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Cells are the Starting Point

All living organisms on Earth are divided into cells
...
Cells are small compartments that hold the
biological equipment necessary to keep an organism alive and
successful
...

There are smaller pieces that make up cells such
as macromolecules andorganelles
...
Cells can also connect to form larger structures
...
However, in the
same way that atoms are the basic unit when you study
matter, cells are the basic unit for biology and organisms
...

Cells hold a variety of pieces and each cell type has a
different purpose
...

If you were only made of one cell, you would be very limited
...
Cells have
problems functioning when they get too big
...
The trillions of cells in your body make
your way of life possible
...
In biology class, you will usually
work withplant-like cells and animal-like cells
...

Biology classes often take out a microscope and look at singlecelled microbes from pond water
...

Plant cells are easier to identify because they have a protective
structure called a cell wall made of cellulose
...
Plants also have organelles such as the
green chloroplast or large, water-filled vacuoles
...


Cells are unique to each type of organism
...

Humans have hundreds of different cell types
...
Even though cells
can be very different, they are basically compartments
surrounded by some type of membrane
...

Whether you are a single cell or a blue
whale with trillions of cells, you are still
made of cells
...
When you think about a
membrane, imagine it is like a big
plastic bag with some tiny holes
...
The holes are there to let some things move in and out of
the cell
...
It is made of millions of
smaller molecules that create a
flexible and porous container
...

The phospholipids make the basic
bag
...
There are also
proteins attached to the inner and
outer surfaces of the membrane
...
The model shows
you that phospholipid molecules are shaped with a head and a
tail region
...
Because
the tails want to avoid water, they tend to stick to each other
and let the heads face the watery (aqueous) areas inside and
outside of the cell
...


Ingrained in the Membrane
What about the membrane proteins? Scientists have shown
that many proteins float in the lipid bilayer
...
Some are only attached to the inner or outer layer
of the membrane while the transmembrane proteins pass
through the entire structure
...


Different Membranes of the Cell
As you learn more about cell organelles, you will find that they
all have a membrane
...
They have
different lipids and proteins that make them unique
...

Some organelles have two membranes
...
The outer membrane contains
the mitochondrion parts
...
While we talk about
membranes all the time, you should remember they all use a
basic phospholipid bilayer structure, but you will find many
variations throughout the cell
...
They are
basic bilayers made of lipids that surround the cell and
organelles
...
You will find millions of embedded protein
molecules when you look at the cell membrane
...
Examples of membrane proteins
include ion channels, receptor proteins, and proteins that allow

cells to connect to each other
...
Peripheral
proteins have weaker and temporary connections to the
membrane
...
When you look at the
entire membrane, there are more peripheral proteins when
compared to the number of integral proteins
...
They are hard
workers and have large sections embedded in
the hydrophobic (middle) layer of the membrane
...
Polytopic transmembrane proteins cross the
membrane several times
...
Ion movement that does not require
work is called passive transport while active
transport systems use work to move molecules
...


Discovering Structures
This structure of the membrane with embedded proteins and a
lipid bilayer was discovered in the early 1970's
...
" They used several different methods, such as
the freeze-fracture technique and electron micrographs, to look

closely at the cell membrane and its structure
...


Cell Wall - What's it for?
Cell membranes surround every cell
you will study
...
Cellulose is a specialized
sugar that is classified as a structural
carbohydrate and not used for
energy
...
The balloon is
protected from the outside world by a structure that provides
protection and support
...
When you eat plant material, you
can’t even digest and break down cellulose for energy
...

While cell walls protect the cells, they also allow plants to grow
to great heights
...
A 100foot tall redwood treedoes not
...
For overall support, dense cells in the core
of the trunk can let a tree grow very high
...

Winds can push them from side to side and they bounce back
...


