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Table of contents

Introduction

• Size of cells
• What is a cell?
• What is the difference between elements?
• What is living?
• What is interesting about cell biology?

Types of cells

• Prokaryotes
  • Bacteria
• Eukaryotes
  • Unique Properties of Plant Cells

Parts of the cell

• Membranes
• Organelles
• Genetic material
• Energy supply (chloroplasts and mitochondria)

Cell division

• Cell cycle
• Meiosis
• Mitosis

Genes

• Expression
• Translation

Introduction

Size of cells

Size of Cells

Cells are so small that even a cluster of these cells from a mouse only measures 50 microns

Although it is generally the case that biological cells are too small to be seen at all without a microscope, there are exceptions as well as considerable range in the sizes of various cell types. Eukaryotic cells are typically 10 times the size of prokaryotic cells (these cell types are discussed in the next Chapter). Plant cells are on average some of the largest cells, probably because in many plant cells the inside is mostly a water filled vacuole.

So, you ask, what are the relative sizes of biological molecules and cells? The following are all approximations:

0.1 nm (nanometer) diameter of a hydrogen atom
0.8 nm Amino Acid
  2 nm Diameter of a DNA Alpha helix
  4 nm Globular Protein
  6 nm microfilaments
 10 nm thickness cell membranes
 11 nm Ribosome
 25 nm Microtubule
 50 nm Nuclear pore
100 nm Large Virus
150-250 nm small bacteria such as Mycoplasma
200 nm Centriole
200 nm (200 to 500 nm) Lysosomes
200 nm (200 to 500 nm) Peroxisomes
800 nm giant virus Mimivirus
  1 µm (micrometer)
       (1 - 10 µm) the general sizes for Prokaryotes
  1 µm Diameter of human nerve cell process
  2 µm E.coli - a bacterium
  3 µm Mitochondrion
  5 µm length of chloroplast
  6 µm (3 - 10 micrometers) the Nucleus
  9 µm Human red blood cell
 10 µm
       (10 - 30 µm) Most Eukaryotic animal cells
       (10 - 100 µm) Most Eukaryotic plant cells
 90 µm small Amoeba
100 µm Human Egg
up to 160 µm Megakaryocyte
up to 500 µm  giant bacterium Thiomargarita
up to 800 µm  large Amoeba
  1 mm (1 millimeter, 1/10th cm)
  1 mm Diameter of the squid giant nerve cell
  120 mm Diameter of an ostrich egg (a dinosaur egg was much larger)
  3 meters Length of a nerve cell of giraffe's neck

Related reading

• Some early history related to the development of an understanding of the existence and importance of cells. The importance of microscopy.

What limits cell sizes?

Prokaryotes - Limited by efficient metabolism

Animal Cells (Eukaryotic) - Limited by Surface Area to Volume ratio

Plant Cells (Eukaryotic) - Have large sizes due to large central vacuole which is responsible for their growth

|What is a cell?

Cells are the organs) and organisms (animals and plants).

File:Biological cefundamental building blocks of life. Cells vary to form individual "single-cell" organisms (bacteria) to "multi-cellular" structures (tissue,ll.svg

Schematic of typical animal cell, showing subcellular components. Organelles: (1) nucleolus (2) nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (ER) (6) Golgi apparatus (7) Cytoskeleton (8) smooth ER (9) mitochondria (10) vacuole (11) cytoplasm (12) lysosome (13) centrioles

Cells are structural units that make up plants and animals; also, there are many single celled organisms. What all living cells have in common is that they are small 'sacks' composed mostly of water. The 'sacks' are made from a phospholipid bilayer membrane. This membrane is semi-permeable (allowing some things to pass in or out of the cell while blocking others). There exist other methods of transport across this membrane that we will get into later.

So what is in a cell? Cells are 90% fluid (called cytoplasm) which consists of free amino acids, proteins, carbohydrates, fats, and numerous other molecules. The cell environment (i.e., the contents of the cytoplasm and the nucleus, as well as the way the DNA is packed) affect gene expression/regulation, and thus are VERY important aspects of inheritance. Below are approximations of other components (each component will be discussed in more detail later):

Elements

• 59% Hydrogen (H)
• 24% Oxygen (O)
• 11% Carbon (C)
• 4% Nitrogen (N)
• 2% Others - Phosphorus (P), Sulphur (S), etc.

