CHAPTER
9
MITOSIS
I. The Cell Cycle
When a cell is preparing for division, it grows larger, the number of organelles
doubles, and the DNA replicates.
A. Function of mitosis: Growth and repair
B. Interphase
1. Most of a cell’s life is spent in interphase, the G0 stage, in which the cell
performs its usual functions.
2. The G1 stage is just prior to DNA replication; a cell grows in size, organelles
increase in number.
3. The S stage is the DNA synthesis (replication) period. At the end of the S stage,
each chromosome has two identical DNA double helix molecules, called sister
chromatids.
4. The G2 stage occurs just prior to cell division. The cell synthesizes proteins such
as microtubules.
C. Stages of Mitosis
1. Prophase
a. Chromatin condenses and become chromosomes.
b. The nuclear envelope disappears.
2.
3.
4.
D.
E.
Metaphase
Chromosomes, attached to kinetochore fibers, are now aligned at the
middle of the cell.
Anaphase
The two sister chromatids separate.
Telophase
a. Spindle disappears in this stage.
b. The nuclear envelope reforms around the daughter chromosomes.
c. Cytokinesis in animal cells: A cleavage furrow indents the plasma
membrane
d. A new cell wall in plant cells:
Control of the Cell Cycle
1. The cell cycle is controlled by both internal and external signals that are
called cell cycle regulatory factors.
3. Growth factors are external signals received at the plasma membrane.
4. Cell Cycle Checkpoints
Apoptosis
1. Apoptosis is programmed cell death, which is done by activating a sequence
of cellular events.
2. Death by apoptosis prevents a tumor from developing.
II. Eukaryotic Chromosomes
A.
DNA in chromosomes of eukaryotic cells is associated with proteins called histones.
B. When a cell is not undergoing division, DNA is relaxed, called chromatin.
C. At cell division, chromatin becomes highly organized and condensed and is now visible
as individual chromosomes.
D. Each species has a characteristic number of chromosomes.
a. The diploid (2n) number includes two sets of chromosomes of each type.
The diploid number is found in all human somatic cells (46).
b. The haploid (n) number (23) is found in all human reproductive cells.
III. The Cell Cycle and Cancer
1. Cancer is a cellular growth disorder that results from the mutation of genes that
regulate the cell cycle; i.e., cancer results from the loss of control and a disruption
of the cell cycle.
2. Carcinogenesis, the development of cancer is gradual—it may take decades
before a cell has the characteristics of a cancer cell. And it is clonal – started
local.
3. Oncogenes = Cell cycle regulatory genes. The mutation of the oncogenes results in
cancer. Dominant oncogenes: Presence of the mutation enhances cell cycle;
Recessive oncogenes: Normally cell cycle inhibitors. Mutation removes the cell
cycle inhibition.
Critical Thinking
Question 1. Human red blood cells develop in the bone marrow from stem cells, and
lose their nucleus before being released into the bloodstream. While this gives a cell that
can be densely packed with hemoglobin molecules, what are the consequences as far as
the longevity of the cell and its ability to replicate?
Question 2. In some birds and true bugs, the number of chromosomes is hard to
determine since chromosomes get smaller and smaller until they are too small to see. Yet,
in animals it is rare to find chromosomes numbering over a hundred pairs. What is the
probable reason for keeping chromosome numbers low?
CHAPTER
10 MEIOSIS AND SEXUAL REPRODUCTION
I. Halving the Chromosome Number
1. Meiosis is reductive cell division, reducing the chromosome number from the diploid
(2n) to the haploid (n).
3. Purpose of meiosis: Gamete formation.
4. A zygote is from fusion of gametes and again has the full or diploid (2n) number of
chromosomes.
II. The Phases of Meiosis: Under microscope, it is the same as mitosis.
A. Prophase I: Important genetic event – meiotic recombination (sister chromatids
recombination; genetic recombination; the cross over)
B. Metaphase I: 2n ways to line up, providing 2n possible gametes.
C. Anaphase I
D. Telophase I
E. Meiosis II : Much like mitosis
III. Genetic Variation
A. Meiotic Recombination
B. Metaphase I
C. Fertilization
1. When gametes fuse at fertilization, chromosomes donated by parents combine.
2. The chromosomally different zygotes from same parents have (223)2 or
70,368,744,000,000 combinations possible without crossing-over.
3. If crossing-over occurs once, then (423)2 or 4,951,760,200,000,000,000,000,000,000
genetically different zygotes are possible for one couple.
Critical Thinking
Question 1. Meiosis, or duplication-division-division, is not the only way to reduce
chromosome numbers by half. It is theoretically possible to simply divide the original diploid
number of chromosomes to produce two haploid cells, and there is reportedly a primitive
organism that does this. What would be a drawback?
Question 2. Bees and ants have a haploid-diploid system for determining the sex of offspring.
The queen can withhold sperm in her seminal receptacle and the unfertilized egg develops into a
female. One species of ant has just two chromosomes in the diploid male and one in the haploid
female. What effect would such a low chromosome number have on the standard “advantages”
of sexual reproduction?
Review Qs:
1. Greek root words meaning "twofold" and "like": ________________.
2. In multicellular organisms, all cells in the body have similar / different amounts of DNA. The
DNA sequences in different cells are also similar / different.
3. Distinguish the following terms: Carcinogenesis / Apoptosis / Metastasis / Angiogenesis
4. The diploid (2n) number of chromosomes for humans is _____________. The haploid (n) number of
chromosomes for humans is _______________.
5. The daughter cells of mitosis have _____ n chromosome and have similar / different DNA
sequences from parent cells.
6. The functions of mitosis: ____________________________.
7. Sequence of the cell cycle: ________________________________________________________.
Sequence of the mitosis phases: ___________________________________________________.
8. Common features of cancer: ___________, _____________, _____________, _____________.
9. True or False: 1). Mutation always resulted in cancer. 2). Cancer is resulted from mutations
accumulated in cell cycle regulatory genes
10. Cancer metastasis mostly happens in these organs: __________, ____________, ___________.
It is because ____________________.
11. Apoptosis, the programmed cell death, is an active / passive process and is good / bad for the
cell.
12. Cloning and asexual reproduction are also through mitosis.
True / False
13. Biological basis of ensuring genetic diversity: ______________; ______________; __________.
14. Meiosis Phases: ________; ________; ________; ________; ________; ________;
________; ________. The cross over occurs at _________ phase. ________ resembles mitosis.
15. Terms: Gemetes, zygotes, chromatin, chromosome, chromatid, sister chromatids, homologous
chromosomes.