Lab: Cell Cycle, Mitosis, Meiosis
Laboratory Exercise Objectives
After completing this lab exercise, you should be able to:
Describe 3 stages of interphase in the cell cycle.
Describe 5 phases of mitosis and final products of mitosis and identify phases of mitosis in pictures and/or drawings.
Describe final products of mitosis and meiosis, respectively.
Describe difference between mitosis and meiosis:
Changes in the number of chromosomes.
Number of divisions involved.
In which type of cells, mitotic cell division and meiotic cell division occur, respectively.
Introduction To Cell Cycle
The vast majority of multi-cellular organisms begin their existence as a single cell, including humans. Therefore, all the cells in an individual are derived from this single cell through the process of cell division cycle. When a cell divides, two daughter cells, which are identical to each other and to the parent cell, are produced. A cell division cycle consists of interphase, mitotic phase and cytokinesis. The cell prepares itself for cell division during interphase before mitotic phase may occur. Mitotic phase is followed by cytokinesis where a daughter cell separates from its mother cell.
Interphase, which takes the longest in a cell cycle is divided into three phases: G1 (first gap), S (synthesis phase), and G2 (second gap). During all three phases, the cell grows by producing proteins and cytoplasmic organelles. However, chromosomes are replicated only during the S phase. Thus, a cell grows (G1), continues to grow as it duplicates its chromosomes (S), grows more and prepares for mitosis (G2), and finally it undergoes mitosis and cytokinesis before restarting the division cycle. During interphase, specifically in G2 phase, metabolic changes assemble the cytoplasmic materials necessary for mitosis and cytokinesis.
In this phase, nucleus is clearly visible under a light microscope, while chromosomes are invisible.
The genetic material in the nucleus, which is duplicated in the middle of interphase (S phase), exist as thin, diffuse fibers called chromatin. Chromatin is a complex consisting of both DNA molecules and specific proteins.
Mitotic Phases & Questions
Mitosis is a phase of the ekukaryotic cell division process whereby the chromosomes (the genetic material) are distributed equally to daughter cells. Chromosomes that have been replicated during the S phase undergo characteristic changes during mitosis in such a way as to ensure that each daughter cell receives a copy of every chromosome. In actively dividing animal cells, the whole process takes about one hour.
It is important to understand that, if the cell division takes place via mitosis, the resulting daughter cells both have the same number of chromosomes as the original cell (e.g., 46 chromosomes in human cells), thus transmitting genetic information from parent cell to daughter cells unchanged and undiminished. This type of cell division occurs in all body cells (somatic cells) in the eukaryotic organism, and allows multicellular organisms to grow and repair damaged tissue.
Although it is a continuous process, mitosis is divided into phases, or stages, based on the characteristic changes in the cell. These phases are prophase, prometaphase, metaphase, anaphase and telophase. The drawings below show chromosome movement and alignment during mitosis in a schematic cell that have 2 chromosomes. Each chromosome has 2 chromatids for a total of 4 chromatids. As you view the drawings, keep in mind that humans have a diploid number of 46.
In mitotic prophase, which is the first stage of mitosis:
At the onset of prophase, chromatin fibers begin to condense into loosely bundled coils and become discernible as individual structures, which are referred to chromatids.
Since the genetic material has already been duplicated earlier in S phase, there are two sister chromatids, bound together at a region termed the centromere.
During this phase, the nucleolus in the nucleus disappears from view and the nuclear membrane begins to break down.
Prometaphase is sometimes included as part of the end of prophase and early metaphase.
The paired chromatids continue to tightly coiled, condensing into discrete structures, which are typically visible at high magnification under a light microscope.
The nuclear membrane breaks down completely and disappears.
Attachment of spindle to the chromatid pair is completed.
• The chromatid pairs start migrating toward the equatorial plane (or, metaphase plate) in the mid-line of the cell.
In mitotic metaphase:
The chromatid pairs convene along the equatorial plane and become aligned.
Metaphase came from the Greek word meaning “after” as it occurs after the spindle is attached to chromatids.
In mitotic anaphase, which is the shortest stage of mitosis:
Sister chromatid of each pair of chromatids are pulled apart moving to the opposite ends of the cell.
The separated sister chromatids are now referred to as daughter chromosomes.
Anaphase came from the Greek word meaning “up,” “against,” “back,” or “re-”: chromosomes are reseparated and pulled back to the opposite ends of the cell.
The final stage of mitosis, telophase (from the Greek word meaning “end”) is marked by the presence of a complete set of chromosomes at each pole of the cell. During this phase, a reversal of prophase and prometaphase events are observed:
The spindle fibers disappear.
A new nuclear membrane forms around each set of separated daughter chromosomes grouped at either pole of the cell.
Both sets of chromosomes, now surrounded by new nuclear membrane, begin to uncoil back into fibrous chromatin, becoming invisible.
The nucleolus reappears, too.
At the end of telophase, mitosis is completed, but cell division is not.
Cytokinesis is not a phase of mitosis, but a separate process, necessary to complete cell division forming two new cells (daughter cells).
Cytokinesis begins at the same time as telophase.
In plants cell, a cell plate is formed, which becomes cell wall later.
In animal cells, a cleavage furrow (pinch) containing a contractile ring develops.