Molecules

• 50% protein
• 15% nucleic acid
• 15% carbohydrates
• 10% lipids
• 10% Other

Components of cytoplasm

• Cytosol - contains mainly water and numerous molecules floating in it- all except the organelles.
• Organelles (which also have membranes) in 'higher' eukaryote organisms:
  • Nucleus (in eukaryotes) - where genetic material (DNA) is located, RNA is transcribed.
  • Endoplasmic Reticulum (ER) - Important for protein synthesis. It is a transport network for molecules destined for specific modifications and locations. There are two types:
    • Rough ER - Has ribosomes, and tends to be more in 'sheets'.
    • Smooth ER - Does not have ribosomes and tends to be more of a tubular network.
  • Ribosomes - half are on the Endoplasmic Reticulum, the other half are 'free' in the cytosol, this is where the RNA goes for translation into proteins.
  • Golgi Apparatus - important for glycosylation, secretion. The Golgi Apparatus is the "UPS" of the cell. Here, proteins and other molecules are prepared for shipping outside of the cell.
  • Lysosomes - Digestive sacks found only in animal cells; the main point of digestion.
  • Peroxisomes - Use oxygen to carry out catabolic reactions, in both plant and animals. In this organelle, an enzyme called catalase is used to break down hydrogen peroxide into water and oxygen gas.
  • Microtubules - made from tubulin, and make up centrioles,cilia,etc.
  • Cytoskeleton - Microtubules, actin and intermediate filaments.
  • Mitochondria - convert foods into usable energy. (ATP production) A mitochondrion does this through aerobic respiration. They have 2 membranes, the inner membranes shapes differ between different types of cells, but they form projections called cristae. The mitochondrion is about the size of a bacteria, and it carries its own genetic material and ribosomes.
  • Vacuoles - More commonly associated with plants. Plants commonly have large vacuoles.
• Organelles found in plant cells and not in animal cells:
  • Plastids - membrane bound organelles used in storage and food production. These are similar to entire prokaryotic cells - for example, like mitochondria they contain their own DNA and self-replicate. They include:
    • Chloroplasts - convert light/food into usable energy. (ATP production)
    • Leucoplasts - store starch, proteins and lipids.
    • Chromoplasts - contain pigments. (E.g. providing colors to flowers)
  • Cell Wall - found in prokaryotic and plant cells; provides structural support and protection.

What is the difference between elements?

The various elements that make up the cell are:

• 59% Hydrogen (H)
• 24% Oxygen (O)
• 11% Carbon (C)
• 4% Nitrogen (N)
• 2% Others - Phosphorus (P), Sulphur (S), etc.

The difference between these elements is their respective atomic weights, electrons, and in general their chemical properties. A given element can only have so many other atoms attached. For instance carbon (C) has 4 electrons in its outer shell and thus can only bind to 4 atoms; Hydrogen only has 1 electron and thus can only bind to one other atom. An example would be Methane which is CH₄. Oxygen only has 2 free electrons, and will sometimes form a double bond with a single atom, which is an 'ester' in organic chemistry (and is typically scented).

Methane	Water	Methanol (Methyl Alcohol)
H | H-C-H | H	H H \ / O	H | H-C-O-H | H

As for the organic molecules that make up a typical cell:

• 50% protein
• 15% nucleic acid
• 15% carbohydrates
• 10% lipids
• 10% Other

Here is a list of Elements, symbols, weights and biological roles.

Element	Symbol	Atomic Weight	Biological Role
Calcium	Ca	40.1	Bone; muscle contraction, second messenger
Carbon	C	12.0	Constituent (backbone) of organic molecules
Chlorine	Cl	35.5	Digestion and photosynthesis
Copper	Cu	63.5	Part of Oxygen—carrying pigment of mollusk blood.
Fluorine	F	19.0	For normal tooth enamel development
Hydrogen	H	1.0	Part of water and all organic molecules
Iodine	I	126.9	Part of thyroxine (a hormone)
Iron	Fe	55.8	Hemoglobin, oxygen caring pigment of many animals
Magnesium	Mg	24.3	Part of chlorophyll, the photosynthetic pigment; essential to some enzymes.
Manganese	Mn	54.9	Essential to some enzyme actions.
Nitrogen	N	14.0	Constituent of all proteins and nucleic acids.
Oxygen	O	16.0	Respiration; part of water; and in nearly all organic molecules.
Phosphorus	P	31.0	High energy bond in ATP.
Potassium	K	39.1	Generation of nerve impulses.
Selenium	Se	79.0	For the working of many enzymes.
Silicon	Si	28.1	Diatom shells; grass leaves.
Sodium	Na	23.0	Part of Salt; nerve conduction
Sulfur	S	32.1	Constituent of most proteins. Important in protein structure: Sulfide bonds are strong.
Zinc	Zn	65.4	Essential to alcohol oxidizing enzyme.

What is living?

The question, "What is life?" has been one of many long discussions and the answer may depend upon your initial definitions.

Some definitions of life are:

1. The quality that distinguishes a vital and functional being from a non-living or dead body or purely chemical matter.
2. The state of a material complex or individual characterized by the capacity to perform certain functional activities including metabolism, growth, and reproduction.
3. The sequence of physical and mental experiences that make up the existence of an individual.

Under these definitions life may or may not include a virus that is only 'alive' if it can insert its genetic material into a living cell. To some, living systems that react to the environment, grow, improve, and reproduce are alive. A more liberal definition would include too much, a narrower one would not include all cells.

What is interesting about cell biology?

What makes particularly interesting is that there is so much that is not fully understood. A cell is a complex system with thousands of molecular components working together in a coordinated way to produce the phenomenon we call "life". During the 20th century these molecular components were identified (for example, see Human Genome Project), but research continues on the details of cellular processes like the control of cell division and cell differentiation. Disruption of the normal control of cell division can cause abnormal cell behavior such as rapid tumor cell growth.

