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October, 2006
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Keystone Species

JoVE 10992

Measures of species biodiversity, such as richness (i.e., the number of species present) and evenness (i.e., their relative abundance), describe an ecological community’s structure. Many factors affect community structure, including abiotic factors (e.g., sunlight and nutrients), disturbances (e.g., fire or flood), species interactions (e.g., predation or competition), and chance events (e.g., foreign species invasion). Certain species—such as keystone species—also play a pivotal role in the structure of a community. Relative to their abundance, keystone species have a disproportionately large impact on community structure. Keystone species exert top-down control over lower trophic level organisms and reduce those organisms’ exploitation of the ecosystem’s resources. The intertidal sea star (Pisaster ochraceus) is a keystone species that influences the biodiversity of the Pacific coast’s kelp forest ecosystem. If the sea star is removed, the population of their prey species (mussels) increases. Left unchecked, mussels overrun the community and displace other organisms—changing the community’s species composition and reducing biodiversity. Recognizing keystone species is important for the maintenance and restoration of ecosystems. The North American gray wolf is a keystone species that affects the biodiv

 Core: Population and Community Ecology

What are Proteins?

JoVE 10677

Proteins are chains of amino acids that are connected by peptide bonds and folded into a 3-dimensional structure. The side chains of individual amino acid residues determine the interactions among amino acid residues, and ultimately the folding of the protein. Depending on the length and structural complexity, chains of amino acid residues are classified as oligopeptides, polypeptides, or proteins. An amino acid is a molecule that contains a carboxyl (–COOH) and an amino group (–NH2) attached to the same carbon atom, the ⍺-carbon. The identity of the amino acid is determined by its side chain or side residue, often called the R-group. The simplest amino acid is glycine, where the residue is a single hydrogen atom. Other amino acids carry more complex side chains. The side chain determines the chemical properties of the amino acid. For example, it may attract or repel water (hydrophilic or hydrophobic), carry a negative charge (acidic), or form hydrogen bonds (polar). Of all known amino acids, only 21 are used to create proteins in eukaryotes (the genetic code encodes only 20 of these). Amino acids are abbreviated using a three letter (e.g., Gly, Val, Pro) or one letter code (e.g., G, V, P). The linear chain of amino acid residues forms the backbone of the protein. The free amino group at one end is called the N-terminus, while t

 Core: Macromolecules


JoVE 10984

Prokaryotes can control gene expression through operons—DNA sequences consisting of regulatory elements and clustered, functionally related protein-coding genes. Operons use a single promoter sequence to initiate transcription of a gene cluster (i.e., a group of structural genes) into a single mRNA molecule. The terminator sequence ends transcription. An operator sequence, located between the promoter and structural genes, prohibits the operon’s transcriptional activity if bound by a repressor protein. Altogether, the promoter, operator, structural genes, and terminator form the core of an operon. Operons are usually either inducible or repressible. Inducible operons, such as the bacterial lac operon, are normally “off” but will turn “on” in the presence of a small molecule called an inducer (e.g., allolactose). When glucose is absent, but lactose is present, allolactose binds and inactivates the lac operon repressor—allowing the operon to generate enzymes responsible for lactose metabolism. Repressible operons, such as the bacterial trp operon, are usually “on” but will turn “off” in the presence of a small molecule called a corepressor (e.g., tryptophan). When tryptophan—an essential amino acid—is abundant, tryptophan binds and activates the tr

 Core: Gene Expression
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