The Genetic Basis of DevelopmentCh. 16

Objective – What you should get from this lecture:

Be able to define genetic equivalence

Be able to describe the experiments that proved it

Identify the 3 processes that turn a single cell zygote into a complex, multicellular organism

BackgroundScientists study how organisms

develop from single celled zygote to complex multicellular organism

Model organisms studied include: Drosophila melanogaster (fruit fly) Caenorhabditis elegans (nematode) Mus musculus (mouse) Danio rerio (zebrafish) Arabidopsis thaliana (common wall cress)

Embryonic developmentMulticellular organisms have great

variety in cell types (cell differentiation) Those cells are further organized into

tissues, organs, organ systemsDuring development of an embryo, the

cells need to not only become different from each other, they need to also be organized in a highly structured manner

Transformation of a zygote to a multicellular organism

3 processes: Cell division

get a ball of undifferentiated cells

Cell differentiation these cells become

specialized in structure & function and are organized into tissues

Morphogenesis the processes that give an

organism it’s shape

All 3 events overlap each otherEarly morphogenesis

Establishes dorsal – ventral ; anterior – posterior

Later morphogenesis Establishes more specific locations –

where limbs will be, etc

The development of plants and animals is different

Animals: cells must move into position in the early embryo to transform it into its characteristic 3-D shape

Plants: morphogenesis and growth occur throughout the life of the plant, not just during embryonic and juvenile development

Plants keep growing…Apical

meristems – the structures on a plant that allow for it’s continual growth and development They are like

regions of the plant that are always embryonic

Found in roots and shoots

Quick ThinkWhat are the 3 processes that occur during embryonic development and how are they different between plants and animals?

Cell DifferentiationDifferences in cells

due to Differences in gene

expression Regulatory

mechanisms that turn genes on or off throughout development

Genomic equivalence Nearly all the cells in Nearly all the cells in

an organism have the an organism have the same genessame genes

Genes unnecessary Genes unnecessary to that cell are turned to that cell are turned offoff Are the genes

permanently disabled, or could they be activated again?

3 experiments helped prove the idea of genomic equivalence

Experiment 1Experiment 1 Can a differentiated plant Can a differentiated plant cell develop into a whole new plant?cell develop into a whole new plant?

Took cells from the root of a carrot plant

Put those cells in nutrient mediumThey grew into a whole new plantThe new plant was a clone of the

original parent plantProved that a differentiated cell

has all the genes necessary to make a new plant

When a cell can reverse its differentiation and generate all the cell types in a new individual, it is said to be totipotent

Experiment 2a: Can animal cells be totipotent?

Took the nucleus from an early embryo cell Not much differentiation in

these cellsRemoved the nucleus from an

egg cellPut donor nucleus into egg

cellMost of these developed into

new organisms

Experiment 2b: Can an animal cell be totipotent?Took the nucleus from a

fully differentiated cell (mature cell)

Removed the nucleus from an egg cell

Inserted the mature nucleus into the egg

Less than 2% of these developed into new organisms Many died prematurely

Experiment 2a & 2b ConclusionThe nucleus from an

animal cell can direct the development of a new individual

Less of an ability to do this as the cell becomes more differentiated The DNA is all the same Chromatin structure is

probably to blameMore methylation

(inactivation of genes) This can be reversible

Experiment 3: can a fully differentiated cell be reprogrammed to become totipotent?

Took mammary cells from sheep 1 “starved” them to stop

their cell cycles Removed the nucleus

from an ovary cell from sheep 2

Fused the 2 cells Grew this new cell in

culture to get an early embryo

Implanted the embryo in sheep 3 It grew and developed

into Dolly

Quick Think

How was the idea of genomic equivalence proven?

DollyGenetically identical to the

donor cell nucleus except for mitochondrial DNA that came from the donated egg cell

Seemed to age more quickly than normal

Died at about 1/2 the normal life expectancy Were her cells somehow

“older” than normal cells?

Reproductive Cloning What happened with Dolly is

called reproductive cloning - creating new individuals

Since then cows, sheep, pigs, cats, mice, & horses have been cloned

Not all clones are exactly alike in appearance or behavior

Copy Cat has a different coat pattern than her mother clone

What about cloning humans?

Random inactivation of the X chromosomes

Problems with CloningDolly was a rare success; the only

survivor out of several hundred triesMost cloned embryos don’t develop

fully to birthThose that do often have birth defects

Why? In normal, differentiated

cells, only certain genes are active

The rest are repressed This has to be reversed in

the donor nucleus to allow all the genes to be transcribed

Misplaced methyl groups (which help regulate gene expression) in the donor nucleus may lead to the problems we see in clones

Quick Think

What do you know or what have you heard about stem cells?

Embryonic Stem Cells A relatively unspecialized cell

that can reproduce itself and differentiate into different kinds of specialized cells

Can be harvested from the blastocyst stage of embryo development

Cloning embryos to the blastocyst stage as a source of stem cells is called therapeutic cloning.

Embryos come from donated embryos from fertility clinics

Adult Stem CellsSome stem cells found in

adult bodiesThey are pluirpotent - can

become several different kinds of cells, but not all Stem cells in the bone

marrow become different kinds of blood cells

Work is being done to try to coax adult stem cells to be more like embryonic stem cells

Why are stem cells valuable?Ultimate goal is to use stem cells to

repair damaged or diseased tissues and organs Brain cells for people with Parkinson’s

disease Pancreatic cells for people with diabetes

https://www.youtube.com/watch?v=5x8e2qsAVGc http://ed.ted.com/lessons/what-are-stem-cells-craig-a-kohn

Quick ThinkSome believe it is wrong to create or

use embryos for stem cell research that will be destroyed. Others, like the scientists who cloned Dolly, believe that “cloning promises such great benefits that it would be immoral not to do it.”

How do you feel about all this? (you don’t have to share if you don’t want

to)

Cell DeterminationDetermination

the development of an embryonic cell into its differentiated state

Irreversible (as far as we know now) Cell expresses genes specific

to the tissue it is a part of If moved to a new tissue, it will

still develop into that kind of cell

Determination starts to occur very early in embryonic development

Determined eye cells moved to a new part of the body - still become eye cells

Differentiated CellsBecome specialized

in producing particular gene products Cells in the eye

produce crystalline proteins

Cells in the liver produce albumin

Muscle cells produce actin & myosin

How cells become different in an embryo The cytoplasm of an

egg cell is not homogeneous mRNA, proteins,

organelles, etc are distributed unevenly

As the zygote divides, the cytoplasm of each new cell contains a different mix of cell components These components

influence the development of the cell - called cytoplasmic determinants

How cells become different in an embryo

The environment around each cell determines cell development Contact with signal

molecules from neighbor cells can cause changes in a cell - called induction

Quick ThinkWould a single stem cell be able to become an embryo all by itself? Why or why not?

So…Quick Write after discussionWhat are the events and processes

that contribute to the development of a single celled zygote into a complex, multicellular organism?