Cells have complex interactions with the surrounding environment. Whether it is the external world of a single celled organism or the other cells of a multicellular organism, a complex web of interactions is present. Study of the mechanisms by which cells respond appropriately to their environments is a major part of cell biology research and often such studies involve what is called signal transduction. For example, a hormone such as insulin interacting with the surface of a cell can result in the altered behavior of hundreds of molecular components inside the cells. This sort of complex and finely tuned cell response to an external signal is required for normal metabolism and to prevent metaboic disorders like Type II diabetes.

Most of the cells of a multi-cellular organism have the same genetic material in every cell, yet, there are over 200 types of cells in the body that are different shapes, sizes and and carry out very different functions. And ALL of these cells were developed from one special cell, a zygote. The study of how the many cell types develop during embryonic development (Developmental Biology) is a branch of Biology that is heavily dependent on the use of microscopy. Much of the control of cell differentiation is at the level of the control of gene transcription, the control of which mRNAs are made. Muscle cells make muscle proteins and nerve cells make brain proteins. Geneticists, molecular biologists and cell biologists are working to discover the details of how cells specialize to accomplish hundreds of functions from muscle contraction to memory storage.

Summary

• Complexity in:
  • inter-relations between cells
  • signal transduction pathways inside cells
  • control of cell death and cell reproduction
  • control of cell differentiation
  • control of cell metabolism.

Types of cells

Prokaryotes

The structures of two prokaryotic cells. The bacterium (shown at the top) is a heterotrophs, organisms that eat other organisms. Cyanophytes are autotrophs, organisms that make their food without eating other organisms.

Most of these prokaryotic cells are small, ranging from 1 to 10 microns with a diameter no greater than 1 micron. The major differences between Prokaryotic and Eukaryotic cells are that prokaryotes do not have a nucleus as a distinct organelle and rarely have any membrane bound organelles [mitochondria, chloroplasts, endoplasmic reticulum, golgi apparatus, a cytoskeleton of microtubules and microfilaments] (the only exception may be a bacterium discovered to have vacuoles). Both types contain DNA as genetic material, have a surrounding cell membrane, have ribosomes[70 s], accomplish similar functions, and are very diverse. For instance, there are over 200 types of cells in the human body, that vary greatly in size, shape, and function.

Prokaryotes are cells without a distinct nucleus.They have genetic material but that material is not enclosed within a membrane. Prokaryotes include bacteria and cyanophytes. The genetic material is a single circular DNA strand and is located within the cytoplasm. Recombination happens through transfers of plasmids (short circles of DNA that pass from one bacterium to another). Prokaryoytes do not engulf solids, nor do they have centrioles or asters. Prokaryotes have a cell wall made up of peptidoglycin.

See also

Eukaryotes

Bacteria

Bacteria are prokaryotic, unicellular organisms. Bacteria are very small; so much so that 1 billion could fit on 1 square centimeter of space on the human gums, and 1 gram of digested food has 10 billion bacteria. Bacteria are the simplest living organisms. Previously they fell under the Kingdom Moneran, but now they fall into two different Kingdoms: Archaebacteria and Eubacteria. There are several differences between the two.

Bacteria also have special structures: Plasmids (a small loop of DNA separate from the nuclear region, which is used for creating genetic variety, inserting into other organisms, and by genetic engineers) and Endospores (hard coat created by some bacteria in extreme conditions--this is why canning jars must be boiled for a long time).

Bacteria Reproduction is either through binary fusion (splitting of a cell with no variety in its genes) or through several other forms that produce genetic variety: Transformation (taking DNA from environment and incorparting it into themselves), Conjugation ("sex" in which cilia hook together and the Plasmids exchange genes), and transduction (viri infect the bacteria and the bacteria infects the virus with its Plasmid to move genes throughout the population).

Archaebacteria

Archaebacteria have no peptidoglycan in their cellular walls. They also have odd lipids in their cell walls. Many are able to, and often do, live in extreme places (like early Earth), and are thus called extremophiles. There are 3 types of Archaebacteria:

1. Methanogens use Carbon dioxide and Hydrogen to make Methane. They are found in sewage, cows, and swamps, and they do not take in oxygen.
2. Extreme Halophiles live in extremely salty places (i.e.: the dead sea and great salt lake).
3. Thermoacidophiles prefer extremely hot, acidic areas (i.e.: hot springs and volcanos).

Eubacteria

Eubacteria have peptidoglycan in their cell walls, and they have no unusual lipids. Bacteria have three shapes:

• bacilli (hot dog shaped)
• cocci (ball shaped)
• spirilli (spring shaped).

Eubacteria can also have prefixes before their names: strepto, indicating chains of the shaped bacteria, and straphylo, indicating clusters of the shaped bacteria. Eubacteria are tested in labratories for Gram stains. Gram stains will determine if antibiotics will work (Gram postive) or if they will not (Gram Negative). There are four major types of Eubacteria:

1. Cyanobacteria are green bacteria that infest fertilizer polluted ponds and lakes and mass produce algae
2. Spirochetes are Gram negative bacteria on which antibiotics do not work
3. Gram Positive (both gram positive that are used to make yogurt, streptthroat is one of these)
4. Proteobacteria (E-coli)

Bacteria produce poisons that can cause sickness: exotoxins, which are given off by the Gram positive bacteria, and endotoxins, which are given off by Gram negative bacteria as they die.

