The term
cell growth is used in the contexts of
cellThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos...
development and
cell division (reproduction). When used in the context of cell division, it refers to growth of cell populations, where one cell (the "mother cell") grows and divides to produce two "daughter cells".
Cell populations
Cell populations go through a type of
exponential growthExponential growth occurs when the growth rate of a mathematical function is proportional to the function's current value...
called doubling. Thus, each
generationGeneration , also known as procreation, is the act of producing offspring. In a more generic sense, it can also refer to the act of creating something inanimate such as electrical generation or cryptographic code generation....
of cells should be twice as numerous as the previous generation. However, the number of generations only gives a maximum figure as not all cells survive in each generation.
Yeast cell size regulation
The relationship between cell size and
cell divisionCell division is a process by which a cell, called the parent cell, divides into two or more cells, called daughter cells. Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing...
has been extensively studied in
yeastYeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with about 1,500 species currently described; they dominate fungal diversity in the oceans. Most reproduce asexually by budding, although a few do so by binary fission...
. For some cells, there is a mechanism by which
cell divisionCell division is a process by which a cell, called the parent cell, divides into two or more cells, called daughter cells. Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing...
is not initiated until a cell has reached a certain size. If the nutrient supply is restricted (after time t = 2 in the diagram, below) and the rate of increase in cell size is slowed, the time period between cell divisions is increased. Yeast cell size mutants were isolated that begin cell division before reaching the normal size (
wee mutants). The
Wee1Wee1 is a nuclear kinase belonging to the Ser/Thr family of protein kinases in the fission yeast Schizosaccharomyces pombe . It has a molecular mass of 96 kDa and it is a key regulator of cell cycle progression....
protein is a
tyrosine kinaseA tyrosine kinase is an enzyme that can transfer a phosphate group from ATP to a tyrosine residue in a protein. Tyrosine kinases are a subgroup of the larger class of protein kinases...
. It normally
phosphorylatesPhosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation turns many protein enzymes on and off, causing or preventing the mechanisms of diseases such as cancer and diabetes....
the Cdc2 cell cycle regulatory protein (cyclin-dependent kinase-1,
CDK1thumb|350px|Schematic of the cell cycle. outer ring: I=[[Interphase]], M=[[Mitosis]]; inner ring: M=Mitosis; G1=[[G1 phase|Growth phase 1]]; S=[[S phase|Synthesis]]; G2=[[G2 phase|Growth phase 2]]...
) on a
tyrosineTyrosine or 4-hydroxyphenylalanine, is one of the 20 amino acids that are used by cells to synthesize proteins. It is a non-essential amino acid with a polar side group...
residue. This covalent modification of the molecular structure of Cdc2 inhibits the enzymatic activity of Cdc2 and prevents cell division. In Wee1 mutants, there is less Wee1 activity and Cdc2 becomes active in smaller cells, causing cell division before the yeast cells reach their normal size. Cell division may be regulated in part by dilution of Wee1 protein in cells as they grow larger.
Cell size regulation in mammals
Many of the signal molecules that convey information to cells during the control of cellular differentiation or growth are called growth factors. The protein mTOR is a serine/threonine
kinaseIn chemistry and biochemistry, a kinase, alternatively known as a phosphotransferase, is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates. The process is referred to as phosphorylation...
that regulates
translationTranslation is the interpreting of the meaning of a text and the subsequent production of an equivalent text, likewise called a "translation," that communicates the same message in another language...
and cell division. Nutrient availability influences mTOR
so that when cells are not able to grow to normal size they will not
undergo cell division.
The details of the molecular mechanisms of mammalian cell size control
are currently being investigated. The size of post-mitotic neurons
depends on the size of the cell body, axon and dendrites. In
vertebrates, neuron size is often a reflection of the number of
synaptic contacts onto the neuron or from a neuron onto other cells.
