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Campbell Biology: Online Textbook Help56 chapters | 510 lessons
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Chris has a master's degree in history and teaches at the University of Northern Colorado.
Which came first, the chicken or the egg? Who cares? What I'm interested in is how the egg became a chicken. In all creatures that reproduce sexually, the egg is just a single cell which when fertilized becomes an entire functional organism. How does this happen? Basically, through cell differentiation, the development of the distinct purpose of a cell. This cell is a brain cell, this is a feather cell, this is a beak cell, so on and so forth, until we have a complete chicken. You may notice that the brain cell in the chicken does not get a lot of activity. Chickens are dumb, that's really what I'm getting at. Anyway, how this egg becomes a chicken, or how any fertilized egg becomes a complex organism, is pretty fascinating. So where should we start? I guess with what comes first.
First, we have two different cells, a sperm cell and an unfertilized egg cell. When they fuse and the genetic information within each combines, together they become a zygote, a single, fertilized cell. That's how this chicken starts out, as one cell containing all of the genetic information from both parents needed to become a functional organism. From this point, the zygote cell will divide through cleavage, or cellular division, and create copies of itself.
Each cell created from the zygote, called a blastomere, therefore contains all of this chicken's genetic information. Every cell that the blastomeres turn into will also have all of this information. In fact, every cell in your body not used for reproduction contains your entire genetic sequence. But, even by the stage of turning from a zygote into blastomeres, these cells are starting to look different, despite having the same genetic information within.
The reason that these blastomeres all seem a bit different is because within each one, different genes on the DNA are starting to be activated. Why did this happen? Because the egg cell was filled with molecules in the cytoplasm that impact gene expression called cytoplasmic determinants. Some cytoplasmic determinants are proteins, others are mRNA, or an assortment of various substances that are randomly distributed across the egg. So, this egg is covered in random cytoplasmic determinants, is then fertilized, and becomes a zygote, and the zygote divides into blastomeres
Now, when this happens, the genetic information within the blastomeres is the same, but the cytoplasmic determinants in the cytoplasm of the cell are different. This means that each nucleus is exposed to different molecules, and as those molecules interact with the genetic material inside the nucleus, they start activating different genes. So, from the first divisions of the zygote into new cells, these cells are focused on certain genes over others.
Now at this point, the cells are starting to activate different genes and figure out what kind of cells they will eventually become, but that process isn't set in stone…yet. Eventually it will be, but for now, each cell still has the potential to change which genes are being expressed. Cytoplasmic determinants within the cell help this, but cells also produce chemicals which they secrete and send to other cells
Some of these chemicals are called signaling molecules, molecules that instruct another cell to change its gene expression. As the embryo starts to develop, this is how it makes sure that skin cells will appear next to skin cells, and not in the middle of an organ. When cell differentiation occurs because a signal molecule has been received, it starts expressing different genes, a process called induction. So, embryonic cells could potentially turn into anything, but thanks to cytoplasmic determinants and signal induction, this zygote becomes all of the different cells needed to make an adorable baby chick.
In all creatures that reproduce sexually, sperm and egg combine to create a single cell containing the full genetic sequence called a zygote. This zygote then divides into blastomeres, but by this stage each cell is already starting down a different path. This is because the egg cytoplasm is full of cytoplasmic determinants, molecules that impact gene expression. Since these are distributed unevenly throughout the zygote, each blastomere has a different collection of cytoplasmic determinants, which impact the genes in the cell differently.
The cell starts to develop a function, but at this point can still be changed through signal induction, when signal molecules from another cell change the genes being expressed. The stage of development during which cells can change the direction of their development is not very long, but through cytoplasmic determinants and inductive signals we start creating all of the diverse cells needed to make an organism. And, that's how the chicken came from an egg.
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Campbell Biology: Online Textbook Help56 chapters | 510 lessons