Download The Cell Cycle - english for biology

Cell Biology-Cell structure and cell functions: Within cells there is an intricate
network of organelles that all have unique functions. These organelles allow the cell to
function properly. Arranged below according to location (nucleus, cytoplasm, and
surface) is a description of common organelles.



One or more per cell
Spherical shape
Denser than surrounding cytoplasm
Chromosomes
- Usually in the form of chromatin
- Contains genetic information
- Composed of DNA
- Thicken for cellular division
- Set number per species (i.e. 23 pairs for human)
Nuclear membrane
- Surrounds nucleus
- Composed of two layers
- Numerous openings for nuclear traffic
Nucleolus
- Spherical shape
- Visible when cell is not dividing
- Contains RNA for protein manufacture




Collective term for cytosol and organelles contained within
Colloidal suspension
Cytosol mainly composed of water with free-floating molecules
Viscosity constantly changes
1
Centrioles
- Paired cylindrical organelles near nucleus
- Composed of nine tubes, each with three tubules
- Involved in cellular division
- Lie at right angles to each other
Chloroplasts
- A plastid usually found in plant cells
- Contain green chlorophyll where photosynthesis takes place
Cytoskeleton
- Composed of microtubules
- Supports cell and provides shape
- Aids movement of materials in and out of cells
Endoplasmic reticulum
- Tubular network fused to nuclear membrane
- Goes through cytoplasm onto cell membrane
- Stores, separates, and serves as cell's transport system
- Smooth type: lacks ribosomes
- Rough type (pictured): ribosomes embedded in surface
Golgi apparatus
- Protein 'packaging plant'
- A membrane structure found near nucleus
- Composed of numerous layers forming a sac
2
Lysosome
- Digestive 'plant' for proteins, lipids, and carbohydrates
- Transports undigested material to cell membrane for removal
- Vary in shape depending on process being carried out
- Cell breaks down if lysosome explodes
Mitochondria
- Second largest organelle with unique genetic structure
- Double-layered outer membrane with inner folds called cristae
- Energy-producing chemical reactions take place on cristae
- Controls level of water and other materials in cell
- Recycles and decomposes proteins, fats, and carbohydrates, and
forms urea
Ribosomes
- Each cell contains thousands
- Miniature 'protein factories'
- Composes 25% of cell's mass
- Stationary type: embedded in rough endoplasmic reticulum
- Mobile type: injects proteins directly into cytoplasm
Vacuoles
- Membrane-bound sacs for storage, digestion, and waste removal
- Contains water solution
- Contractile vacuoles for water removal (in unicellular organisms)
Cell wall
- Most commonly found in plant cells
- Controls turgity
- Extracellular structure surrounding plasma membrane
- Primary cell wall: extremely elastic
- Secondary cell wall: forms around primary cell wall after growth is complete
3
Plasma membrane
- Outer membrane of cell that controls cellular traffic
- Contains proteins (left, gray) that span through the membrane and allow passage of materials
- Proteins are surrounded by a phospholipid bi-layer.
4
The Cell Cycle |
Despite differences between prokaryotes and eukaryotes, there are several common
features in their cell division processes. Replication of the DNA must occur. Segregation
of the "original" and its "replica" follow. Cytokinesis ends the cell division process.
Whether the cell was eukaryotic or prokaryotic, these basic events must occur.
Cytokinesis is the process where one cell splits off from its sister cell. It usually occurs
after cell division. The Cell Cycle is the sequence of growth, DNA replication, growth and
cell division that all cells go through. Beginning after cytokinesis, the daughter cells are
quite small and low on ATP. They acquire ATP and increase in size during the G1 phase
of Interphase. Most cells are observed in Interphase, the longest part of the cell cycle.
After acquiring sufficient size and ATP, the cells then undergo DNA Synthesis
(replication of the original DNA molecules, making identical copies, one "new molecule"
eventually destined for each new cell) which occurs during the S phase. Since the
formation of new DNA is an energy draining process, the cell undergoes a second
growth and energy acquisition stage, the G2 phase. The energy acquired during G2 is
used in cell division (in this case mitosis).
The cell cycle. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and
WH Freeman (www.whfreeman.com), used with permission.
Regulation of the cell cycle is accomplished in several ways. Some cells divide rapidly
(beans, for example take 19 hours for the complete cycle; red blood cells must divide at
a rate of 2.5 million per second). Others, such as nerve cells, lose their capability to
divide once they reach maturity. Some cells, such as liver cells, retain but do not
normally utilize their capacity for division. Liver cells will divide if part of the liver is
removed. The division continues until the liver reaches its former size.