Another Hole in the Wall
A cell wall is not an impenetrable
fortress around the delicate plant cell
...
The cell
membranes of neighboring cells are
able to connect through these holes
...
Molecules can also pass
through the spaces within the cell
walls, avoiding the cells completely (apoplastic pathways)
...
Cells can lose water
...
When the air heats up and the water vapor pressure
decreases, plants lose water through the process of
transpiration
...
Even when the plant cells lose
water, the basic shape is maintained by the cell walls
...
It will look just the same as when it started
...
Bacteria have
a structure called a cell wall
...
They are not the same as the plant cell walls
made of cellulose
...
Chitin is another structural

carbohydrate
...


Cell Connections and Communication
All living things communicate in one way or another
...
Cellular communication is
on a molecular level
...
We don't cover the
communication between single-celled organisms
...


Gap Junctions
Gap junctions are one type of cell connection
...
A gap junction is an opening from one
cell to another
...
The connections
are called channels and they act like tunnels for the movement
of molecules
...
They
physically connect cells like the gap junction, but no opening is
created
...
You will find desmosomes in
your skin cells
...

The distance between the cells, however small, is about 10
times wider than the gap junction connections
...
Tight junctions happen when two membranes actually
bond into one
...
Cells have some distance with a desmosome
...
Tight junctions form solid
walls
...


Cytoplasm - Filling Fluid
Cytoplasm is the fluid that fills a cell
...
Early on, they didn't know about the
many different types of fluids in the cell
...
The only two 'plasms' left
arecytoplasm (the fluid in the cell also called cytosol)
and nucleoplasm (the fluid in the nucleus)
...

The cell organelles are suspended in the cytosol
...
The cytoplasm has
many different moleculesdissolved in solution
...
Waste products are also dissolved before
they are taken in by vacuoles or sent out of the cell
...
Nucleoplasm can
only be found inside of the nucleus
...
The nucleoplasm is the suspension
fluid that holds the cell's chromatinand nucleolus
...
When the cell divides, the
nuclear membrane dissolves and the nucleoplasm is released
...


More than Filling
The cytosol in a cell does more than just suspend the
organelles
...
The products can then be used by the
organelles of the cell
...
The cytosol has enzymes that
break glucose down into pyruvate molecules that are then
sent to the mitochondria
...
It helps control
eating, movement, and reproduction
...
The nucleus is not
always in the center of the cell
...

You probably won't find it near the edge of a cell because that
might be a dangerous place for the nucleus to be
...


Life Before a Nucleus
Not all cells have a nucleus
...
You may
have heard of chromatin and DNA
...
If you don't have a defined nucleus, your DNA is
probably floating around the cell in a region called
the nucleoid
...


Important Materials in the Envelope
The things that make a eukaryotic cell are a defined nucleus
and other organelles
...
The nuclear envelope is a
membrane similar to the cell membrane around the whole
cell
...

When the cell is in a resting state there
is something called chromatin in the
nucleus
...
DNA and
RNA are the nucleic acids inside of the
cell
...
It
condenses
...
You will also find
the nucleolus inside of the nucleus
...
It is
made of RNA and protein
...


Chromosomes - Pull up Those Genes
Chromosomes are the things that make organisms what they
are
...
Chromosomes are made up of DNA
...
Your

genes make you who you are
...
Inprokaryotes,
DNA floats in the cytoplasm in an area called the nucleoid
...
They usually sit around
uncoiled and as loose strands called chromatin
...
The tightly wound DNA is the chromosome
...
They
are usually found in pairs
...
They count
individuals not every organism has pairs
...
Peas only have 12
...
The
number of chromosomes is NOT related to the intelligence or
complexity of the creature
...
Does that make a crayfish five times smarter or
more complex than you are? No
...
You
will often find plants of the same species with multiple sets of
chromosomes
...
You will most likely
find mRNA in the nucleus with the DNA
...
When the chromosomes are
visible, cells with two complete sets of chromosomes are
called diploids (46 in a human)
...
Cells
with only one set (23 in a human) are called haploid
cells
...
Haploid cells are
created in cell division termed meiosis
...
They are there to help the cell when it
comes time to divide
...
You will usually find
them near the nucleus but they cannot be seen when the cell
is not dividing
...


