Chapter 21- Development and Gene Expression

1. Chapter 21- Development and Gene Expression Central questions: How are cells in various locations of the body different -is the variation in their genome or in the proteins they express? Can differentiated cells be coaxed to retrace their steps and become de-differentiated? How do cells become different from one another to form different body parts when they all develop from ONE cell - the zygote? Are there difference or similarities in the way genes behave during development between various species?

2. Figure 21.1 From early embryo to tadpole: what a difference a week makes

3. Types of stem cells: Embryonic totipotent Embryonic pluripotent Adult stem cell (misnomer)

4. Embryonic development— single-celled zygotes 2 cell stage 4 cell stage Blastula (hollow ball with cells on the outside). Humans - this is called blastocyst A cell from the blastula/blastocyst is also an embryonic stem cell - it is able to differentiate into many types of cells but not a full organism - pluripotent! A cell upto the 8 cell stage is an embryonic stem cell - it is able to differentiate into a full organism - totipotent! Embryonic blastocyst stem cells in animals are pluripotent because epigenetic modifications are minimal- that is = DNA methylation/histone acetylation (turning off its genes) is minimal

6. Embryonic development— Blastula Gastrula (Ectoderm, Mesoderm, Endoderm) Tissues Organs Organ Systems Once the cell is committed to its fate during gastrula formation, they become differentiated - they lose their pluripotency (genes turned off)! Adults/babies (humans ) have adult stem cells - in special places like the bone marrow which are also pluripotent but to a lesser degree than embryonic stem cells!

8. Stem Cells In The Human Adult • Bone Marrow Cells – make blood cells all through life • Brain Stem Cells – can make neurons and glial cells • Skin stem cells – keratinocytes, hair follicles, epidermis • Are human stem cells PLURIPOTENT? (-can differentiate into multiple cell types) • Yes, but to a limited extent

9. Stem Cells - another property • Stem Cells have telomerase (immortal) - capable of self-renewal

10. Cell division (mitosis) of the zygote increases the number of cells in an organism. Differentiation is when each cell becomes specialized in both structure and function (genes turned off by epigenetic processes). Morphogenesis is when the eventual shape (body plan) of the organism forms - head-tail axes; top-down axes….

11. If pluripotent - this means some genes are still off (epigenetics)!

13. Carrot cells are totipotent. • In plants, cells remain totipotent— • all genes can be activated, and any cell • can form any part of the organism

14. Review Questions: • Are there differences in gene number or type between undifferentiated (stem) cells and differentiated (mature/adult) cells? • Are there stem cells in an adult human body? Where? • Can adult differentiated cells be induced to make any (and all) types of body cells - retrace and become ‘embryo like’? NO! And Yes! And Yes!

15. Are there differences in genes between different cells in the body? • No = Genomic Equivalence • Are there differences in genes expression between undifferentiated (stem) cells and differentiated (mature/adult) cells? • Yes. Genes get inactivated/activated through processes like methylation (epigenetics) • When cells differentiate, are genes • inactivated irreversibly? Can they retrace to become de-differentiated? • Well, it depends on the organism! Dec 2007 - scientists de-differentiated the human skin cells by using transcription factors!

16. Can adult skin cells retrace steps to become de-differentiated? Somatic Cell Nuclear transfer (SCNT) Take egg cell and remove nucleus (haploid) - throw it away Take skin cell and remove nucleus (diploid) - save this Insert skin cell nucleus (somatic cell) into egg cell cytoplasm. No need for sperm! Why? Allow egg cell to divide and become blastocyst Now you can extract stem cells (THERAPEUTIC CLONING) OR carry out REPRODUCTIVE CLONING - implant blastocyst in a surrogate mom and grow a clone!

17. Dolly and Bonny! • Reproductive Cloning is an offshoot of stem cell research

18. SCNT made Dolly the sheep! -Mammary gland cells from donor arrested in G0 phase apparently “dedifferentiated.” Review: Dolly’s mitochondrial DNA is from the egg donor sheep.

19. Remember in cloning - an egg nucleus is replaced with a nucleus of a differentiated cell. Ability of differentiated nucleus to support normal development is related to its age - Dolly may have died prematurely and developed arthritis at a young age! (epigenetics controls this)

20. Therefore… in animals - review • Nuclei change as cells differentiate • The DNA sequence usually doesn’t change, but chromatin structure may be altered • Nuclear “potency” is restricted as cells develop and become more differentiated.

