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IB Biology

Notes for Cells

The information in this document is meant to cover Topic 2 and Topic 4.2.1-4.2.3 of the IB Syllabus.
Characteristics of Living Things (summary)

Living organisms obtain and use energy to power activities such as movement and growth.
 heterotrophs consume other organisms or their products
 autotrophs convert sunlight to food
Living organisms try to maintain a constant internal environment.
 e.g. body temperature, water balance
Living organisms reproduce.
Living organisms are made of cells.

The Cell Theory was originally proposed by Matthias Schleiden & Theodor Schwann (~1840)
The cell theory states:
1. All life forms are made from one or more cells.
2. Cells only arise from pre-existing cells.
3. The cell is the smallest form of life.
Note: Some biologists consider unicellular organisms to be acellular. Skeletal muscle and fungal hyphae
are multinucleate – not technically organised into cells.
Evidence for the Cell Theory
Cells had been observed as early as the 1600’s (by Robert Hooke in 1662 and Anthony van
Leeuwenhoek in 1680) by using simple microscopes. By the 1800’s there was sufficient evidence that
biologists were able to state the cell theory:


Living things are made of cells

The branch of biology which studies cells specifically is called
histology. Histologists can observe cells using light and electron
microscopes, and have studied many unicellular and multicellular
organisms to learn about cellular structure.

Cells are the smallest units of life

Viruses are crystalline, non-cellular particles that may only
reproduce themselves by infecting a cell and taking over its
metabolic processes. They are made of components that are found
in cells (DNA or RNA and protein), but are not cells themselves.
Organelles are sub-structures found in cells. Some (e.g.
chloroplasts) have been shown to survive outside the cell for only
brief periods of time in laboratory experiments.

Cells come from pre-existing cells

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Louis Pasteur demonstrated that spontaneous generation (life from
non-living substances) does not occur, using a simple experiment.
Sterilised broth only grew bacteria when it had been exposed to air
– meaning that the microbes had to have come from the air itself,
and couldn’t just magically appear. 

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Notes for Cells
Some microorganisms have a dormant spore phase as part of their
life cycles – this allows for survival during unfavourable conditions,
and could account for living things “magically” appearing when
conditions are favourable again.
Observations of cells during mitosis and meiosis; one cell becomes
two (or more).

Cells contain a blueprint for
growth, development and

Observations on the behaviour of chromosomes provided insight
into the nature of genes (DNA) and both day-to-day activity and
Experimental evidence of the effect of gene transfer between
organisms (genetic engineering).

Cells are the site of the chemical
reactions of life

Enzymes are biological catalysts responsible for almost all cellular
processes, and most importantly those involved in extracting energy
from food (cellular respiration, fermentation).
Biochemical synthesis of macromolecules such as proteins (from
amino acids) and complex carbohydrates (from sugars).
Cell ultra-structure, the presence of discrete organelles with specific
functions. This also includes the discovery that certain biochemical
processes are restricted to particular regions or organelles within the

Unicellular Organisms carry out all the functions of life. This includes metabolism, response (to stimuli),
homeostasis, growth, reproduction and nutrition.
Multicellular Organisms have cells that are organized into tissues and organs. During early
development, cells differentiate by turning off certain genes and leaving other genes turned on. All body
cells of a multicellular organism still have the same DNA, but a skin cell doesn't need to have the insulin
gene turned on. 

Tissues are groups of cells that develop in the same way, with the same structure and function.
(e.g. heart muscle)
Organs are groups of tissues that have combined to form a single structure. In an organ the
tissues work together to perform an overall function. (e.g. the heart)
Organ systems are groups of organs within an organism that together carry out a process.
(e.g. cardiovascular system)

