UNIT 4: Cell Cycle, Mitosis, Meiosis
What you should know:
All living things are made up of cells, and depending
on how complex the organism is, it may have one cell, like a bacterium,
or trillions of cells, like humans. The question is, how does a fertilized
human embryo with one cell develop into an adult with 5 trillion cells?
The answer is in a process referred to as cell division. A single fertilized
human egg cell will divide to produce two cells. The two cells have
the same genetic information as each other and the parent cell. The
cells will continue to divide until trillions of cells are produced.
This is how a fertilized egg grows into a baby, and then an adult. This
division of cells is referred to as mitosis.
During the cell cycle, a cell grows, prepares for division, and divides
to create two daughter cells. The three main stages of the cell cycle
are interphase, mitosis, and cytokinesis. Cells spend most of their
lives, about 78 percent of the time, not dividing in a phase called
interphase. During interphase the cells get larger, replicate their
chromosomes for cell division, synthesize substances that are essential
to cell division, and build materials for growth and development. Cells
in interphase have an easily seen nucleus and nucleolus, and the chromosomes
appear as short strands within the nucleus.
During interphase the chromosomes, which are composed of DNA, are replicated
before the cell divides so an exact copy of every chromosome is ready
for each daughter cell. During mitosis the chromosomes in a cell are
split apart when the cell divides. Chromosomes contain the genetic information
that is passed from one generation to the next. The genetic information
is DNA which is like the blueprints for a cell. The DNA is identical
in all cells of the body, but it is expressed differently in different
types of cells. In hair cells the DNA codes for the color of the hair.
In eye cells it codes for the color of your eyes. DNA also instructs
each well when to divide, and how to function.
Mitosis occurs in four phases. The first phase is prophase.
Cells in prophase have an enlarged nucleus, the nucleolus is no longer
visible, and the chromosomes appear as short jumbled strands within
the nucleus. Cells spend about 14 percent of the cell cycle in prophase.
This is the next largest amount of time spent in a phase after interphase.
This phase takes longer than the others because the nuclear envelope
fragments and the microtubules have to attach to the chromosomes. Metaphase
follows prophase. Cells in metaphase have the chromosomes, which appear
as long thin strands under the microscope, lined up along the center
of the cell. Metaphase takes about 4 percent of the time required for
the completion of a cell cycle. This is one of the quickest phases because
all that occurs is the lining up of the chromosomes at the center of
the cell. The third phase is anaphase. This phase only takes about 0.8
percent of the cell cycle to complete, which is the shortest time required
out of all the phases. During anaphase the sister chromatids, or the
two identical parts of a chromosome, are pulled apart to opposite ends
of the cell by the spindle fibers. This allows each daughter cell to
have an identical copy of each of the original cell’s chromosomes.
The last stage of mitosis is telophase. Telophase takes up about 3 percent
of the cell cycle. This is also a short phase. During telophase the
chromosomes appear at the opposite ends of the cell and a new nuclear
membrane begins to form around the chromosomes in each half of the cell.
Cytokinesis occurs at the end of mitosis as the cytoplasm divides and
two distinct cells are formed. In plant cells, a cell plate forms halfway
between the divided nuclei, and then a cell wall appears in the cell
plate. In animal cells, the cell membrane moves inward until the cytoplasm
in pinched in half. Each half has its own nucleus and cytoplasmic organelles.
Meiosis reduces the chromosomal number by half to form four haploid gametes, or reproductive cells, that are genetically different. When gametes combine during fertilization the cell becomes diploid. During the first half of meiosis, homologous chromosomes separate. During prophase I crossing-over occurs. This results in the exchange of genetic material between homologous chromosomes. During meiosis I the number of chromosomes is split in half and the resulting cells are haploid. During meiosis II the two chromatids of each chromosomes split and the resulting cells are haploid. At the end of meiosis there are four haploid cells. Meiosis increases genetic variation in organisms through independent assortment, crossing-over, and random fertilization.
How you make sense of things:
Observations or experiences (examples,
Patterns (laws, generalizations,
graphs, tables, categories)
Explanations (models, theories)
Initial Student EPE
A baby grows into an adult
When I cut my hand it heals
Living things grow and can repair wounds
Lizards shed their skin
Onion root tip cells observed through a microscope
look different from one another.
Observation that cancer can kill an organism
One cell can split into two cells.
New cells are formed when old ones die.
Many onion root tip cells looked similar to the others.
Only a few looked differently.
It is hard to stop cancer from spreading.
Mitosis - Cells divide to produce new cells
Microscopic observation that all onion root cells
have a nucleus filled with DNA.
Microscopic observation that many onion root tip cells
have a solid nucleus with loose chromatin inside
Microscopic observation that only a few onion root
tip cells did not have a distinct, intact nucleus.
Microscopic observation that many cells have condensed
chromosomes in an enlarged nucleus which is fragmenting at the nuclear
Microscopic observation that very few cells appear
to have chromosomes at opposite ends of the cell.
Microscopic observation that very few cells have chromosomes
lined up in the middle.
Microscopic observation that few cells appear to be
splitting into two new cells.
Data table showing number of minutes it takes normal
chicken stomach cells to complete each phase of mitosis, and number
of minutes it takes cancerous chicken cells to complete each phase
Each cell has two copies of each chromosome before it divides.
Many cells in one stage of mitosis means that that
stage takes a long time to complete.
During interphase the nucleus of the cell is intact.
This phase takes about 80 percent of the time required to complete
the cell cycle.
During cell division, the nucleus disappears after
prophase. Cell division does not take a long time.Prophase is the
longest phase of mitosis, but it occurs faster than interphase.
Anaphase is the shortest phase of mitosis. In anaphase,
the sister chromatids are pulled apart to opposite ends of the cell.
In metaphase, chromosomes line up along the metaphase
plate.Metaphase takes a short amount of time to complete.
Telophase is the next to the shortest phase of mitosis
after anaphase. In telophase, the chromosomes appear at opposite
ends of the cell, and the middle of the cell starts to pinch inward.
Cytokinesis is the splitting of the cytoplasm after
Cancer cells go through each phase of mitosis faster
than normal cells, except for anaphase.
Each cell needs instructions from DNA in order to
grow and divide.
DNA must replicate before it divides to conserve
the number of chromosomes in each cell. The two daughter cells
produced by mitosis have identical DNA to their parents and to
Interphase is the time between cell divisions, and
this phase takes a long time because the cell has to grow, replicate
it’s DNA and prepare for cell division. (The phase of interphase
is a pattern, but I put this information in this column because
this is the explanation of why interphase takes the longest amount
of time to complete.)
Mitosis is the process of cell division. It consists
of four stages: prophase, metaphase, anaphase, telophase. The
four stages of mitosis occur quickly, except for prophase.
Cancer cells have an uncontrolled rate of cell growth
and continue to divide when normal cells would have stopped.
Meiosis is similar to mitosis but it divides the chromosomes twice and results in the formation of four haploid cells that are genetically different.
What you should be able to do:
1. Explain how multicellular organisms grow based on how cells grow
2 . Students will use their knowledge of cell division to explain the
difference between normal
cells and cancer cells by comparing data tables on normal and cancerous
3 . Students will use their knowledge of cell division to explain the
process of mitosis and how a
single celled embryo develops into a multicellular adult.
4. Students will compare mitosis and meiosis.