Bio 100 LAB 3 Chapter 3 UPDATED

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Bio 100 LAB 3 Chapter 3

DNA Isolation from Plant Nuclei & Cheek Cells

Introduction to DNA Isolation

DNA (deoxyribonucleic acid) is a fundamental macromolecule found in all living cells, carrying the genetic instructions for cellular function. This laboratory exercise focuses on isolating DNA from two eukaryotic sources: wheat germ (plant) nuclei and human cheek cells. Understanding the structure of cells and the properties of DNA is essential for mastering the techniques of DNA extraction.

Eukaryotic cells contain membrane-bound organelles, including a nucleus where DNA is stored.
Prokaryotic cells lack a nucleus; their DNA is found in a nucleoid region.
DNA is composed of nucleotides, each consisting of a phosphate group, deoxyribose sugar, and one of four nitrogenous bases: adenine (A), guanine (G), thymine (T), or cytosine (C).
In eukaryotes, DNA is linear and organized into chromosomes; in prokaryotes, it is typically circular.
DNA is packaged with proteins (histones) into chromatin, forming nucleosomes as the basic unit of DNA packing.

Principles of DNA Isolation

Isolating DNA from cells involves several key steps to break open the cell and nuclear membranes, protect DNA from degradation, and separate it from proteins and other cellular components.

Cell Lysis: Disruption of the plasma membrane to release cellular contents. Methods include mechanical action (e.g., grinding with mortar and pestle), detergents (e.g., SDS, Triton X-100), or osmotic shock.
Nuclear Isolation: Separation of nuclei from other cell components by low-speed centrifugation, forming a pellet.
DNase Inhibition: DNase enzymes can degrade DNA once regulatory mechanisms are disrupted. EDTA is added to chelate divalent cations (Mg2+, Ca2+) required for DNase activity, thus protecting DNA.
Protein Removal: Detergents like SDS denature proteins, including histones, unwrapping DNA from nucleosomes. Sodium chloride (NaCl) precipitates proteins, which are then removed by centrifugation.
DNA Precipitation: DNA is insoluble in alcohols such as ethanol. Adding cold ethanol causes DNA to precipitate, forming a visible layer at the interface.

Laboratory Protocols

Isolation of DNA from Wheat Germ Nuclei

This protocol involves mechanical disruption, chemical lysis, and sequential centrifugation to isolate DNA from plant nuclei.

Homogenization: Wheat germ is ground with extraction buffer using a mortar and pestle to break open cells and release nuclei.
Filtration: The homogenate is filtered through cheesecloth to remove debris.
Centrifugation: The filtered suspension is centrifuged to pellet nuclei; the supernatant is discarded.
Resuspension and Washing: The pellet is resuspended in extraction buffer and centrifuged again to further purify nuclei.
DNase Inhibition: The pellet is resuspended in EDTA to inhibit DNase enzymes.
Nuclear Lysis: SDS is added to disrupt the nuclear envelope and denature histone proteins, releasing DNA.
Protein Precipitation: NaCl is added to precipitate proteins, followed by centrifugation to remove them.
DNA Precipitation: The supernatant containing DNA is mixed with cold ethanol, causing DNA to precipitate at the interface.

Flowchart Summary

Wheat germ + extraction buffer → grind → filter → centrifuge → decant supernatant → add extraction buffer → resuspend → centrifuge → decant → add EDTA → resuspend = mixed nuclear suspension
Mixed nuclear suspension + SDS → mix → heat → cool → add NaCl → mix → incubate on ice → centrifuge → transfer supernatant → add ethanol → DNA precipitates

Isolation of DNA from Human Cheek Cells

This protocol uses enzymatic and chemical lysis to extract DNA from human epithelial cells collected from the mouth.

Cell Collection: Cheek cells are collected using cytology brushes and suspended in SDS lysis buffer.
Protein Digestion: Proteinase K is added to digest proteins, including histones and other cellular proteins.
Salt Addition: Sodium chloride is added to help precipitate proteins and facilitate DNA extraction.
DNA Precipitation: The solution is transferred to a clean tube, and cold ethanol is gently layered on top. DNA precipitates at the interface and can be observed as a stringy, cloudy layer.

Key Reagents and Their Functions

Reagent	Function	Application
Extraction Buffer	Maintains osmotic balance, facilitates cell lysis	Wheat germ protocol
Triton X-100	Detergent, disrupts plasma membrane	Wheat germ protocol
SDS (Sodium Dodecyl Sulfate)	Denatures proteins, disrupts membranes	Both protocols
EDTA	Inhibits DNase by chelating Mg2+ and Ca2+	Wheat germ protocol
NaCl (Sodium Chloride)	Precipitates proteins	Both protocols
Proteinase K	Digests proteins	Cheek cell protocol
Ethanol (cold)	Precipitates DNA	Both protocols

Scientific Principles and Explanations

Why Use Specific Techniques and Reagents?

Mortar and Pestle: Used for wheat germ to mechanically break tough plant cell walls; not needed for cheek cells, which lack rigid walls.
Homogenate: The mixture after cell lysis contains cytoplasmic components, organelles, and membrane fragments.
SDS: Disrupts lipid membranes and denatures proteins, facilitating release of DNA from nuclei and removal of histones.
Cold Ethanol: DNA is insoluble in alcohol, especially when cold, causing it to precipitate out of solution for collection.

Potential Issues in DNA Isolation

Insufficient cell lysis (e.g., inadequate grinding or mixing)
Loss of DNA during transfer steps
Degradation by DNases if EDTA is not used or is insufficient
Improper precipitation (e.g., ethanol not cold, incorrect layering)

Summary Table: Comparison of Plant and Animal DNA Isolation Protocols

Step	Wheat Germ (Plant)	Cheek Cells (Animal)
Cell Disruption	Mechanical (mortar & pestle), detergent	Detergent only
Protein Removal	SDS, NaCl, centrifugation	Proteinase K, NaCl
DNase Inhibition	EDTA	Not specified
DNA Precipitation	Cold ethanol	Cold ethanol

Key Terms and Definitions

Homogenate: The mixture of cell contents after lysis.
Pellet: The solid material collected at the bottom of a tube after centrifugation.
Supernatant: The liquid above the pellet after centrifugation.
DNase: Enzyme that degrades DNA.
EDTA: A chelating agent that binds divalent cations, inhibiting DNase activity.
SDS: An anionic detergent that denatures proteins and disrupts membranes.
Proteinase K: An enzyme that digests proteins, aiding in DNA purification.
Precipitation: The process by which a dissolved substance becomes insoluble and forms a solid.

Example Applications

Genetic Analysis: Isolated DNA can be used for PCR, sequencing, or restriction enzyme analysis.
Forensics: Human DNA extraction is foundational for forensic identification.
Biotechnology: Plant DNA isolation is essential for genetic engineering and crop improvement studies.