Centriole Structure
A centriole is a small set of microtubules arranged in a specific
way
...
When two
centrioles are found next to each other, they are usually at
right angles
...
During division, you may also see groups
of threads attached to the centrioles
...


Relaxing When There's no Work
We already mentioned that you would find centrioles near the
nucleus
...
You will see a condensed and darker area of
the cytoplasm called the centrosome
...
During division you will see four centrioles
...


Interphase is the time when the cell is at rest
...
During
prophase, the centrioles move to opposite ends of the nucleus
and a mitotic spindle of threads begins to appear
...

During anaphase, the chromosomes are split and pulled
towards each centriole
...
The centrioles have done their job
...
Enzymes made of proteins are
used to help speed up biological processes
...

Proteins even make up most of your hair
...
Ribosomes are the
protein builders or the protein synthesizers of the cell
...

Ribosomes are special because they are
found in both prokaryotes
and eukaryotes
...
Since there
are no membrane-bound organelles in
prokaryotes, the ribosomes float free in
the cytosol
...

You might find them floating in the cytosol
...

Other ribosomes are found on the endoplasmic reticulum
...
It looks bumpy under a microscope
...
There are also
ribosomes attached to the nuclear envelope
...


Two Pieces Make the Whole
There are two pieces or subunits to every
ribosome
...
Even though ribosomes have
slightly different structures in different
species, their functional areas are all very
similar
...
The 60-S/ 40-S model works fine for
eukaryotic cells while prokaryotic cells have ribosomes made of
50-S and 30-S subunits
...
Scientists
have used this difference in ribosome structure to develop
drugs that can kill prokaryotic microorganisms which cause
disease
...


Mixing and Matching Amino Acids

When are ribosomes used in the process of protein synthesis?
When the cell needs to make a protein, mRNA is created in the
nucleus
...
When it is time to make the protein, the two
subunits come together and combine with the mRNA
...

The process of making proteins is quite simple
...
Another nucleic acid that lives in the cell
is transfer RNA
...
With the mRNA offering instructions, the
ribosome connects to a tRNA and pulls off one amino acid
...
The ribosome builds a long amino acid
(polypeptide) chain that will eventually be part of a larger
protein
...
They
areorganelles that act like a digestive system which takes in
nutrients, breaks them down, and creates energy rich
molecules for the cell
...
Many of the reactions involved

in cellular respiration happen in the mitochondria
...

Mitochondria are small organelles floating free throughout the
cell
...
Muscle cells need a lot of energy so they
have loads of mitochondria
...
If a cell feels it is not getting
enough energy to survive, more mitochondria can be created
...
It all depends on the needs of the cell
...
They are
made of two membranes
...
The inner
membrane folds over many times and
creates layered structures
called cristae
...

The folding of the inner membrane
increases the surface area inside the organelle
...
If you have more space to work, you can get more work
done
...

What’s in the matrix? It's not like the movies at all
...
There are also

structures called granules which may control concentrations of
ions
...


Using Oxygen to Release Energy
How does cellular respiration occur in mitochondria? The matrix
is filled with water and proteins (enzymes)
...
Proteins embedded in the inner
membrane and enzymes involved in the citric acid
cycle ultimately release water (H2O) and carbon dioxide (CO2)
molecules from the breakdown of oxygen (O2) and glucose
(C6H12O6)
...

Mitochondria are also involved in controlling the concentration
of calcium (Ca2+) ions within the cell
...