Cancer cells are those which undergo a series of rapid divisions such that the daughter
cells divide before they have reached "functional maturity". Environmental factors such
as changes in temperature and pH, and declining nutrient levels lead to declining cell
division rates. When cells stop dividing, they stop usually at a point late in the G1 phase,
the R point (for restriction).
Prokaryotic Cell Division |
Prokaryotes are much simpler in their organization than are eukaryotes. There are a
great many more organelles in eukaryotes, also more chromosomes. The usual method
5
of prokaryote cell division is termed binary fission. The prokaryotic chromosome is a
single DNA molecule that first replicates, then attaches each copy to a different part of
the cell membrane. When the cell begins to pull apart, the replicate and original
chromosomes are separated. Following cell splitting (cytokinesis), there are then two
cells of identical genetic composition (except for the rare chance of a spontaneous
mutation).
The prokaryote chromosome is much easier to manipulate than the eukaryotic one. We
thus know much more about the location of genes and their control in prokaryotes.
One consequence of this asexual method of reproduction is that all organisms in a
colony are genetic equals. When treating a bacterial disease, a drug that kills one
bacteria (of a specific type) will also kill all other members of that clone (colony) it comes
in contact with.
Eukaryotic Cell Division |
Due to their increased numbers of chromosomes, organelles and complexity, eukaryote
cell division is more complicated, although the same processes of replication,
segregation, and cytokinesis still occur.
Mitosis |
Mitosis is the process of forming (generally) identical daughter cells by replicating and
dividing the original chromosomes, in effect making a cellular xerox. Commonly the two
processes of cell division are confused. Mitosis deals only with the segregation of the
chromosomes and organelles into daughter cells.
Eukaryotic chromosomes occur in the cell in greater numbers than prokaryotic
chromosomes. The condensed replicated chromosomes have several points of interest.
The kinetochore is the point where microtubules of the spindle apparatus attach.
Replicated chromosomes consist of two molecules of DNA (along with their associated
histone proteins) known as chromatids. The area where both chromatids are in contact
with each other is known as the centromere the kinetochores are on the outer sides of
the centromere. Remember that chromosomes are condensed chromatin (DNA plus
histone proteins).
Structure of a eukaryotic chromosome. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
6
During mitosis replicated chromosomes are positioned near the middle of the cytoplasm
and then segregated so that each daughter cell receives a copy of the original DNA (if
you start with 46 in the parent cell, you should end up with 46 chromosomes in each
daughter cell). To do this cells utilize microtubules (referred to as the spindle apparatus)
to "pull" chromosomes into each "cell". The microtubules have the 9+2 arrangement
discussed earlier. Animal cells (except for a group of worms known as nematodes) have
a centriole. Plants and most other eukaryotic organisms lack centrioles. Prokaryotes, of
course, lack spindles and centrioles; the cell membrane assumes this function when it
pulls the by-then replicated chromosomes apart during binary fission. Cells that contain
centrioles also have a series of smaller microtubules, the aster, that extend from the
centrioles to the cell membrane. The aster is thought to serve as a brace for the
functioning of the spindle fibers.
Structure and main features of a spindle apparatus. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer
Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
The phases of mitosis are sometimes difficult to separate. Remember that the process is
a dynamic one, not the static process displayed of necessity in a textbook.
Meiosis |
Sexual reproduction occurs only in eukaryotes. During the formation of gametes, the
number of chromosomes is reduced by half, and returned to the full amount when the
two gametes fuse during fertilization.
Ploidy |
Haploid and diploid are terms referring to the number of sets of chromosomes in a cell.
Gregor Mendel determined his peas had two sets of alleles, one from each parent.
Diploid organisms are those with two (di) sets. Human beings (except for their gametes),
most animals and many plants are diploid. We abbreviate diploid as 2n. Ploidy is a term
referring to the number of sets of chromosomes. Haploid organisms/cells have only one
set of chromosomes, abbreviated as n. Organisms with more than two sets of
chromosomes are termed polyploid. Chromosomes that carry the same genes are
termed homologous chromosomes. The alleles on homologous chromosomes may
differ, as in the case of heterozygous individuals. Organisms (normally) receive one set
of homologous chromosomes from each parent.