Eukaryotes

An animal Cell

Eukaryotes house a distinct nucleus, a structure in which the genetic material (DNA) is contained, surrounded by a membrane much like the outer cell membrane. Eucaryotic cells are found in most algae, protozoa, all multicellular organisms (plants and animals) including humans. The genetic material in the nucleus forms multiple chromosomes that are linear and complexed with proteins that help the DNA 'pack' and are involved in regulation of gene expression.

The cells of higher plants differ from animal cells in that they have large vacuoles, a cell wall, chloroplasts, and a lack of lysosomes, centrioles, pseudopods, and flagella or cilia. Animal cells do not have the chloroplasts, and may or may not have cilia, pseudopods or flagella, depending on the type of cell.

See also

Prokaryotes

Unique Properties of Plant Cells

Biology_Cell_biology | Types of cells

<< Eukaryotes | Unique Properties of Plant Cells

Plant Cells have a number of important differences compared to their animal counterparts. The major ones are the Chloroplasts, Cell walls and Vacuoles. Unlike animal cells, plant cells do not have centrioles.

Chloroplasts

The chloroplasts are an organelle similar to the mitochondria in that they are self reproducing and they are the energy factories of the cell. There most of the similarities ends. Chloroplasts capture light energy from the sun and convert it into ATP and sugar. In this way the cell can support itself without food.

Vacuoles

Plants often have large structures containing water surrounded by a membrane in the center of their cells. These are vacuoles and act as a store of water and food (in seeds), a place to dump wastes and a structural support for the cell to maintain turgor. When the plant loses water the vacuoles quickly lose their water, and when plants have a lot of water the vacuoles fill up. In mature plants there is usually one large vacuole in the centre of the cell.

Cell walls

Plant cells are not flaccid like animal cells and have a rigid cell wall around them made of fibrils of cellulose embedded in a matrix of several other kinds of polymers such as pectin and lignin. The cellulose molecules are linear and provide the perfect shape for intermolecular hydrogen bonding to produce long, stiff fibrils. It is the cell wall that is primarily responsible for ensuring the cell does not burst in hypertonic surroundings.

Parts of the cell

Membranes

Cell biology | Parts of the cell

Plasma membrane bilayer

The phospholipid bilayer which the cell membrane is an example of, is composed of various cholesterol, phospholipids, glycolipids and proteins. Below is an example of a simple phospholipid bilayer.

The smaller molecules shown between the phospholipids are Cholesterol molecules. They help to provide rigidity or stability to the membrane. The two main components of phospholipids are shown in these figures by blue circles representing the hydrophilic head groups and by long thin lines representing the hydrophobic fatty acid tails.

Both the interior of the cell and the area surrounding the cell is made up of water or similar aqueous solution. Consequently, phospholipids orient themsleves with respect to the water and with each other so that the hydrophilic ("water loving") head groups are grouped together and face the water, and the hydrophobic ("water fearing") tails turn away from the water and toward each other. This self-organization of phospholipids results in one of just a few easily recognizable structures. Cell membranes are constructed of a phospholipid bilayer as shown above.

Smaller structures can also form, known as 'micelles' in which there is no inner layer of of phospholipid. Instead, the interior of a micell is wholly hydrophobic, filled with the fatty acid chains of the phospholipids and any other hydrophobic molecule they enclose. Micelles are not so important for the understanding of cellular structure, but are useful for demonstrating the principles of hydrophilicity and hydrophobicity, and for contrasting with lipid bilayers.

At least 10 different types of lipids are commonly found in cell membranes. Each type of cell or organelle will have a different percentage of each lipid, protein and carbohydrate. The main types of lipids are:

• Cholesterol
• Glycolipids
• Phosphatidylcholine
• Sphingomyelin
• Phosphatidylethnolamine
• Phosphatydilinositol
• Phosphatidylserine
• Phosphatidylglycerol
• Diphosphatidylglycerol (Cardiolipin)
• Phosphatidic acid

Important aspects of Membranes

1. Phospholipids
2. Cholesterol
3. Semi-permeability and Osmosis
4. Proteins and channels
5. Hydrophobicity
6. Self-assembly

Organelles

Cell biology | Parts of the cell

Schematic of typical animal cell, showing subcellular components.

Organelles: (1) nucleolus (2) nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (ER) (6) Golgi apparatus (7) Cytoskeleton (8) smooth ER (9) mitochondrion (10) vacuole (11) cytoplasm (12) lysosome (13) centrioles

Nucleus

The nucleus contains genetic material or DNA in the form of chromatin. It is a double membraned structure, with pores on it. These pores act as a "gateway" to help the nucleoplasm to maintain continuity with the cytoplasm.

Mitochondria

A mitochondrian is the organelle responsible for a cell's metabolism. It synthetizes ATP through a protein called ATP synthase. Mitochondria have a double membrane. An outer membrane and a folded inner membrane. The internal membrane, called the 'cristaeAWDQS' is invaginated (folded or creased), to maximize surface area enabling it to hold more ATP synthases.

Chloroplasts

The inside of a chloroplast with the granum circled.

Chloroplasts are found only in photosynthesizing cells; e.g. plant cells. Chloroplasts carry out photosynthesis by using several photosystem proteins. Chloroplasts also give a cell its green colour and are widely believed to have evolved from symbiotic prokaryotes that adapted to live inside eukaryotic cells. Physiologically, chloroplasts are flat discs usually 2-10 micrometer in diameter and 1 micrometer thick. The chloroplast has a two membrane envelope termed the Inner & Outer membrane respectively. Between these two layers is the intermembrane space.