For example, the size of motoneurons usually reflects the size of
the
motor unitA motor unit is a single α-motor neuron and all of the corresponding muscle fibers it innervates; all of these fibres will be of the same type . When a motor unit is activated, all of its fibers contract...
that is controlled by the motoneuron.
Invertebrates often have giant neurons and axons that provide
special functions such as rapid
action potentialAn action potential is a transient alteration of the transmembrane voltage across an excitable membrane generated by the activity of voltage-gated ion channels embedded in the membrane. Action potentials play multiple roles in several types of excitable cells such as neurons, myocytes, and...
propagation.
Mammals also use this trick for increasing the speed of signals in the
nervous system, but they can also use
myelinMyelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system...
to accomplish this, so
most human neurons are relatively small cells.
Other experimental systems for the study of cell size regulation
One common means to produce very large cells is by cell fusion to form
syncytiaIn biology, a syncytium is a large cell-like structure filled with cytoplasm containing many nuclei. Most cells in all organisms have a single nucleus; syncytia are specialized forms used by various organisms in normal tissue.- Formation :A syncytium can form in two ways...
. For example, very long (several inches)
skeletal muscleSkeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system. It is one of three major muscle types, the others being cardiac and smooth muscle. As its name suggests, skeletal muscle is linked to bone by bundles of collagen fibers known as...
cells are formed by fusion of thousands of
myocyteA myocyte is the type of cell found in muscles. They arise from myoblasts.Each myocyte contains myofibrils, which are long chains of sarcomeres, the contractile units of the cell....
s. Genetic studies of the fruit fly
DrosophilaDrosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is commonly known as the common fruit fly or vinegar fly. Starting from Charles W...
have revealed several genes that are required for the formation of multinucleated muscle cells by fusion of
myoblastA myoblast is a type of progenitor cell that gives rise to myocytes. Myocyte, skeletal muscle cell and muscle fiber are synonymous terms.Skeletal muscle fibers are made when myoblasts fuse together; muscle fibers therefore have multiple nuclei .The fusion of myoblasts is specific to skeletal muscle...
s. Some of the key proteins are important for
cell adhesionCellular adhesion is the binding of a cell to a surface, extracellular matrix or another cell using cell adhesion molecules such as selectins, integrins, and cadherins.- Process :...
between myocytes and some are involved in adhesion-dependent cell-to-cell
signal transductionIn biology, signal transduction refers to any process by which a cell converts one kind of signal or stimulus into another. Most processes of signal transduction involve ordered sequences of biochemical reactions inside the cell, which are carried out by enzymes and activated by second messengers,...
that allows for a cascade of cell fusion events.
OocyteAn oocyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is part of the ovary development. The germ cells produce a primordial germ cell which becomes an oogonium which marks the start of mitosis...
s can be unusually large cells in species for which embryonic development takes place away from the mother's body. Their large size can be achieved either by pumping in cytosolic components from adjacent cells through cytoplasmic bridges (
Drosophila) or by internalization of nutrient storage granules (yolk granules) by
endocytosisEndocytosis is the process by which cells absorb molecules from outside the cell by engulfing it with their cell membrane. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma membrane or cell membrane...
(
frogFrogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by long hind legs, a short body, webbed digits , protruding eyes and the absence of a tail...
s).
Increases in the size of
plant cellPlant cells are eukaryotic cells that differ in several key respects from the cells of other eukaryotic organisms. Their distinctive features include:...
s is complicated by the fact that almost all plant cells are inside of a solid
cell wallA cell wall is a tough, flexible and sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to act as a...
. Under the influence of certain plant hormones the cell wall can be remodeled, allowing for increases in cell size that are important for the growth of some plant tissues.
