Transcript Cells and Development - NIU Department of Biological Sciences

Cells and Development
Domains of Life
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Current classification scheme has the
largest division into 3 groups: Bacteria,
Archaea, and Eukarya. Based on 16S
ribosomal RNA sequence similarities.
Bacteria (also called Eubacteria) and
Archaea are prokaryotes. Eukarya are
eukaryotes.
Prokaryote: no membrane-bound nucleus
containing the cell’s DNA. Eukaryotes are
defined by having a membrane-bound
nucleus that holds the DNA.
Eukaryotes also have other membrane
bound organelles, and eukaryotes have
linear chromosomes.
Prokaryotes don’t have membrane bound
organelles, although some prokaryotes
have some internal membranes. Most
prokaryotes have circular chromosomes,
but some have linear chromosomes or
even a mixture of circular an linear
chromosomes.
Eukaryotic Cell Structures
Eukaryotic Cell Structures
• nucleus: holds the chromosomes, surrounded by the
double membrane nuclear envelope, which has
nuclear pores in it--traffic is controlled, but ribosomes
(big) can get out for example. The nucleolus is an
area of the nucleus where ribosomal RNA is made in
large quantities. Other structures in the nucleus
have also been defined, including area for
transcription and for RNA processing.
• mitochondria: makes most of the ATP by aerobic
respiration: Krebs cycle and electron
transport/oxidative phosphorylation. Two
membranes separate 2 different regions of the
mitochondria. Mitochondria have their own circular
DNA with about 30 genes on it: derived from
bacterial DNA (endosymbiont hypothesis).
Eukaryotic Cell Structures
• endoplasmic reticulum (ER): series of membrane-bound channels
and vesicles in the cell. The rough ER is studded with ribosomes:
for translation of proteins that get secreted or get inserted into the
cell membrane. The smooth ER is where sugars are added to the
proteins (glycosylation); membrane lipids are also synthesized in the
smooth ER.
• Golgi apparatus: takes proteins from the ER and packages them for
secretion from the cell. Movement between the ER, the Golgi, and
the plasma membrane occurs in small membrane-bound bodies
called vesicles.
• Lysosomes: intracellular digestion. Low internal pH and full of
various hydrolytic enzymes. Vesicles full of extracellular material
get transported from plasma membrane to the lysosomes; also
involved in apoptosis (programmed cell death).
• Peroxisomes: use superoxide and peroxide (very toxic to the cell) to
oxidize various compounds.
Eukaryotic Cell Structures
• Plasma membrane: the outer surface of the cell. Composed of a
phospholipid bilayer: by itself only lets oxygen, carbon dioxide,
water, as few other small molecules in or out. All other molecules
are transported down the electrochemical gradient by channel
proteins, or pumped up the gradient by ATP-driven pumps. Also,
plasma membrane has adhesion proteins that connect to other cells
or extracellular matrix, and receptors for hormones and other
signaling molecules.
• Cytoskeleton: microtubules, microfilaments, intermediate filaments:
structure and transport within the cell.
– microfilaments are made of actin, which interacts with various forms of
myosin to provide cell movement and changes in cell shape.
– microtubules are made of tubulin monomers. Mitotic spindle is made of
microtubules. Also movement of organelles occurs with the interaction
of microtubules and motor proteins. Cilia and flagella are made of
microtubules as well.
– intermediate filaments are structural only.
Eukaryotic Cells
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Size is limited by the need for oxygen, nutrients, control signals, etc., to
diffuse from one end of the cell to the other. For more-or-less round cells,
10-30 µm is typical. Most bacterial cells are 1-2 µm.
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Many different cell types in humans. All have same basic DNA, with minor
changes due to random mutations plus a few cell types (e.g. immune
system cells) have major DNA changes.
Most human cells are diploid (2n). However:
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– the gametes, sperm and egg, are haploid (1n).
– some are polyploid due to endomitosis, DNA replication without cell division.