Ribosomes

Ribosomes are responsible for protein synthesis; they are composed of two subunits that to elongate an aminoacid sequence. And they were dicovered by 'Chris Jennings'.

Endoplasmic Reticulum

The Endoplasmic Reticulum (ER) acts as a transport from the nucleus and ribosomes to the Golgi apparatus. There are two types of endoplasmic reticulum:

Smooth ER

Smooth ER act as transport for various things, mainly the RNA from the nucleus to the ribosomes (RNA is a small piece of the DNA code specifically designed to tell the ribosomes what to make). Smooth ER appears smooth in texture, hence the name.

Rough ER

Rough ER are "rough" because of the ribosomes embedded in them. The rough ER take the protein to the Golgi apparatus to be packaged into vacuoles

Golgi Complex

The Golgi Complex bonds functional groups to different biomolecules to direct them to their respective destinations. It basically "packages" the stuff into vacuoles. The Golgi Complex looks like pieces of pita bread stacked on top of each other.They are the ones that have their origin from the ER.They basically function as the delivery system of the cell.

Vacuole

Vacuoles are storage places. They store water, food or cell waste products.

Central Vacuole

The central vacuole is found only in plant cells. It is filled with water and is pressurised, like a balloon. This forces all the other organelles within the cell out toward the cell wall. This pressure is called turgor pressure and is what gives plants their "crisp" and firm structure.

Peroxisomes

Peroxisomes perform a variety of metabolic processes and as a by-product, produce hydrogen peroxide. Peroxisomes use peroxase enzyme to break down this hydrogen peroxide into water and oxygen.

Lysosomes

Lysosomes are vacuoles containing digestive and destructive membranes. In white blood cells, these are used to kill the bacteria or virus, while in tadpole-tail cells they kill the cell by separating the tail from the main body.

They also do much of the cellular digestion involved in apoptosis, the process of programmed cell death.

A day in the life of a cell

The nucleus has transcribed a piece of its DNA, called RNA, and sent it to the ribosomes embedded in the rough endoplasmic reticulum. These ribosomes then translates the RNA into some valuable protein, which is then transported to the Golgi Apparatus. The Golgi Apparatus "packages" the protein into a secretory vesicle where it eventually reaches the cell membrane ready for secretion out of the cell and into the external environment.

Links

For more info go to http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/cells.htm

Genetic material

Cell biology | Parts of the cell

1. Genetic material of Prokaryotes
2. Genetic material of Eukaryotes
3. Nucleus
4. Nuclear membrane
5. Nucleolus
6. Codons
7. RNA polymerase
8. Histones

Energy supply (chloroplasts and mitochondria)

Cell biology | Parts of the cell

Chloroplasts are the organelles used for photosynthesis (a process that incorporates light energy into storage as chemical energy) whereas mitochondria used in respiration (a process that releases stored chemical energy). It assumed that you already know the information about these organelles explained in the organelles section. If you have not read the entries on chloroplasts and mitochondria from there yet, please go back and read them now.

Chemicals to know

1. Chemicals to familiarize yourself with in advance.

Photosynthesis

1. Light Dependent Reactions
2. Calvin-Benson Cycle

Cellular Respiration

1. Glycolysis
2. Krebs cycle
3. Electron transport

Cell division

Cell cycle

The normal cell cycle consists of 2 major stages. The first is Interphase, during which the cell lives and grows larger. The second is Mitotic Phase. Interphase is composed of three subphases. G₁ phase (first gap), S phase (synthesis), and G₂ phase (second gap). The interphase is the growth of the cell. The normal cell functions of creating proteins and organelles. The Mitotic Phase is composed of Mitosis and Cytokinesis. Mitosis, when the cell divides. Mitosis can be further divided into multiple phases. Cytokinesis, which is when the two daughter cells complete their separation. There is some overlap between there two sub phases.

From Wikipedia

The cell cycle is the cycle of a biological cell, consisting of repeated mitotic cell division and interphase (the growth phase). A cell spends the overwhelming majority of its time in the interphase(about 90% of time).

Overview

Schematic of the cell cycle. I=Interphase, M=Mitosis.
The duration of mitosis in relation to the other phases has
been exaggerated in this diagram.

The cell cycle consists of

• G₁ phase, the first growth phase
• S phase, during which the DNA is replicated, where S stands for the Synthesis of DNA.
• G₂ phase is the second growth phase, also the preparation phase for the
• M phase or mitosis and cytokinesis, the actual division of the cell into two daughter cells

The cell cycle stops at several checkpoints and can only proceed if certain conditions are met, for example, if the cell has reached a certain diameter. Some cells, such as neurons, never divide once they become locked in a G₀ phase.

Details of mitosis

Schematic of interphase (brown) and mitosis (yellow).

Meiosis

Meiosis is a special type of cell division that is designed to produce gametes. Before meiosis occurs, the cell will be double diploid and have a pair of each chromosome, the same as before mitosis.

