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