Multicellular organisms show emergent properties, which is a result of the interactions between
component parts. In essence, the whole is greater than the sum of its parts. Life itself can be seen as an
emergent property.
Stem Cells
Most of the cells in a multicellular organism are highly specialised, and may not continue to divide once
they have reached maturity. Some specialised cells never divide (e.g. brain neurons), and others only
divide when damaged (e.g. liver cells). Stem cells are unspecialised cells that may continue to divide
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throughout the life of the organisms. Adult stem cells can divide an unlimited number of times,
producing a new stem cell and a body tissue cell each time. This is how blood cells are produced in the
bone marrow.
Stem cell researchers use embryonic stem cells, which may grow into any type of cell, unlike adult stem
cells. These researchers hope that stem cell therapy will be used to treat diseases such as Parkinson’s,
Alzheimer’s and Type I diabetes. It is also possible that stem cell therapy could be used to treat spinal
cord injuries.
Prokaryotic and Eukaryotic Cells
All cells can be classified as either prokaryotic or eukaryotic.
 Prokaryotic cells do not have a nucleus. Instead they have a loop of naked DNA (nucleoid).
 Eukaryotic cells’ DNA is contained within a membrane, forming a nucleus. Eukaryotic
chromosomes are linear, not loops.
 Both prokaryotic and eukaryotic cells are organized into discrete structures called organelles,
which have specific functions within the cell. Eukaryotic cells have more, and they are more
Note: Any eukaryotic cell is more similar to any other eukaryotic cell than any prokaryotic cell.

Prokaryotic Cells
These cells do not have a membrane-bound nucleus, just a simple loop of DNA. They also have very few
Functions of Prokaryotic Cell Structures:

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http://www.biologyforlife.com/IB Biology/Units/Classification
and Diversity/bacteria diagram.jpg

cell wall: forms a protective
outer layer that prevents
damage from outside and
bursting if internal pressure is
too high
plasma membrane: controls
exchange of substances
(nutrients and waste) between
cytoplasm and extra-cellular
environment; some may be
pumped in by active transport
mesosome: increases the area
of the membrane (internally) for
ATP production; may be
involved in moving DNA to the
cell's poles before cell division
cytoplasm: contains enzymes
that catalyze the chemical
reactions of metabolism and
DNA in a region called the

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ribosomes: synthesize proteins by translating messenger RNA; some proteins stay in the cell
while others may be secreted
naked DNA: stores the genetic information that controls the cell and is passed on to the daughter
pili: hair-like structures that enable attachment to surfaces and to other bacteria

Prokaryotes (bacteria) may be classified according to their metabolism:

Photosynthesis: Blue-green bacteria make their own food by photosynthesis – making them

Nitrogen fixation: Nitrogen-fixing bacteria convert nitrogen gas from the air into nitrogen

Fermentation: Many bacteria absorb organic substances, convert them into other organic
substances and release them. For example, the bacteria which is used to produce yoghurt
converts lactose (sugar) into lactic acid.

All prokaryote cells are capable of extremely rapid growth when conditions are favourable to them – this
occurs by binary fission and can result in doubling every 20 minutes.
Eukaryotic Cells
Eukaryotic cells have a nucleus which is enclosed by a membrane, and many organelles, some of which
are also membrane-bound. DNA is enclosed within the nucleus. The liver cell below is a typical
eukaryotic cell.

Image from http://library.thinkquest.org/C004535/media/liver_cell.gif

Functions of Eukaryotic Cell Structures

ribosomes: may be free-floating in the cytoplasm, or attached to the endoplasmic reticulum; they
perform protein synthesis by translating information on mRNA
Golgi apparatus: modifies proteins which are being exported from the cell; produces lysosomes

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lysosomes: contain digestive enzymes; used to digest food particles brought in to the cell or may
break open to digest the cell when it becomes damaged (“suicide sac”)
rough endoplasmic reticulum (RER): is the site of protein synthesis for any proteins which are
being exported from the cell (free-floating ribosomes make proteins for use within the cell)
mitochondrion: is the site of aerobic cellular respiration
nucleus: contains the cell's genetic material (chromosomes) within the nuclear membrane
nucleolus: makes ribosomes for the cell

In addition to these internal components, there may also be extra-cellular structures such as
glycoproteins for support and adhesion (in animal cells) and a cell wall for support and maintaining
shape (in plant cells).
Comparing Plant & Animal Cells



Cell wall

Not present. Animal cells only have a
plasma membrane.

