Students should be able to:
All cells go through a series of events that are collectively termed the Cell Cycle. The cell cycle is divided into stages for the convenience of study. Before the concept of the cell cycle biologists were of the impression that there were no events that were part of cell division and unless the cell was dividing, mitosis, then cells were considered to be in Interphase. As investigations of the cell and its functions progressed it was found that the nuclear material, DNA was replicated and chromosomes were duplicated. Other cell division preparatory process were also found. The cell organelles are duplicated too. Biologists decided they needed a better way to describe the events of cell division and the concept of the cell cycle was used.
The cell cycle consists of four stages. They are the G1, S, G2 and mitosis. Check your textbook or other resources (pre-lab) for an explanation of the cell cycle.
The preparation for the mitotic part of the cell cycle occurs during the S stage and G2 stages. During the S stage the DNA is replicated and the chromosomes are duplicated. During the G2 stage the organelles are duplicated and the cell readies itself for division.
The chromosome is formed from chromatin, which is uncondensed DNA and proteins. Each cell contains a set of chromosomes (two chromosomes of each kind) and is said to be diploid or 2N. Half of this set, or one of each kind of chromosome is haploid or N. Cells can be either haploid or diploid. Chromosome formation is a complexed and involves the formation of nucleosomes which are produced when the DNA is wound onto histones. The nucleosomes coil and loop forming the condensed chromosomes seen during cell division.
Mitosis consists of two parts. The division of the nuclear material or karyokinesis and the division of the cytoplasmic material or cytokinesis. Karyokinesis consists of the following phases:
Cytokinesis occurs during telophase and consists of the formation of a cleavage furrow (animal cells) or a cell plate (plant cells). The formation of the furrow or plate divides the cell into two daughter cells.
You must remember that biologists determined the phases for convenience of study. the process is continuous and does not stop at the various phases. When certain events in the phases begin or end, they determine the phase. The following is an illustration of the events that occur in each phase. The first is prophase. Prophase requires the most time of any of the phases. It is complicated and many events characterize it. Prophase can be subdivided into early and late phases.
In early prophase the chromosomes begin condensing from the chromatin, the spindle formation begins as the centromeres, which contains two centrioles, move to the poles of the cell (see late prophase photo also). The nuclear membrane starts to fragment. The photograph below illustrates some of this.
Late prophase is characterized by the disintegration of the nuclear membrane, the complete condensation of the chromosomes, and the disappearance of the nucleolus. The chromosomes are scattered randomly at this point. Each chromosome now consists of two sister chromatids held together at a centromere. See photograph below.
In metaphase the sister chromatids are attached to the spindle and the chromosomes are lined up at the equatorial plate.
Anaphase begins when the sister chromatids of the chromosomes separate at the centromere, thus forming two daughter chromosomes. Each daughter chromosomes consists of a centromere and one chromatid. The chromosome number at the centrosome (pole) is dipoid or 2N.
Telophase begins when a new nuclear membrane begins to form around the daughter chromosomes. The chromosomes begin to reverse the process of condensation and become the chromatin. The nucleolus reforms. A cleavage furrow begins "cutting" the cell into the two daughter cells via a constriction termed a contractile ring.
Plant mitosis has the same phases as animal mitosis except their spindle is void of the asters and centrioles. Cytokinesis is by the formation of a cell plate (plasma membrane), which "divides" the cell into two daughter cells by expanding outward until it reaches the cell membrane. A new cell wall then forms on either side of the cell plate.
Meiosis is reduction division. In this form of cell division the chromosome number is reduced by one half, diploid to haploid (2N to N, so the new daughter cells have the haploid (N) number of chromosomes instead of the diploid (2N) number as seen in mitosis. This form of cell division is seen in all sexually reproducing organisms because the fertilization of an egg by a sperm must result in a diploid (2N) individual. The only way this is accomplished is by having the sperm and egg by haploid (N). The production of sperm and egg is accomplished by the process of meiosis. If meiosis did not occur then the chromosome number would double each generation, and the cell could not survive with this increasingly larger number of chromosomes.
In meiosis the overall process is duplicate (S stage of cell cycle) divide (meiotic division I) and divide (meiotic division II). In mitosis the overall process is duplicate, divide, duplicate, divide, ad infinitum.
Meiosis: ----> = division
2N cell duplicates to form 4N cell ----> two 2N daughter cells ----> four 1N daughter cells which ends the process
Mitosis: ----> = division
2N cell duplicates to form 4N cell ----> two 2N daughter cells and mitosis starts again
In meiosis the chromosomes form homologues (consisting of two sister chromatids) during prophase I. Each homologue pairs with its other homologue to form a homologous pair. These homologous pairs synapse or line up side by side, however, they do not join. Following synapsis the chromosomes line up at the equatorial plate and this starts metaphase I. The homologous pair separate during anaphase I, migrating toward the poles. Each daughter nucleus contains a homologue, which is one of the homologous pair, which consists of two sister chromatids, and is thus 2N. Crossing-over also occurs during meiosis I. Crossing over is an exchange of genetic material by non-sister chromatids of a homologous pair of chromosomes. This exchange of genetic material is the basis for additional variation in the offspring (another being the recombination of genetic material from two different genetic sources, the parents). This can lead to better adaptations and hence is a part of the process of evolution.
These daughter cells now undergo a second meiotic division without duplication of the DNA. In prophase II and metaphase II the events are the same as prophase I and metaphase I, except the homologue sister chromatids separate during anaphase II becoming daughter chromosomes. Telophase II is the same as telophase I and the sister chromosomes are separated, thus forming four daughter cells, with the haploid number of chromosomes (N). A review of the material in Chapter 10 (Meiosis and Sexual Reproduction) of your text and the completion of the Pre-Lab Activities will clarify the events of meiosis.
Pre-Lab Activity #1 - Mitosis
Pre-Lab Activity #2 - Meiosis
Viewing the following materials will help you in understanding meiosis and how it compares to mitosis.
Mitosis / Meiosis Comparison:
Pre-Lab Activity #3 - Gametogenesis
Diagram: Cross section of an ovary
Human Ovary, XS
Resolution: 1207x1064x2 48,319 bytes
Mammalian Testis XS FeH x100slide 19-372 B5 Meiosis many stages
of meiosis are visible in this view G1.8
Pre-Lab Activity #4 - Other Helpful Materials for Studying Cell Division
Below are several sites that will help you understand mitosis. They have animation and / or movies. It is highly suggested that you "visit" these sites and go through the tutorials and / or view the materials.
Pre-Lab Activity #5 - Data Collection Sheet
Download the Data Collection Sheet and bring it to the laboratory session.
The Laboratory Activities and Data Collection
Lab Activity #1 - Identification of the Mitotic Phases of Plant Cells (Allium)
Lab Activity #2 - Identification of the Mitotic Phases of Animal Cells (Whitefish blastula)
Lab Activity #3 - Mitosis and Meiosis Models
Lab Activity #4 - Gametogenesis (Spermatogensis and Oogenesis)
Post-lab Activity and Data Analysis
Results and Analysis: