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DNA Replication: The process by which a DNA molecule is duplicated.

Replication in Eukaryotes

JoVE 10789

In eukaryotic cells, DNA replication is highly conserved and tightly regulated. Multiple linear chromosomes must be duplicated with high fidelity before cell division, so there are many proteins that fill specialized roles in the replication process. Replication occurs in three phases: initiation, elongation, and termination, and ends with two complete sets of chromosomes in the nucleus.

Eukaryotic replication follows many of the same principles as prokaryotic DNA replication, but because the genome is much larger and the chromosomes are linear rather than circular, the process requires more proteins and has a few key differences. Replication occurs simultaneously at multiple origins of replication along each chromosome. Initiator proteins recognize and bind to the origin, recruiting helicase to unwind the DNA double helix. At each point of origin, two replication forks form. Primase then adds short RNA primers to the single strands of DNA, which serve as a starting point for DNA polymerase to bind and begin copying the sequence. DNA can only be synthesized in the 5’ to 3’ direction, so replication of both strands from a single replication fork proceeds in two different directions. The leading strand is synthesized continuously, while the lagging strand is synthesized in short stretches 100-200 base pairs in length, called Okazaki fragments. Once the bu

 Core: DNA Structure and Function

Replication in Prokaryotes

JoVE 10788

DNA replication has three main steps: initiation, elongation, and termination. Replication in prokaryotes begins when initiator proteins bind to the single origin of replication (ori) on the cell’s circular chromosome. Replication then proceeds around the entire circle of the chromosome in each direction from two replication forks, resulting in two DNA molecules.

Replication is coordinated and carried out by a host of specialized proteins. Topoisomerase breaks one side of the double-stranded DNA phosphate-sugar backbone, allowing the DNA helix to unwind more rapidly, while helicase breaks the bonds between base pairs at the fork, separating the DNA into two template strands. Proteins that bind single-stranded DNA molecules stabilize the strands as the replication fork travels along the chromosome. DNA can only be synthesized in the 5’ to 3’ direction, so one strand of the template—the leading strand—is elongated continuously, while the other strand—the lagging strand—is synthesized in shorter pieces of 1000-2000 base pairs called Okazaki fragments. Much of the research to understand prokaryotic DNA replication has been performed in the bacterium Escherichia coli, a commonly-used model organism. E. coli has 5 DNA polymerases: Pol I, II, III, IV, and V. Pol III is responsible for the majority of DN

 Core: DNA Structure and Function

Recombinant DNA

JoVE 10808

Scientists create recombinant DNA by combining DNA from different sources—often, other species—in the laboratory. DNA cloning allows researchers to study specific genes by inserting them into easily manipulated cells, such as bacteria. Organisms that contain recombinant DNA are known as genetically modified organisms (GMOs). Recombinant DNA technology produces organisms with new genes that can benefit science, medicine, and agriculture. Creation of recombinant DNA involves inserting a gene of interest into a vector—a vehicle that carries foreign DNA into host cells for DNA replication and protein expression. The most commonly used cloning vectors are plasmids, small circular pieces of DNA that replicate independently from the host’s chromosomal DNA. To create recombinant DNA, both the donor DNA, including the gene of interest, and the vector are cut at specific nucleotide sequences—called restriction sites—using restriction enzymes. The enzyme DNA ligase seals the sugar-phosphate backbone where the gene of interest and plasmid connect. The result is a recombinant DNA molecule consisting of a vector with an integrated piece of donor DNA—called an insert. A scientist may then introduce this hybrid DNA molecule into a host organism—typically bacteria or yeast—where it easily and rapidly replicat

 Core: Biotechnology

DNA Ligation Reactions

JoVE 5069

In molecular biology, ligation refers to the joining of two DNA fragments through the formation of a phosphodiester bond. An enzyme known as a ligase catalyzes the ligation reaction. In the cell, ligases repair single and double strand breaks that occur during DNA replication. In the laboratory, DNA ligase is used during molecular cloning to join DNA fragments of inserts with vectors…