Cell wall and plasma membrane are
both present. Cell wall is composed of
cellulose (polysaccharide) fibres.


Not present.

Present in plant cells involved in
photosynthesis (i.e. Not found in root

Carbohydrate storage Carbohydrates are stored as glycogen.

Carbohydrates are stored as starch


Small and temporary. May not be
present at all.

Large and permanent – the fluid-filled
vacuole helps support the plant.


Usually rounded, but able to change

Usually square, and does not change

Cell division

Centrioles present – form the spindle
apparatus during mitosis. Cleavage
furrow forms during cytokinesis and
cells pinch apart.

Centrioles absent – spindle apparatus
formed from cytoskeleton. New cell
wall forms between daughter cells.

Comparing Prokaryotic and Eukaryotic Cells




Extremely small, 5-10 µm

Larger, 50-150 µm

Type of genetic

A naked loop of DNA.

Chromosomes consisting of strands of
DNA associated with protein
(chromatin). Four or more
chromosomes present.

Location of genetic

In the cytoplasm, in a region called the In the nucleus inside a double nuclear
membrane called the nuclear envelope.

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Cell wall

Generally present, made of

Present only in plants (cellulose) and
fungi (chitin)


Not present. The plasma membrane
and mesosomes are used for cellular

Always present.


Smaller (70S)

Larger (80S)

Organelles bounded Few or none are present.
by a single membrane *Usually none

Many are present including ER, Golgi
apparatus and lysosomes

Motile organelles

Some may have cilia or flagella with
internal structures, 200 nm in diameter

Some may have simple flagella, 20 nm
in diameter

Most cells are too small to see with the naked eye.
How Small is Small?

Eukaryotic Cell
Prokaryotic Cell

10 – 100 µm
1 – 5 µm
10 – 20 µm
2 – 10 µm
0.5 – 5 µm

Large Virus (HIV)
Cell Membrane
DNA Double Helix
H atom

100 nm
25 nm
7.5 nm thick
2 nm thick


In order to observe cells, we can use magnifying lenses (hand lens, microscopes) to bring them into
Light microscopes were the first type to be developed, and are in wide use still today. They use light and
glass lenses to magnify the image of a cell or tissue being observed.
Electron microscopes use narrow beams of electrons instead of light, and produce highly magnified
images. There are two types of electron microscope:

Transmission Electron Microscope (TEM): an electron beam passes through a very thin section of
material. An image is formed because some electrons pass through and others do not, similar to
how light microscopes work.

Scanning Electron Microscope (SEM): a beam of electrons is scanned in a series of lines across
the surface of the specimen. This results in a three-dimensional image of the specimen.

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Notes for Cells

Limitations to Cell Size
Cells cannot continue growing indefinitely. Once they reach a maximum size, they usually divide. If a
cell becomes too large, it develops problems related to its surface area-to-volume ratio: as the overall
size of the cell increases, its surface area-to-volume ratio decreases. Since the surface area represents the
cell's plasma membrane (through which all nutrients and oxygen enter, and wastes exit) and the volume
represents the cell's cytoplasm & organelles (which require nutrients and oxygen, and produce wastes), it
is important for cells to have a high surface area-to-volume ratio.
One 2 cm x 2 cm x 2 cm “cell” VS. eight 1 cm x 1 cm x 1 cm “cells”:

SA = 6(2 cm x 2 cm)


SA = 8 x 6(1 cm x 1 cm)

= 6(4 cm2)

= 8(6 cm2)

= 24 cm2

= 48 cm2

= 2 cm x 2 cm x 2 cm
= 8 cm


SA:V = 24:8
= 3:1


= 8(1 cm x 1 cm x 1 cm)
= 8 cm3
SA:V = 48:8
= 6:1

The eight smaller “cells” have the same total volume, but double the membrane, and so overall have a
better surface area-to-volume ratio. This is a recurring theme in biology, and not strictly related to the
size of cells.
Calculating Magnification
When viewing images of microscopic objects such as cells, it is often useful to know the magnification at
which you are viewing said object. The linear magnification is one way to indicate the size of an object:
1. Measure the cell's length or diameter on your drawing.
2. Measure the cell's actual length or diameter (may involve estimating size using FOV).
3. Make sure both measurements are in the same units.
4. Divide the diagram size by the actual or estimated size – this value is your magnification.