7
Meiosis is a special type of nuclear division which segregates one copy of each
homologous chromosome into each new "gamete". Mitosis maintains the cell's original
ploidy level (for example, one diploid 2n cell producing two diploid 2n cells; one haploid n
cell producing two haploid n cells; etc.). Meiosis, on the other hand, reduces the number
of sets of chromosomes by half, so that when gametic recombination (fertilization)
occurs the ploidy of the parents will be reestablished.
Most cells in the human body are produced by mitosis. These are the somatic (or
vegetative) line cells. Cells that become gametes are referred to as germ line cells. The
vast majority of cell divisions in the human body are mitotic, with meiosis being restricted
to the gonads.
Life Cycles |
Life cycles are a diagrammatic representation of the events in the organism's
development and reproduction. When interpreting life cycles, pay close attention to the
ploidy level of particular parts of the cycle and where in the life cycle meiosis occurs. For
example, animal life cycles have a dominant diploid phase, with the gametic (haploid)
phase being a relative few cells. Most of the cells in your body are diploid, germ line
diploid cells will undergo meiosis to produce gametes, with fertilization closely following
meiosis.Plant life cycles have two sequential phases that are termed alternation of
generations. The sporophyte phase is "diploid", and is that part of the life cycle in which
meiosis occurs. However, many plant species are thought to arise by polyploidy, and the
use of "diploid" in the last sentence was meant to indicate that the greater number of
chromosome sets occur in this phase. The gametophyte phase is "haploid", and is the
part of the life cycle in which gametes are produced (by mitosis of haploid cells). In
flowering plants (angiosperms) the multicelled visible plant (leaf, stem, etc.) is
sporophyte, while pollen and ovaries contain the male and female gametophytes,
respectively. Plant life cycles differ from animal ones by adding a phase (the haploid
gametophyte) after meiosis and before the production of gametes.
Many protists and fungi have a haploid dominated life cycle. The dominant phase is
haploid, while the diploid phase is only a few cells (often only the single celled zygote, as
in Chlamydomonas ). Many protists reproduce by mitosis until their environment
deteriorates, then they undergo sexual reproduction to produce a resting zygotic cyst.
Phases of Meiosis |
Two successive nuclear divisions occur, Meiosis I (Reduction) and Meiosis II (Division). Meiosis
produces 4 haploid cells. Mitosis produces 2 diploid cells. The old name for meiosis was
reduction/ division. Meiosis I reduces the ploidy level from 2n to n (reduction) while Meiosis II
divides the remaining set of chromosomes in a mitosis-like process (division). Most of the
differences between the processes occur during Meiosis I.
Comparison of Mitosis and Meiosis
Mitosis maintains ploidy level, while meiosis reduces it. Meiosis may be considered a reduction
phase followed by a slightly altered mitosis. Meiosis occurs in a relative few cells of a multicellular
organism, while mitosis is more common.
8
Mitosis
This process of cellular division is unique to eukaryotic cells. The
following illustrations depict only the replication of chromosomes
and their division. Keep in mind that the process of mitosis
involves the entire cell which includes a multitude of organelles.
Interphase
Time between divisions
Protein synthesis carried out
Chromatin present
Nucleolus present
DNA replicated towards division
time
Metaphase
Anaphase
Prophase
Chromatin thickens into
chromosomes
Nuclear membrane disintegrates
Centriole pairs move to opposite
ends of the cell
Spindle fibers begin to form
Guided by the spindle fibers, the
chromosome pairs
line up along the center of the
spindle structure
The chromosome pairs (sisters)
begin to pull apart
Once separated, they are called
daughter chromosomes
Due to pull, many chromosomes
bend
Groove in plasma membrane
present
Telophase
Chromosomes return to
chromatin
Spindle disintegrates
Nuclear membrane takes shape
again
Centrioles replicate
Membrane continues to pinch
inward
(in plant cells a new cell wall is
laid)
When the process is complete, each cell will have the same
genetic material that the original cell had before replication. Each
of the daughter cells is also identical to each other. Note that once
telophase is complete, the cell returns to interphase.
9
Meiosis
All gametes have half the number of chromosomes that regular
cells have. Gametes are created through the process of meiosis.
Meiosis involves two divisions which create four haploid cells.
Phase I
INTERPHASE I
PROPHASE I
METAPHASE I
ANAPHASE I
TELOPHASE I
In Phase I, all chromatin material makes a copy of itself. Spindle fibers begin to develop as the nuclear
membrane disintegrates. Some crossing over may occur as the chromosomes thicken and group into
tetrads. The tetrads line up and pull apart. Once the chromosomes pairs are at opposite poles the cytoplasm
material divides. Note how the two cells formed do not have the same genetic material, yet they have the
normal number of chromosomes.
