Cell Biology

Step 1: Understanding the Concept:
Contrast in microscopy refers to the difference in light intensity between the image and the adjacent background.
Many biological specimens, such as tissue sections, are largely transparent and have very little natural contrast, making them difficult to see under a standard bright-field microscope.
To visualize specific structures, the contrast must be enhanced.
Step 2: Detailed Explanation:


Staining: This is the most common method to increase contrast in light microscopy.
Dyes are used that selectively bind to specific cellular components (like the nucleus, cytoplasm, or cell membranes), making them colored and thus easily distinguishable from the unstained background.

Changing magnification: Increasing magnification makes the image appear larger, but it does not increase the contrast.
In fact, excessively high magnification without sufficient contrast and resolution can result in a blurry, unclear image.

Changing microscope resolution: Resolution is the ability to distinguish between two closely spaced points.
While improving resolution enhances the detail that can be seen, it does not directly increase the contrast of an unstained specimen.

Use of fluorescence dyes: This is a specific type of staining used in fluorescence microscopy, not typically in a standard compound (bright-field) microscope.
While it dramatically increases contrast, "staining" is the broader and more general correct answer applicable to compound microscopy.

Step 3: Final Answer:
Therefore, staining is the primary method to increase contrast for observing tissue sections with a compound microscope.

Step 1: Understanding the Concept:
The question describes a process called differential centrifugation, a common technique used in cell biology to separate organelles and other subcellular components from a cell homogenate based on their size, shape, and density.
The principle is that larger and denser particles sediment (form a pellet at the bottom of the tube) at lower centrifugal forces, while smaller and less dense particles remain in the supernatant and require higher forces to pellet.
Step 2: Detailed Explanation:
The standard order of fractionation by increasing centrifugal force is:


Low-speed centrifugation (e.g., 600-1000g for 10 min): This force is sufficient to pellet the largest and densest components, which are the intact cells (if any remain), the cytoskeleton, and primarily the nuclei. This first pellet is often called the nuclear pellet.

Medium-speed centrifugation (e.g., 15,000g for 20 min): The supernatant from the first step is centrifuged again at a higher speed. This pellets organelles of intermediate size and density, such as mitochondria, chloroplasts (in plant cells), lysosomes, and peroxisomes.

High-speed centrifugation (e.g., 100,000g for 60 min): The supernatant from the previous step is subjected to even higher forces, which pellets smaller components like microsomes (fragments of the endoplasmic reticulum and Golgi apparatus).

Ultracentrifugation (e.g., >150,000g for several hours): The final supernatant is centrifuged at very high speeds to pellet the smallest components, such as ribosomes, large macromolecules, and viruses.

Step 3: Final Answer:
Based on this scheme, centrifugation at 1000g for 10 minutes will specifically pellet the nuclei.

Step 1: Understanding the Concept:
The resting membrane potential is the electrical potential difference across the plasma membrane of a nerve cell when it is not stimulated or excited. It is a fundamental property of neurons that allows them to transmit signals.
Step 2: Detailed Explanation:


The resting membrane potential is established and maintained by the differential permeability of the membrane to various ions, primarily potassium (K ) and sodium (Na ), and the action of the Na /K pump.

In a resting state, the neuron's membrane is much more permeable to K than to Na , due to the presence of open K "leak" channels.

K ions, which are concentrated inside the cell, diffuse out down their concentration gradient, leaving behind unpaired negative charges (anions like proteins) inside the cell.

This movement of positive charge out of the cell makes the inside of the membrane negative relative to the outside.

The typical value for the resting membrane potential in most neurons ranges from -60 to -80 millivolts (mV), with a commonly cited average value of -70 mV.

The other ranges provided are not representative of a typical neuron's resting potential.

Step 3: Final Answer:
The membrane potential of a resting neuron is typically in the range of -60 to -80 millivolts.

Step 1: Understanding the Concept:
The title "guardian of the genome" is a well-known epithet for a specific tumor suppressor protein that plays a central role in maintaining genetic stability and preventing cancer.
Step 2: Detailed Explanation:


The protein p53 is a transcription factor that is activated in response to various cellular stresses, most importantly, DNA damage.

Once activated, p53 can halt the cell cycle (cell cycle arrest) to allow time for DNA repair.

If the DNA damage is too severe to be repaired, p53 will trigger apoptosis (programmed cell death), eliminating the potentially cancerous cell.

By preventing cells with damaged DNA from proliferating, p53 maintains the integrity of the genome and suppresses tumor formation. This critical role has earned it the nickname "the guardian of the genome."

The other options are different types of proteins: Rb is another tumor suppressor that controls the G1/S checkpoint. Myc is a proto-oncogene that promotes cell proliferation. PFU stands for "plaque-forming unit" and is a measure used in virology, not a protein name in this context.

Step 3: Final Answer:
The protein p53 is referred to as "the guardian of the genome."

Concept: Lipid rafts are specialized microdomains in the plasma membrane that are more ordered and tightly packed than the surrounding membrane. They serve as platforms for cell signaling, membrane trafficking, and protein sorting.
Step 1:Structure of the plasma membrane.
The plasma membrane mainly consists of phospholipids, proteins, and cholesterol arranged in a fluid mosaic structure. However, some regions form distinct microdomains called lipid rafts.
Step 2:Composition of lipid rafts.
Lipid rafts are enriched in: [ \text{Cholesterol} \quad \text{and} \quad \text{Sphingolipids} ] These molecules pack tightly together, creating a more ordered and less fluid region of the membrane.
Step 3:Functional importance.
These domains help organize membrane proteins involved in signaling pathways and play roles in processes such as endocytosis and immune responses. Thus, the primary composition of lipid rafts is cholesterol and sphingolipids.

Concept: Cellular fractionation is a laboratory technique used to separate cellular components based on their size and density using differential centrifugation. During centrifugation, heavier and larger organelles sediment faster and form pellets at lower centrifugal speeds.
Step 1:Principle of differential centrifugation.
When a homogenized cell extract is centrifuged at increasing speeds, organelles sediment in the order of their size and density. Larger organelles pellet first, while smaller components remain in the supernatant.
Step 2:Low-speed centrifugation.
At approximately (1000g) for about 10 minutes, the largest and heaviest cellular structures sediment. These typically include: [ \text{Nuclei, unbroken cells, and large cell debris} ]
Step 3:Subsequent centrifugation steps.
Higher centrifugal forces are required to pellet smaller organelles:

• (10,000g): mitochondria and lysosomes
• (100,000g): microsomes and fragments of endoplasmic reticulum
• (>100,000g): ribosomes

Thus, during centrifugation at (1000g) for 10 minutes, the organelle that forms part of the pellet is the nucleus.