Handouts-in-Gen-Bio-Microscope-and-cell-divisions.docx

Handouts in Gen. Bio. 1 Microscopes and Cell Cycle Microscope: Parts and Functions **Microscope** is a laboratory instrument use to examine objects that are too small to be seen by the naked eye. -A tool used by biologist in their experimental analysis. -Used in biology to study cells, tissues...

Handouts in Gen. Bio. 1 Microscopes and Cell Cycle Microscope: Parts and Functions **Microscope** is a laboratory instrument use to examine objects that are too small to be seen by the naked eye. -A tool used by biologist in their experimental analysis. -Used in biology to study cells, tissues and other substances. **Parts of Compound Microscope** 1\. Stand/Base- Horse-shoe shaped iron portion that supports the entire microscope. 2\. Inclination Joint- Located between the pillar and the arm for tilting the upper part of the microscope to a desired position. 3\. Substage- Located immediately below the stage containing the iris diaphragm and condenser. 4\. Stage - Platform with an opening to let the light pass and where the slide with the specimen is place. 5\. Arm or Handle- Connect the base with the body tube. 6\. Stage Clips- Keep the glass slide in place. 7\. Fine Adjustment Knob- Smaller wheel to see the finer detail of the object for precise focusing. 8\. Coarse Adjustment Knob- Large wheel used along with LPO for rough focus of the specimen. 9\. Revolving Nosepiece- Below the draw tube for attachment and shifting of objectives. 10\. Body Tube (Head)- Supports the objective and the ocular. 11\. Draw Tube- A place where the eyepiece or ocular is inserted. 12\. Mirror- Used to focus the rays of the light source from the sun toward the object. 13\. Iris Diaphragm -- Regulates the amount of light passing through the condenser. 14\. Eyepiece- Fits into the draw tube. 15\. Objectives (Magnifying Lenses)- Small tubes containing lenses attached to the nosepiece which magnify the object or the specimen.\ -Has 3 magnifying power:\ 1. LPO- shorter tube marked 10X\ 2. HPO-longer tube marked 40X, 43X or 45X.\ 3. Oil Immersion Objective- marked 97X or 100X. **Why is it important to handle the microscope with the utmost care?** For both experienced and inexperienced users, microscopes should always be handled with care. Proper microscope use will help prevent damage to the equipment and prevent laboratory accidents such as breaking slides. **Cell Cycle** **Stages of the cell cycle** In eukaryotic cells, or cells with a nucleus, the stages of the cell cycle are divided into two major phases: **interphase** and the **mitotic (M) phase**. -During *interphase*, the cell grows and makes a copy of its DNA. -During the *mitotic (M) phase*, the cell separates its DNA into two sets and divides its cytoplasm, forming two new cells. **Interphase** Preparation for division happens in three steps: **1. G\_11​start subscript, 1, end subscript phase.** During G\_11​start subscript, 1, end subscript phase, also called the first gap phase, the cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in later steps.  [[\[Do cells always grow before they divide?\]]](javascript:void(0)) **2. S phase.** In S phase, the cell synthesizes a complete copy of the DNA in its nucleus. It also duplicates a microtubule-organizing structure called the centrosome. The centrosomes help separate DNA during M phase. **3. G\_22​start subscript, 2, end subscript phase.** During the second gap phase, or G\_22​start subscript, 2, end subscript phase, the cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis. G\_22​start subscript, 2, end subscript phase ends when mitosis begins. The G\_11​start subscript, 1, end subscript, S, and G\_22​start subscript, 2, end subscript phases together are known as **interphase**. The prefix *inter*- means between, reflecting that interphase takes place between one mitotic (M) phase and the next. Image of the cell cycle. Interphase is composed of G1 phase (cell growth), followed by S phase (DNA synthesis), followed by G2 phase (cell growth). At the end of interphase comes the mitotic phase, which is made up of mitosis and cytokinesis and leads to the formation of two daughter cells. Mitosis precedes cytokinesis, though the two processes typically overlap somewhat. Mitosis Cell Division **What is mitosis?** ***Mitosis is a type of cell division in which one cell (the mother) divides to produce two new cells (the daughters) that are genetically identical to itself.*** In the context of the cell cycle, mitosis is the part of the division process in which the DNA of the cell\'s nucleus is split into two equal sets of chromosomes. **Phases of mitosis** Mitosis consists of four basic phases: prophase, metaphase, anaphase, and telophase.  