 Basic Methods in Cellular and Molecular Biology


JoVE 10790

Synthesis of new DNA molecules starts when DNA polymerase links nucleotides together in a sequence that is complementary to the template DNA strand. DNA polymerase has a higher affinity for the correct base to ensure fidelity in DNA replication. The DNA polymerase furthermore proofreads during replication, using an exonuclease domain that cuts off incorrect nucleotides from the nascent DNA strand. Genomic DNA is synthesized in the 5’ to 3’ direction. Each cell contains a number of DNA polymerases that play different roles in synthesizing and correcting mistakes in DNA; DNA polymerase delta and epsilon possess proofreading ability when replicating nuclear DNA. These polymerases “read” each base after it is added to the new strand. If the newly-added base is incorrect, the polymerase reverses direction (moving from 3’ to 5’) and uses an exonucleolytic domain to cut off the incorrect base. Subsequently, it is replaced with the correct base. Proofreading is important for preventing mutations from occurring in newly-synthesized DNA, but what happens when the proofreading mechanism fails? When a mutation alters the exonuclease domain of DNA polymerase, it loses the ability to remove incorrect nucleotides. In consequence, mutations can accumulate rapidly throughout the genome. This type of mutation has been linked to

 Core: DNA Structure and Function


JoVE 10819

The polymerase chain reaction, or PCR, is a widely used technique for copying segments of DNA. Due to exponential amplification, PCR can produce millions or billions of DNA copies within just a few hours. In a PCR reaction, a heat-resistant DNA polymerase enzyme amplifies the original DNA through a series of temperature changes inside an automated machine called a thermocycler.

Kary Mullis developed PCR in 1983, for which he was awarded the 1993 Nobel Prize in Chemistry. Being a relatively fast, inexpensive, and precise way of copying a DNA sequence, PCR became an invaluable tool for numerous applications, including molecular cloning, gene mutagenesis, pathogen detection, gene expression analysis, DNA quantitation and sequencing, and genetic disease diagnosis. PCR mimics the natural DNA replication process that occurs in cells. The reaction mixture includes a template DNA sequence to be copied, a pair of short DNA molecules called primers, free DNA building blocks called deoxynucleotide triphosphates (dNTPs), and a specialized DNA polymerase enzyme. PCR involves a series of steps at high temperatures, requiring a DNA polymerase enzyme that is functional at such temperatures. The most commonly used DNA polymerase is Taq polymerase, named after Thermus aquaticus, the bacterium from which the polymerase was initially isolated. DNA

 Core: Biotechnology

Enzyme Activity- Concept

JoVE 10585

Biological Catalysts

All living organisms continuously perform numerous biochemical reactions to sustain their presence. Most of these reactions require an input of energy to start, which is called the activation energy. Catalysts are chemicals that lower the activation energy. Even though catalysts facilitate a chemical reaction, they are not consumed by it. This means a catalyst …

 Lab Bio

Mismatch Repair

JoVE 10791

Organisms are capable of detecting and fixing nucleotide mismatches that occur during DNA replication. This sophisticated process requires identifying the new strand and replacing the erroneous bases with correct nucleotides. Mismatch repair is coordinated by many proteins in both prokaryotes and eukaryotes.

The human genome has more than 3 billion base pairs of DNA per cell. Prior to cell division, that vast amount of genetic information needs to be replicated. Despite the proofreading ability of the DNA polymerase, a copying error occurs approximately every 1 million base pairs. One type of error is the mismatch of nucleotides, for example, the pairing of A with G or T with C. Such mismatches are detected and repaired by the Mutator protein family. These proteins were first described in the bacteria Escherichia coli (E. coli), but homologs appear throughout prokaryotes and eukaryotes. Mutator S (MutS) initiates the mismatch repair (MMR) by identifying and binding to the mismatch. Subsequently, MutL identifies which strand is the new copy. Only the new strand requires fixing while the template strand needs to remain intact. How can the molecular machinery identify the newly synthesized strand of DNA? In many organisms, cytosine and adenine bases of the new strand receive a methyl group some time after replication. Therefore,

