Mitosis & Meiosis Lab

3A:

Purpose: The purpose of this lab was to observe the different stages of mitosis and approximate how long the cell spends in each stage.

Introduction: Mitosis is a process that enables the replication of somatic, or body, cells. When mitosis occurs, 2 identical cells are produced.Each of these cells have 46 chromosomes. In order for mitosis to occur, it must go through a series of steps: prophase, metaphase, anaphase, and telophase. Interphase is a step before all of these and it contains a g1 step, where the cell spends most of its life growing, the S phase where replication of DNA occurs and the g2 phase where the cell prepares for prophase. Interphase also contains a G0 phase where a cell makes the decision to not undergo mitosis. During prophase, the chromatin within the cells begin to form chromosomes. In metaphase, the chromosomes formed line up in a single file line down the center of the cell. Anaphase is when the chromosomes split at the centromere, and walk along the spindle fibers to opposite sides of the cell. Finally, during telophase the cell pinches off to create two, new, identical cells.

Method: In order to see the amount of time that the cell spends in each phase, we looked at an onion root tip through a microscope.

Under the microscope we were able to clearly see the individual cells separated by the cell plate. By looking carefully at each cell, we had to determine which stage it was in by looking at where the chromosomes were located.

Discussion
From the graph above, we can see that the cell spent the longest time in interphase. The cell spent seventy three percent of the time in interphase. This is because all cells start off in interphase and some may continue on through the first checkpoint in the cell cycle and decide to divide. The cell also has to grow and get ready to did divide in interphase Other cells may not go on past the first checkpoint. The least amount of time a cell spent was in  anaphase and telophase. This may be due to the last checkpoint which occurs in metaphase. If the cell does not pass this checkpoint it will not go onto to anaphase and telophase. Usually the cell spends around ninety percent of the time in interphase. We may have had some error in the not counting all of the cells or mistaking a cell in interphase as one in prophase. The cell spent about 11% of its time in prophase, in which parent cell chromosomes condense and compact. Our data is not entirely accurate since a cell spends majority of its time in interphase. This error could have been because we didn’t count all the cells or because we mistakenly placed some in a different category. From the graph above, we can also see that telophase takes the least amount of time because it is the last phase of mitosis. During telophase, the chromosomes simply assemble at opposite ends of the cell and cytokinesis, or division of the cytoplasm, usually occurs and two identical cells are created.

Conclusion:

In conclusion, from this portion of the lab we were able to see that a cell spends the majority of its life cycle in interphase. Interphase results in the growth of cells and DNA replication in order for the cell to continue through the other steps of mitosis. A cell has to grow and have all the things it needs: organelles, DNA, correct number of chromosomes, etc. in order to go through the rest of the stages. It makes sense that each stage decreases in time because most of the growth and formation occurs in the beginning and the remaining steps do not need nearly as long because only a few things have to form and attach correctly.

3B.1

Purpose: The purpose of this lab was to show the stages of meiosis using chromosome models. These models represented a chromosome going through both Meiosis I and Meiosis II. It also is used to represent how the crossing over of genes occurs.

Introduction:

Meiosis is different from mitosis in that it produces 4 unique haploid cells rather than 2 identical diploid cells. During meiosis I, the chromosome number changes from diploid (2n) to haploid (n). In Meiosis II, the sister chromatids are separated which creates 4 haploid cells as the final product. The homologous chromosomes needed to complete this process are brought together through synapsis. After this crossing over can occur, where the different chromosomes are able to  exchange their genetic material. Calculating the the distance between two genes is based on how much crossing over occurs.

Method: For this section of the lab we used a model with beads to represent a chromosome. We were able to walk through each step of both meiosis I and II with the beads.We were also able to simulate the action of crossing over with these beads.  

Discussion:

In G1 of interphase, the chromatin is coiled within the nuclear envelope. The centrosomes are located within the plasma membrane and are close together without having formed spindles.

In prophase we begin to see the chromosomes we see the early mitotic spindle within the plasma membrane. There are still fragments of a nuclear envelope that has began to dissolve away. The sister chromatids begin to cross over and the mitotic spindle begins to form. The homologous genes cross over and the site of this is the chiasmata.

In metaphase the chromosomes begin to line up horizontally against the imaginary metaphase plate. The chromosomes are attached to the kinetochores and the mitotic spindle becomes more prominent and the centrosomes are at the top and the bottom of the cell.

In anaphase the homologous chromosomes separate and one pulls to the top while the other pulls to the bottom. The sister chromatids remain attached leading to a haploid.

In telophase the cell begins to spit in half causing a cleavage furrow. The mitotic spindles dissipate and the nuclear envelope fragments come back since there are going to be two cells and they need a new envelope.

In prophase II there are two cells with a nuclear envelope and the mitotic spindles form again in the cells. There is no DNA replication from telophase I to prophase II.

In metaphase II, the chromosomes line up vertically against the imaginary metaphase plate. The mitotic spindles form horizontally connecting to the centrosomes. The nuclear envelope dissolves.

In Anaphase II, the sister chromatids separate.

In telophase II, the two individual cells begin to split forming daughter haploid cells. The nuclear envelope forms again to protect the four new daughter cells and the mitotic spindles disappear.

Conclusion:

In conclusion, from this lab we are now able to visualize the stages of meiosis and pick out the differences between meiosis and mitosis. We also know that no chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated. Meiosis results in four haploid cells because the DNA splits and only half of it is taken to form cells that have combination of genes inherited from 2 parents. We are able to see that there is a strong amount of variation between people because independent assortment and crossing over which results in unique combinations of genes.

3.15

Purpose: The purpose of this portion of the lab is to examine how much crossing over occurs during meiosis.

Introduction: Crossing over is when non-sister chromatids trade parts of their DNA segments. This can happen multiple times while meiosis occurs. Because crossing over is possible, there is a greater chance for variance among gametes The crossing over of genes happens during the first part of meiosis, Meiosis I.

Method: Using the given slides, we counted and kept track of which chromosomes crossed over and which did not. When there were alternating colors, we knew that crossing over had occurred. When there were  of the same color in a row, we knew that no crossing over had occurred.

Discussion: Crossing over is dependent on the amount of distance between the genes. In order to determine the relative distance we need to use a map unit, which is an arbitrary unit that describes the distance between the linked genes. Based on the information gathered, we can understand that linked genes that are further apart have a high chance of crossing over compared to those genes that are closer together on a chromosome. We counted the ones that had four in a row which indicated no crossing over as the table indicated. The ones that showed crossing over we counted as 106. When we took the total number of Asci, which was the ones that crossed over and the ones that didn’t the total was 195. We took 106, the number that showed crossing over, and divided it by 195, the total number, and cut that number in half. We did this to find the percent showing cross over divided by two in order to find map units. The map units provided us with a distance. We drew this above to indicate the percentage of crossing over within a map unit.

Conclusion:

Genes that are further apart have a higher chance of crossing over. This is because the map unit percentage goes up and the gene is able to cross over properly because the amount of distance has increased. This leads to genetic variation because there are many different possible traits when things cross over.