Examples: hepatocytes (liver cells) range from 2n to 8n, and bone marrow
megakaryocytes range from 16n to 64n.
– some cells, notably red blood cells, have no nucleus.
– some cells fuse together to form a multinuclear syncytium, notably muscle cells.
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In most animals, the germ line cells, cells which become the sperm and
egg, are separated from the somatic cells (all other cells) early in
development.
Cell Culture
• How to study cell behavior. Can do it in whole organisms,
sometimes called in situ studies. This can have lots of complicating
factors as many tissues and organs interact. Also, can’t see or
access many cells.
• Tissue explants: cut out a piece, culture it in a nutrient medium
• Primary cell culture: dissociate a tissue into individual cells and grow
in nutrient medium. Problem: cells are mortal, after about 60
divisions they stop dividing.
• Permanent cell line: Easy to grow and maintain. No limit to cell
divisions, immortalized (transformed) by mutations equivalent to
cancer induction. Can be done spontaneously, or with known
mutagens, or by transfection with DNA containing oncogenes
(mutated genes which cause cancer).
– an important resource is cell lines from interesting patients. To keep a
stable source of their genetic material, lymphoblastoid cells can be
immortalized by Epstein-Barr virus, which remains separate from the
chromosomal DNA.
Principles of Development
• How to get from a single cell, the zygote, to a
multicellular organism.
• General events:
– cell division and growth
– differentiation: cells develop different phenotypes
– pattern formation: overall development of body axes,
the general body plant, and structure of individual
organs
– morphogenesis: changes in shape
Canalization of Development
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Cells become increasingly specialized
during development. Their range of possible
fates (final cell type) decreases. This is
called the canalization of development.
Initially, cells of the embryo are totipotent:
can develop into any embryonic cell. After a
while, embryo is divided into a trophoblast
and an inner cell mass. Inner cell mass
become the embryo while trophoblast
becomes outer membranes and placenta.
Cells in ICM can become any embryonic
tissue, but they can’t become trophoblast
cells: these cells are pluripotent. As
development proceeds, embryonic cells
become increasingly specialized and can no
longer become any final cell type: they
become multipotent and finally unipotent
when they can only become one final cell
type.
At some point, a cell is determined to be a
particular cell type. Determination is
followed by differentiation, changes of form
and function, into that cell type.
decisions about determination are caused
by a cell’s lineage: previous decisions, by its
position in the embryo, and by signals
passed between the cells.
Stem Cells
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Stem cells are self-renewing cells that
differentiate into a variety of cell types. After
a stem cell divides, one daughter cell
typically remains a stem cell, while the other
one starts to differentiate into a final cell
type: this is called asymmetric cell division.
There are many types of stem cell in adults,
and they are generally rare and hard to find.
Some differentiate into a single cell type,
while others can have multiple fates.
Embryonic stem cells are the pluripotent
cells of the inner cell mass, which can
become any kind of embryonic cell.
Pattern Formation
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How does the basic body plan get formed
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axis development: based partly an uneven
distribution of components in the egg and
partly on external events.
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axes: dorsal-ventral (back-front), cranialcaudal (head-tail), left-right.
position within an organ: e.g. how to get 5
different fingers on a hand
sperm entry point determines boundary
between trophoblast and inner cell mass,
which in turn determines the dorsal-ventral
axis
cranial-caudal axis probably determined by
position of second polar body exit relative to
sperm entry point
morphogen gradients. Certain cells secrete
chemicals that act as morphogens: signals
that allow other cells in that tissue to
determine their position in the tissue. The
farther a cell is from the morphogen
secretion site, the lower the concentration of
the morphogen.
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a well known example is the zone of
polarizing activity, which occurs in limb buds.
Cells nearest the ZPA become the little finger
or toe, while those farthest away become the
thumb or big toe.
Hox genes
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Animal development
along an axis, from
Drosophila to humans,
is largely determined
by clusters of
homeobox (Hox)
genes.