Phase II
INTERPHASE II
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
In Phase II, no duplication of genetic material occurs. As the chromatin thickens to form chromosomes and
group into pairs, the spindle forms and the nuclear membrane disintegrates. As in mitosis, the pairs of
chromosomes line up at the center and pull apart to opposite poles. The cytoplams then divides. In male
organisms the four new cells are all the same size. In females one of the four cells receives the bulk of the
cytoplasm material. This becomes the functioning egg while the other three smaller cells disintegrate.
10
Cell theory:
Throughout time, thoughts and ideas of life have been formed, stretching from
abiogenesis and spontaneous generation to the modern cell theory. Here is an overview of
the progression of thought that has contributed to today's cell theory. You'll relive key
events in scientific history and explore profiles of influential scientists and philosophers.
The Cell Theory
ANAXIMANDER
A member of the Greeks in the sixth century B.C. who resided on the Ionian Islands. He is credited with
coming up with the primary thoughts of evolution. His perspective was that creatures from the sea were
forced to come ashore, thereby evolving into land creatures.
PLATO
Plato did not directly aid in the progress of biological thinking. His view was not experimental, but more
philosophical. Many of his students went on to influence the progression of biological studies in the field of
classification.
THE ATOMISTS
The most noted of this group of Greek philosophers was Democritus (460 - 370 B.C.). He followed
Anaximander's view of evolution. Democritus is credited as being the father of atomic theory which
connects directly to biology. One important theory of his was simply that if you have nothing, nothing may
be created out of it.
ARISTOTLE
Aristotle (384 - 322 B.C.) was known for his experimental approach and numerous dissections. He was
drawn to animal classification in order to discover aspects of connection between the soul and the human
body. Some of his animal classifications still stand today. One of his famous thoughts is a foreshadowing of
Mendelian genetic concepts:
"It is evident that there must be something or other really existing, corresponding to what we call by the
name of Nature. For a given germ does not give rise to any random living being, nor spring from any
chance one, but each germ springs from a definite parent and gives rise to a predictable progeny. And thus
it is the germ that is the ruling influence and fabricator of the offspring." THE DARK AGES
Following the Greeks, there was a downfall in scientific thought. This decline is usually attributed to the
Christian Church, and the power shift to the "barbaric" tribes.
LEONARDO DA VINCI, RENE DESCARTES, AND WILLIAM HARVEY
These three scientific figures, thought not all living during the same time period, can be accredited with
much of the advancement of anatomical thought following the Dark Ages, such as discovering the
circulation of blood.
THE MICROSCOPE
This instrument opened up new doors in the field of biology, by allowing scientists to gaze into a new
world: the cellular world. Galileo is credited with the invention of the microscope. Two of the main
pioneers in microscope usage were Athanasius Kircher and Antonie von Leeuwenhoek.
ROBERT HOOKE
This English naturalist (1635 - 1703) coined the term "cell" after viewing slices of cork through a
11
microscope. The term came from the Latin word cella which means "storeroom" or "small container". He
documented his work in the Micrographia, written in 1665.
JEAN-BAPTISTE DE LAMARCK
The majority of this Frenchman's work (1744 - 1829) dealt with animal classification and evolution. He is
credited with taking steps towards the creation of the cell theory with this saying:
"Every step which Nature takes when making her direct creations consists in organizing into
cellular tissue the minute masses of viscous or mucous substances that she finds at her disposal
under favorable circumstances."
THE CELL: AN INDIVIDUAL UNIT OF LIFE
In 1824, Rene Dutrochet discovered that "the cell is the fundamental element in the structure of living
bodies, forming both animals and plants through juxtaposition." However, the first sightings of the internal
action of the cell were made by Robert Brown. In Berlin, Johannes Muller created connections between
biology and medicine, prompting the connective thinking of his students, such as those of Theodore
Schwann. Schwann created the term "cell theory" and declared that plants consisted of cells. This
declaration was made after that of Matthias Schlieden's (1804 - 1881) that animals are composed of cells.
