Lecture Notes

Mechanism of development
Learning objective:

1. Reminder of mechanisms for regulating gene expression- an essential component of development
2. To Review key concepts of developmental biology and approaches used to study development

LO2. REVIEW KEY CONCEPT IN DEVELOPMENTAL BIOLOGY

The principle of development

 Main question in developmental biology: how can fertilised eggs generate a complex organism?

Detailed questions of developmental biology

 Differentiation
- How can fertilised eggs generate so many different cell types?
 Morphogenesis
- How do cells move and change shape to create ordered structures?
 Growth
- How do cells know when to stop dividing?
 Reproduction
- How are gametes set aside
 Evolution
- How do evolutionary changes in development create new body forms?
 Environmental integration (‘’extended phenotype’’)
- How is development of organism integrated into the larger context of its habitat (environmental factors)?

The question of how eggs can generate whole organism goes back to the 6 th century

2 theories Preformationism Epigenesis
 Adult is performed in egg or sperm  Organism progressively built anew in each
 All organisms were generated at the same generation
time  No premade man
 ‘humunculus’= v.small human curled up in  Aristotle supported epigenesis
the head of each sperm




DNA gives the instruction

 The nucleus of the zygote and every cell that is produced during ontogenesis contains the complete genome
- Ontogenesis = development of individual organism or behavioural features from the earliest stage of maturity
 Every cell (somatic and germ line cells) can be used to make a new animal
- Example is dolly the sheep
- Somatic cell = any cell of living organism other than reproductive cells (egg and sperm cells)
- Germ line cells = sperm and egg cells (gametes)

1. REMINDER OF MECHANISMS FOR REGULATING GENE EXPRESSION- AN ESSENTIAL COMPONENT OF DEVELOPMENT

, Differentiation: making cells different by regulation of gene expression

 How are different cells types established during development?
- All cells contain the same genes
- Not all genes are expressed in all cells
- Expression of different sets of genes in different cell types

Factors that control gene expression regulate:

1. Chromatin structure
2. Initiation of transcription
3. RNA processing
4. Initiation of translation
5. Post-translational modification

1. Regulation of chromatin structure
 DNA associated with proteins called histones
 There are 8 histones that form an octomer
 DNA wraps around the octamer and are tightly packed
 Therefore, DNA not easily accessible to enzymes (RNA polymerase ll)

Genes within highly packed heterochromatin are usually not expressed

 Heterochromatin = tightly packed form of DNA, which comes in
multiple varieties
 ~1.5 % DNA of human genomes codes for proteins

What affects gene regulation:

 Location of gene promoter relative to nucleosome (DNA segment
+ histone octamer) and site where DNA is attached to the
chromosome scaffold or nuclear laminar can affect whether a
gene is transcribed
 Nuclear laminar
- Filamentous protein network that coats the inner nuclear
membrane.
- Consists of intermediate filaments and membrane-associated
protein.
- Provides mechanical support
- Regulate cellular events i.e. DNA replication and cell division