A scale bar (
) may also be used to indicate size – similar to the scale on a map, it represents a
specific distance on the diagram or photograph.
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Biological Membranes
Biological membranes, whether they are the plasma (cell) membrane, or a part of the endoplasmic
reticulum, all have a similar structure. They are composed of amphiphilic fats called phospholipids in a
bilayer. There are also various proteins embedded within the bilayer. This model of the plasma
membrane is called the Fluid Mosaic Model.

Image from http://bio.winona.edu/berg/ILLUST/memb-mod.jpg

phospholipids are amphiphilic, which means that they are both hydrophobic (the fatty acids) and
hydrophilic (the phosphate)
 it is because of this unique structure, that the phospholipid molecules arrange themselves in a
bilayer – the hydrophilic “heads” arrange themselves facing the watery extracellular fluid and
cytoplasm, while the hydrophobic “tails” hide in the middle, away from water

membrane proteins may be peripheral (attached to the surface) or integral (embedded within the
bilayer) – some integral proteins, called transmembrane proteins, pass all the way through the
 hormone receptor sites allow the hormone to bind on the surface of the cell, and transmit a
signal to the inside of the cell
 enzymes located in the membrane catalyse reactions inside or outside the cell, depending on
the location of the active site
 electron carriers are arranged in chains and pass electrons from one to the next in a series of
redox reactions (chemiosmosis)
 cell adhesion proteins allow cells to stick together (cell-cell recognition sites)
 pumps are used for active transport, using energy to move substances across the membrane
 channels are usually gated (to motion, binding of a ligand, or changes in voltage) and allow
facilitated diffusion through the membrane

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Membrane Transport
There are various mechanisms by which substances pass through the membrane.
Passive Transport is the movement of particles across a membrane with the concentration gradient with
no additional input of energy.

Diffusion is the passive movement of particles from a region of higher concentration to a region of
lower concentration, as a result of the random motion of particles.
Particles in a liquid or gas are in constant motion – this kinetic energy is the driving force
behind diffusion. This is also known as Brownian movement and increases with temperature.
 Particles can diffuse across a membrane that is permeable to them. Plasma membranes are
permeable to oxygen gas, carbon dioxide, and water.

Dialysis is the diffusion of solutes across a semi-permeable membrane. Oxygen and carbon
dioxide move across the cell membrane by dialysis.

Osmosis is the passive movement of water molecules from a region of lower solute concentration
(i.e. higher water concentration) to a region of higher solute concentration (i.e. lower water
concentration) across a partially permeable membrane. Osmosis occurs in response to a high
concentration of a substance that cannot cross the membrane freely, and water moves to that side
of the membrane in an attempt to achieve equilibrium. Because water is a polar molecule,
special pores called aquaporins allow water through.
Hypertonic solutions have a lower water concentration (higher solute concentration) than
cytoplasm. Salt water is generally hypertonic to cytoplasm.
 Hypotonic solutions have a higher water concentration (lower solute concentration) than
cytoplasm. Distilled water is hypotonic to cytoplasm.
 Isotonic solutions have the same water concentration (and therefore solute concentration) as
cytoplasm. Extracellular fluid (ECF) and blood plasma are both isotonic to cytoplasm.

Facilitated diffusion occurs when a transmembrane channel protein forms a fluid-filled
passageway for ions and small hydrophilic molecules such as glucose to pass through. These
channels are not constantly open – they open in response to a trigger such as a substance other
than the one passing through the channel binding to a receptor site. Opening the channel may
require some energy, due to changing the protein's shape, but transport through occurs passively,
by diffusion.

Active Transport

Active Transport is the movement of substances across membranes using energy from ATP. Active
transport can move substances against the concentration gradient (i.e. from low to high
concentration). Protein pumps in the membrane are used for active transport. Each pump only
transports a specific substance or substances. The sodium/potassium ion (Na+/K+) pump is a
typical example. (See next page for diagram.)