Meiosis consists of 2 cell divisions, and results in four cells. The first division is when genetic crossover occurs and the traits on the chromosomes are shuffled. The cell will perform a normal prophase, then enter metaphase during which it begins the crossover, then proceed normally through anaphase and telophase.

The first division produces two normal diploid cells, however the process is not complete. The cell will prepare for another division and enter a second prophase. During the second metaphase, the chromosome pairs are separated so that each new cell will get half the normal genes. The cell division will continue thorough anaphase and telophase, and the nuclei will reassemble. The result of the divisions will be 4 haploid gamete cells.

Crossover

Crossover is the process by which two chromosomes paired up during prophase I of meiosis exchange a distal portion of their DNA. Crossover occurs when two chromosomes, normally two homologous instances of the same chromosome, break and connect to each other's ends. If they break at the same locus, this merely results in an exchange of genes. This is the normal way in which crossover occurs. If they break at different loci, the result is a duplication of genes on one chromosome and a deletion on the other. If they break on opposite sides of the centromere, this results in one chromosome being lost during cell division.

Any pair of homologous chromosomes may be expected to cross over three or four times during meiosis. This aids evolution by increasing independent assortment, and reducing the genetic linkage between genes on the same chromosome.

Mitosis

Mitosis is the normal type of cell division. Before the cells can divide, the chromosomes will have duplicated and the cell will have twice the normal set of genes.

The first step of cell division is prophase, during which the nucleus dissolves and the chromosomes begin migration to the midline of the cell. (Some biology textbooks insert a phase called "prometaphase" at this point.)The second step, known as metaphase, occurs when all the chromosomes are aligned in pairs along the midline of the cell. As the cell enters anaphase, the chromatids, which form the chromosomes, will separate and drift toward opposite poles of the cell. As the separated chromatids, now termed chromosomes, reach the poles, the cell will enter telophase and nuclei will start to reform. The process of mitosis ends after the nuclei have reformed and the cell membrane begins to separate the cell into two daughter cells, during cytokinesis.

Mitosis divides genetic information during cell division.

From Wikipedia

In biology, Mitosis is the process of chromosome segregation and nuclear division that follows replication of the genetic material in eukaryotic cells. This process assures that each daughter nucleus receives a complete copy of the organism's genetic material. In most eukaryotes, mitosis is accompanied with cell division or cytokinesis, but there are many exceptions, for instance among fungi. There is another process called meiosis, in which the daughter nuclei receive half the chromosomes of the parent, which is involved in gamete formation and other similar processes, which makes the parent cell still active.

Mitosis is divided into several stages, with the remainder of the cell's growth cycle considered interphase. Properly speaking, a typical cell cycle involves a series of stages: G1, the first growth phase; S, where the genetic material is duplicated; G2, the second growth phase; and M, where the nucleus divides through mitosis. Mitosis is divided into prophase, prometaphase, metaphase, anaphase and telophase.

The whole procedure is very similar among most eukaryotes, with only minor variations. As prokaryotes lack a nucleus and only have a single chromosome with no centromere, they cannot be properly said to undergo mitosis.

Prophase

Prophase: The two round objects above the nucleus are the centrosomes. Note the condensed chromatin.

The genetic material (DNA), which normally exists in the form of chromatin condenses into a highly ordered structure called a chromosome. Since the genetic material has been duplicated, there are two identical copies of each chromosome in the cell. Identical chromosomes (called sister chromosomes) are attached to each other at a DNA element present on every chromosome called the centromere. When chromosomes are paired up and attached, each individual chromosome in the pair is called a chromatid, while the whole unit (confusingly) is called a chromosome. Just to be even more confusing, when the chromatids separate, they are no longer called chromatids, but are called chromosomes again. The task of mitosis is to assure that one copy of each sister chromatid - and only one copy - goes to each daughter cell after cell division.

The other important piece of hardware in mitosis is the centriole, which serves as a sort of anchor. During prophase, the two centrioles - which replicate independently of mitosis - begin recruiting microtubules (which may be thought of as cellular ropes or poles) and forming a mitotic spindle between them. By increasing the length of the spindle (growing the microtubules), the centrioles push apart to opposite ends of the cell nucleus. It should be noted that many eukaryotes, for instance plants, lack centrioles although the basic process is still similar.

Prometaphase

Some biology texts do not include this phase, considering it a part of prophase. In this phase, the nuclear membrane dissolves in some eukaryotes, reforming later once mitosis is complete. This is called open mitosis, found in most multicellular forms. Many protists undergo closed mitosis, in which the nuclear membrane persists throughout.

Now kinetochores begin to form at the centromeres. This is a complex structure that may be thought of as an 'eyelet' for the microtubule 'rope' - it is the attaching point by which chromosomes may be secured. The kinetochore is an enormously complex structure that is not yet fully understood. Two kinetochores form on each chromosome - one for each chromatid.

When the spindle grows to sufficient length, the microtubules begin searching for kinetochores to attach to.

Metaphase

Metaphase: The chromosomes have aligned at the metaphase plate.

As microtubules find and attach to kinetochores, they begin to line up in the middle of the cell. Proper segragation requires that every kinetochore be attached to a microtubule before separation begins. It is thought that unattached kinetochores control this process by generating a signal - the mitotic spindle checkpoint - that tells the cell to wait before proceeding to anaphase. There are many theories as to how this is accomplished, some of them involving the generation of tension when both microtubules are attached to the kinetochore.