Chloroplasts - Show Me the Green
Chloroplasts are the food
producers of the cell
...
Animal cells do not
have chloroplasts
...
The entire process is
called photosynthesis and it all
depends on the little green
chlorophyll molecules in each

chloroplast
...
They are classified as
the producers of the world
...
The oxygen
released by the chloroplasts is the same oxygen you breathe
every day
...
They
use oxygen in the process of releasing chemical energy from
sugars
...
Two membranes contain and protect
the inner parts of the chloroplast
...
The
inner membrane surrounds the stroma and the grana (stacks
of thylakoids)
...

Chlorophyll molecules sit on the surface of each thylakoid and
capture light energy from the Sun
...

The stacks of thylakoid sacs are connected by stroma
lamellae
...

If all of the thylakoids were overlapping and
bunched together, there would not be an
efficient way to capture the Sun’s energy
...
Photosynthesis is the process of a plant
taking energy from the Sun and creating sugars
...

Those energy-rich compounds move into the stroma where
enzymes fix the carbon atoms from carbon dioxide (CO2)
...

Plants and animals then use the sugars (glucose) for food and
energy
...


Different Chlorophyll Molecules
Not all chlorophyll is the same
...
You will hear about chlorophyll a
and b most often
...

There are other molecules that are also photosynthetic
...
While these compounds might
be involved in photosynthesis, they are not all green or the
same structure as chlorophyll
...


Endoplasmic Reticulum - Wrapping it Up

Another organelle in the cell is the endoplasmic
reticulum (ER)
...
It works closely with the Golgi
apparatus, ribososmes, mRNA, and tRNA
...
The membranes are slightly different from cell to cell
and a cell’s function determines the size and structure of the
ER
...
Cells that synthesize and
release a lot of proteins would need a large amount of ER
...


Rough and Smooth
There are two basic types of ER
...
Rough ER looks like sheets or disks of bumpy
membranes while smooth ER looks more like tubes
...

The double membranes of smooth and rough ER
form sacs called cisternae
...
When enough proteins have been
synthesized, they collect and are pinched off
invesicles
...

Smooth ER (SER) acts as a storage organelle
...
Steroids are a type of ringed organic molecule
used for many purposes in an organism
...
Cells in your
body that release oils also have more SER than most cells
...
It
is able to store many ions in solution that the cell will need at
a later time
...
It is easier to have
them stored in a pack for easy use
...
The SR can release those ions immediately
...


Rough ER (RER) was also mentioned in the section on
ribosomes and is very important in the synthesis and packaging
of proteins
...
” The RER is also attached to the nuclear
envelope that surrounds the nucleus
...

The process of protein synthesis starts when mRNA moves

from the nucleus to a ribosome on the surface of the RER
...
When the proteins are
complete, they collect and the RER pinches off a vesicle
...
Some of the proteins will
be used in the cell and some will be sent out into intercellular
space
...

It is another packaging organelle like the endoplasmic
reticulum (ER)
...
It is pronounced GOL-JI in the same way you would
say squee-gie, as soft a "G" sound
...


Foundation of Vesicles
The Golgi apparatus gathers simple molecules and combines
them to make molecules that are more complex
...
It is
also the organelle that builds lysosomes (cell digestion
machines)
...
The
vesicles are created in the same way the ER does it
...

The Golgi apparatus is a series of membranes shaped like
pancakes
...
The membrane surrounds an area of
fluid where the complex molecules (proteins, sugars, enzymes)

are stored and changed
...


Working with the Rough ER

The Golgi complex works closely with the rough ER
...
This vesicle or sac floats through the
cytoplasm to the Golgi apparatus and is absorbed
...
From there, the vesicle moves to the cell
membrane and the molecules are released out of the cell
...
They are found in both animal
and plant cells but are much larger
in plant cells
...

They can even store waste products so the rest of the cell is
protected from contamination
...

The structure of vacuoles is fairly simple
...
In that fluid are
nutrients or waste products
...
Those tiny water bags help to support the plant
...