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Image from http://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-13/0812.jpg

Protons (H+) are also transported actively across the membrane, during chemiosmosis in mitochondria
(electron transport chain) Image from and chloroplasts (light-dependent reactions).


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Bulk Transport
During bulk transport, substances are transported across the membrane, or within the cell, enclosed
within a bubble of membrane called a vesicle. The fluidity of the membrane allows vesicles to form from
the plasma membrane or join with it (or the Golgi apparatus) quite easily.

Endocytosis is the transport of materials from the ECF to the cytoplasm by wrapping a portion of
the plasma membrane around it, and bringing it inside the cell. There are two forms of

Phagocytosis “cell eating” occurs when the membrane engulfs relatively large particles.
Some white blood cells (e.g. macrophages), and Amoeba use phagocytosis.

Pinocytosis “cell drinking” occurs when the membrane engulfs droplets of ECF and dissolved
particles. All cells use pinocytosis.

Exocytosis is the reverse of endocytosis. Materials made inside the cell (hormones or
neurotransmitters, for example) are brought to the membrane in a vesicle, which joins the plasma
membrane, releasing the contents to the ECF.

Substances can also be transported within the cell inside vesicles. Proteins made on the rough
endoplasmic reticulum bud off inside a vesicle and move to the Golgi apparatus. Here, they fuse
with the Golgi apparatus, and become modified – sugars or lipids may be added. Then, another
vesicle buds off the Golgi apparatus and moves to the plasma membrane.

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The Cell Cycle

Image from http://bhs.smuhsd.org/bhsnew/academicprog/science/vaughn/Student%20Projects/Paul%20&%20Marcus/cycle.jpg

The cell cycle involves three distinct phases:

Interphase is a period when the cell is not actively dividing. It is a period of growth for the cell.
Protein synthesis, metabolism of food, and other biochemical processes occur during this time.
Replication of DNA in preparation for cell reproduction also occurs during this time. It is divided
into three stages:

G1 is a period of growth and normal metabolic functions, which occurs directly after

S is a period of synthesis, or DNA replication in preparation for mitosis.

G2 is a period of further growth and preparation for mitosis.

Mitosis is the replication of a cell's nucleus in preparation for cell division. It is during this time
that chromosomes become visible with a light microscope. Mitosis produces two genetically
identical nuclei.
Prophase is the first stage of mitosis in cells.
■ The centrioles separate and move towards the poles of the cell, while extending
microtubules that form the spindle apparatus.
■ Chromatin super-coils around itself (like winding up a ball of yarn), resulting in visible
bodies called chromosomes – these are identical sister chromatids joined by a
■ The nuclear membrane begins to break down.
 Metaphase is the second stage of mitosis.
■ The chromosomes are attached to the spindle by their centromeres, and align along the
cell's equatorial plate.
■ The nuclear membrane is no longer present.

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Anaphase is the third stage of mitosis.
■ The centromeres have split, and the spindle fibres begin to contract, pulling the identical
sister chromatids to opposite ends of the cell (the poles).
 Telophase is the final stage of mitosis.
■ The sister chromatids have reached the poles of the cell.
■ The spindle fibres begin to break down, and the nuclear membrane reforms.
■ Chromosomes begin to uncoil and are no longer visible – once again they are called
■ Telophase is followed by cytokinesis.

Image from http://www.dartmouth.edu/~cbbc/courses/bio4/bio4-1997/images/mitosis.JPG

Cytokinesis is the splitting of the original (parent) cell into two new (daughter) cells. Cytokinesis
is different in plant and animal cells due to the presence (or absence) of a cell wall.
In animal cells, the plasma membrane begins to pull inwards at the equatorial plate after
anaphase. By the end of telophase, the membrane has met in the middle, and two cells are
formed as the membrane pinches off from itself.
 In plant cells, a new cell wall begins to form after anaphase, at the equatorial plate. Formation
of the cell wall divides the original cell into two.

