Cell division. The cell cycle. Cloning

Cell Division
The Cell Cycle
Cloning 411
cellular differentiation
is the process by which a less specialized cell becomes a more specialized cell type.
occurs numerous times as the organism changes from a single zygote to a complex system of tissues and cell types.
a common process in adults as well: adult stem cells divide and create fully-differentiated daughter cells during tissue repair and during normal cell turnover.

Cell Division
The Cell Cycle
causes a cells size, shape, polarity, metabolic activity, and responsiveness to signals to change dramatically.
these changes are largely due to highly-controlled modifications in gene expression.
different cells can have very different physical characteristics despite having the same genome.
a cell that is able to differentiate into many cell types is known as pluripotent.
called stem cells in animals
called meristematic cells in higher plants

Cell Division
The Cell Cycle
a cell that is able to differentiate into all cell types is known as totipotent.
in mammals, only the zygote and early embryonic cells are totipotent, while in plants (and in animals), many differentiated cells can become totipotent with simple laboratory techniques.

Cell Division
The Cell Cycle
What is Cloning?
cloning is the process of forming identical offspring from a single cell or tissue of a parent organism.
both the clone and the parent have identical or near identical DNA (random mutations occur)
does not result in variation of traits
considered a form of asexual reproduction
clones occur naturally
example)
Hydra undergoing mitosis during the process of budding

Cell Division
The Cell Cycle
example:
Hydra undergoing mitosis during the process of budding
Runner of a strawberry plant
Monozygotic twins (zygote undergoes mitosis and splits into two)

Cell Division
The Cell Cycle
Plant Cloning
In 1958 Fredrick Stewart produced a carrot plant from a single carrot cell
now cloning is widespread in the agriculture/horticulture industries.
it is desirable (profitable) to have plants of predictable characteristics
Easy to clone plants: carrots, tobacco, lettuce
Hard to clone plants: grasses, legumes.

Cell Division
The Cell Cycle
Animal Cloning
Robert Biggs and Thomas King
investigated nuclear transplants in frogs.
first to clone a frog.

Cell Division
The Cell Cycle
the cloning of the sheep “Dolly” by Dr. Ian Wilmut’s team was the first to clone an animal using adult cells.
the nucleus of an udder cell of an adult sheep was placed in the enucleated egg cell from another sheep.
the egg developed in a Petri dish until an early embryo stage.
then the egg was placed into the womb of another sheep.

Cell Division
The Cell Cycle
DNA donor: adult Finn Dorsett Sheep
Egg donor: Poll Dorsett Sheep
Womb provider: a third sheep
Clone: Dolly was a clone of the adult Finn Dorsett Sheep
http://en.wikipedia.org/wiki/Dolly_(sheep)

Are scientists able to clone human beings?
CLONING

Cell Division
The Cell Cycle
Cell Aging

Cell Division
The Cell Cycle
Cell Aging
Telomeres
are caps at the ends of chromosomes.
they reduce in length each time a cell undergoes the cell cycle
have a role in cell aging and cancer cells.
the length of telomeres is affected by the enzyme telomerase.

Cell Division
The Cell Cycle
as cells go through the cell cycle their telomeres become shorter.
eventually the telomeres become very short and the cell stops going through the cell cycle and dies.
telomerase is an enzyme that keeps the telomeres long but is only found at limited levels in somatic cells.
embyronic stem cells have a high level of telomerase.
telomere length acts as a biological clock.

Cell Division
The Cell Cycle
the case of “Dolly” the prematurely aging sheep.
Dolly was cloned using an adult nucleus with telomeres that had already began to shorten.
Dolly developed arthritis and died of a lung disease at only half her life expectancy. (controversy)

Cell Division
The Cell Cycle
the case of Cancer
cancer cells never seem to lose their ability to divide.
the telomeres of cancer cells do not shorten.
telomerase is reactivated in cancer cells allowing the cancer cells to maintain telomere length and keep on dividing.

DEVELOPMENT OF AND GAMETES
Formation of sex cells during meiosis is gametogenesis
Cytoplasm of female gametes does not divide equally after each division- oogenesis.
One of daughter cells, ootid, receives most of cytoplasm
Other cells, polar bodies, die, and nutrients are absorbed
Only one egg cell is produced from meiosis

Meiosis II- splits 1st polar body and forms 3rd polar body- all disintegrate.
Sperm fuses with ovum (egg cell)
Egg does not move
Egg cells require nutrients
Fuel future cell divisions in event that egg cell becomes fertilized

- born with 400 000 egg cells (only 400 mature)
1000 egg cells mature within ovary every 28 days
Only 1 egg cell leaves ovary, the rest break down and are absorbed by body
Oocyte - cell that produces egg cells
Does not continue to divide after a woman reaches puberty
As a ages:
# of egg cells in ovary declines until about age 50 or 60
No eggs remain in ovary = Menopause

At puberty
400,000 primary oocytes

Diploid spermatocytes
Cells that give rise to sperm cells
Are capable of many mitotic divisions before meiosis ever begins
Males can produce 1 billion sperm cells every day
Spermatogenesis
Sperm cells show equal division of cytoplasm
Because of their function, sperm cells have much less cytoplasm than egg cells
streamlined and cannot carry excess weight

SUMMARY OF GAMETOGENESIS

Differences across kingdoms
Not all organisms use haploid & diploid stages in same way
which one is dominant (2n or n) differs
but still alternate between haploid & diploid
have to for sexual reproduction

How About Plants?
Plants also form gametes, by meiosis
Pollen cells are sex cells
Egg cells are stored in a variety of structures
Contain a haploid chromosome number
Fusion of and gametes restores diploid chromosome number

Plants vs. Fungi Reproduction
Generalized Plant Life Cycle
Alternation of Generations
Sporophyte and gametophyte take turns reproducing each other