Cell Biology - Biology

G0 phase is a post-mitotic phase. It may be a permanent (as often is the case for neurons) or semi-permanent (for some liver, kidney, and stomach cells) phase. Epithelial cells do not enter G0, allowing these cells to continue to divide throughout an organisms life.

The cell pictured above is in prophase. Key indicators that the cell is in this stage are that the nuclear envelope is still intact and chromosomes have become condensed and paired up with sister chromatids.

Most cells spend about 90% of their time in interphase. Note that mitosis and meiosis comprise only about 10% of the cell cycle.

Cell division, or mitosis, is a rather small portion of a cell's life and includes prophase, metaphase, anaphase and telophase. The majority of a cell's life is spent in interphase.

DNA is synthesized in the S phase only. The G phases are stages of cellular growth while the M phase consists of mitotic cellular division.

During anaphase, chromosomes are split and sister chromatids move to opposite ends of the cell. This separation is caused by the shortening of a pair of spindle fibers attached to the kinetochore of each sister chromatid. The other end of the spindle fibers is attached to a centriole at each respective end of the cell, which pulls the sister chromatids apart as it shortens the fiber.

Mitochondrial DNA is much smaller and simpler than nuclear DNA, meaning it does not need histones or introns. Mitochondrial DNA is also circular, like bacterial DNA, rather than linear like nuclear DNA. As the ovum is the only contributor of mitochondria to an offspring, all mDNA must therefore be inherited from the mother.

The mitochondria is responsible for making the energy for the cell by cellular respiration. It does this by taking the major breakdown products of glucose (pyruvate and NADH) and converting them into ATP by the citric acid cycle and electron transport chain. This is an oxygen-dependent process.

Lysosomes are specialized vesicles that bud off of the Golgi apparatus and house digestive enzymes within the cell. Lysosomes have only one outer membrane, made of a phospholipid bilayer, similar to the plasma membrane surrounding the cell.

In contrast, the nucleus, mitochondria, and chloroplasts all have two outer membranes. Each of these is made of a lipid bilayer, for a total of four layers of phospholipids around these structures.

Lysosomes are membrane-bound organelles that contain digestive enzymes which break down all four types of macromolecules. The mitochondria is responsible for synthesis of the majority of the cell's energy in the form of ATP. The cytoskeleton maintains cell shape and provides a "freeway" for transporting substances within the cell. Flagella are proteinaceous structures that aid in cell motility.

Ribosomes are composed of ribosomal RNA (rRNA) and proteins. Their primary function is to bind mRNA and tRNA to build proteins. Ribosomes are the fundamental structure necessary for translation and protein formation.

The nucleus houses the DNA of the cell, the cytoskeleton and specialized proteins transport compounds within the cell, and the smooth endoplasmic reticulum breaks down toxic substances, such as alcohol.

A cell wall is a tough, and rigid layer of polysaccharides lying outside of the plasma membrane in plants, some fungi, and some protists. The cell wall provides these cells with structural support and protection. It also prevents over-expansion when water enters the cell. Animal cells lack a cell wall, and only have a plasma membrane.

Hydrogen peroxide is an example of a reactive oxygen species, made as a byproduct of metabolism. Peroxisomes use the enzyme catalase to catalyze the following reaction: . Hydrogen peroxide can be toxic to a cell if it builds up, thus converting it to harmless substances like water and oxygen gas is an important metabolic function of peroxisomes. Also, note that the breakdown of fatty acids (beta oxidation) occurs in both peroxisomes and mitochondria. Proteins, carbohydrates, and nucleic acids are broken down by the lysosome.

The plant central vacuole is a membrane bound organelle containing water and other enzymes. The central vacuole functions as a storage space in the center of the plant cell, and is responsible for many functions including digestion, isolating harmful materials, maintaining internal hydrostatic pressure (turgor), storage of water and small molecules, cell rigidity, and more. Aerobic respiration occurs in the mitochondria, where:

The fluid-mosaic model for the cell membrane is a double layer of phospholipids with embedded proteins. The proteins do active transport in pumping molecules across the membrane. The phosphates are on the outside and the lipid chains are on the inside of the membrane.

The cell wall provides support and protection for the cell. Most are made from carbohydrate and protein. They are not made of phospholipids or lipids.

The nucleus is the site of DNA replication, and transcription, which is the process by which DNA is converted to RNA. Enzymes are a class of proteins. All protein synthesis occurs in the ribosomes, some of which are on the endoplasmic reticulum (rough ER), and some are floating in the cytoplasm. Detoxification of harmful substances is carried out by the smooth endoplasmic reticulum.

Nuclear pores are complex structures made up of proteins which allow movement of certain molecules into and out of the nucleus. Flagella are long tail-like projections found on some cells that function in cell motility. The rough endoplasmic reticulum has ribosomes on it, which are the sites of protein synthesis. Lysosomes are organelles that contain hydrolytic enzymes used to recycle/destroy macromolecules.

Prokaryotes have a single circular DNA molecule, and eukaryotes have multiple linear chromosomes. The four nucleotides for DNA are in both prokaryotes and eukaryotes.