lecture 3 - Multicellularity and cell specialisation Flashcards
What are the advantages of multicellularity?
•Multicellular organisms are bigger and better protected
from predation.
•Larger organisms are buffered more effectively from the
external environment.
•Multicellularity allows the development of cell-types with
specialised functions within organisms.
•In primitive colonial forms there are few specialised
cells
•Humans contain over 200 different cell types.
What is the Flagellar Synthesis Constraint Hypothesis?
Cells with flagella allow a simple multicellular organism
to move.
•The microtubule organising machinery needed for the
formation of flagella is also required for the formation of
the spindle apparatus used in cell division.
•There is competition for the use of this machinery for
the processes of cell movement and cell division.
•Cells with functional flagella do not divide.
•Presence of both specialised flagellated and nonflagellated cells in a simple colony allows the organism to move and grow at the same time.
Balance between differentiated cell-types is very
important e.g. flagellated versus non-flagellated cells
•Flagellated cells required for locomotion.
•Non-flagellated cells required for growth and
reproduction
•Maintaining balance requires cell communication.
In the primitive alga Volvox nonflagellate cells divide and give rise to juveniles which are released from
the adult.
Flagellated cells around the outside
are required for motility.
Describe the importance of cell communication for multicellular life
.Environmental Sensing
•Multicellularity requires a complex ability to sense the
extracellular environment e.g. recognition of self and
non-self
Cell Adhesion
•Multicellular organisms require a complex system for
mediating cell-to-cell adhesion and cell to extracellular
matrix interactions
What is the “toolkit” for multicellularity?
Cell-to-Cell (Intercellular ) Signalling Multicellularity requires signalling between cells within an organism to co-ordinate behaviour. 1) Homeostasis and maintenance of internal state of the adult. 2) The complex process of development. i. Cell cycle control ii. Cell movement iii. Differentiation iv. Patterning
Describe how Intercellular Signalling Pre-dates The
Origin of Multicellular Organisms
•Genome analysis indicates a close relationship
between animals and choanoflagellates.
•Choanoflagellates have many genes found in
animals, including genes coding for components
of signalling pathways.
Yeast mating type
•Mating factor peptide is secreted by haploid individual cells
•Signals to cells of opposite mating type to stop proliferating
•Cells form protrusions in preparation for mating and fusion
to form a diploid cell which then produces haploid spores
What is differential gene expression?
Cell lineage restriction is dependent upon differential gene expression.
•The process of development generates different patterns of gene activity in the cells of the embryo
What is gene expression?
“The process by which a gene’s coded
information is converted into the structures
present and operating in the cell.”
Expressed genes include:
•Genes that are transcribed into mRNAs, which
are then translated into protein
•Genes that are transcribed into RNA but not
translated into protein e.g. transfer RNAs,
ribosomal RNAs and various RNAs with
regulatory activity
How is gene expression regulated?
The activity of a protein coding gene can be regulated at multiple levels Regulation of gene expression • Gene transcription • Processing of pre-mRNA to produce mature mRNA • Transport of mRNA from nucleus • Translation of mRNA to produce protein • Post-translational modification of activity and stability
What is mosaic development?
•In 19th century it was proposed that cell nuclei
and chromosomes contained heritable genetic
information.
•August Weisman proposed that the nucleus
contained “determinants”.
•Unequal division of determinants to daughter
cells would then account for differences between
cells.
What is regulative development?
•Hans Driesch’s experiments with sea urchin
argued against mosaic development.
•Both cells must contain all the necessary
information for normal development
Sea urchin larva develops normally from two cell stage
Driesch’s separation of cells at two cell stage resulted in death of one cell and surviving cell developed into normal larva
What is gene constancy or nuclear equivalence?
Somatic cells generally all contain the same
genetic information
Cloning by transfer of nuclei from differentiated somatic
cells demonstrates that genetic information is not lost
during development and differentiation.