When chromosomes are bivalently attached - when both kinetochores are attached to microtubules emanating from each centriole - they line up in the middle of the spindle, forming what is called the metaphase plate. This does not occur in every organism - in some cases chromosomes move back and forth between the centrioles randomly, only roughly lining up along the midline.

Anaphase

Early anaphase: Kinetochore microtubules shorten

Anaphase is the stage of meiosis or mitosis when chromosomes separate and move to opposite poles of the cell (opposite ends of the nuclear spindle). Centromeres are broken and chromatids rip apart.

When every kinetochore is attached to a microtubule and the chromosomes have lined up along the middle of the spindle, the cell proceeds to anaphase. This is divided into two phases. First, the proteins that bind the sister chromatids together are cloven, allowing them to separate. They are pulled apart by the microtubules, towards the respective centrioles to which they are attached. Next, the spindle axis elongates, driving the centrioles (and the set of chromosomes to which they are attached) apart to opposite ends of the cell. These two stages are sometimes called 'early' and 'late' anaphase.

At the end of anaphase, the cell has succeeded in separating identical copies of the genetic material into two distinct populations.

Telophase

Telophase is the opposite of prophase. A nuclear membrane reforms around each of the daughter cells; nucleoli reappear. The spindles and asters (in animals) disappear. The chromatids start to elongate and become less condensed, changing their form to the long and thin chromatin.

Cytokinesis

Cytokinesis refers to the physical division of one eukaryotic cell. Cytokinesis generally follows the replication of the cell's chromosomes, usually mitotically, but sometimes meiotically. Except for some special cases, the amount of cytoplasm in each daughter cell is the same. In animal cells, the cell membrane forms a cleavage furrow and pinches apart like a balloon. In plant cells, a cell plate forms, which becomes the new cell wall separating the daughters. Various patterns occur in other groups.

Genes

Expression

Gene expression is the first stage of a process that decodes what the DNA holds in a cell. It is the expression of a gene that gives rise to a protein.

How does gene expression occur?

Genetic expression is a wide complex process. It must be regulated by a series of mechanisms.

It starts of with transcription that gives rise to the RNA messenger (RNAm) from DNA. The RNAm in prokariotes is coupled with several ribosomes which are responsibles of translating proteins.

In eukariotes RNAm that is made from DNA is immature, and it is called preRNAm. PreRNAm loses non-coding sections (called exons), becoming onto a RNAm mature. RNAm is coupled to ribosomes on Rough Endoplasmatic Reticle (RER) where translation happens. Translation is made when a new polypeptide is formed. The genetic code indeed says the order of pe polypeptides, but it doen't give us a clue about it tridimensional structure. Tridimensional structure is given by post-translational processes.

Translation occurs following transcription wherein the protein synthesis machinery gets into action and uses its tools to read out the message that the RNA holds.

There are some genes without coding proteins, they works a regulation sequences. These sequences can enhance (called "enhancers") or inhibit (called "represors") when a protein is coupled with them and a substrate or a hormone are joined together.

In pluricellular organisms only few cell are allowed to produce a certain type of protein; e.g: Haemoglobin is encoded in every cell of a mammal organism (it includes human), but only precursors of red blood cell are allowd to express it (red blood cell are not allowed to express it, because they lose it nucleus). However, the enhancers and represors are present in every cell of a mammal

Genetic Information

In nature, there is information found in all living cells. Different cultures have often studied this information and used various forms of recording techniques to display it. Ancient Egyptians, in particular, referred to this information and its records as "provider of attributes" and determined it ||| to mean several, and that was earlier in human history of recording something that was known about nature.

There were often other signs as well that accompanied Egyptian writings on the source of this "information key of life". Among them were double, water and wick of twisted flax. But the most central one, for modern science, of course, was the snake like determinative that meant a worm or serpent in the limit of life. This limit, water, was "N" meaning that something or someone is, the essence which would be referred to by the Greeks as "esse" or "ens", and in today's English terms, the "essence".

In anthropology, the language of gene expression is rooted in the sources of knowledge that Odhiambo Siangla of Kenya has called "rieko" and Jeremy Narby of Switzeland has termed the "cosmic serpent". Both Siangla and Narby are not only experts in cultures but are trained in communication and expression. And from both the key has been the "three letter word".

In the alphabet of the three letter word found in cell biology are the organic bases, which are adenine (A), guanine (G), cytosine (C) and thymine (T). It is the triplet recipe of these bases that make up the ‘dictionary’ we call in molecular biology genetic code.

The codal system enables the transmission of genetic information to be codified, which at molecular level, is conveyed through genes.

What is gene ? A gene is a region of DNA that produces a functional RNA molecule. If a region of DNA is not functional, that region is not a transmissible form of information for protein synthesis. And because the information is not transmissible, it is not readily functional. There are various sizes of gene. The first recorded attempts to imagine the very small was the Horus Eye, which is also a pristine idea of limit. Today we talk about bases. The insulin gene, for example, has 1.7 x $10^3$, about 1700 nucleotides. There exists a receptor gene known as low-density lipoprotein (LDL). This protein has 4.5 x$10^4$ nucleotides. In terms of nucleotides this (LDL) approximates to 45,000 nucleotides. Now, with the dystrophic gene as another example, we find the nucleotides to be around 2.0 x$10^8$, approximately 200,000,000 nucleotides in number.