In plant cells, the vacuoles are much larger than in animal
cells
...
Sometimes that vacuole can take up
more than half of the cell's volume
...
Don't forge that vacuoles can also
hold the plant waste products
...
Vacuoles
hold onto things that the cell might need, just like a backpack
...
Plants
usecell walls to provide support and surround cells
...
Plant cells do not shrink because of
changes in the amount of cytoplasm
...


Those vacuoles gain and lose water depending on how much
water is available to the plant
...
It still maintains its
basic structure because of the cell walls
...


Microfilaments - Stringy Proteins
You will find microfilaments in most cells
...
They are long, thin, and stringy
proteins (mainly actin) compared to the rounder, tube-shaped
microtubules
...
They work with microtubules
to form the structure that allows a cell to hold its shape, move
itself, and move its organelles
...
The cytoskeleton is different
from cytoplasm (cytosol)
...
Cytoplasm is just a fluid
...

Think about an amoeba
...

Then lysosomes and peroxisomes are sent to begin
digestion
...


You will also find many microfilaments in muscle tissue
...
The two
proteins myosin and actin work together to help the muscle
cells relax and contract
...
Combine those protein
threads with some ions in the muscle cell and you get a huge
contraction
...
All of the muscle cells work together to make a
muscle contract
...
We just talked about the
contraction of a muscle cell
...
When
you learn about single-celled organisms, you will understand
that they need to move
...
The microfilaments are often found anchored to
proteins in the cell membrane
...
Those binding proteins allow the microfilaments to
push and pull on the cell membrane to help the cell move
...

Although they are both proteins that help define cell structure
and movement, they are very different molecules
...
Those subunits are made of the
protein called tubulin
...


Elements of the Cytoskeleton
All of the microfilaments and microtubules combine to form the
cytoskeleton of the cell
...
The cytoskeleton provides
structure
...
The cytoskeleton connects
to every organelle and every part of the cell membrane
...
All of the pieces work together so that
the foot might reach out towards the food
...
The movement of the cell membrane, organelles,
and cytoplasm is all related to the tubules and filaments
...

They are also very important in cell division
...
They are a part of a small pair of
organelles called centrioles that have the specific purpose to
help a cell divide
...


Moving Organisms
Beyond the role they play in internal cell movement,
microtubules also work together to form larger structures that
work on the outside of the cells
...
Cilia are little
hairs you might see on the outside of a paramecium or
otherprotists
...

Flagella are long, thick tails
...


Lysosomes - Little Enzyme Packages
You will find organelles called lysosomes in
nearly every animal-like eukaryotic cell
...
The purpose of the lysosome is
todigest things
...
What
creates a lysosome? You'll have to visit
the Golgi complex for that answer
...
The enzyme proteins are first created in
the rough endoplasmic reticulum
...
The
Golgi then does its final work to create the digestive enzymes
and pinches off a small, very specific vesicle
...
From there the lysosomes float in the cytoplasm
until they are needed
...


Lysosome Action

Since lysosomes are little digestion machines, they go to work
when the cell absorbs or eats some food
...
The enzymes break down complex molecules that
can include complex sugars and proteins
...
When the signal is sent out,
lysosomes will actually digest the cell organelles for nutrients
...
If the
lysosome holds many types of enzymes, how can the lysosome
survive? Lysosomes are designed to break down complex
molecules and pieces of the cell
...
They are small vesicles found around the cell
...
They differ from
lysosomes in the type of enzyme they hold
...

Lysosomes have enzymes that work in oxygen-poor areas and
lower pH
...
They
are very well known for digesting fatty acids
...
Because
they do that job, you would expect liver cells to have more
peroxisomes than most other cells in a human body
...


Creating Hydrogen Peroxide
Peroxisomes work in a very specific way
...
One of the byproducts of the digestion is hydrogen
peroxide (H2O2)
...
The water is harmless
to the cell and the oxygen can be used in the next digestive
reaction
...
The protein enzymes are usually created
by lysosomes floating in the cell
...
Peroxisomes continue to grow until
they split in two
...
It may come from

the endoplasmic reticulum, but it may be created in a way
different from lysosomes
...
If they don't
do anything productive, they are not
needed anymore
...
They had their
purpose long before they started
working together in groups and
building more advanced organisms
...