 Core: DNA Structure and Function

Mitosis and Cytokinesis

JoVE 10762

In eukaryotic cells, the cell's cycle—the division cycle—is divided into distinct, coordinated cellular processes that include cell growth, DNA replication/chromosome duplication, chromosome distribution to daughter cells, and finally, cell division. The cell cycle is tightly regulated by its regulatory systems as well as extracellular signals that affect cell proliferation. The processes of the cell cycle occur over approximately 24 hours (in typical human cells) and in two major distinguishable stages. The first stage is DNA replication, during the S phase of interphase. The second stage is the mitotic (M) phase, which involves the separation of the duplicated chromosomes into two new nuclei (mitosis) and cytoplasmic division (cytokinesis). The two phases are separated by intervals (G1 and G2 gaps), during which the cell prepares for replication and division. Mitosis can be divided into five distinct stages—prophase, prometaphase, metaphase, anaphase, and telophase. Cytokinesis, which begins during anaphase or telophase (depending on the cell), is part of the M phase, but not part of mitosis. As the cell enters mitosis, its replicated chromosomes begin to condense and become visible as threadlike structures with the aid of proteins known as condensins. The mitotic spindle apparatus b

 Core: Cell Cycle and Division

Nucleotide Excision Repair

JoVE 10792

Exposure to mutagens can damage DNA and result in bulky lesions that distort the double-helix structure or impede proper transcription. Damaged DNA can be detected and repaired in a process called nucleotide excision repair (NER). NER employs a set of specialized proteins that first scan DNA to detect a damaged region. Next, NER proteins separate the strands and excise the damaged area. Finally, they coordinate the replacement with new, matching nucleotides. Cells are regularly exposed to mutagens—factors in the environment which can damage DNA and generate mutations. UV radiation is one of the most common mutagens and is estimated to introduce a significant number of changes to DNA. These include bends or kinks in the structure which can block DNA replication or transcription. If these errors are not fixed, the damage can cause mutations which in turn can result in cancer or disease depending on which sequences are disrupted. Nucleotide excision repair relies on specific protein complexes to recognize damaged regions of DNA and flag them for removal and repair. In prokaryotes, the process involves three proteins—UvrA, UvrB, and UvrC. The first two proteins work together as a complex, traveling along the DNA strands to detect any physical aberrations. Once identified, the strands at the damaged location are separated, and endon

 Core: DNA Structure and Function

Cell Division- Concept

JoVE 10571

Cell division is fundamental to all living organisms and required for growth and development. As an essential means of reproduction for all living things, cell division allows organisms to transfer their genetic material to their offspring. For a unicellular organism, cellular division generates a completely new organism. For multicellular organisms, cellular division produces new cells for…

 Lab Bio


JoVE 10761

The cell cycle occurs over approximately 24 hours (in a typical human cell) and in two distinct stages: interphase, which includes three phases of the cell cycle (G1, S, and G2), and mitosis (M). During interphase, which takes up about 95 percent of the duration of the eukaryotic cell cycle, cells grow and replicate their DNA in preparation for mitosis.

Following each period of mitosis and cytokinesis, eukaryotic cells enter interphase, during which they grow and replicate their DNA in preparation for the next mitotic division. During the G1 (gap 1) phase, cells grow continuously and prepare for DNA replication. During this phase, cells are metabolically active and copy essential organelles and biochemical molecules, such as proteins. In the subsequent S (synthesis) phase of interphase, cells duplicate their nuclear DNA, which remains packaged in semi-condensed chromatin. During the S phase, cells also duplicate the centrosome, a microtubule-organizing structure that forms the mitotic spindle apparatus. The mitotic spindle separates chromosomes during mitosis. In the G2 (gap 2) phase, which follows DNA synthesis, cells continue to grow and synthesize proteins and organelles to prepare for mitosis. In human cells, the G1 phase spans approximately 11 hours, the S phase takes about

 Core: Cell Cycle and Division

What are Nucleic Acids?