Most unicellular organisms are microscopic in size, but there are some giant bacteria and
protozoaProtozoa or Cornelius protozoans Protozoa or Cornelius protozoans Protozoa or Cornelius protozoans (from Greek πρῶτον proton "first" and ζῷα zoa "animals"; singular protozoon; (the word "protozoan" is originally an adjective, used as a noun) are microorganisms classified as unicellular eukaryotes....
that are visible to the naked eye. See:
Table of cell sizes - Dense populations of a giant sulfur bacterium in Namibian shelf sediments - Large protists of the genus Chaos, closely related to the genus Amoeba
Cell division
Cell reproduction is
asexualAsexual reproduction is reproduction which does not involve meiosis, ploidy reduction, or fertilization. Only one parent is involved in asexual reproduction. A more stringent definition is agamogenesis which refers to reproduction without the fusion of gametes...
. For most of the constituents of the cell, growth is a steady, continuous process, interrupted only briefly at M phase when the nucleus and then the cell divide in two.
The process of cell division, called
cell cycleThe cell cycle, or cell-division cycle, is the series of events that take place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...
, has four major parts called phases. The first part, called
G1 phase is marked by synthesis of various
enzymeEnzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, called the products. Almost all processes in a biological cell need enzymes to occur at...
s that are required for DNA replication.
The second part of the cell cycle is the
S phase, where
DNA replicationDNA replication, the basis for biological inheritance, is a fundamental process occurring in all living organisms to copy their DNA. This process is "semiconservative" in that each strand of the original double-stranded DNA molecule serves as template for the reproduction of the complementary strand...
produces two identical sets of chromosomes. The third part is the
G2 phase where significant
protein synthesis occurs during this phase, mainly involving the production of microtubules, which are required during the process of division, called
mitosisMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two daughter nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells containing...
.
The fourth phase,
M phase, consists of nuclear division (karyokinesis) and cytoplasmic division (
cytokinesisCytokinesis is the process in which the cytoplasm of a single eukaryotic cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation...
), accompanied by the formation of a new
cell membraneThe cell membrane is the biological membrane separating the interior of a cell from the outside environment....
. This is the physical division of "mother" and "daughter" cells. The M phase has been broken down into several distinct phases, sequentially known as
prophaseProphase is a stage of mitosis in which the chromatin condenses into a highly ordered structure called a chromosome in which the chromatin becomes visible. Prophase accounts for approximately 3% of the cell cycle. This process, called chromatin condensation, is mediated by the condensin complex...
,
prometaphasePrometaphase is the phase of mitosis following prophase and preceding metaphase, in eukaryotic somatic cells.The nuclear envelope breaks into fragments and disappears. Microtubules emerging from the centrosomes at the poles of the spindle reach the chromosomes, now highly condensed. At the...
,
metaphaseMetaphase, from the ancient Greek μετά and φάσις , is a stage of mitosis in the eukaryotic cell cycle in which condensed & highly coiled chromosomes, carrying genetic information, align in the middle of the cell before being separated into each of the two daughter cells...
,
anaphaseAnaphase, is from the ancient Greek ἀνά and φάσις , is the stage of mitosis when chromosomes separate in a eukaryotic cell. Each chromatid moves to opposite poles of the cell, the opposite ends of the mitotic spindle, near the microtubule organizing centers...
and
telophaseTelophase, from the ancient Greek "τελος" and "φασις" , is a stage in either meiosis or mitosis in an eukaryotic cell reversing the effects of prophase and prometaphase events...
leading to cytokinesis.
Cell division is more complex in
eukaryoteA eukaryote is an organism whose cells contain complex structures enclosed within membranes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried...
s than in other organisms.
ProkaryoticThe prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. They differ from the eukaryotes, which have a cell nucleus. Most are unicellular, but a few prokaryotes such as myxobacteria have multicellular stages in their life cycles...
cells such as
bacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
l cells reproduce by
binary fissionBinary fission, or prokaryotic fission, is the form of asexual reproduction and cell division used by all prokaryotic and some eukaryotic organisms...
, a process that includes DNA replication, chromosome segregation, and cytokinesis. Eukaryotic cell division either involves
mitosisMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two daughter nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells containing...
or a more complex process called
meiosisIn biology, meiosis is a process of reductional division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores. As with mitosis, before meiosis begins, the DNA in the original...