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Comparison of Cytokinesis in Plant & Animal Cells:

Image from http://fig.cox.miami.edu/~cmallery/150/mitosis/c7.12.9cytokinesis.jpg

Mitosis is used in eukaryotes whenever it is necessary to produce identical cells:

growth of multicellular organisms (e.g. bone cells, muscle cells)

embryonic development

repair of damaged tissues (e.g. new skin cells to repair a wound)

asexual reproduction

How Mitosis ensures that Identical Cells are produced:

During interphase (S), an exact copy of each chromosome is made by DNA replication, forming
two identical sister chromatids.
The sister chromatids remain attached to each other by their centromeres during metaphase,
when each gets attached to a spindle fibre.
In anaphase, the centromeres split and one chromatid from each pair moves towards opposite
poles of the cell.
The chromosomes at the poles become the nuclei of the daughter cells, each with identical sets
of chromosomes.
Cytokinesis splits the parent cell in between the two new nuclei, forming two cells with exact
copies of the original nucleus.

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When Mitosis Fails
Mitosis, like any other process in cells, needs to be controlled. Normally, cells only undergo mitosis
when new cells are needed (e.g. for growth or repair). Sometimes, the control mechanisms fail, and cells
begin to divide uncontrollably. Repeated divisions cause the number of cells to quickly increase, forming
a mass of cells called a tumour. This can occur in any organ or tissue in the body.
Tumours can grow to a large size, and if cells break off from the tumour, they can spread to other parts of
the body and form more tumours (this is called metastasis). Cancer is a disease caused by the growth of
tumours. Tumours are harmful to the body because they use up nutrients and oxygen required by healthy
cells, killing healthy cells as they take over.

Image from http://www.medicalook.com/diseases_images/cancer.gif

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Meiosis – Reduction Division
In mitosis, the daughter cells produced have the same number of chromosomes as the parent cell – two
of each type. This condition is called diploidy – cells with two copies of each chromosome are diploid
(2n). Mitosis is useful for growth, development and repair – even asexual reproduction – sexual
reproduction requires another method of cell division to produce gametes.
Gametes (sex cells) combine with other gametes during fertilization. Because of this, they must have
only one set of chromosomes – a condition known as haploidy. When two haploid (n) gametes combine,
the resulting cell is diploid. Meiosis is the process by which gametes are formed, and is called a
reduction division because the parent cells are diploid, but the resulting gametes are haploid.
How does meiosis work?
Meiosis involves two divisions of the nucleus, known as meiosis I and meiosis II. These divisions are
similar to the process of mitosis.
Both mitosis and meiosis begin with duplication of a cell’s chromosomes:

From this point, a cell may progress through mitosis (to regenerate diploid cells) or meiosis (to produce
Meiosis I is the first stage of meiosis. It begins with prophase I, in which homologous chromosomes pair
up (forming tetrads). At this time, crossing over may occur, a process in which pieces of non-sister
chromatids are exchanged.

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Prophase I is also when the centrioles separate and move to opposite poles of the cell.
In metaphase I, the chromosomes migrate to the cell’s equatorial plate, attaching to the spindle fibres.

Homologous chromosomes are pulled apart by the shortening of the spindle fibres in anaphase I. The
centromeres remain intact.

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During telophase I, the chromosomes are divided into two separate cells. The centrioles and spindle
fibres disappear. Each cell has one homologous pair (bivalent).

There is no additional replication of DNA between meiosis I and meiosis II. Meiosis II very closely
resembles mitosis, and occurs in both cells formed by cytokinesis after telophase I. The end result after
telophase II is four haploid cells:

All images in this section from:

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Two processes ensure that the four daughter cells produced by meiosis are genetically different:
• independent assortment of maternal and paternal homologous chromosomes
o bivalents line up randomly at the cell’s equator in metaphase I
o separation of homologous chromosomes is independent – the new cells will have a
mixture of maternal and paternal chromosomes
• crossing over of segments of non-sister chromatids (between maternal and paternal
o this results in new combinations of genes on the chromosomes of the gametes produced


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