JoVE 10684

Nucleic acids are long chains of nucleotides linked together by phosphodiester bonds. There are two types of nucleic acids: deoxyribonucleic acid, or DNA, and ribonucleic acid, or RNA. Nucleotides in both DNA and RNA are made up of a sugar, a nitrogen base, and a phosphate molecule.

A cell’s hereditary material is comprised of nucleic acids, which enable living organisms to pass on genetic information from one generation to next. There are two types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA and RNA differ very slightly in their chemical composition, yet play entirely different biological roles. Chemically, nucleic acids are polynucleotides—chains of nucleotides. A nucleotide is composed of three components: a pentose sugar, a nitrogen base, and a phosphate group. The sugar and the base together form a nucleoside. Hence, a nucleotide is sometimes referred to as a nucleoside monophosphate. Each of the three components of a nucleotide plays a key role in the overall assembly of nucleic acids. As the name suggests, a pentose sugar has five carbon atoms, which are labeled 1o, 2o, 3o, 4o, and 5o. The pentose sugar in RNA is ribose, meaning the 2o carbon carries a hydroxyl group. The sugar in DNA is deoxyribose, meaning the 2o

 Core: Macromolecules


JoVE 10793

Mutations are changes in the sequence of DNA. These changes can occur spontaneously or they can be induced by exposure to environmental factors. Mutations can be characterized in a number of different ways: whether and how they alter the amino acid sequence of the protein, whether they occur over a small or large area of DNA, and whether they occur in somatic cells or germline cells.

Mutations that occur at a single nucleotide are called point mutations. When point mutations occur within genes, the consequences can vary in severity depending on what happens to the encoded amino acid sequence. A silent mutation does not change the amino acid identity and will have no effect on an organism. A missense mutation changes a single amino acid, and the effects might be serious if the change alters the function of the protein. A nonsense mutation produces a stop codon that truncates the protein, likely rendering it nonfunctional. Frameshift mutations occur when one or more nucleotides are inserted into or deleted from a protein-coding DNA sequence, affecting all of the codons downstream of the location of the mutation. The most drastic type of mutation, chromosomal alteration, changes the physical structure of a chromosome. Chromosomal alterations can include deletion, duplication, or inversion of large stretches of DNA within a single chromosome, or integration o

 Core: DNA Structure and Function

Electrophoretic Mobility Shift Assay (EMSA)

JoVE 5694

The electrophoretic mobility shift assay (EMSA) is a biochemical procedure used to elucidate binding between proteins and nucleic acids. In this assay a radiolabeled nucleic acid and test protein are mixed. Binding is determined via gel electrophoresis which separates components based on mass, charge, and conformation.

This video shows the concepts of EMSA and a general procedure, …


Protein Crystallization

JoVE 5689

Protein crystallization, obtaining a solid lattice of biomolecules, elucidates protein structure and enables the study of protein function. Crystallization involves drying purified protein under a combination of many factors, including pH, temperature, ionic strength, and protein concentration. Once crystals are obtained, the protein structure can be elucidated by x-ray diffraction and…


Phosphodiester Linkages

JoVE 10685

Phosphodiester linkage is created when a phosphoric acid molecule (H3PO4) is linked with two hydroxyl groups (–OH) of two other molecules, forming two ester bonds and removing two water molecules. Phosphodiester linkage is commonly found in nucleic acids (DNA and RNA) and plays a critical role in their structure and function.

