Do NowIdentify each of the cells in the diagram and indicate the Human chromosome number of each.

Sperm (23)

Egg (23) Zygote (46)

How do we go from a zygote (a single cell) to a multicellular organism? (hint: think about a process used to increase cell#)

STAGES OF EMBRYONIC DEVELOPMENT

• Series of steps leading from zygote to organism

1. Cleavage – increase in cell# by rapid mitosis, no cell growth

2. Differentiation – cells get their jobs by “reading” different parts of DNA, form tissues and organs

3. Growth

Stage 1

• Egg and sperm are produced by MEIOSIS (gametogenesis).• They contain half as much genetic material as parent cells (in humans = 23)

• The joining of the two nuclei is called fertilization

• This can occur internally or externally

The spermapproachesthe egg

12

3

The sperm’senzymesdigest the egg’s jelly coat

4 The spermnucleus enters the egg cytoplasm

The plasma membranesof sperm and egg fuse

SPERM

Nucleus

Spermhead

Jellycoat

EGG CELL

Spermnucleus

Eggnucleus

5 The nucleiof spermand egg fuse

Zygotenucleus

Stage 2• The

fertilized egg or Zygote now has how many chromosomes?

46

•Normal fertilization: http://www.stanford.edu/group/Urchin/nfert.htm

Stage 3

• The fertilized egg divides into two new cells by MITOSIS to form identical cells

• This is the beginning of cleavage: rapid increase in cell #’s by mitosis (stages 3-4)

• NO CELL GROWTH!• # of chromosomes in each cell: 46

2-cell stage

http://fig.cox.miami.edu/~cmallery/150/devel/blastula.gif

Stage 4

• Four Cell Stage: still increasing in numbers by mitosis (cleavage)• Number of chromosomes in each cell: 46

Stage 5

• Blastula: hollow ball stage

• Number of chromosomes in each cell: 46

• The layer of cells is generally one cell thick and the cavity is filled with fluid

• Cell growth starts

Stage 6

• Gastrulation: inward pushing of blastula to form two layers (indentation)

• The cell is beginning to grow before dividing

Stage 7

• Differentiation: formation of three layers which will develop into organs and organ systems

• Development can take place internally or externally

Twins• Fraternal Twins –

two separate eggs fertilized by two separate sperm (dizygotic)

•Identical Twins – single egg is fertilized , zygote splits early (monzygotic), have identical DNA

• Indian girl - born fused at the pelvis to a "parasitic twin" that stopped developing in the womb

• She had absorbed the organs and body parts of the other fetus, a condition that occurs once in 50,000 conjoined twin births.

Fertilization and Cleavage Video Clip

• http://trc.ucdavis.edu/biosci10v/bis10v/media/ch27/cleavage_implant.html

• http://www.unc.edu/~fconlon/DanielResearch.htm

• http://raven.zoology.washington.edu/celldynamics/events/workshops/archive/2003/cytomod_abstracts/GvD_VDF/images/GvD-VEF-fig1.jpg

• http://www.howstuffworks.com/adam-200048.htm

Stem cells – cells without a “job”, removed from blastocyst before they differentiate