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Transcript of Lesson 03.01
2. Glass microscope slide with prepared onion root tip specimen Interphase 12-16
Cytokinesis 3 -6 1. Always handle microscopes and glass slides carefully.
2. Wash your hands after handling the prepared specimens. Cell Cycle Cell Cycle Onion cell
Interphase is the longest phase of cell cycle. It lasts for the 90% time of the total time of cell cycle.
Cell prepare itself for the division in this phase. All the necessary proteins and duplication of organelles occur in this phase.
Anaphase is the shortest phase of mitosis. It lasts for only a couple of minutes. 2. What stages were the longest and shortest? Give a brief explanation of why these stages may have that time period. 3. Describe the stages of the cell cycle The cell cycle is divided into two main parts: interphase and mitosis. During interphase, the cell grows and replicates its chromosomes. Interphase accounts for all but an hour or two of a 24-h cell cycle, and is subdivided into three phases: gap phase 1 (G1), synthesis of DNA(S), and gap phase 2 (G2). Interphase is followed by mitosis (nuclear division) and cytokinesis (cell division). Step by step:
1. Interphase- when the cell is going about regular activities
2. Prophase- when the DNA chromosomes first become visible
3. Metaphase- When the chromos line up in the middle
4. Anaphase- when the chromos form pairs and are apart
5. Telophase/cytokinesis- when the cell has two separate nuclei and is about to split into two different cells 1. explain the process of mitosis and its role in the formation of new cells. The root tips of onion plants are commonly used for observing cell division because:
1. The onion plants are easy to grow in large numbers.
2. The cells at the tip of the onion roots are actively dividing, so many of the cells will be in stages of mitosis.
3. The root tips can be prepared in a way that allows them to be flattened on the microscope slide so that the chromosomes of individual cells can be observed.
4. This species has a relatively small number of chromosomes, and they are fairly large and distinct. Background Information: 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.
The region of cell division is where cells are actively dividing but not increasing significantly in size.
In the region of cell elongation, cells are increasing in size but not dividing.
We will be observing cells in the region of cell division to ensure that we observe all stages of mitosis and the cell cycle. For this lab activity, you will be examining a section of an onion root tip under a compound light microscope. These slides were prepared by slicing the roots into thin sections, mounting them on glass microscope slides, staining so that the chromosomes are more visible, and then covering the specimen with a cover slip.
The images below give examples of what you might see in each of the stages of the cell cycle. Lab Instructions: Humans are diploid creatures, meaning that each of the chromosomes in our body are paired up with another.
Haploid cells possess only one set of a chromosome. For example, a diploid human cell possesses 46 chromosomes and a gamete created by a human is haploid possesses 23 chromosomes.
Tetraploid organisms possess more than 3 sets of a particular chromosome.
Reproduction occurs in humans with the fusion of two haploid cells (gametes) that create a zygote. The nuclei of both these cells fuse, bringing together half the genetic information from the parents into one new cell, that is now genetically different from both its parents.
This increases genetic diversity, as half of the genetic content from each of the parents brings about unique offspring, which possesses a unique genome presenting unique characteristics. Meiosis as a process can increase genetic variation in many ways, explained soon.
The Process of Meiosis
The process of meiosis essentially involves two cycles of division, involving a gamete mother cell (diploid cell) dividing and then dividing again to form 4 haploid cells. These can be subdivided into four distinct phases which are a continuous process
* Prophase - Homologous chromosomes in the nucleus begin to pair up with one another and then split into chromatids (one half of a chromosome) where crossing over can occur. Crossing offer can increase genetic variation.
* Metaphase - Chromosomes line up at the equator of the cell, where the sequence of the chromosomes lined up is at random, through chance, increasing genetic variation via independent assortment.
* Anaphase - The homologous chromosomes move to opposing poles from the equator
* Telophase - A new nuclei forms near each pole alongside its new chromosome compliment.
At this stage two haploid cells have been created from the original diploid cell of the parent.
* Prophase II - The nuclear membrane disappears and the second meiotic division is initiated.
* Metaphase II - Pairs of chromatids line up at the equator
* Anaphase II - Each of these chromatid pairs move away from the equator to the poles via spindle fibres
* Telophase II - Four new haploid gametes are created that will fuse with the gametes of the opposite sex to create a zygote.
Overall, this process of meiosis creates gametes to pass genetic information from parents to offspring, continuing the family tree and the species as a whole. 1. Interphase looks like a circle with a bunch of dots and squiggly lines.
2.Prophase looks like an undefined line almost like - - - - the scissorss icon you get on a piece of paper, and has X's and lines all over it.
3.Metaphase is a cricle with chromosomes/X's in the middle of the cells nucleus.
4.Anaphase is a circle with chromosomes on the opposite side each other and look like sideways V's.
5. Telophase is when membranes form around each set of chromosomes while the chromosomes spread out into chromatin.
6. Cytokinesis is when the cell divides into two separate cells. Record any observations about the cells you observed (what does the cell look like for each stage):
Be sure to answer the following reflection questions in the conclusion of your lab report:
Based on your data, what can you infer about the length of time spent in each stage of the cell cycle?
What stages were the longest and shortest? Give a brief explanation of why these stages may have that time period.
What is a distinguishing visible feature of each stage of the cell cycle?
What differences can you see when you compare the nucleus of a dividing cell with that of a nondividing cell?
If your observation had not been restricted to the tip of the onion root, how would the results be different?