. Mitosis and meiosis are sometimes called the two "
nuclearIn cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as...
division" processes. Binary fission is similar to eukaryotic cell reproduction that involves mitosis. Both lead to the production of two daughter cells with the same number of chromosomes as the parental cell. Meiosis is used for a special cell reproduction process of diploid organisms. It produces four special daughter cells (
gameteA gamete is a cell that fuses with another gamete during fertilization in organisms that reproduce sexually...
s) which have half the normal cellular amount of DNA. A
maleMale refers to the sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization...
and a
femaleFemale is the sex of an organism, or a part of an organism, which produces mobile ova .- Defining Characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...
gamete can then combine to produce a
zygoteA zygote , or zygocyte, is the initial cell formed when a new organism is produced by means of sexual reproduction. A zygote is synthesized from the union of two gametes, and constitutes the first stage in a unique organism's development...
, a cell which again has the normal amount of chromosomes.
The rest of this article is a comparison of the main features of the three types of cell
reproduction that either involve binary fission, mitosis, or meiosis. The diagram below depicts the
similarities and differences of these three types of cell reproduction.
Comparison of the three types of cell division
The DNA content of a cell is duplicated at the start of the cell reproduction process. Prior to
DNA replicationDNA replication, the basis for biological inheritance, is a fundamental process occurring in all living organisms to copy their DNA. This process is "semiconservative" in that each strand of the original double-stranded DNA molecule serves as template for the reproduction of the complementary strand...
, the DNA content of a cell can be represented as the amount
Z (the cell has Z ribomosomes). After the DNA replication process, the amount of DNA in the cell is
2Z (multiplication: 2 x Z = 2Z). During Binary fission and mitosis the duplicated DNA content of the reproducing parental cell is separated into two equal halves that are destined to end up in the two daughter cells. The final part of the cell reproduction process is
cell divisionCell division is a process by which a cell, called the parent cell, divides into two or more cells, called daughter cells. Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing...
, when daughter cells physically split apart from a parental cell. During meiosis, there are two cell division steps that together produce the four daughter cells.
After the completion of binary fission or cell reproduction involving mitosis, each daughter cell has the same amount of DNA (
Z) as what the parental cell had before it replicated its DNA. These two types of cell reproduction produced two daughter cells that have the same number of chromosomes as the parental cell. After meiotic cell reproduction the four daughter cells have half the number of chromosomes that the parental cell originally had. This is the haploid amount of DNA, often symbolized as
N. Meiosis is used by diploid organisms to produce haploid gametes. In a diploid organism such as the human organism, most cells of the body have the diploid amount of DNA,
2N. Using this notation for counting chromosomes we say that human
somaticThe term somatic refers to cells of the body, rather than gametes . In humans, somatic cells contain two copies of each chromosome , whereas gametes only contain one copy of each chromosome...
cells have
46 chromosomesA karyotype is the charcteristic chromosome complement of a eukaryote species. The term may also be used of a cell, or an individual. The preparation and study of karyotypes is part of cytogenetics....
(2N = 46) while human
spermA sperm, from the ancient Greek word σπέρμα and and more commonly known as a sperm cell, is the haploid cell that is the male gamete. It joins an ovum to form a zygote. A zygote is a single cell, with a complete set of chromosomes, that normally develops into an embryo.Sperm cells contribute...
and
eggAn ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
s have 23 chromosomes (N = 23). Humans have 23 distinct types of chromosomes, the 22
autosomeAn autosome is a chromosome that is not a sex chromosome – that is to say there are an equal number of copies of the chromosome in males and females. For example, in humans, there are 22 pairs of autosomes, but the X and Y chromosomes are sex chromosomes....
s and the special category of
sex chromosomesA sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most sexual organisms have two sexes. In many cases, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual...
