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The Cell Nucleus

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					The Cell Nucleus
by James Niemasik
The nucleus is the brain of eukaryotic cells. It is only present in eukaryotic cells (which are eukaryotic because they have a nucleus) and there is only one of these organelles in each cell. Usually the nucleus is round and is the largest organelle in the cell. It is surrounded by a membrane, called the nuclear envelope, which is similar to the cell membrane that encloses the entire cell. The envelope is riddled with holes, called nuclear pores, that allow specific materials to pass in and out of the nucleus, just like proteins in the cell membrane regulate the movement of molecules in and out of the cell The nucleus is the pink half-sphere itself. Attached to the nuclear envelope is the in the center of the cell. endoplasmic reticulum. The nucleus is Source: Audesirk, Gerald, and surrounded by the cytoplasm inside a cell. Audesirk, Teresa. Biology: Life on Earth. The nucleus houses the DNA (deoxyribonucleic acid) which stores genetic information for a cell. The DNA contains instructions for the production of the cell's proteins and for reproduction. To construct proteins, the DNA is copied to messenger RNA (ribonucleic acid) in the process called transcription. The mRNA goes to the ribosomes, either in the nucleus or in the endoplasmic reticulum, where the actual construction of the proteins takes place. Structurally, the nucleus is composed of three main parts, the nucleolus, the nuclear envelope, and the chromatin. The nucleolus contains ribosomes, RNA, DNA, and proteins. The nucleolus has some of the ribosomes that

DNA to protein Source: http://cellbio.utmb.edu/cellbio/ribosome.htm

synthesize proteins (others are in the endoplasmic reticulum). The chromatin (meaning "colored substance") contains DNA and proteins formed into packets of code called chromosomes. When the cell divides, the chromosomes fold up on themselves, getting wider. The nuclear envelope is important because it allows the nucleus to control the rest of the cell, such as by sending out ATP. The envelope will let molecules like ATP through but will keep other things in or out, so the nucleus is isolated from the cytoplasm.

Structure of the Nucleus:

Membrane Structure
The cell is highly organized with many functional units or organelles. Most of these units are limited by one or more membranes. To perform the function of the organelle, the membrane is specialized in that it contains specific proteins and lipid components that enable it to perform its unique roles for that cell or organelle. In essence membranes are essential for the integrity and function of the cell. Membrane components may: be protective regulate transport in and out of cell or subcellular domain allow selective receptivity and signal transduction by providing transmembrane receptors that bind signaling molecules allow cell recognition provide anchoring sites for cytoskeletal filaments or components of the extracellular matrix. This allows the cell to maintain its shape and perhaps move to distant sites. help compartmentalize subcellular domains or microdomains provide a stable site for the binding and catalysis of enzymes. regulate the fusion of the membrane with other membranes in the cell via specialized junctions ) provide a passageway across the membrane for certain molecules, such as in gap junctions. allow directed cell or organelle motility

Prokaryotes, Eukaryotes, & Viruses Tutorial
Prokaryotes
Characteristics of prokaryotic cells. As mentioned in the previous page, prokaryotes include the kingdoms of Monera (simple bacteria) and Archaea. Simply stated, prokaryotes are molecules surrounded by a membrane and cell wall. Prokaryotic cells lack characteristic eukaryotic subcellular membrane enclosed "organelles," but may contain membrane systems inside a cell wall. Prokaryotic cells may have photosynthetic pigments, such as is found in cyanobacteria ("blue bacteria"). Some prokaryotic cells have external whip-like flagella for locomotion or hair like pili for adhesion. Prokaryotic cells come in multiple shapes: cocci (round), baccilli (rods), and spirilla or spirochetes (helical cells). Bacteria & antibiotics Pseudomonas bacteria The cell wall is the target for antibiotics, as well as for carbohydrates that our immune system uses to detect infection. A major threat to humankind is the antibiotic-resistant strains of bacteria have been selected by overuse of antibiotics.

Sympathy for the life of bacteria If you were bacteria:
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You have 0.001 times as much DNA as a eukaryotic cell. You live in a medium which has a viscosity about equal to asphalt. You have a wonderful "motor" for swimming. Unfortunately, your motor can only run in two directions and at one speed. In forward, you are propelled in one direction at 30 mph. In reverse your motor makes you turn flips or tumble. You can only do one or the other. You cannot stop. While you can "learn", you divide every twenty minutes and have to restart your education. You can have sex, with males possessing a sexual apparatus for transferring genetic information to receptive females. However, since you are both going 30 mph it is difficult to find each other. Furthermore, if you are male, nature gave you a severe problem. Every time you mate with a female, she turns into a male. In bacteria, "maleness" is an infective venereal disease.

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Also, at fairly high frequencies, spontaneous mutations cause you to turn into a female. Eukaryotes have enslaved some of your "brethren" to use as energy generating mitochondria and chloroplasts. They are also using you as a tool in a massive effort to understand genetics. The method of recombinant DNA is designed to exploit you for their own good. There is no SPCA to protect you. The last laugh may be yours. You have spent three and a half billion years practicing chemical warfare. Humans thought that antibiotics would end infectious diseases, but the overuse of drugs has resulted in the selection of drug resistant bacteria. They didn't realize that this was only the first battle, and now the war is ready to begin. Humans think this is their era. A more truthful statement would be that we all live in the age of bacteria.

