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Cytoplasm: The part of a cell that contains the Cytosol and small structures excluding the Cell nucleus; Mitochondria; and large Vacuoles. (Glick, Glossary of Biochemistry and Molecular Biology, 1990)

Cytoplasm

JoVE 10967

The cytoplasm consists of organelles, an aqueous solution called the cytosol, and a framework of protein scaffolds called the cytoskeleton. The cytosol is a rich broth of ions, small organic molecules such as glucose, and macromolecules such as proteins. Several cellular processes including protein synthesis occur in the cytoplasm.

The composition of the cytosol promotes protein folding such that hydrophobic amino acid side chains are oriented away from the aqueous solution and towards the protein core. However, cellular stressors such as aging and changes in pH, temperature, or osmolarity cause protein misfolding. Misfolded proteins may aggregate to form insoluble deposits in the cytoplasm. Insoluble protein aggregates are implicated in neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis. The eukaryotic cytoskeleton consists of three types of filamentous proteins: microtubules, microfilaments, and intermediate filaments. Microtubules–the largest type of filament–are made up of the protein tubulin. Microtubules are dynamic structures that can grow or shrink by adding or removing tubulin molecules from the ends of their strands. They provide structural stability and provide tracks for the transport of proteins and vesicles within the cell. In addition, microtu

 Core: Biology

Prokaryotic Cells

JoVE 10690

Prokaryotes are small unicellular organisms in the domains Archaea and Bacteria. Bacteria include many common organisms such as Salmonella and Escherichia coli, while the Archaea include extremophiles that live in harsh environments, such as volcanic springs.

Like eukaryotic cells, all prokaryotic cells are surrounded by a plasma membrane and have DNA that contains the genetic instructions, cytoplasm that fills the interior of the cell, and ribosomes that synthesize proteins. However, unlike eukaryotic cells, prokaryotes lack a nucleus or other membrane-bound intracellular organelles. Their cellular components generally float freely within the cytoplasm, although their DNA—usually consisting of a single, circular chromosome—is clustered within a region called the nucleoid. Inside the cytoplasm, many prokaryotes have small circular pieces of DNA called plasmids. These are distinct from the chromosomal DNA in the nucleoid and tend to have just a few genes—such as genes for antibiotic resistance. Plasmids are self-replicating and can be transmitted between prokaryotes. Most prokaryotes have a cell wall made of peptidoglycan that lies outside of their plasma membrane, which physically protects the cell and helps it maintain osmotic pressure in different environments. Many prokaryotes also have a sticky capsule layer that covers

 Core: Biology

Fertilization

JoVE 10907

During fertilization, an egg and sperm cell fuse to create a new diploid structure. In humans, the process occurs once the egg has been released from the ovary, and travels into the fallopian tubes. The process requires several key steps: 1) sperm present in the genital tract must locate the egg; 2) once there, sperm need to release enzymes to help them burrow through the protective zona pellucida of the egg; and 3) the membranes of a single sperm cell and egg must fuse, with the sperm releasing its contents—including its nucleus and centrosome—into the egg’s cytoplasm. If these steps are successful, the genetic material of the male and female gametes combine, and mitotic cell division commences, giving rise to a diploid embryo. The binding of the sperm and egg cell brings about various changes, among them the production of waves of calcium ions (Ca2+) pulsing through the egg cell. Such oscillations are initiated by sperm-egg fusion and result from both the release and uptake of endogenous Ca2+ in the endoplasmic reticulum of an egg cell and the simultaneous discharge and intake of such ions from the egg’s extracellular environment. Importantly, calcium signaling modifies the egg by causing vesicles, called cortical granules, that lay directly below its plasma membrane to release their contents into the open space bene

 Core: Biology

The Central Dogma

JoVE 10798

The central dogma of biology states that information encoded in the DNA is transferred to messenger RNA (mRNA), which then directs the synthesis of protein. The set of instructions that enable the mRNA nucleotide sequence to be decoded into amino acids is called the genetic code. The universal nature of this genetic code has spurred advances in scientific research, agriculture, and medicine. In the early 1900s, scientists discovered that DNA stores all the information needed for cellular functions and that proteins perform most of these functions. However, the mechanisms of converting genetic information into functional proteins remained unknown for many years. Initially, it was believed that a single gene is directly converted into its encoded protein. Two crucial discoveries in eukaryotic cells challenged this theory: First, protein production does not take place in the nucleus. Second, DNA is not present outside the nucleus. These findings sparked the search for an intermediary molecule that connects DNA with protein production. This intermediary molecule, found in both the nucleus and the cytoplasm, and associated with protein production, is RNA. During transcription, RNA is synthesized in the nucleus, using DNA as a template. The newly-synthesized RNA is similar in sequence to the DNA strand, except thymidine in DNA is replaced by uracil i

 Core: Biology

Cell Structure- Concept

JoVE 10587

Background

Cells represent the most basic biological units of all organisms, whether it be simple, single-celled organisms like bacteria, or large, multicellular organisms like elephants and giant redwood trees. In the mid 19th century, the Cell Theory was proposed to define a cell, which states:



Every living organism is made up of one or more cells.
The cells…

 Lab Bio

Internal Receptors

JoVE 11011

Many cellular signals are hydrophilic and therefore cannot pass through the plasma membrane. However, small or hydrophobic signaling molecules can cross the hydrophobic core of the plasma membrane and bind to internal, or intracellular, receptors that reside within the cell. Many mammalian steroid hormones use this mechanism of cell signaling, as does nitric oxide (NO) gas.