Let's start by looking at a cell right before it begins mitosis. This cell is in interphase (late G\_22​start subscript, 2, end subscript phase) and has already copied its DNA, so the chromosomes in the nucleus each consist of two connected copies, called **sister chromatids**. You can't see the chromosomes very clearly at this point, because they are still in their long, stringy, decondensed form. This animal cell has also made a copy of its **centrosome**, an organelle that will play a key role in orchestrating mitosis, so there are two centrosomes. (Plant cells generally don't have centrosomes with centrioles, but have a different type of **microtubule organizing center** that plays a similar role.) Early prophase. The mitotic spindle starts to form, the chromosomes start to condense, and the nucleolus disappears. In early **prophase**, the cell starts to break down some structures and build others up, setting the stage for division of the chromosomes. - The chromosomes start to condense (making them easier to pull apart later on). - The **mitotic spindle** begins to form. The spindle is a structure made of microtubules, strong fibers that are part of the cell's "skeleton." Its job is to organize the chromosomes and move them around during mitosis. The spindle grows between the centrosomes as they move apart. - The **nucleolus** (or nucleoli, plural), a part of the nucleus where ribosomes are made, disappears. This is a sign that the nucleus is getting ready to break down.  In late prophase (sometimes also called **prometaphase**), the mitotic spindle begins to capture and organize the chromosomes. - The chromosomes become even more condensed, so they are very compact. - The nuclear envelope breaks down, releasing the chromosomes. - The mitotic spindle grows more, and some of the microtubules start to "capture" chromosomes. Anatomy of the mitotic spindle. Diagram indicating kinetochore microtubules (bound to kinetochores) and the aster. The aster is an array of microtubules that radiates out from the centrosome towards the cell edge. Diagram also indicates the centromere region of a chromosome, the narrow \"waist\" where the two sister chromatids are most tightly connected, and the kinetochore, a pad of proteins found at the centromere. Microtubules can bind to chromosomes at the kinetochore, a patch of protein found on the centromere of each sister chromatid. (Centromeres are the regions of DNA where the sister chromatids are most tightly connected.) Microtubules that bind a chromosome are called kinetochore microtubules. Microtubules that don't bind to kinetochores can grab on to microtubules from the opposite pole, stabilizing the spindle. More microtubules extend from each centrosome towards the edge of the cell, forming a structure called the aster.  In **metaphase**, the spindle has captured all the chromosomes and lined them up at the middle of the cell, ready to divide. - All the chromosomes align at the **metaphase plate** (not a physical structure, just a term for the plane where the chromosomes line up). - At this stage, the two kinetochores of each chromosome should be attached to microtubules from opposite spindle poles. https://cdn.kastatic.org/ka-perseus-images/aa3eddb779598a813e45ad6fd780cb82655758ed.png In **anaphase**, the sister chromatids separate from each other and are pulled towards opposite ends of the cell. - The protein "glue" that holds the sister chromatids together is broken down, allowing them to separate. Each is now its own chromosome. The chromosomes of each pair are pulled towards opposite ends of the cell. - Microtubules not attached to chromosomes elongate and push apart, separating the poles and making the cell longer. All of these processes are driven by **motor proteins**, molecular machines that can "walk" along microtubule tracks and carry a cargo. In mitosis, motor proteins carry chromosomes or other microtubules as they walk.  In **telophase**, the cell is nearly done dividing, and it starts to re-establish its normal structures as cytokinesis (division of the cell contents) takes place. - The mitotic spindle is broken down into its building blocks. - Two new nuclei form, one for each set of chromosomes. Nuclear membranes and nucleoli reappear. - The chromosomes begin to decondense and return to their "stringy" form https://cdn.kastatic.org/ka-perseus-images/f099142d1f9c234ff397699b9085a84f678116bc.png **Cytokinesis**, the division of the cytoplasm to form two new cells, overlaps with the final stages of mitosis. It may start in either anaphase or telophase, depending on the cell, and finishes shortly after telophase. In animal cells, cytokinesis is contractile, pinching the cell in two like a coin purse with a drawstring. The "drawstring" is a band of filaments made of a protein called actin, and the pinch crease is known as the **cleavage furrow**. Plant cells can't be divided like this because they have a cell wall and are too stiff. Instead, a structure called the **cell plate** forms down the middle of the cell, splitting it into two daughter cells separated by a new wall.  When cytokinesis finishes, we end up with two new cells, each with a complete set of chromosomes identical to those of the mother cell. The daughter cells can now begin their own cellular "lives," and -- depending on what they decide to be when they grow up -- may undergo mitosis themselves, repeating the cycle.