The cell divides into two daughter cells. Each daughter cell has a complete copy of the genome (total genetic material; i.e., a complete set of chromosomes) of its parent cell.
The end of cytokinesis marks the end of the mitotic cell division, and the daughter cells then enter interphase.
Q 1. How many times does the parent cell divide during mitotic cell division?
Q 2. How many daughter cells are produced at the end of mitotic cell division?
Q 3. An onion cell contains 8 pairs of chromosomes or 16 total number of chromosomes. How many chromosomes does each daughter cell contain after mitotic cell division?
Q 4. Identify each mitotic phase in the following diagram and label using the terms of mitotic phases and cytokinesis:.
A. _____________________; B-D. _____________________
E. _____________________; F. ________________________
G. _____________________; H-I. ______________________
Introduction To Meiosis & Questions
The chromosomes in normal somatic cells occur in homologous pairs, each containing homologues (homologous chromosome). Homologues are morphologically similar, and the loci (locations of genes on chromosomes) on one homologue are duplicated on the other homologue. The total number of chromosomes in a cell is referred to as the diploid number (diploid number of human body cell is 46). To maintain this characteristic number of chromosomes during reproduction to produce gametes, the homologues of a chromosomal pair must be separated to produce half the diploid number, the haploid number, in the gametes,
or sex cells. Thus, meiosis occurs in germ cells that give rise to gametes such as eggs and sperms.
Meiosis involves a mechanism similar to that involved in mitosis, but the outcome is quite different: mitosis results in equal division of a replicated set of chromosomes between two identical, diploid daughter cells, whereas meiosis results in the division of a set of replicated chromosomes among four haploid cells. As in mitosis, individual chromosomes are already duplicated before meiosis begins.
The unique feature of meiosis occurs during the prophase of the first meiotic division (Prophase I), the members of each pair of homologous chromosomes come together, or synapse, after condensation of the DNA-protein complex. This pairing of homologues does not occur in mitosis and is the major mechanistic difference between mitosis and meiosis. After this paring process (synapsis), in which two sister chromatids are closely associated with the two homologous sister chromatids, the homologues then repel one another.
During this paring-repelling process, there are joining between two non-sister chromatids (chromatids from different homologues) and crossing over or exchange of genetic material (recombination) between non-sister chromatids occurs where homologous chromosomes physically exchange pieces which produce new gene combinations in the chromosomes which end up in the gametes.
And then, much like metaphase of mitosis, the homologous chromosomes, each composed of two sister chromatids, move apart toward either end of the cell without the sister chromatids being separated.
It is when the sister chromatids are move apart in the Meiosis II. In Meiosis II, the sister chromatids are separated as in mitosis. Since there is no duplication of genetic material before Meiosis II, each of four resulting cells contains one chromatid.
In comparison to mitosis, meiosis results in the reproductive cells where the chromosome number is halved (haploid or 1N). This reduction in chromosome number allows an organism to maintain a constant chromosome number after sexual reproduction. During sexual reproduction each parent contributes a haploid set of chromosomes to the new individual which will then have the diploid number of chromosomes (2N).
Meiosis differs from mitosis in the following aspects:
Meiosis only takes place in reproductive cells (germ cells).
Meiosis reduces the number of chromosomes from the diploid number (2N) to the haploid number (1N).
During meiosis, two homologous pairs of chromosomes recombine to create new combination of genetic information (crossing over).
Meiosis involves two divisions producing a total of four daughter cells.
The process of second division in meiosis is the same as mitosis.
NOTE: Refer to the textbook for further details on meiosis.
Q 1. How many times does the parent cell divide during meiosis cell division?
Q 2. How many daughter cells are produced from one parent cell by the end of meiosis cell division?
Q 3. An onion cell contains 8 pairs of chromosomes or 16 total number of chromosomes. How many chromosomes does each daughter cell contain after cell division in meiosis?
A. Observation of Mitosis in Onion Root Tip in a plant and from cells in a whitefish
There are three cellular regions near the tip of an onion root:
The root cap contains cells that cover and protect the underlying growth region as the root pushed through the soil. Outer layers of the root cap is composed of mostly dead cells.
The region of cell division is where cells are actively dividing but not increasing significantly in size. The cell division is most active in the peripheral zone of the meristem.
In the region of cell elongation, cells are increasing in size, but not dividing. frequently. However, as a root also grow in diameter, there are columns of actively dividing cells along the region of cell elongation.
Chromosomes generally are not visible as distinct entities in non-dividing cells, since the DNA is in fibrous form (chromatin), but the process of mitosis is facilitated by supercoiling of the chromatin into a highly compacted form, chromosome. Supercoiled chromosomes can be visualized in cells, particularly if they are treated with a DNA-specific stain, such as one used to prepare the slide we use today. Once supercoiling occurs we can see the sister chromatids.
Mitosis in Onion Root Tip:
Good view of embryonic cell mitosis in onion roots & white fish at 1000X magnification using oil technique:
Draw the phases of mitosis below. Start with 3 pairs of chromosomes .
B. Modeling Meiosis
Show how of 3 pairs of chromosomes with a total of 6 chromosomes will form 4 sperm cells. Do this by verbally explaining or by making a drawing.