Pregnancy {gestation}| has embryonic and fetal stages. Human embryonic and fetal development takes 280 days or 40 weeks [Winick, 1978].
cells
Approximately 50 cell divisions happen from zygote to newborn, making 256 or so cell types and 10^15 cells.
DNA
DNA one-dimensional molecules can encode embryo development in three spatial dimensions and one time dimension, because embryonic development uses relative times and positions.
development genes
Development genes have many and long introns and many regulatory regions. Development genes express in same order as order on chromosome, which is also spatial order of organism tissues and organs. Development genes turn on and off in cascades. Successive stages depend on previous stages. Transcription factors and receptors cause different gene-regulation and gene-expression patterns and so different cell types.
fertilization
At fertilization, maternal-effect genes code transcription factors that establish top-to-bottom embryo polarity. Bicoid morphogen, at one pole, sets up top-to-bottom gradient. Follicle-cell maternal-effect genes code transcription factors that establish front-to-back embryo polarity. Nanos morphogen, at other pole, sets up gradient across embryo.
Dorsal protein transcription factor, similar to rel protein and NF-kappaB, concentrates in cell nucleus ventrally, and cytoplasm dorsally, in all embryo cells. Cactus and Toll genes can partition dorsal protein to cell locations. Perhaps, Toll proteins are receptors.
gap genes
After first cell divisions, gap genes code transcription factors, with zinc fingers, that make bands along embryo by working with maternal-effect genes and by repressing each other. Gap genes are hunchback, Kruppel, knirps, and hunchback-maternal. Gap genes also regulate genes expressed later. Transcription-factor binding sites are high-affinity or low-affinity, so transcription-factor concentration affects which genes transcribe and how much, leading to gradients and bands.
segmentation genes
After gap-gene expression, segmentation genes code transcription factors that make number of segments, pair segments, and give polarity to segments. Segmentation genes work with gap-gene products and interact with each other, using autofeedback, to sharpen segment boundaries. Segmentation genes include pair-rule genes, such as fushi tarazu gene, even-skipped gene, hairy gene, runt gene, and eve gene.
homeotic genes
After segmentation-gene expression, homeotic genes, such as vertebrate HOX genes, code transcription factors that determine body-part type, such as antenna or thorax. Homeotic genes work with pair-rule pairs to make segments differentiate. All homeotic genes evolved from one gene by gene duplication. Homeotic genes have homeoboxes, which make homeodomains, which bind to DNA promoter sequences to control transcription. All animals have homeotic genes.
torso and polehole genes
After segmentation-gene expression, head and tail develop. For transcription factors only in head and tail, torso gene makes protein-tyrosine-kinase membrane receptors. Polehole gene, similar to raf proto-oncogene, makes protein-serine, threonine kinase that acts on head and tail growth-factor receptors.
proneural genes
After head and tail develop, proneural genes, such as daughterless and achaete-scute, code for transcription factors, with helix-loop-helix, that make neural precursor cells to start brain development. da enhances achaete-scute. enc inhibits achaete-scute.
neurogenic genes
Then neurogenic genes, such as notch, split enhancer, big brain, mastermind, and neuralized, make cell-to-cell signal proteins for cell adhesion, signal transduction, membrane channels, and transcription factors. Neurogenic genes develop cells and inhibit nearby cells.
selector genes
Then selector genes, such as cut gene, code for homeobox transcription factors that make neuron types.
neurotrophic genes
After neurons creation, neurotrophic genes code for secreted neurotrophic factors that keep neurons alive, differentiate neurons, and make neurotransmitters, such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), CNTF, and NT-3. Other genes code neurotrophic-factor receptor proteins.
information from mother
Fetus can use information about mother and environment, such as orientation information. Fetus needs to know relation to mother to aid survival.
Biological Sciences>Zoology>Development>Stages>Gestation
4-Zoology-Development-Stages-Gestation
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Date Modified: 2022.0224