BIOGENESIS
German pathologist Rudolf Virchow (1821 - 1902) altered the thought of cellular biology with his
statement that "every cell comes from a cell". Not even twenty years after this statement,
processes of cell reproduction were being described--Virchow had completed the thought behind
the basic cell theory
Useful Terms of Cellular Biology
















Anaphase - phase of mitosis in which the chromosomes begin to pull to opposite poles of the cell
Abiogenesis - spontaneous generation, nonliving matter yielding life
Animal - kingdom composed of multicellular organisms divided into two divisions: vertebrates
and invertebrates, who obtain their food from external sources and reproduce sexually or asexually
Asexual - a type of reproduction that does not require the union of female and male gametes
Biogenesis - the concept that all life arises from living matter
Cell - the smallest unit of life that carries out its own processes
Cellulose - a carbohydrate that is found in cell walls
Cell wall - multi-layered, sturdy structure composed of cellulose that provides plants and other
organisms with their rigidity
Centrioles - essential tubular organelles found near the nucleus in pairs that aid in cellular
division
Chlorophyll - the green material found in chloroplasts that is active in photosynthesis
Chloroplasts - membrane-bound organelles containing chlorophyll that is found in photosynthetic
organisms
Chromosomes - condensed form of chromatin visible during cellular division
Cytoplasm - collective term for cytosol and all the organelles contained in it (outside the nucleus
and within the plasma membrane)
Cytoskeleton - network of microtubules that support and give structure to cell while aiding in
intracellular transport
Cytosol - jelly-like material that contains the organelles between the nucleus and the plasma
membrane
DNA - deoxyribonucleic acid is the double-helix molecule holding the genetic information of
organisms that, along with protein, composes the chromatin
12






























Endoplasmic reticulum - the cell's highway transport system composed of tubes and membranes
connected to the nuclear membrane and extending through the cytoplasm that is also utilized for
storing and separating
Enzyme - a protein utilized in chemical reactions
Eukaryotes - advanced cell type with a nuclear membrane surrounding genetic material and
numerous membrane-bound organelles dispersed in a complex cellular structure
Flagellum - an extension on many unicellular organisms that enables movement through whiplike motions
Fungi - kingdom in which organisms may be unicellular or multicellular, obtain their food from
external sources, and reproduce sexual or asexually
Golgi apparatus - multi-layered organelle near the nucleus used for packaging of materials to be
transported out of the cell
Interphase - time period between cellular divisions in which cellular processes such as protein
synthesis are carried out
Lysosomes - the digestive plants of food for the cell, changes shape from task to task
Meiosis - cellular division that yields four gametes through two cellular divisions
Metaphase - phase of mitosis in which the chromosome pairs line up at the equator of the cell
Mitochondria - genetically independent organelles that produce energy for the cells along their
many internal folds, called cristae
Mitosis - cellular division that yields two identical cells from one cell through a five-step process
Moneran - simplest kingdom containing prokaryotic cells (mostly bacteria), some of which can
create their own food
Nuclear membrane - membrane surrounding the nucleus that is covered with pores and controls
nuclear traffic
Nucleolus - spherical structure within the nucleus that contains RNA
Nucleus - spherical organelle that is the cell's control center
Phagocyte - collective term for cells that engulf other cells or microorganisms
Plant - kingdom containing a variety of multicellular organelles that reproduce sexually or
asexually, and also carry out photosynthesis
Plasma membrane - outer membrane of cells composed of proteins and a phospholipid bi-layer
that controls cellular traffic
Plastid - vital organelle that aids in the metabolism of unicellular organisms and plant cells
(chloroplasts, chromoplasts, leucoplasts are examples)
Prokaryotes - primitive cell type that lacks a nuclear membrane and membrane-bound organelles
Protein - a complex molecule found in numerous cellular structures that is composed of amino
acids
Protist cell - kingdom containing organisms with eukaryotic cells that are unicellular and
multicellular and reproduce sexually and asexually
Prophase - phase of mitosis in which the chromatin duplicates itself and thickens into
chromosomes, the spindle fibers form, and the nuclear membrane disintegrates
Ribosomes - extremely small grain-like organelle that provides the sites for protein synthesis
(they may be free in the cytoplasm or attached to the endoplasmic reticulum)
RNA - ribonucleic acid, a molecule that is a necessary component of the protein synthesis process
Sexual - reproduction involving the union of female and male gametes to form a zygote
Spontaneous generation - abiogenesis, the creation of life from nonliving material
Telophase - phase of mitosis in which the chromosome pairs have separated and reached opposite
poles of the cell as the spindle begins to disintegrate, the nuclear membrane reappears, and the
cytoplasm begins to divide
Vacuoles - membrane-bound organelles in the cytoplasm that are used for storage and digestion
13