. There are two distinct sex chromosomes, the X chromosome and the Y chromosome. A diploid human cell has 23 chromosomes from that person's father and 23 from the mother. That is, your body has two copies of human chromosome number 2, one from each of your parents.
Immediately after DNA replication a human cell will have 46 "double chromosomes". In each double chromosome there are two copies of that chromosome's DNA molecule. During mitosis the double chromosomes are split to produce 92 "single chromosomes", half of which go into each daughter cell. During meiosis, there are two chromosome separation steps which assure that each of the four daughter cells gets one copy of each of the 23 types of chromosome.
Sexual reproduction
Main article: Evolution of sexScientists currently have developed several competing hypotheses to explain the evolution of sexual reproduction. Many groups of organisms, notably the majority of animals and plants, reproduce sexually. The evolution of sex contains two related, yet distinct, themes: its origin and its maintenance...
Though cell reproduction that uses mitosis can reproduce eukaryotic cells, eukaryotes bother with the more complicated process of meiosis because
sexual reproductionSexual reproduction is characterized by processes that pass a combination of genetic material to offspring, resulting in diversity. The main two processes are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the restoration...
such as meiosis confers a
selective advantageNatural selection is the process by which heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations...
. Notice that when meiosis starts, the two copies of sister chromatids number 2 are adjacent to each other. During this time, there can be
genetic recombinationGenetic recombination is the process by which a strand of genetic material is broken and then joined to a different DNA molecule. In eukaryotes recombination occurs in mitosis as a common mechanism of DNA repair and in meiosis as a way of facilitating chromosomal crossover...
events. Parts of the chromosome 2 DNA gained from one parent (red) will swap over to the chromosome 2 DNA molecule that received from the other parent (green). Notice that in mitosis the two copies of chromosome number 2 do not interact. It is these new combinations of parts of chromosomes that provide the major advantage for sexually reproducing organisms by allowing for new combinations of genes and more efficient
evolutionIn biology, evolution is change in the genetic material of a population of organisms from one generation to the next. Though changes produced in any one generation are normally small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a...
.
However, in organisms with more than one set of chromosomes at the main life cycle stage, sex may also provide an advantage because, under random mating, it produces homozygotes and heterozygotes according to the Hardy-Weinberg ratio.
Cell growth disorders
A series of growth disorders can occur at the cellular level and these underpins much of the subsequent course in
cancerCancer is a class of diseases in which a group of cells display uncontrolled growth , invasion , and sometimes metastasis...
, in which a group of cells display uncontrolled growth and division beyond the normal limits,
invasion (intrusion on and destruction of adjacent tissues), and sometimes
metastasisMetastasis , or metastatic disease, sometimes abbreviated mets, is the spread of a disease from one organ or part to another non-adjacent organ or part...
(spread to other locations in the body via lymph or blood).
See also
- Bacterial growth
250px|right|thumb|Growth is shown as L = log where numbers is the number of colony forming units per ml, versus T Bacterial growth is the division of one bacterium into two daughter cells in a process called binary fission. Providing no mutational event occurs the resulting daughter cells are...
- Cancer
Cancer is a class of diseases in which a group of cells display uncontrolled growth , invasion , and sometimes metastasis...
- Clone (genetics)
- Developmental biology
Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis," which is the process that gives rise to tissues, organs and anatomy.Developmental biology is that...
- Stem cell
Stem cells are cells found in most, if not all, multi-cellular organisms. They are characterized by the ability to renew themselves through mitotic cell division and differentiating into a diverse range of specialized cell types. Research in the stem cell field grew out of findings by Canadian...
- Cell cycle
The cell cycle, or cell-division cycle, is the series of events that take place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...
- Binary fission
Binary fission, or prokaryotic fission, is the form of asexual reproduction and cell division used by all prokaryotic and some eukaryotic organisms...
- Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two daughter nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells containing...
- Meiosis
In biology, meiosis is a process of reductional division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores. As with mitosis, before meiosis begins, the DNA in the original...
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