Bio lec 11

One source briefly highlights the Rainbow Darter (Etheostoma caeruleum) as the Random Animal of the Week. This gorgeous fish is found in clean, fast-moving streams with rocky bottoms, measuring about 2-3 inches long. Females select mates based on showy males, making them the “peacocks” of the fish world. They are classified as insectivores, eating aquatic invertebrates like insects and crayfish. Rainbow Darters are sensitive to pollution and live in clean streams. Their biggest threats are water pollution, including sediment from developments, nutrients (fertilizers), and pesticides.

The sources explain fundamental biological principles, starting with the fact that an individual organism begins as a single cell, a zygote. This single cell divides by mitosis, creating identical copies of itself. As the organism grows through mitosis, individual cells become differentiated. Although every cell in the body is genetically identical, different cell types like skin cells, neurons, and muscle cells look and behave differently because some genes become inactivated while others are activated. This process allows cells to become specialized. Gene regulation refers to how a cell turns on or off different genes. Genes that are "turned on" are transcribed into mRNA and then translated into proteins, while genes that are "turned off" are not transcribed or translated.

Gene regulation mechanisms differ between bacteria and eukaryotes.

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In bacteria, like E. coli, gene regulation allows them to conserve resources by only expressing the genes they need. For example, E. coli can use lactose if available, requiring three enzymes whose genes are located next to each other. Genes with related functions that can be simultaneously regulated form an operon. E. coli has a lac operon. Control of an operon involves a promoter, a DNA sequence near the operon where RNA polymerase binds, and an operator, a sequence between the promoter and genes where repressors can bind to turn off transcription. In the lac operon, the presence of lactose interferes with the attachment of the lac repressor to the operator, thus turning the operon on. Defining "operon" and explaining how lactose turns on the lac operon are included as activities.

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In eukaryotes (organisms other than bacteria and archaea), several methods are used for gene regulation. One method is DNA packing; when DNA is in a condensed state, such as when preparing for cell division, it cannot be transcribed or translated. X chromosome inactivation in female cats is provided as an example. The initiation of transcription is considered the most important stage for regulating gene expression in eukaryotes. Most eukaryotic genes are typically turned "off" by default. Many proteins, including transcriptional factors, are involved in enhancing transcription by binding to enhancers and promoters to activate multiple genes simultaneously. Repressor proteins can also bind to DNA sequences called silencers to inhibit transcription. Gene expression can also be modified through RNA processing and breakdown. The initial mRNA transcript is edited to remove introns and link exons before leaving the nucleus. Alternative RNA splicing, where exons of one gene are linked in different ways, can create different polypeptides. Not all RNAs code for protein; some, like microRNAs (miRNAs), perform important functions. miRNAs can bind to proteins to form a complex with target mRNAs; if the complementarity is complete, the target mRNA is degraded, but if only partial, the complex blocks translation. Other ways genes can be regulated include breaking down mRNA in the cytoplasm, regulating translation, altering the proteins produced, or breaking down the produced proteins. Gene regulation opportunities exist at each step of the process of forming proteins from DNA.

In multicellular organisms, cell signaling is crucial for communication between cells. Chemicals like hormones can be secreted by one cell to signal activity in another. The signal molecule binds to a receptor protein on the target cell, starting a signal transduction pathway. This pathway involves a series of relay proteins that ultimately activate a transcription factor, triggering specific gene transcription.

Homeotic genes are described as “master control genes” that regulate the expression of other gene groups. They are vital in embryonic development, where cells use chemical signals to coordinate development. Fruit flies provide a well-studied example, where specific homeotic genes determine where structures like legs and antennae develop. If a homeotic gene is active in the mid-body, it triggers legs to form there, but it is inactive elsewhere. A mutation in one of these genes can cause significant changes, such as flies having legs growing out of their heads. An activity asks how a single gene mutation can cause such a big change and what these genes are called.

The sources also discuss cell differentiation in the context of cloning. Cells start undifferentiated with the potential to become any cell type. As an organism grows, selective gene activation/inactivation leads to differentiation. Sometimes, differentiated cells can “dedifferentiate,” allowing them to grow into a new organism or part, as seen with vegetative propagation in plants or limb regeneration in amphibians. Growing a new organism from the somatic (body) cells of an adult results in a clone, which is genetically identical to the source organism. Cloning is the production of identical copies of a cell, organism, or DNA molecule, and it doesn't only refer to whole organisms; cloning cells or molecules is more common. There are different types of cloning for different purposes.

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Reproductive cloning involves extracting a female animal's egg, removing its DNA, and inserting DNA from a somatic cell of the desired organism. The resulting embryo is implanted into a female to grow. This can be used for desirable farm animals or endangered species. Dolly the sheep, the first mammal cloned (1997), died in 2003 from a lung disease, though it's difficult to definitively link this to cloning. The first primates (macaques) were cloned in January 2018.

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Therapeutic cloning aims to generate embryonic stem cells (ES cells). ES cells can divide indefinitely in culture and have the potential to develop into any cell type. Specific growth factors can induce these cells to differentiate. They may potentially treat or cure diseases or injured organs, with some success reported in treating stroke victims or Parkinson’s disease by inserting stem cells into the brain. The controversy surrounding ES cells lies in their derivation from embryos. Adult stem cells are partially differentiated and less flexible than ES cells, but are less controversial as they can be harvested from adults. Examples include bone marrow stem cells becoming blood cell types. They are generally more difficult to culture. New sources include induced pluripotent stem cells created from somatic cells.

Finally, the sources touch upon the Genetics of Cancer. Cancer is the second-leading cause of death in industrialized countries. Oncogenes are genes that cause cancer. Humans carry many proto-oncogenes which can become oncogenes if changed, for example, by mutation or viral insertion. Many proto-oncogenes code for growth factors that stimulate cell division; too much growth factor from a mutated gene can cause cancerous growth. Tumor-suppressor genes normally inhibit cell division; if mutated, they can also cause cancer. Carcinogens are substances causing DNA mutations that lead to cancer, such as UV radiation, tobacco smoke, and alcohol. Prevention includes consuming compounds in fruits and vegetables, like antioxidants, which can reduce and repair DNA damage. Examples of cancer-fighting foods are listed