Programming of Development Flashcards
When does development begin?
-development commences with the production of a zygote following fertilisation
Understanding Development in Humans
Problems
- humans are very complex
- ordered progression of development is dependent on the expression of the genetic information encoded in the genome
- how is this information expressed at the right time, in the right places to lead to the formation of a normally functioning organism
Understanding Development in Humans
Solution
- don’t start with studying humans as they are too complex
- identify simple model organisms amenable to experiment and relate their information to more complex organisms
Animal Development
Model Organisms
- fruit fly - Drosophilia melanogaster
- nematode worm - C. elegans
- mouse - Mus musculus
- zebra fish - Danio rerio
- cress - Arabidopsis thaliana
Questions in Understanding Development
- what are the mechanisms that control development?
- how is an organism programmed to develop shape and form characteristic of its species?
- how do the individual parts of organisms develop to perform different functions
- genetic analysis and molecular biology combined could provide answers
Cellular Division and Differentiation
- a parent cell divides to form two daughter cells
- in development, the 2 daughter cells are different in function because they express different sets of genes in the genome
- but the different genes expressed are mostly consequences of differentiation, not the cause
- we can identify the genes that regulate developmental decisions by isolating mutants in which the process is disrupted i.e. a parent cell that divides to produce to identical daughter cells
Bacteriophage Development
- a simple prokaryotic model
- bacteriophage development requires programming of gene expression in time
- in many bacteriophages, expression of some genes occurs early in the maturation process, in other genes it is late
- this is achieved by a cascade mechanism, expression of early genes is required to allow the expression of late genes
- mechanisms controlling the switch between early and late gene expression vary in different viruses
Examples of Early-Late Gene Switches
- in phage λ , the product of an early gene, Q, is a transcription factor required to allow the transcription of later genes
- in phage T7, gene 1 encodes an RNA polymerase which can only transcribe from late gene promoters, RNA polymerase produced by the host (E.coli) is used to transcribe the early genes
- in phage T4, early genes 33 and 55 produce transcription factors needed for the transcription of later genes
Bacteriophage Morphogenesis
- assembly of phage T4 capsid is an excellent example of how mutants can establish the order of events in a morphogenesis pathway
- mutants can be isolated that are blocked in capsid assembly
- these need to be conditional mutants, no capsid = no virus
Morphology of Bacteriophage T4
- capsid assembly can be divided into three major steps, head, tail and tail fibres
- the tail is composed of the base plate, core and sheath
- the head is attached to the top of the tail
- the tail fibres are connected to the base plate
Bacteriophage T4 Assembly
Stages
1) the empty head is assembled, DNA is inserted, more proteins are added to complete the head
2) base plate is synthesised, core forms, sheath forms and is built up around the core
3) tail fibres are assembled in stages from precursor molecules
- steps 1, 2, and 3 take place concurrently, there is evidence for the order of assembly and independence of the three steps from mutant analysis
- the head is attached to the tail, and the tail fibres are attached to the base plate
Bacteriophage T4 Assembly
Extra Information
- not all gene products are structural components
- some of the non-structural proteins are assembly enzymes
- the non-structural proteins are less abundant
- historically they were called minor proteins as less of them is made but this is misleading as their role in assembly is NOT minor
Bacteriophage T4 Assembly
Head
- 24 gene products are required to make a head
- 10 are structural
- gene 23 encodes a major head capsid protein
- mutants blocked in head formation are still able to assemble tails and tail fibres
- some capsid associated proteins are nucleases responsible for cutting up DNA into ‘headful lengths’ as DNA is synthesised as a concatemer
Bacteriophage T4 Assembly
Analysis Approach - Description
- identification of temperature sensitive mutants blocked in the different stages (head, tail and fibre) of capsid assembly
- use complementation analysis (double infections) to determine how many genes control each stage
- use biochemical analysis (SDS-polyacrylamide gel electrophoresis) to identify proteins defective in each mutant
Bacteriophage T4 Assembly
Analysis Approach - Head Example
- gene 22 is required to assemble the empty head
- if gene 22 is blocked, a headless phage is produced
- under the microscope, clumps or protein 22 are visible as well as assembled tail and fibre complexes
Bacteriophage T4 Assembly
Base Plate and Tail Assembly
- assembly of the base plate and tail involves similar principles to head assembly
- 32 genes are required, 26 of which are structural
- mutants blocked in tail sheath production can make heads but heads cant be joined to the defective tail
- similarly mutants can also make tail fibres but cant join them to defective tails