DNA and RNA are polynucleotides, or long chains of nucleotides, linked together. Nucleotides are composed of a nitrogen base (adenine, guanine, thymine, cytosine, or uracil), a pentose sugar and a phosphate molecule (PO 3−4). In a polynucleotide chain, nucleotides are linked together by phosphodiester bonds. A phosphodiester bond occurs when phosphate forms two ester bonds. The first ester bond already exists between the phosphate group and the sugar of a nucleotide. The second ester bond is formed by reacting to a hydroxyl group (–OH) in a second molecule. Each formation of an ester bond removes a water molecule. Inside the cell, a polynucleotide is built from free nucleotides that have three phosphate groups attached to the 5o carbon of their sugar. These nucleotides are thus called nucleotide triphosphates. During the formation of phosphodiester bonds, two phosphates are lost, leaving the nucleotide with one phosphate group that is attached to t

 Core: Macromolecules

Cell Cycle Analysis

JoVE 5641

Cell cycle refers to the set of events through which a cell grows, replicates its genome, and ultimately divides into two daughter cells through the process of mitosis. Because the amount of DNA in a cell shows characteristic changes throughout the cycle, techniques known as cell cycle analysis can be used to separate a population of cells according to the different phases …

 Cell Biology

What is the Cell Cycle?

JoVE 10757

The cell cycle refers to the sequence of events occurring throughout a typical cell’s life. In eukaryotic cells, the somatic cell cycle has two stages: interphase and the mitotic phase. During interphase, the cell grows, performs its basic metabolic functions, copies its DNA, and prepares for mitotic cell division. Then, during mitosis and cytokinesis, the cell divides its nuclear and cytoplasmic materials, respectively. This generates two daughter cells that are identical to the original parent cell. The cell cycle is essential for the growth of the organism, replacement of damaged cells, and regeneration of aged cells. Cancer is the result of uncontrolled cell division sparked by a gene mutation. There are three major checkpoints in the eukaryotic cell cycle. At each checkpoint, the progression to the next cell cycle stage can be halted until conditions are more favorable. The G1 checkpoint is the first of these, where a cell’s size, energy, nutrients, DNA quality, and other external factors are evaluated. If the cell is deemed inadequate, it does not continue to the S phase of interphase. The G2 checkpoint is the second checkpoint. Here, the cell ensures that all of the DNA has been replicated and is not damaged before entering mitosis. If any DNA damage is detected that cannot be repaired, the cell may undergo apoptosis, or

 Core: Cell Cycle and Division

Positive Regulator Molecules

JoVE 10763

To consistently produce healthy cells, the cell cycle—the process that generates daughter cells—must be precisely regulated.

Internal regulatory checkpoints ensure that a cell’s size, energy reserves, and DNA quality and completeness are sufficient to advance through the cell cycle. At these checkpoints, positive and negative regulators promote or inhibit a cell’s continuation through the cell cycle. Positive regulators include two protein groups that allow cells to pass through regulatory checkpoints: cyclins and cyclin-dependent kinases (CDKs). These proteins are present in eukaryotes, ranging from yeast to humans. Cyclins can be categorized as G1, G1/S, S, or M cyclins based on the cell cycle phase or transition they are most involved in. Generally, levels of a given cyclin are low during most of the cell cycle but abruptly increase at the checkpoint they most contribute to (G1 cyclins are an exception, as they are required throughout the cell cycle). The cyclin is then degraded by enzymes in the cytoplasm and its levels decline. Meanwhile, cyclins needed for the next checkpoints accumulate. To regulate the cell cycle, cyclins must be bound to a Cyclin-dependent kinase (CDK)—a type of enzyme that attaches a phosphate group to modify the activity of a target protein.