Even if you were a single cell, you would have a purpose
...
You would be moving around (probably
in a liquid) and just trying to stay alive
...
If you were missing a piece you
needed to survive, you would die
...
Organelles are groups of complex
molecules that help a cell survive
...
The
larger a cell becomes the more
organelles it will need
...
If you are

a big cell, you will need to eat more than a little cell
...
A larger cell would
need to eat more and may wind up having
moremitochondria to process that food into energy
...
A cell from
your brain could not survive in a Petri dish for long
...
It does have the ability
to transmit electrical systems around your body
...
On the other hand, that amoeba will never help you
transmit electrical impulses
...
Simpler cells have a
better chance of surviving on their own while complex cells can
accomplish tasks that are more advanced
...

There are several different types of this
easy movement of molecules
...
You may
also see proteins in the cell
membrane that act aschannels to
help the movement along
...

Sometimes, proteins are used to help move molecules more
quickly
...
It could be
as simple as bringing in a glucose molecule
...
The cell might notice outside fluids rushing by with
free glucose molecules
...
That's an easy situation of passive transport because
the glucose is moving from higher to lower concentration
...
If you needed to
remove glucose, the cell would require energy
...
For
example, oxygen molecule
concentrations could be very high outside of the cell and very
low inside
...
There is no
energy needed for this process
...
It can also happen
with other molecules that can kill a cell
...
This is a water specific process
...
Because concentrations like to be the
same, the cell can pump ions in an out to stay
alive
...

For a cell to survive, ion concentrations need to be the same on

both sides of the cell membrane
...
This can be very bad
...
The classic example of this type of swelling
happens when red blood cells are placed in water
...


Active Transport - Energy to Transport
Active transport describes what happens when a cell
uses energy to transport something
...

We're talking about the movement of
individual molecules across the cell
membrane
...
Sometimes a
cell has to work and use some energy to
maintain a proper balance of ions and
molecules
...
There
are thousands of proteins
embedded in the cell's lipid bilayer
...
They are
positioned to cross the membrane so one part is on the inside
of the cell and one part is on the outside
...
The membrane proteins are very specific
...

There are hundreds of types of these membrane proteins in the
many cells of your body
...
That term means they are pumping something
(usually ions) from areas of lower to higher concentration
...
The membrane proteins are
constantly pumping ions in and out to get the membrane of the
neuron ready to transmit electrical impulses
...
There are poisons that stop the
membrane proteins from transporting their
molecules
...

Sometimes the proteins are destroyed and
other times they are just plugged up
...
What if a poison came along and
blocked eight of them? You could not survive with just two
pumps working and would slowly die
...


Phagocytosis - Time to Eat!
Simply put, cells need to eat and drink just like you
...
It is one type of
endocytosis
...
Phagocytosis is the situation when it gets
a solid
...
The

whole cell works during the process
...

Phagocytosis is a cell taking in a large object that it will
eventually digest
...
First, the cell senses the bacterium because of
chemicals in the environment
...
Once contact is made, the amoeba slowly wraps its cell
membrane around the object
...
When the cell
membrane surrounds the object, the object has actually been
sealed in a new vesicle
...
If a cell does not act
quickly, the bacterium could begin to
reproduce
...
The enzyme action is called
an oxidative burst
...
Waste particles remain in the
vesicle and can then be sent out of the cell
...
Once the process was refined,
unicellular organisms became able to eat other objects that
were almost as big as they were
...
Overall, cells could grow faster and larger
...


Pinocytosis - One More Time

Simply put, cells need to eat and drink just like
you
...
It is one
type of endocytosis
...
Phagocytosis is the situation when
it gets a solid
...