Now, the introns. It is the noncoding regions of DNA that are called introns meaning the “intervening sequences”. Introns make up a greater part of the nucleotide sequences of a gene. The coding regions are called exons to mean “expression sequences”. They constitute a minority of the nucleotide progression of a DNA and they instruct cellular workshops for the formation of proteins via amino acids.

Through proteins, the expression of genetic information is achieved. In particular are the enzymes. Even during the ancient time the enzymes were understood and utilized well. The enzymes catalyze the chemical reactions of anabolic kind, that is, the building of cellular food and those of catabolic type, the braking down of food. The two processes are collectively termed metabolism. What, further, can we add about proteins?

We can further say that proteins are concentration of heteropolymers manufactured from amino acids. There are 20 amino acids used in synthesizing natural proteins. It is clear that a protein may consist of many, in fact, several hundred amino acid sediments. It is essentially unlimited in number to speak about how many different proteins we can make from combinations of amino acids. Mathematics explains it well. There is therefore a diverse set of proteins whose forms and functions can be achieved by means of a coding system explained below.

Genetic information flows unidirectional, from DNA to protein and with messenger RNA (mRNA) as intermediate. First, DNA encodes genetic information into RNA molecule. This is called transcription (TC) of the information. Then the information gets converted into proteins, being named here translation (TL). It is this concept of information current that is called the Central Dogma of molecular biology. The Central Dogma is the fundamental theme in our exploration on gene articulation.

In order to complete the picture, we can add two further aspects of information flow. We can add duplication of the genetic material, which occurs prior to cell division. And that a DNA, in this case, represents duplication process, —DNA transfer. Wherefore in this case it is known as DNA replication. But where some viruses have RNA instead of DNA as their genetic material, we speak about reverse transcription (RT). With this transcription, we get a DNA molecule as a copy of the viral RNA genome.

In other words, genetic information, whether historically traced world wide (Narby,1998) or particularly assigned to ancient Africa (Siangla,1997) involves gene expression. Both DNA and RNA are polynucleotides, where nucleotides are the monomer—building units, which are composed of three basic subunits called nitrogenous base, sugar, and phosphoric acid. Genetic information is contained in DNA. The genetic code in DNA expresses the connection between the polynucleotide alphabet of four bases and 20 amino acids. In one strand of the parental DNA molecule, there is a dictated amino acid sequence strictly for protein production.

We will discuss in the next few postings, a relatively detailed understanding of the polymerization of amino acids sequence as directed by base sequences of messenger RNA.

At the moment, though, let us note that protein synthesis is an expression of genetic information. Protein synthesis is the cellular procedure, as we have said, of making proteins and involves two main processes: Transcription and Translation. The two processes mean that the direction of the synthesis is from DNA to RNA and then from RNA to protein respectively. Is this true to all organisms?

Yes. With a few exceptions, which are in mitochondria, and as stated above, some viruses become exceptions to this order because in their genetic material, they have RNA instead of DNA as their initial information source. However it is true that in all organisms, methods that relate the nucleotide sequence in messenger RNA to the amino acid sequence in proteins (genetic code proper) are the same. For in the given exceptions there occurs reverse transcription (RT). With that viral example of transcription noted, we get DNA molecular information being copied from the genome of viral RNA.

Building on this clue that is provided by transcription processes, we can readily see that a three-nucleotide sense codon denotes each amino acid. For example, UUU specifies phenylalanine, UCU specifies serine and GCA specifies alanine. But UAC and UAU both specify tyrosine. We will speak more about this tyrosine when expanding cell biology in the study of melanin.

Here now are other ways to see the remaining three properties of the genetic code. One is the contiguous property. With this property the codons do not overlap and at the same time they do not separated by spacers. The other is degenerate property in which there is more than one codon for some amino acids as exemplified by tyrosine in the above paragraph. And finally, there is the unambiguous property. With this genetic code of unambiguity, each codon specifies only one amino acid.

Translation

The Translation Phase of Genetic Expression is divided into 2 Steps Transcription and Translation. During Transcription RNA Polymerase unzips the two halfs of the DNA where it needs to transcript. Then free RNA bases Attach to the DNA bases with the Polymerase starting at the promoter and ending at the Termination signal. From this the RNA can become mRNA, rRNA, or tRNA. The mRNA is a ribbon like strand that takes the genetic information from the nucleus of the cell to the ribosome. rRNA forms a globular ball that attaches to the rough E.R. to help make ribosomes. finally the tRNA forms a hair shaped landing base that reads the genetic information to make proteins. Translation happens when mRNA is pulled through a ribosome and tRNA reads the RNA bases on the mRNA to make anti-codons of 3 bases and brings amino-acids to form the protein. This starts with the condon AUG and ends at UAG. When done the protein forms the correct shape and does the task it was created for. This brings the genetic code from the nucleus, which it never leaves, to the cytoplasm of the cell where proteins are produced to upkeep the body.

License

GNU Free Documentation License

Version 1.2, November 2002

Copyright (C) 2000,2001,2002  Free Software Foundation, Inc.
51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
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