Download The Cell Cycle

BIOLOGY 3201
Unit 2: Reproduction and Development
NOTES
PAGE : 1
CELL CYCLE/MITOSIS/MEIOSIS
The Cell Cycle
Cell Cycle:
A continuous series of cell growth and division for a cell. All cells go
through a cell cycle of some sort.
The cell cycle consists of two stages.
a.
b.
A.
Growth Phase
Division Phase
Diagram of the Cell Cycle
Growth Phase (longest part of cell cycle)
ƒ Also known as Interphase.
ƒ Consists of three segments.
o G1 or Gap 1 phase – Cell undergoes rapid growth and normal
metabolic activities. (A Cell spends most of its time in this phase).
o S Phase – The cell replicates(duplicates) the DNA (Chromosomes)
o G2 or Gap 2 Phase – The centrioles replicate in preparation for cell
division.
B.
Division Phase (Shortest part of the cell cycle)
ƒ This is the stage where the cell divides into two new cells.
ƒ Consists of two Stages.
i. Mitosis – The process of dividing the nucleus.
ii. Cytokinesis – The process of dividing the cytoplasm.
BIOLOGY 3201
NOTES
PAGE : 2
Unit 2: Reproduction and Development
CELL CYCLE/MITOSIS/MEIOSIS
How long is the cell cycle?
The cell cycle varies in length depending on the type of cell and its environment. Some
cell cycles last for a few hours whereas others last for a number of years.
Ex:
Cells in stomach lining Æ cell lasts for about 2 days before dividing.
Brain cells Æ lasts for up to 50 years.
BIOLOGY 3201
Unit 2: Reproduction and Development
NOTES
PAGE : 3
CELL CYCLE/MITOSIS/MEIOSIS
Mitosis
Mitosis:
The series of stages a cell goes through in order to divide the nucleus
(DNA material).
Purpose of Mitosis:
1.
To ensure genetic continuity by creating a copy of the DNA. I.e. make sure
the DNA is carried on by maintaining the same number of chromosomes
from one cell to the next.
2.
To create new cells for growth and repair
Where does mitosis occur?
ƒ
Mitosis occurs within the body (somatic) cells of an organism.
Ex: skin, muscle, bone, etc.
All cells EXCEPT the Gonads (Ovary and testes)
Results of Mitosis when finished
ƒ
ƒ
Two New cells called Daughter cells.
Each Cell has the same number of chromosomes as the parent cell.
Chromosomes
ƒ
ƒ
ƒ
ƒ
Strands of DNA that carry out the genetic code.
The genetic code determines your traits such as hair colour, height etc.
Chromosomes are found within the nucleus of a cell.
Chromosomes are normally Single Stranded (SS) but become Double Stranded
(DS) during interphase when they are replicated.
BIOLOGY 3201
Unit 2: Reproduction and Development
NOTES
PAGE : 4
CELL CYCLE/MITOSIS/MEIOSIS
The Stages of Mitosis
There are four stages to Mitosis. In order, they are;
A.
B.
C.
D.
Prophase
Metaphase
Anaphase
Telophase
Prophase
The following happens during this stage.
<
<
<
<
The Chromatin (Fine threads of DNA in the nucleus) condenses to reveal Double
Stranded (DS) Chromosomes.
The Centrioles begin to move to the opposite ends of the cell.
The nuclear membrane breaks down and disappears.
The centrioles grow filaments called spindles and are now called asters.
Spindles
Disappearing Nuclear
Envelope
Centriole
DS Chromosomes
BIOLOGY 3201
Unit 2: Reproduction and Development
NOTES
PAGE : 5
CELL CYCLE/MITOSIS/MEIOSIS
Metaphase
During Metaphase, the following events happen.
<
<
The Chromosomes line up at the middle of the cell.
The spindles attach to the chromosomes at the Centromeres.
Centromere:
The middle portion of a chromosome that holds the
chromatids together.
Centromere
Spindles
BIOLOGY 3201
Unit 2: Reproduction and Development
NOTES
PAGE : 6
CELL CYCLE/MITOSIS/MEIOSIS
Anaphase
During Anaphase, the following events happen.
<
<
The Spindles pull the chromosomes apart and to the opposite ends of the cell.
There are now Single Stranded (SS) chromosomes at each end of the cell.