Different members of
the Hox clusters are
activated in different
parts of the morphogen
gradient, in an
overlapping pattern
The Hox gene
products then stimulate
the activity of other
genes that cause the
cells to differentiate
into the proper type.
Fertilization
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Egg is surrounded by two layers of
extracellular matrix, the vitelline
membrane and the zona pellucida.
The sperm cells must dissolve their
way through these layers to get to the
egg. Sperm contain an acrosome at
their tips that contains the necessary
enzymes.
When a sperm reaches the egg
membrane, the membranes fuse,
putting teh sperm nucleus inside the
egg.
Egg membrane then depolarizes and
cortical granules release their contests
to push all other sperm cells away.
Meiosis 2 occurs and the second polar
body exits opposite the sperm entry
point.
the male and female pronuclei then
undergo mitosis together, and the
resulting nuclei fuse.
Fertilization occurs in the Fallopian
tubes. After fertilization, the embryo
takes about a week to reach the
uterus.
Early Development
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The early cell divisions of the embryo occur without any overall growth. These divisions, the
cleavage divisions, result in the morula, a ball of 16 or more cells. Each cell is called a
blastomere.
After a few more divisions, cells on the outside of the morula flatten out, and the inside develops
into a hollow ball, the blastocyst.
On one side of the blastocyst a clump of cells, the inner cell mass, forms. The inner cell mass
develops into the embryo and the amnion, the inner membrane.
The other cells of the blastocyst are called the trophoblast (trophoectoderm), The trophoblast
forms the chorion, the outer membrane of the embryo, and the embryonic part of the placenta
(which is also composed of maternal tissues).
At about 5 days afer fertilization, the blastocyst hatches by releasing itself from the zona pellucida
that surrounded the egg, Then implantation into the uterine wall occurs, about 6 days postfertilization.
Gastrulation
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Lewis Wolpert: “It is not birth, marriage, or
death, but gastrulation, which is truly the
important event in your life.”
About 3 weeks after fertilization, the cells of
the inner cell mass undergo a series of
movements that end up producing the three
fundamental germ layers of the body:
ectoderm, mesoderm, and endoderm. Also,
body orientation gets established.
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ectoderm turns into skin and nervous system
mesoderm turns into muscle, bone, circulatory
system, kidneys
endoderm turns into gut lining, endocrine
glands, most internal organs
Cells in one area of inner cell mass develop a
primitive streak, an area where the cells start
to move inward. The cells that end up inside
become the endoderm, while the cells that
remain outside become the ectoderm. The
mesoderm develops last, from cells near the
primitive streak.
The primitive streak is replaced by the
notochord, a rod of cartilage that is the
defining characteristic of the chordates
(which includes eh vertebrates).
Neurulation
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After gastrulation finishes, about 4
weeks after fertilization, the
nervous system starts to form,
The first event is the induction of
the neural tube (beginning of the
spinal cord) by the notochord
interacting with the ectoderm
above it. If the tube fails to close,
spina bifida or anencephaly
(absence of a brain) results.
Neural crest cells form at the
margins of the neural tube. These
cells migrate laterally, forming the
peripheral nervous system,
melanocytes (pigment cells).
This period ends after about 8
weeks, after which the embryo is
called a fetus, which grows and
develops further.
Twinning
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2 basic types: dizygotic (fraternal):
develop from 2 separate eggs fertilized
by separate sperm. Nothing more
than siblings who happen to share a
womb.
monozygotic (identical): develop from
one fertilized egg, with the embryo
splitting into 2 early in development.
Cause is unknown.
Cells up to the 4 cell stage are
totipotent: any single cell can develop
into a whole person. This limits
identical siblings to quadruplets.
splitting the embryo after about 12
days of development can be
incomplete, resulting in conjoined
twins. The joined region can include
almost any area of the body and any
degree of completeness.