BIOL 202 Lecture Notes - Lecture 5: Cell Membrane, Genetic Screen, Multicellular Organism
Prof Moon Lecture 5:
Developmental Genetics
We all start from a single cell but develop into a multicellular organism with specialized
cells. How does this happen? By transcriptional regulation.
Embryonic development in Drosophila can be used to observe specialization of cells by
transcriptional regulation.
There is no cytokinesis i.e. the nucleus keeps dividing after it is fertilized and a diploid is
formed. No cellular plasma membrane is formed and the number of nuclei keep increasing
as part of the embryogenesis. After 9 divisions the replication slows down and the nuclei
start to migrate to the periphery of the embryo. After 4 more divisions at the periphery, the
plasma membrane forms as the periphery (shell). This separation and division etc. are all
controlled by the proteins and RNA provided by the mother as early on in embryogenesis,
there is no transcription and translation i.e. the baby has not yet produced any
transcriptional factors f its own. After 2 hrs of embryogenesis specification of cells starts to
happen (first separation of anterior, ventral and posterior). Later specification of each
portion takes place.
A forward genetic analysis was done to identify genes required for organizing the drosophila
embryo. There were 2 screens done, one for the maternal genes and one for zygotic genes.
Maternal genes screening: The phenotype
determined by the genotype of the mother. The
fathers genotype doesn’t matter. As long as the
mother has even 1 single wild type copy, the
offspring will be normal.
Zygotic genes: phenotype determined by
genotype of embryo. The inheritance shows
regular Mendellian pattern.
Many genes found from the maternal genetic screen were found to be transcriptional
factors which are very crucial for embryogenesis.
Two genes important for basic positioning along the anterior or posterior axes are bicoid
and nanos. Bicoid is important for anterior formation. Mutants lacks anterior structures
(head, thoracic segments). Nanos is important for posterior segments (abdomen). Both
bicoid and nanos encode for transcription factors.
Bicoid mRNA from the mother localizes to anterior of embryo and the mRNA gets translated
forming the bicoid protein which starts diffusing towards the posterior side forming a
diffusion gradient and so providing information about distance from the anterior. Bicoid is
required to form anterior structures, is it sufficient? Yes! We know this because when bicoid
mrna is injected anywhere in the embryo, t starts forming the anterior structures right then
and there.
Nanos: it is a gene responsible for posterior formation and the nanos proteins diffuses from
posterior to anterior. Another thing nanos does is, that it inhibits translation of uniformly
distributed maternal hunchback mRNA. Towards the P, the hunchback protein is very less
where the nanos protein is high.
5 types f genes found from the maternal screening:
1- Gap genes: translate maternal A-P gradients into broad sub-domains. They are
involved in development of segmented embryos e.g. drosophilla. Gap genes are