Single Stranded
Chromosomes
BIOLOGY 3201
Unit 2: Reproduction and Development
NOTES
PAGE : 7
CELL CYCLE/MITOSIS/MEIOSIS
Telophase
During Telophase, the following events happen.
<
<
<
<
The chromosomes uncoil and become chromatin again.
The nuclear membrane begins to form around each set of chromosomes.
The Spindle and Aster disappear
Cytokinesis begins.
Cytokinesis:
The process of dividing the Cytoplasm of a cell to create two new
cells.
The cytoplasm is divided equally among the two new daughter
cells.
This occurs when the cell membrane in animal cells and the cell
plate (plant cells) pinches together in the middle and eventually
closes off from each other.
Nuclear membrane
reappearing
Chromosomes
uncoiling
Cytokinesis occurring
BIOLOGY 3201
Unit 2: Reproduction and Development
NOTES
PAGE : 8
CELL CYCLE/MITOSIS/MEIOSIS
Definitions:
1. Chromatid:
In a DS chromosome, an exact copy of DNA.
2. Nuclear Membrane:
The membrane that surrounds the nucleus of a cell. Keeps
its contents together.
3. Spindle:
Long fibre created from the aster that attaches to the centromere of
chromosomes. Pulls chromosomes apart.
4. Aster:
The star-like structure at each end of a dividing cell that helps pull
chromosomes apart.
5. Cell Plate:
A plate of tissue that grows between dividing plant cells during
telophase. The cell plate separates the two new plant cells.
6. Cytoplasm:
The fluid of the cell. Important in giving structure to the cell.
7. Chromosome: A long strand of DNA (Deoxyribonucleic Acid), a protein, which
carries genes to express traits.
8. Gene:
A small segment of DNA on a chromosome that is responsible for
the expression of traits.
9. Centrioles:
Cylindrical shaped bodies near the nucleus of a cell that becomes
the aster during cell division.
10. Chromatin:
Fine strands of DNA within the nucleus of a cell that makes up the
chromosomes.
BIOLOGY 3201
Unit 2: Reproduction and Development
NOTES
PAGE : 9
CELL CYCLE/MITOSIS/MEIOSIS
Cell Division and Cancer
Mutation:
A permanent change in the DNA molecule resulting in a change to genes.
Mutations are normally caused by either of the following:
Causes of Mutations:
i)Chemicals
ii)Radiation
iii)Viruses
Implications of Mutations
If a mutation occurs within a gene responsible for cell division within somatic cells, cancer may
result.
CANCER:
The uncontrolled, rapid growth and division of cells.
How does cancer operate?
Within cells are certain genes that control the rate at which mitosis occurs. These genes are like
switches. When a cell finishes mitosis certain genes are “turned on” that produces proteins that
stop mitosis. When the genes are “off” mitosis can continue and the cell can divide to produce
new cells.
Mutations can permanently inactivate the “off” switch or gene resulting in no stop to mitosis.
The result is uncontrollable growth and division of cells.
Other genes start mitosis and if these are “switched on” permanently, then cancer or
uncontrollable cell division results.
Oncogenes:
These are genes that when mutated can cause cancer in a cell.
BIOLOGY 3201
Unit 2: Reproduction and Development
NOTES
PAGE : 10
CELL CYCLE/MITOSIS/MEIOSIS
TREATMENTS FOR CANCER
Cancer cells divide much more quickly than do normal cells so anything that interferes with cell
division will harm these cells more readily than normal cells. Cancer treatments are based on this
fact.
1.
Radiation Therapy
Radiations such as X-rays or gamma rays are directed at the affected area of the
body.
Radiation works by damaging the chromosomes within a cell making it unable to
grow or divide.
Radiation affects normal cells too, but they are usually able to repair themselves.
Radiation is often directed at a very specific target to minimize damage in other
areas.
Radiation therapy is often used to treat:
Skin, Breast, larynx and cervical tumors.
Side affects of Radiation Therapy:
i)
ii)
iii)
iv)
2.
Fatigue
Skin inflammation
Hair loss (if treatment on the brain)
Sterility in males (if treatment on testicles)
Chemotherapy
The use of drugs to treat cancer.
Sometimes used in conjuction with radiation therapy.
Used to treat cancers found throughout the body.
