with what are called
homologous chromosomes (or homologues)
because they both carry genes controlling the
same inherited characteristics
 These diploid cells divide to form 4 haploid
cells
8. Meiosis
 Any cell with two homologous (the same) sets of
chromosomes is called a diploid cell
 the total number of chromosomes is called the diploid
number (abbreviated 2n)
 For humans, the diploid number is 46; that is 2n=46
 Almost all human cells are diploid
8. Meiosis
 The exception are the egg and sperm cells,
collectively known as gametes.
 A cell with a single chromosome set is called a
haploid cell.
 For humans, the haploid number (abbreviated n) is
23; that is n=23
8. Meiosis
 Prophase I –
 Each chromosome pairs with its corresponding
homologous chromosome to form a tetrad. The
tetrads overlap and exchange some of their genetic
material – crossing-over.
8. Meiosis
 Crossing over in Prophase I results in great
diversity because new genetic variations can
result from it.
8. Meiosis
 Metaphase I –
 Spindle fibers attach to the chromosomes.
8. Meiosis
 Anaphase I –
 The fibers pull the homologous chromosomes
toward opposite ends of the cell.

The cells are now containing half of the genetic information from the original parent cell and are
thus considered HAPLOID!
8. Meiosis
 Telophase I and cytokinesis –
 Nuclear membranes reforms, the cell separates into
two cells.
8. Meiosis
 Prophase II –
 Meiosis I results in two haploid (N) daughter cells,
each with half the number of chromosomes as the
original cell.
8. Meiosis
 Metaphase II –
 The chromosomes line up in a similar way to the
metaphase stage of mitosis.
 Anaphase II –
 The sister chromatids separate and move toward
opposite ends of the cell.
8. Meiosis
 Telophase II and cytokinesis –
 Meiosis II results in four haploid (N) daughter cells.
http://www.sumanasinc.com/webcontent/animations/content/meiosis.html
http://www.pbs.org/wgbh/nova/baby/divi_flash.html
9.Why do cells need two types of cell
division?
 Mitosis
 Provides growth, tissue repair, and asexual reproduction
 Produces daughter cells genetically identical to the parent cell
 Involves one division of the nucleus, and is usually accompanied
by cytokinesis, producing two diploid daughter cells.
 Meiosis
 Need for sexual reproduction human egg and sperm cells
 Entails two nuclear and cytoplasmic divisions
 Yields four haploid daughter cells, with one member of each
homologous chromosome pair.
 Form tetrads; crossing over occurs.
10. Gamete Formation
 In females:
10. Gamete Formation
 In males:
11. Karyotype
 The term karyotype refers to the chromosome
complement of a cell or a whole organism.
 A karyotype is an ordered display of magnified
images of an individual’s chromosomes arranged
in pairs, starting with the longest.
 In particular, it shows the number, size, and shape
of the chromosomes as seen during metaphase of
mitosis.
 Chromosome numbers vary considerably among
organisms and may differ between closely related
species.
11. Karytype
 Karyotypes are prepared from the nuclei of
cultured white blood cells that are ‘frozen’ at the
metaphase stage of mitosis.
 Shows the chromosomes condensed and doubled
 A photograph of the chromosomes is then cut up
and the chromosomes are rearranged on a grid
so that the homologous pairs are placed
together.
 Homologous pairs are identified by their general
shape, length, and the pattern of banding
produced by a special staining technique.
11. Karyotype
 Male karyotype
 Has 44 autosomes, a single X chromosome, and a
Y chromosome (written as 44 + XY)
 Female karyotype
 Shows two X chromosomes (written as 44 + XX)
11. Karyotype- Normal
11. Karyotype- Abnormal
12a. Mutations- Chromosome
Number
 Nondisjunction
 Members of a chromosome fail to separate.
 Can lead to an abnormal chromosome number in
any sexually reproducing diploid organism.
 For example, if there is nondisjunction affecting
human chromosome 21 during meiosis I, half the
resulting gametes will carry an extra chromosome
21.
 Then, if one of these gametes unites with a normal
gamete, trisomy 21 (Down Syndrome) will result.
12a. Mutations- Chromosome
Number
12b. Mutations- Chromosome
Structure
 Abnormalities in chromosome structure:
 Breakage of a chromosome can lead to a variety
of rearrangements affecting the genes of that
chromosome:
 1. deletion: if a fragment of a chromosome is lost.
 Usually cause serious physical and mental problems.
 Deletion of chromosome 5 causes cri du chat
syndrome: child is mentally retarded, has a small head
with unusual facial features, and has a cry that sounds
like the mewing of a distressed cats. Usually die in
infancy or early childhood.
12b. Mutations- Chromosome
Structure
 2.duplication: if a fragment from one chromosome
joins to a sister chromatid or homologous
chromosome.
 3.inversion: if a fragment reattaches to the original
chromosome but in the reverse direction.
 Less likely than deletions or duplications to produce
harmful effects, because all genes are still present in
normal number
 4. translocation: moves a segment from one
chromosome to another nonhomologous
chromosome
 Crossing over between nonhomologous
chromosomes!
12b. Mutations- Chromosome
Structure