One Zygote Divides Into Many Identical Cells That Then SpecialiseAt fertilisation a sperm cell fuses with an egg cell to form a single zygote that carries the full genome of the new organism.The zygote divides many times by mitosis, producing many genetically identical unspecialised cells.These early cells can still give rise to many different cell types, then progressively commit as development continues.Each unspecialised cell then differentiates into a specialised cell suited to a particular job.AnalogyEvery cell holds the same library of songs, which is the genome.Each cell type plays a different playlist by expressing a different subset of those genes.Differentiation: Same Genome, Different Genes Switched OnDefinitionSee other references2DifferentiationDifferentiation is the process where pluripotent cells (unspecialized cells) develop into specialized cells with unique structures and functions.Every cell in the body carries the same genome, so cells differ only in which genes they use.A cell becomes specialised through differential gene expression, meaning it expresses some genes and not others.A red blood cell expresses the haemoglobin genes and switches off genes for other cell functions, giving it its oxygen-carrying role.Common MistakeDifferentiation does not delete any genes from the cell.All genes are retained, and only some are expressed in each cell type.Position Tells a Cell Which Genes to ExpressSignalling molecules spread through the embryo and form concentration gradients that vary from one region to another.A cell senses its position from the concentration it detects, which is called positional information.That position sets its differential gene expression, which fixes the cell's fate so neighbouring regions form different tissues.ExampleIn the fruit fly, gradients of signalling molecules such as Bicoid make cells in the head region develop differently from cells in the abdomen.Putting It TogetherFertilisation and mitosis produce many identical unspecialised cells from one zygote.Position and differential gene expression then turn those cells into specialised cell types.Active recallWhat forms at fertilisation and by which process does it divide into many cells?Why are the many cells produced from the zygote genetically identical?What is differential gene expression?Does differentiation remove genes from a cell, and why or why not?How does a cell's position, sensed via concentration gradients, decide its fate?
