11. • Together, these various types of human cells enable a person to function as a whole.
How can an organism have different cell types even though they contain the same DNA?
12. The Generation of Different Cell Types
• Four of the many different types of human cells…
-Each has a unique cellular
structure that appropriate for
carrying out its function.
Do they have different genes?
…Do they all have the same
genome?
(a) Three muscle cells (partial) (b) A nerve cell (partial)
-Remember that every somatic
cell in our body was produced
by repeated rounds of mitosis
that started with the zygote.
What makes them different? (c) Sperm cells (d) Blood cells
13. •The only way that cells with the same genetic information can develop into cells
with different structures and function is if gene activity is regulated.
•Cell contains mechanisms that determine what genes will be turned on and
which will be turned off in a particular cell.
What does it mean to say that genes are turned on or off?
•They are regulated to express only few but not all.
•- Cell differentiation…The process whereby cells become specialized in structure
and function.
•Gene Regulation: Cell Differentiation.
•During embryonic development, groups of cells follow diverging developmental
pathways, and each group develops into a particular kind of tissue.
14. Different Types of Cells Express Different
Combinations of Genes
• Shown: The patterns of gene expression in 3
different types of human cells.
Certain type of cells will express same protein
(house-keeping protein) because all cell need it to
function properly Pancreas
cell
Eye lens cell
(in embryo)
Nerve cell
Glycolysis
• constitutive /House-keeping genes: they are active
in all metabolizing cells.
enzyme
genes
Crystallin
gene
Which gene(s) are house-keeping genes?
ribosomes, RNA polymerase, and GTF (general
transcription factor).
Insulin
gene
Hemoglobin
gene
• The specialized proteins (expressed in only
particular types of cells):
-cystallin: forms the lens of the eye
-insulin: a hormone made in the pancreas
-hemoglobin: oxygen transport protein
-Proteins specific to nerve cells are not shown. Key:
Active
gene
Inactive
gene
15. Do all cells contain the same DNA?
• Clones…genetically identical organisms.
-Nuclear transplantation: The replacement of an egg’s nucleus with one
from a differentiated cell.
• Dolly the sheep (Scotland, 1997)…Reproductive cloning.
Who did Dolly resemble?…The egg donor, nucleus donor, or the
surrogate mother.
- Demonstrates that the nuclei of cells contain a complete complement
of genetic information
-Dolly died in 2003 (6 yrs old). It died from progressive lung
disease, but also suffered from arthritis. Cloned animals tend
to suffer maladies, including premature aging.
Reproductive cloning
Donor
cell Nucleus from
donor cell
Implant embryo in
surrogate mother
Clone of donor is
born
Therapeutic cloning
Remove Add somatic Grow in culture
to produce an
early embryo
nucleus cell nucleus
from adult
donor to
from egg cell
Remove embryonic
stem cells from
embryo and grow in
culture
Induce stem cells to
form specialized
cells for therapeutic
use
enucleated egg
cell
16. • All cells contain the same genetic material; however, different
types of cells contain different kinds of proteins. Therefore, the
expression of genes must be regulated/controlled in a cell
specific manner
• Expression of genes must also be controlled temporarily during
development and in response to certain environmental factors.
• In addition, the level of gene expression is also
regulated…certain proteins are present in higher levels than
others in a particular type of cell.
17. Do all cells express the same
genes?
• Within an organism, each cell type has a unique protein expression profile.
Human brain cells Human liver cells
• Blue: Cell-type specific proteins
• Red: Common proteins (not necessarily expressed at the same levels in different cell types)
– Some common genes such as rRNA genes are referred to as “housekeeping” genes
because they are expressed at all times in all cell types.
18. • The living cells possess a remarkable property to adapt to changes in the
environment by regulating the gene expression.
• For instance, insulin is synthesized by specialized cells of pancreas and not
by cells of other organs (say kidney, liver), although the nuclei of all the cells
of the body contain the insulin genes.
• Molecular regulatory mechanisms facilitate the expression of insulin gene in
pancreas, while preventing its expression in other cells.
• More over all the proteins/RNA are not required by the cell all the time.
• Some proteins are required at some time and yet other proteins are
required at another time.
• Moreover these proteins are required in lesser quantities at one time, yet at
other times they may be required in higher quantities.
• There are yet another class of proteins which are constantly (always)
present in the cell, like the enzymes of the TCA cycle.
• Therefore genes can be conveniently grouped under two classes:
• Constitutive genes and Inducible genes:
19. • Constitutive ( house keeping) genes:
• Are expressed at a fixed rate, irrespective to the cell condition.
• The products (proteins) of these genes are required all the time in a
cell.
• Their structure is simpler.
• Are not controllable.
• housekeeping genes are typically constitutive genes that are required
for the maintenance of basic cellular function, and are expressed in all
cells of an organism
• Controllable genes/ regulatable genes/ inducible genes:
• Are expressed only as needed.
• Their amount may increase or decrease with respect to their basal
level in different condition.
• Their structure is relatively complicated with some
response elements
20. DIFFERENT GENE CATEGORIES
Genes turned “on” in all cells
at all times (e.g. transcription
machinery, translation
machinery, energy
Housekeeping genes
conversion, etc.).
Genes that are turned “on” in
each cell that give a cell its
special properties and
function.
Cell type specific genes
Genes specific to certain
stages during growth &
development of a person.
Developmental regulatory
genes
Genes not normally
expressed but can be in
response to external stimuli
(e.g. hormone).
Inducible genes
21. Transcription Factors
• proteins which bind to
promoter & enhancer regions
of DNA to turn on (or off)
genes
• ability to be turned on is
inducible
• ability to be turned off is
repressible
• genes are most often
regulated as a group
• located next to one another
on a chromosome
• these genes along with their
regulatory sequences of DNA
are called an operon