The Archaea are becoming more understood The methanogenic archaeon, Metanococcus jannaschii:
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is found 3 km down, at 85 deg C has 1738 genes, 56% of which are new to science has bacteria-like genes and operons but with eukaryotic-like information processing and secretion systems and eukaryotic protein synthesis These findings represent the scientific equivalent of opening a new porthole on Earth and discovering a wholly new view of the universe. In decoding the genetic structure of archaea, we were astounded to find that two-thirds of the genes do not look like anything we've ever seen in biology before. This brings to closure the question of whether archaea are separate and distinct life forms. -Dr. J. Craig
Venter

Prokaryotes, Eukaryotes, & Viruses Tutorial
Eukaryotes
Basic structure The basic eukaryotic cell contains the following: plasma membrane glycocalyx (components external to the plasma membrane) cytoplasm (semifluid) cytoskeleton - microfilaments and microtubules that suspend organelles, give shape, and allow motion 5. presence of characteristic membrane enclosed subcellular organelles Characteristic biomembranes and organelles Plasma Membrane A lipid/protein/carbohydrate complex, providing a barrier and containing transport and signaling systems. 1. 2. 3. 4.

Nucleus Double membrane surrounding the chromosomes and the nucleolus. Pores allow specific communication with the cytoplasm. The nucleolus is a site for synthesis of RNA making up the ribosome.

Mitochondria Surrounded by a double membrane with a series of folds called cristae. Functions in energy production through metabolism. Contains its own DNA, and is believed to have originated as a captured bacterium.

Chloroplasts (plastids) Surrounded by a double membrane, containing stacked thylakoid membranes. Responsible for photosynthesis, the trapping of light energy for the synthesis of sugars. Contains DNA, and like mitochondria is believed to have originated as a captured bacterium.

Rough endoplasmic reticulum (RER) A network of interconnected membranes forming channels within the cell. Covered with ribosomes (causing the "rough" appearance) which are in the process of synthesizing proteins for secretion or localization in membranes. Ribosomes Protein and RNA complex responsible for protein synthesis. Smooth endoplasmic reticulum (SER) A network of interconnected membranes forming channels within the cell. A site for synthesis and metabolism of lipids. Also contains enzymes for detoxifying chemicals including drugs and pesticides.

Golgi apparatus A series of stacked membranes. Vesicles (small membrane surrounded bags) carry materials from the RER to the Golgi apparatus. Vesicles move between the stacks while the proteins are "processed" to a mature form. Vesicles then carry newly formed membrane and secreted proteins to their final destinations including secretion or membrane localization. Lysosymes A membrane bound organelle that is responsible for degrading proteins and membranes in the cell, and also helps degrade materials ingested by the cell.

Vacuoles Membrane surrounded "bags" that contain water and storage materials in plants.

Peroxisomes or Microbodies Produce and degrade hydrogen peroxide, a toxic compound that can be produced during metabolism.

Cell wall Plants have a rigid cell wall in addition to their cell membranes

Cell Membranes Problem Set
Problem 4: Osmosis
A sample of cells is placed in a salt solution. The cells shrink and the membrane is distorted. Relative to the cell, the solution is probably: A. isotonic B. hypotonic C. osmotic D. hypertonic

Cell Membranes Problem Set
Problem 4: Osmosis Tutorial to help answer the question
A sample of cells is placed in a salt solution. The cells shrink and the membrane is distorted. Relative to the cell, the solution is probably: A. isotonic. B. hypotonic.

C. osmotic. D. hypertonic.

Tutorial Cells in aqueous solutions
The figures show what can happen when animal or plant cells are placed in an aqueous solution. Water can move across membranes, but polar solutes dissolved in water cannot. The net movement of water (osmosis) is in the direction of increased solute concentrations. An easy way to visualize this rule is simply that the net water movement is from an area of high water concentration (little dissolved solute) to an area of low water concentration (high levels of solute). Animal cells Plant cells

If the solution is isotonic relative to the cell, then the solute concentrations are the same on both sides of the membrane and water moves equally in both directions

A hypertonic solution has increased solute, and a net movement of water outside causing the cell to shrink.

A hypotonic solution has decreased solute concentration, and a net movement of water inside the cell, causing swelling or breakage.

Problem 4 | Answer | Problem 5

Meiosis Tutorial

Meiosis I & II
What is meiosis I? In meiosis I, chromosomes in a diploid cell resegregate, producing four haploid daughter cells. It is this step in meiosis that generates genetic diversity. The phases of meiosis I & II Prophase I DNA replication precedes the start of meiosis I. During prophase I, homologous chromosomes pair and form synapses, a step unique to meiosis. The paired chromosomes are called bivalents, and the formation of chiasmata caused by genetic recombination becomes apparent. Chromosomal condensation allows these to be viewed in the microscope. Note that the bivalent has two chromosomes and four chromatids, with one chromosome coming from each parent. Prometaphase I The nuclear membrane disappears. One kinetochore forms per chromosome rather than one per chromatid, and the chromosomes attached to spindle fibers begin to move.

Metaphase I Bivalents, each composed of two chromosomes (four chromatids) align at the metaphase plate. The orientation is random, with either parental homologue on a side. This means that there is a 5050 chance for the daughter cells to get either the mother's or father's homologue for each chromosome. Anaphase I Chiasmata separate. Chromosomes, each with two chromatids, move to separate poles. Each of the daughter cells is now haploid (23 chromosomes), but each chromosome has two chromatids.

Telophase I

Nuclear envelopes may reform, or the cell may quickly start meiosis II.

Cytokinesis Analogous to mitosis where two complete daughter cells form.

Meiosis I Animation (360 kb) Meiosis II Animation (360 kb)
Meiosis II is similar to mitosis. However, there is no "S" phase. The chromatids of each chromosome are no longer identical because of recombination. Meiosis II separates the chromatids producing two daughter cells each with 23 chromosomes (haploid), and each chromosome has only one chromatid


				
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