Similar to membrane-bound receptors, binding of a ligand to a receptor located in the cytoplasm or nucleus of a cell causes a conformational change in the receptor. Like transcription factors, the active receptor can bind to receptor-specific DNA binding sites to increase or decrease the transcription of target genes. In the case of an intracellular receptor located in the cytoplasm, the receptor-ligand complex must first cross the nuclear membrane. Many steroid hormones, including estrogen and testosterone, use intracellular receptors to induce specific effects. As an example, estrogen can diffuse across the membrane; binding of estrogen to its receptor results in dimerization of the receptors and transport of the ligand-receptor complex to the nucleus. Once in the nucleus, the complex can bind to DNA sequences called Estrogen-Response Elements (EREs). Depending on the other transcription factors and co-activators, binding of activated estrogen receptors (ERs) to EREs may cause an incre

 Core: Biology

Oogenesis

JoVE 10906

In human women, oogenesis produces one mature egg cell or ovum for every precursor cell that enters meiosis. This process differs in two unique ways from the equivalent procedure of spermatogenesis in males. First, meiotic divisions during oogenesis are asymmetric, meaning that a large oocyte (containing most of the cytoplasm) and minor polar body are produced as a result of meiosis I, and again following meiosis II. Since only oocytes will go on to form embryos if fertilized, this unequal distribution of cell contents ensures that there are enough cytoplasm and nutrients to nourish the early stages of development. Second, during oogenesis, meiosis “arrests” at two distinct points: once during embryonic growth and a second time during puberty. In mammals, oocytes are suspended in prophase I until sexual maturation, at which point meiosis I continues under hormonal influence until an egg precursor cell is released into a fallopian tube. At ovulation, the precursor exits the ovary and, only if fertilization occurs, is stimulated to complete meiosis II and form a complete egg. Defects during oogenesis can result in severe consequences. In particular, problems with chromosome segregation during either meiosis I or meiosis II may lead to an embryo being aneuploid, meaning that it contains an abnormal number of chromosomes. Increased age elevates a woman

 Core: Biology

Meiosis II

JoVE 10768

Meiosis II is the second and final stage of meiosis. It relies on the haploid cells produced during meiosis I, each of which contain only 23 chromosomes—one from each homologous initial pair. Importantly, each chromosome in these cells is composed of two joined copies, and when these cells enter meiosis II, the goal is to separate such sister chromatids using the same microtubule-based network employed in other division processes. The result of meiosis II is two haploid cells, each containing only one copy of all 23 chromosomes. Depending on whether the process occurs in males or females, these cells may form eggs or sperm, which—when joined through the process of fertilization—may yield a new diploid individual. Although the goal of meiosis II is the same in both males and females—to produce haploid egg or sperm cells—there are some critical differences in this process between the sexes. For example, in a woman’s egg precursor cells, the meiotic spindle apparatus responsible for separating sister chromatids forms off to one side, near the periphery. This asymmetry allows for two cells of unequal sizes to be produced following meiosis II: a large egg, and a smaller polar body that dissolves. This division of cytoplasm ensures that the egg contains enough nutrients to support an embryo. The position of the meiotic spind

 Core: Biology

Ribosomes

JoVE 10692

Ribosomes translate genetic information encoded by messenger RNA (mRNA) into proteins. Both prokaryotic and eukaryotic cells have ribosomes. Cells that synthesize large quantities of protein—such as secretory cells in the human pancreas—can contain millions of ribosomes.

Ribosomes are composed of ribosomal RNA (rRNA) and proteins. Ribosomes are not surrounded by a membrane (i.e., despite their specific cell function, they are not an organelle). In eukaryotes, rRNA is transcribed from genes in the nucleolus—a part of the nucleus that specializes in ribosome production. Within the nucleolus, rRNA is combined with proteins that are imported from the cytoplasm. The assembly produces two subunits of a ribosome—the large and small subunits. These subunits then leave the nucleus through pores in the nuclear envelope. Each one large and small subunit bind to each other once mRNA binds to a site on the small subunit at the start of the translation process. This step forms a functional ribosome. Ribosomes may assemble in the cytosol—called free ribosomes—or while attached to the outside of the nuclear envelope or endoplasmic reticulum—called bound ribosomes. Generally, free ribosomes synthesize proteins used in the cytoplasm, while bound ribosomes synthesize proteins that are inserted into membranes, packaged into org

 Core: Biology

Types of Hormones

JoVE 10988

Hormones can be classified into three main types based on their chemical structures: steroids, peptides, and amines. Their actions are mediated by the specific receptors they bind to on target cells.

Steroid hormones are derived from cholesterol and are lipophilic in nature. This allows them to readily traverse the lipid-rich cell membrane to bind to their intracellular receptors in the cytoplasm or nucleus. Once bound, the cytoplasmic hormone-receptor complex translocates to the nucleus. Here, it binds to regulatory sequences on the DNA to alter gene expression. Peptide hormones are made up of chains of amino acids and are hydrophilic. Hence, they are unable to diffuse across the cell membrane. Instead, they bind to extracellular receptors present on the surface of target cells. Such binding triggers a series of signaling reactions within the cell to ultimately carry out the specific functions of the hormone. Amine hormones are derived from a single amino acid, either tyrosine or tryptophan. This class of hormones is unique because they share their mechanism of action with both steroid as well as peptide hormones. For example, although epinephrine and thyroxine are both derived from the amino acid tyrosine, they mediate their effects through diverse mechanisms. Epinephrine binds to G-protein coupled receptors present on the surface of the plasma

 Core: Biology
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