22. Ques. 3) How do cells become different from one another to form different body parts when they all develop from ONE cell - the zygote? How does a stem cell make a differentiated cell? Determination & differentiation of muscle cells What turns on the Master control gene? Master control gene => codes for transcription factors => turned on (determination) => transcription factors => turs on other genes=> more transcription factors => muscle protein genes turned on => muscle protein (myosin) made => cell has differentiated (These are INTERNAL SIGNALS)

23. Figure 21.9 Determination and differentiation of muscle cells (Layer 1)

24. Figure 21.9 Determination and differentiation of muscle cells (Layer 2)

25. Figure 21.9 Determination and differentiation of muscle cells (Layer 3)

26. Model organisms for development studies— • observable embryos • short generation times • relatively small genomes • knowledge about the organism and its genes • * Drosophila, C. elegans, mouse, zebrafish, • Arabidopsis

27. What tells a cell (and triggers the master gene) what its fate will be? • Cytoplasmic determinants (from the mom - internal signals in egg) • Induction—signal molecules from cells nearby (neighbors)

28. 1) Cytoplasmic determinants include mRNA, proteins, chemicals, and organelles and how they are distributed in the egg. They are distributed unevenly - and this can set up gradients that says ‘head’ side, ‘tail’ side , etc.

29. Cytoplasmic determinants are coded for by maternal effect genes (or egg-polarity genes) Example: --Bicoid mRNA is present at the anterior end of the egg -Bicoid protein is essential for head formation.

30. Background on Drosophila: Cytokinesis does not occur in the early Drosophila embryo. Nuclei migrate to the periphery in the blastula.

32. Bicoid is a morphogen—a substance that establishes an organism’s axis or other 3D features. Bicoid helps create the anterior/posterior axis. It’s a transcription factor that activates expression of segmentation genes

33. 3 types of segmentation genes: • Gap genes map out basic subdivisions along anterior/posterior axis • Pair-rule genes define smaller regions • Segment-polarity genes determine the anterior/posterior axis of each specific segment.

34. 2) Induction = signals impinging on an embryonic cell from other nearby embryonic cells. These can be transcription factors - remember cell communication?

35. Our friend Drosophila ! Has 3 parts—head, thorax, and abdomen has an anterior/posterior axis and a dorsal/ventral axis Cytoplasmic determinants and induction together lead to PATTERN FORMATION Dorsal Anterior Posterior Ventral

36. Pattern formation is the development of the • spatial organization of an organism. • Molecular clues (positional information) tell cells • where they’ll be located in the body • who their neighbors will be • how to respond to other molecular signals

37. Cell Lineage of all 959 C. elegans cells!

38. Homeotic genes determine the segment on which appendages or other structures will form The expression of these genes is activated by Transcription factors coded by segmentation genes.

40. All homeotic genes contain a homeobox domain. Homeobox domains have been found in many other animals besides flies, and most genes with a homeobox are related to development. The homeobox domain is actually a DNA- binding domain! So proteins containing it are likely to be transcription factors!!

41. Flies and mice have homologous genes coding for proteins involved in development.

43. Induction is when cells signal other cells to change in a specific way—mostly activating or inactivating transcription. Induction has been studied most in the nematode, C. elegans.

44. Vulva precursor cells can develop into 3 different types of cells. Signals from the anchor cell induce the determination of each cell. Effects of inducers can vary depending on concentration.

45. Apoptosis is programmed cell death—occurs at various stages of development. Suicide proteins are activated - cell blebs (becomes multilobed), nucleus condenses, and then slowly degrades due to nucleases and proteases….how painful! It is then eaten by neighboring cells.

46. Apoptosis is programmed cell death—occurs at various stages of development ex: ‘web retraction’ between digits/fingers (textbook activity)

47. MUTANT MICE GALLERY!In the name of Science…..Are you ready for the gore?

48. Figure 21.x2a Laboratory mice: brachyury mutant

49. Figure 21.x2b Laboratory mice: eye-bleb mutant

50. Figure 21.x2c Laboratory mice: Hfh11 mutant