The whole cell works during the process
...


You Gotta Hydrate
Almost all cells do some form of pinocytosis
...
It's about taking in drops of fluid that are outside
of the cell
...
There are even different
styles of pinocytosis
...

Basic pinocytosis involves a cell taking in very small drops
of extracellular fluids
...
Pinocytosis sees the cell membrane wrap around
a drop and pinch it off into the cell
...

Pinocytosis is a process that is happening all of the time
...
A cell
with many microvilli (like in your intestine) is constantly
pinching off little vesicles filled with fluid
...


Mitosis - When Cells Split Apart
Eventually cells need to duplicate
...
This tutorial will talk

about mitosis
...
It duplicates
its DNA and the two new cells (daughter cells) have the
same pieces and genetic code
...
Start with one; get two that are the same
...

Beyond the idea that two identical cells are created, there are
certain steps in the process
...
You should remember the
term PMATI (pronounced PeeMahtEee)
...
It breaks down to
...
Always
remember - PMATI!

The Phases
Prophase: A cell gets the idea that it is time to divide
...
You need to duplicate DNA, get
certain pieces in the right position (centrioles), and generally
prepare the cell for the process of mitotic division
...
The DNA lines up along a central axis and
the centrioles send out specialized tubules that connect to the
DNA
...
Two strands of a chromosome are connected
at the center with something called a centromere
...

Anaphase: Here we go! The separation
begins
...
When the chromosomes
get to the side of the cell, it's time to
move on to telophase
...
This is the time when
the cell membranecloses in and splits
the cell into two pieces
...

Interphase: This is the normal state of a cell
...
It's just going about its daily business of
surviving and making sure it has all of the nutrients and energy
it needs
...
It is duplicating its nucleic acids, so when it's
time for prophase again, all the pieces are there
...
Mitosis has one division and meiosis has two
divisions
...
You also need to remember that four cells are created
where there was originally one
...
When a cell goes through
meiosis, it's not concerned about creating another working
cell
...
The

steps of meiosis are very simple
...
Scientists say Meiosis I and Meiosis
II, but it's just two PMATIs
...

As we said, meiosis happens when it's time to reproduce
...
Plants do it, animals do it, and even fungi do
it (sometimes)
...

That second division divides the number of chromosomes in
half
...
Haploid means half the regular
number
...
Normal cells are
considered to be diploid cells
...
Pairs of chromosomes are lined up at the center of the
cell and then pulled to each side
...

This crossing over is an exchange of genes
...
The
cell divides, leaving two new cells with a pair of chromosomes
each
...
Since this is meiosis, there is a very short
interphase and division begins again
...
Each
chromosome pair has acentromere
...
In Metaphase II all of
the chromosomes line up along the center of the cell and the
centrioles are in position for the duplication
...

Each one splits into two pieces
...

Each daughter cell will get one-half of the DNA needed to
make a functioning cell
...
When it's all over, you are
left with four haploid cells that are called gametes
...
When they do, they will form a
new organism
...


The common features of prokaryotic and eukaryotic cells are:
1
...
Plasma membrane, a phospholipid bilayer with proteins that separates the cell from the
surrounding environment and functions as a selective barrier for the import and export of materials
3
...
Ribosomes, the organelles on which protein synthesis takes place

Concept 2: Features of Prokaryotic Cells
Prokaryotes, which include all bacteria and archaea (archaebacteria), are the simplest
cellular organisms
...
Notably, prokaryotic cells lack a nucleus and membranous
organelles
...
g
...

Prokaryotic cells are fundamentally different in their internal organization from
eukaryotic cells
...

The nucleus is bounded by the nuclear envelope, a double membrane with
many nuclear pores through which material enters and leaves
...
Eukaryotic cells are more complex than prokaryotic cells and are
found in a great many different forms
...

Many materials are moved around the cell by the endomembrane system, including some proteins

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