Ex: Leukemia
Chemotherapy durgs work by attacking cells that are dividing as well as
preventing others from dividing.
Chemotherapy affects both cancer and normal cells.
BIOLOGY 3201
NOTES
PAGE : 11
Side effects of Chemotherapy
1.
2.
3.
Hair loss
Nausea
diarrhea
Unit 2: Reproduction and Development
CELL CYCLE/MITOSIS/MEIOSIS
BIOLOGY 3201
Unit 2: Reproduction and Development
NOTES
PAGE : 12
CELL CYCLE/MITOSIS/MEIOSIS
Meiosis
Meiosis:
A type of cell division that results in gametes (sperm and egg) being created with
half the number of chromosomes (haploid – n) as the parent cell.
Purpose of Meiosis:
1.
2.
Genetic continuity
Genetic variability
Where does meiosis occur: Meiosis occurs in the gonads (Testes and Ovaries).
There are two types of Meiosis.
1.
2.
Ôogenesis:
Spermatogenesis
(See below for explanation)
(See below for explanation)
The Stages of Meiosis
NOTE:
Meiosis occurs in two stages. Stage 1 is called Meiosis 1 and Stage 2 is called
Meiosis 2.
MEIOSIS 1 Æ Has 4 stages
a) Prophase 1
b) Metaphase 1
c) Anaphase 1
d) Telophase 1
Meiosis 2 Æ Has 4 stages
a) Prophase 2
b) Metaphase 2
c) Anaphase 2
d) Telophase 2
BIOLOGY 3201
Unit 2: Reproduction and Development
NOTES
PAGE : 13
CELL CYCLE/MITOSIS/MEIOSIS
Stages of Meiosis 1
Meiosis 1 is also known as Reduction Division
Reduction Division:
The process of reducing the number of chromosomes in a cell by
half (1/2). In other words, going from diploid (2n) to haploid (n)
Ex: 46 chromosomes Æ 23 chromosomes
Prophase 1
ƒ
ƒ
ƒ
During Prophase 1 of meiosis, homologous chromosomes (similar
chromosomes) line up next to each other as pairs.
Pairs of homologous chromosomes are called TETRADS.
Crossing over of non-sister chromatids occurs.
Crossing Over:
The overlapping of non-sister chromatids resulting in an exchange
of genes.
Homologous Chromosomes
Tetrad (pair of homologous chromosomes)
BIOLOGY 3201
NOTES
PAGE : 14
Unit 2: Reproduction and Development
CELL CYCLE/MITOSIS/MEIOSIS
Crossing over explained
Metaphase 1
ƒ
ƒ
During Metaphase 1 Homologous chromosomes line up at the middle of the cell.
The spindle from one end of the cell attaches to one pair of sister chromatids while a
spindle from the other end attaches to the other pair of sister chromatids.
BIOLOGY 3201
NOTES
PAGE : 15
Unit 2: Reproduction and Development
CELL CYCLE/MITOSIS/MEIOSIS
Anaphase 1
ƒ
During Anaphase 1, the spindle pulls homologous chromosomes to separate ends of the
cell.
Telophase 1
During Telophase 1, the following events happen.
ƒ
ƒ
ƒ
ƒ
Nuclear membrane reappears around each set of separated chromosomes.
Spindle disappears
Cytokinesis (division of the cytoplasm) occurs.
Two new cells are created with ½ (haploid) the chromosome number of the original cell.
o i.e 46 DS chromosomes Æ 23 DS chromosomes per cell.
Or 96 chromatids Æ 46 chromatids per cell.
BIOLOGY 3201
Unit 2: Reproduction and Development
NOTES
PAGE : 16
CELL CYCLE/MITOSIS/MEIOSIS
Stages of Meiosis II
NOTE:
The stages of Meiosis 2 are identical to the stages of Mitosis.
Prophase 2
During prophase 2 the following events occur.
ƒ
ƒ
ƒ
Nuclear membrane disappears.
Spindle forms.
Chromsomes become visible.
Metaphase 2
During Metaphase 2 the following events occur.
ƒ
ƒ
Chromosomes line up at the middle of the cell.
Spindle attaches to centromere of each chromosome.
Anaphase 2
During Anaphase 2 the following events occur.
