The egg is a large cell; the volume is 1 million larger than normal cells when fertilised it is called a Zygote. During fertilisation, the zygote forms ab embryo containing millions of cells of the same volume. when deposited ready for fertilisation, the haploid ovum is at metaphase of meiosis II
The zygote is polarised along the vertical axis
In Humans, the entrance of the sperm initiates a cascade of events; firstly there is an increase in the concentration of calcium which then causes the activation of egg and release from meiotic II block. Meiosis II is the completed, the haploid nuclei of the sperm and ovum fuse to form a diploid Zygote then there is a rapid division of cells a process called cleavage.
This is basic machinery of the way animals forms across all species
The three germ layers
The gradients establish main areas along the axis, gastrulation, dorsal-ventral, anterior-posterior left, right. Done by gradients of maternal mRNAs and proteins deposited in the egg. This means that for example, the molecules of the cell found in the animal pole and vegetal pole are different in concentration. The graded activation of the zygote genes in segmentation and tissue specification depends on the position of each cell in the embryo exposed to the graded expression of maternal morphogens, which in turn activates embryonic/zygote genes. These genes are activated differently across the zygote.
The positional information within the regions establishes the main body system, for example, the central nervous system and skeletal system are determined by position within the gradient of expressions of morphogens. Similar positional information within the local areas happens to build the various organs and appendages of the animal.
Patterning example- Drosophila
- In Drosophila and other insects, cleavage involves repeated mitosis without cytokinesis
- daughter cell is suspended in one egg
- Thousands of nuclei migrate to the egg margins
- Plasma membrane form around the each nucleus
- Gene expression in daughter cells is regulated by the gradient of cytoplasmic constituents of mother egg.
Bicoid and NANOS
- Drosophila egg has a gradient of maternal mRNA of the bicoid gene with high levels at the anterior and lower levels at the posterior. After fertilisation, m RNA in the zygote is translated into bicoid protein. High levels at the anterior lead to the formation of lava head. On the other, the poster has high levels of the Nanos which will form the tail.
An experiment was done to prove this; scientists removed some of the bicoid rich from the anterior of the fertilised egg. Then replaced with NANo rich cytoplasm from another egg, the result was non-viable two-tailed Drosophila.
Morphogens are substances whose non-uniform distribution governs the pattern of tissue development in the process called morphogenesis or patterning of the embryo. Morphogens establish the positions of various specialised cells types within the tissue. More specifically a morphogen is a signalling molecule that acts directly on a cell to produce specific cellular responses depending on it local location, recent studies indicate that morphogens also help in the activation of transcription factors that help in the expression of genes.
Morphogenesis is the process by which specialised cell divides to give rise to seemingly identical cells. Over time, cells take different routes and eventually become organised into functional organ system.
If all the cells in our body have the same DNA, then why aren’t they all the same cell?
Different genes that code for particular proteins switch on and off at different times in the developmental process. In human beings, different genes are expressed at different times as cells divide and grow. The DNA contains all the information that is needed to form a human being, all the cells in the human body have the same the number of genes, but it is up to the individual cell to determine which gene it wants to activate.
To the get, the information needed a gene must be turned on through the use of transcription factors. Transcription proteins help in the expression of specific genes. For example, the SRY gene ( sex-determining region on the Y chromosome) at the Y chromosomes that determine whether the baby will be a boy will only get switched at about 21 weeks. There are several ways a gene can be switched “ON” or off, for example through the use of transcription factors, methylation and environmental factors. In short, we are all born equal, but our environmental and cultural experiences have an impact on our biology. So one cannot say it is all biology or it is all environmental.
Another example is, the genetic makeup of humans does not create language. However, it directs the organisation of human brain and other peripheral organs that are an important prerequisite in the formation of language, this could account for the differences in language skills.
Finally, who we are biology half environmental, we might have the gene that cord sex, but it all depends on whether that particular gene is expressed. And A child that is not stimulated enough to talk will take longer to develop cognitively, even though all the genetic material is there and is expressed.
Hori, S., Nomura, T., & Sakaguchi, S. (2003). Control of regulatory T cell development by the transcription factor Foxp3. Science, 299(5609), 1057-1061.
Kugler, J. M., & Lasko, P. (2009). Localization, anchoring and translational control of oskar, gurken, bicoid and nanos mRNA during Drosophila oogenesis. Fly, 3(1), 15-28.
Mansouri, A., & Gruss, P. (2016). Pax Genes.
Nishi, Y., Zhang, X., Jeong, J., Peterson, K. A., Vedenko, A., Bulyk, M. L., … & McMahon, A. P. (2015). A direct fate exclusion mechanism by Sonic hedgehog-regulated transcriptional repressors. Development, 142(19), 3286-3293.
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