Lab 3
Cells and Cell Replication

Learning Objectives
In this lab students will:
- Learn how to use the microscope to observe cells from multicellular organisms.
- View and compare prepared human and frog blood smears.
- Identify the 4 stages of mitosis in a magnified onion root tip.
- View magnified cells of the aquatic plant Elodea before and after undergoing plasmolysis.

Cells

As we saw in Lab 2, the Cell Theory states that all living things are made up of cells. All cells share basic common characteristics. They share these common characteristics because there is an evolutionary link between simple prokaryotic cells and more complex eukaryotic cells.
All cells are composed of four macromolecules; proteins, lipids, carbohydrates, and nucleic acids (DNA and RNA).
All cells are surrounded by a cell (or plasma) membrane made up of lipids and proteins.
All cells store genetic information in the form of DNA.
All cells contain ribosomes, the site where proteins are made.
All cells contain a gel like cytosol.
Although all cells share the above characteristics, there are major differences between prokaryotic cells and eukaryotic cells.
"Prokaryote" means "before (pro) the nucleus (karyon). Prokaryotic cells lack a large organelle called the nucleus. This is the most obvious difference between prokaryotic cells and eukaryotic cells. Prokaryotic cells are also much smaller (about 10 times smaller then a typical eukaryotic cell) and have fewer internal structures (organelles).
"Eukaryote" means true (Eu) nucleus (karyon). All eukaryotic cells have a large nucleus that stores the genetic information (DNA). Eukaryotic cells are much larger and have membrane-bound organelles. Organelles are internal structures that carry out specific functions for the cell.
Plants, animals, fungi and protists all have eukaryotic cells. You are a eukaryote!

A diagram of an oval shaped 2-dimensional animal cell showing nucleus and various organelles within. From inside to outside of the cell; at the center of the cell lies the membrane covered circular nucleus. At the periphery of the nuclear membrane which covers the nucleus, there are flat stacked endoplasmic reticulums (ER). The ones with ribosomes are the rough ER are without are the smooth ER. The golgi appears as flat stacked structures with ends facing towards the plasma membrane. The lysosomes are round structures scattered in the cytoplasm. Many bean-like structures with seriated inside are the mitochondria. The centrosomes are two tube-like structures in the cytoplasm. Bean like fluid filled structures are the vacuoles. In the cytoplasm the thread and string like structures scattered around are intermediate filaments and microtubules.

This figure shows the organelles found in a typical animal cell.
The nucleus stores the DNA and is surrounded by a double membrane called the nuclear envelope.
• Inside the nucleus is one or more darker staining regions. This is the nucleolus, the site where ribosomes are made.
The endoplasmic reticulum (ER) is a membranous organelle that may have ribosomes associated with it (rough ER) or no ribosomes associated with it (smooth ER). The rough ER is the site for the synthesis of proteins that will leave the cell to carry out their functions elsewhere.
The smooth ER is where lipids are made.
The Golgi complex is a collecting, packaging and sorting center for the cell. Materials made by the cell and destined for use outside the cell are processed and modified in the Golgi complex.
Lysosomes are sites where large macromolecules or worn out organelles are broken down.
The mitochondria are the organelles where responsible for making energy (the mighty mitochondria). When cells receive nutrients from their environment, the mitochondria convert the potential energy in the nutrients into a form that can be used by the cell to carry out its functions.
Chloroplasts (not shown in the image because they are only found in plant cells and some protists) are organelles responsible for converting energy from the sun into chemical energy that plants and other photosynthetic organisms can use to drive the synthesis if glucose. Chloroplasts are found in photosynthetic cells.

• In this exercise you will investigate the structures of the various multicellular eukaryotes: human cheek epithelial cells, human and frog blood cells, cells of an aquatic plant, Elodea, and actively dividing cells from an onion root tip.

Cell Division

New cells arise from the division of previously existing cells.
Cell division is necessary for an organism to grow, heal itself, and replenish old or damaged cells.
There are two types of cell division; mitosis, which produced 2 daughter cells identical to one another, and meiosis, which produced 4 genetically unique daughter cells.
In this exercise, you will focus on mitotic cell division.
In order for each new daughter cell to have a full set of chromosomes, the first step of cell division is to copy the DNA.
This is called DNA replication.

A diagram of cell cycle depting life of a cell in two rings. The outer ring shows life in interphase and mitosis followed by cytokinesis. The inner ring shows cell cycle phases like G1,  S and G2 followed by mitosis phases.

The cell cycle diagram shown here depicts the life cycle of a cell. As the diagram shows, the majority of the cell's life is spent in interphase. This phase is subdivided into three phases: G1, S and G2. During G1 phase the cell grows larger (G stands for "gap", but you can think of it as 'growth").
During S phase the DNA copies (replicates) itself (S stands for synthesis, a new copy of DNA is being synthesized).
Finally, in the G2 phase the cell prepares to divide by mitosis.





A diagram of mitosis in an animal cell showing how a single cell with a diploid nucleus goes through a series of phases including prophase, metaphase, anaphase and the telophase leading to a formation of two daughter cells with similar diploid nuclei. During prophase we see the nuclear envelope disappearing and the chromosomes condensing. The spindle apparatus begins to form and the chromosomes, composed of two sister chromatids become condensed. During Metaphase we see the chromosomes are completely condensed and line up at the equator of the cell. During Anaphase we see the chromatids are separated and move to opposite ends of the cell. During telophase we see the nuclear envelope reforming around each cluster of chromosomes, the Spindle fibers and centrioles disappear and the Chromosomes begin to uncoil.

Mitosis
• Mitosis, which is the division of the nucleus to produce two diploid nuclei, occurs after interphase.
• Prophase, metaphase, anaphase and telophase are the 4 phases of mitosis. Each phase can be identified by observing unique changes in the chromosomes.
This is an image of a dividing animal cell. You will need to become familiar with what is happening at each stage of mitosis and be able to distinguish cells that are in each stage of mitosis from those in interphase for this lab.

• Prophase: The nuclear envelope disappears and the chromosomes condense. The spindle apparatus begins to form; the chromosomes, composed of two sister chromatids become condensed.

• Metaphase: The chromosomes line up at the equator of the cell, called the metaphase plate. Both plants and animal cells form spindles, but only animal cells have centrioles.

• Anaphase: The chromatids are separated and move to opposite ends of the cell.

• Telophase: The nuclear envelope reforms around each cluster of chromosomes. Spindle fibers and centrioles disappear. Chromosomes begin to uncoil, and cytokinesis soon follows to divide the cytoplasm.




In this lab you will complete several activities to study the structure of cells and the processes that occur in them. These include identifying the phases of mitosis in a magnified onion root tip.

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Non-majors College Biology Lab Manual © 2021 by Marie McGovern Ph.D. is licensed under CC BY-NC 4.0