ƒ
Spindle pulls one chromatid from each chromosome to opposite ends of cell.
Telophase 2
During Telophase 2 the following events occur.
ƒ
ƒ
ƒ
ƒ
Nuclear membrane reappears around each group of chromatids (now called SS
chromosomes)
Spindle disappears.
Cytokinesis occurs.
Two new cells are created, each having the haploid number of chromosomes.
BIOLOGY 3201
Unit 2: Reproduction and Development
NOTES
PAGE : 17
CELL CYCLE/MITOSIS/MEIOSIS
Ôogenesis
This is meiosis that occurs in the ovaries of females.
Result:
The creation of an egg with ½ the normal number of chromosomes.
Explanation of Oogenesis
Within the ovaries of females, a diploid (2n) cell called a Oogonium undergoes meiosis.
Before Meiosis begins, the oogonium replicates (doubles) its chromosomes so that it has 46 DS
chromosomes or 92 sister Chromatids.
During the first Meiotic division, the Homologous (similar) chromosomes are separated. During
this stage cytokinesis occurs unevenly and as a result one large and one small cell are created.
The large cell is called a Primary Oocyte and the small cell is called a Polar Body.
Each of these cells will have 23 DS chromosomes or 46 SS chromosomes (chromatids).
During the second meiotic division, the cells divide again creating 4 cells, but this time the new
cells created each have 23 SS chromosomes. Again, cytokinesis has resulted in an unequal
distribution of cytoplasm.
NOTE:
The three polar bodies will die because they do not have enough cytoplasm to
keep them running.
BIOLOGY 3201
Unit 2: Reproduction and Development
NOTES
PAGE : 18
CELL CYCLE/MITOSIS/MEIOSIS
Spermatogenesis
This is meiosis that occurs in the testes of males.
Result:
The creation of FOUR Sperm, each with ½ (haploid) the number of chromosomes
as the original cell.
Explanation of Spermatogenesis
Within the testicles of a male, a spermatogonium replicates its chromosomes to contain 46 DS
chromosomes or 92 chromatids.
During the first meiotic division, the cells divide and the homologous chromosomes separate.
This results in 2 new cells, each with 23 DS chromosomes or 46 chromatids each.
During the second division, two new cells are created, each with 23 SS chromosomes.
These cells, called primary spermatids, will mature into sperm.
NOTE:
Each spermatid matures into sperm in the Epididymis of the testicles.
BIOLOGY 3201
Unit 2: Reproduction and Development
NOTES
PAGE : 19
CELL CYCLE/MITOSIS/MEIOSIS
Meiosis Definitions
1.
Diploid (2n):
each of its cells.
This is the normal number of chromosomes an organism has in
Ex:
2.
Haploid/Monoploid (n):
has in each cell.
humans - Diploid(2n) = 46
This is ½ the normal number of chromosomes an organism
Ex:
humans - monoploid (n) = 23
3.
Tetrad:
A term meaning a group of 2 homologous DS chromosomes.
A term meaning a group of 4 chromatids
4.
Synapsis:
The pairing of the homologous chromosomes.
5.
Crossing Over:
The exchange of genes between non-sister chromatids in
homologous pairs of chromosomes.
6.
Nondisjunction:
meiosis.
7.
Reduction Division: The stage where the number of chromosomes in each cell is
reduced by ½.
The failure of homologous chromosomes to separate during
BIOLOGY 3201
Unit 2: Reproduction and Development
NOTES
PAGE : 20
CELL CYCLE/MITOSIS/MEIOSIS
End Result of Meiosis
The end result of Meiosis is as follows:
ƒ
ƒ
ƒ
4 new cells created.
Each cell has ½ or Haploid (n) the number of chromosomes as the parent/original cell.
Crossing over has resulted in a new combination of genes on chromosomes leading to
variety in organisms.
MITOSIS/MEIOSIS COMPARED
Event
Mitosis
Meiosis
Somatic Cells
Gonads
Number of Cells Created
per complete cycle
2
4
Number of Divisions
1
2
Same as parent cell
½ of parent cell
Does Crossing over
happen?
No
Yes, possibly
Do Homologous
Chromosomes line up?
No
Yes
Genetic Continuity, growth
Genetic variability
Where does it occur?
Number of Chromosomes
per new cell created
Purpose