 Core: Cell Cycle and Division

An Introduction to Cell Division

JoVE 5640

Cell division is the process by which a parent cell divides and gives rise to two or more daughter cells. It is a means of reproduction for single-cell organisms. In multicellular organisms, cell division contributes to growth, development, repair, and the generation of reproductive cells (sperms and eggs). Cell division is a tightly regulated process, and aberrant cell…

 Cell Biology

Genetic Screens

JoVE 5542

Genetic screens are critical tools for defining gene function and understanding gene interactions. Screens typically involve mutating genes and then assessing the affected organisms for phenotypes of interest. The process can be “forward”, where mutations are generated randomly to identify unknown genes responsible for the phenotypes, or it can be…


An Introduction to Aging and Regeneration

JoVE 5337

Tissues are maintained through a balance of cellular aging and regeneration. Aging refers to the gradual loss of cellular function, and regeneration is the repair of damaged tissue generally mediated by preexisting adult or somatic stem cells. Scientists are interested in understanding the biological mechanisms behind these two complex processes. By doing so, researchers may be able to use…

 Developmental Biology

Laser Microirradiation to Study In Vivo Cellular Responses to Simple and Complex DNA Damage

1Department of Biological Chemistry, School of Medicine, University of California, Irvine, 2Beckman Laser Institute and Medical Clinic, University of California, Irvine, 3Department of Developmental and Cell Biology, School of Biological Sciences, University of California, Irvine, 4Department of Biomedical Engineering and Surgery, University of California, Irvine

JoVE 56213


Detection of the Genome and Transcripts of a Persistent DNA Virus in Neuronal Tissues by Fluorescent In situ Hybridization Combined with Immunostaining

1Virus and Centromere Team, Centre de Génétique et Physiologie Moléculaire et Cellulaire, CNRS UMR 5534, 2Université de Lyon 1, 3Laboratoire d'excellence, LabEX DEVweCAN, 4Institut de Virologie Moléculaire et Structurale, CNRS UPR 3296, 5Centre de Recherche en Cancérologie de Lyon, INSERM U1052, CNRS UMR 5286

JoVE 51091


A Primary Neuron Culture System for the Study of Herpes Simplex Virus Latency and Reactivation

1Department of Microbiology, New York University School of Medicine, 2Molecular Neurobiology Program, Skirball Institute for Biomolecular Medicine, New York University School of Medicine, 3Department of Otolaryngology, New York University School of Medicine, 4Department of Cell Biology, New York University School of Medicine, 5Department of Physiology and Neuroscience, New York University School of Medicine, 6Department of Psychiatry, New York University School of Medicine, 7Center for Neural Science, New York University School of Medicine

JoVE 3823

 Immunology and Infection

Quantitation and Analysis of the Formation of HO-Endonuclease Stimulated Chromosomal Translocations by Single-Strand Annealing in Saccharomyces cerevisiae

1Irell & Manella Graduate School of Biological Sciences, 2Department of Molecular and Cellular Biology, City of Hope Comprehensive Cancer Center and Beckman Research Institute, 3Department of Biochemistry and Molecular Biology, University of Southern California, Norris Comprehensive Cancer Center

JoVE 3150


Modified Terminal Restriction Fragment Analysis for Quantifying Telomere Length Using In-gel Hybridization

1Departments of Pathology and Infectious Diseases and Microbiology, University of Pittsburgh, 2University of Pittsburgh Cancer Institute, 3Department of Environmental and Occupational Health, University of Pittsburgh, 4Departments of Psychiatry, Psychology, Behavioral & Community Health Sciences, University of Pittsburgh

JoVE 56001

 Cancer Research

Quantification of Protein Interaction Network Dynamics using Multiplexed Co-Immunoprecipitation

1Center for Integrative Brain Research, Seattle Children's Research Institute, 2Graduate Program in Neuroscience, University of Washington, 3Department of Cancer Immunology and Virology, Dana-Farber Cancer Institute, Department of Medicine, Harvard Medical School, 4Broad Institute of Harvard and MIT, 5Department of Mathematics, University of Houston, 6Department of Molecular Microbiology and Immunology, School of Medicine, University of Missouri, 7Department of Surgery, School of Medicine, University of Missouri, 8Department Bioengineering, College of Engineering, University of Missouri, 9Department of Pediatrics, University of Washington

JoVE 60029

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