Development

Development involves systematic gene-directed changes throughout an organism's life cycle.
Divided into four subprocesses:
- Growth (cell division)
- Differentiation
- Pattern formation
- Morphogenesis

Post-fertilization, the diploid zygote undergoes rapid mitotic divisions:
- In animals, this period is known as cleavage.
- Controlled by cyclins and cyclin-dependent kinases (Cdks).
Cleavage results in smaller cells called blastomeres, with G1 and G2 phases shortened or eliminated.

In zebrafish, blastomeres divide every several minutes, creating an embryo of thousands of cells in just under 3 hours.
Blastomeres are non-differentiated, potentially giving rise to any tissue.

Totipotent: Can give rise to any cell type (zygote, blastomeres).
Pluripotent: Can develop into all cell types (e.g., inner cell mass of blastocyst).
Multipotent: Can develop into a limited number of cell types (e.g., hematopoietic stem cells).
Unipotent: Give rise to only one tissue (e.g., spermatogonia).

Cleavage in mammals continues for 5-6 days forming the blastocyst:
- Outer layer (trophoblast): forms the placenta.
- Inner cell mass (embryoblast): forms the embryo; source of embryonic stem cells (ES cells).

Human bodies contain over 210 major types of differentiated cells.
Cell determination commits a cell to a developmental pathway; this can be experimentally observed by relocating cells.
- If cells adapt to their new positions, they are not determined.

The standard test for cell determination involves transplanting tail cells to a head position, showing whether they maintain original fate or adopt new one based on location.
Differentiation is initiated through transcription factors altering gene expression patterns.

Cytoplasmic Determinants: Differentiation influenced by inherited material within cells.
Induction: Changes in cell fate due to interactions with adjacent cells.

Development in multicellular animals involves no changes in DNA sequences, suggesting epigenetic processes.
Nuclear transfer has demonstrated that determination is reversible.
Totipotency can be seen in early cleavage stages in mammalian embryos, allowing for identical twins' natural formation.

In 1984, the first successful nucleus transplant led to cloning a sheep, followed by Dolly in 1996, proving that differentiation can be reversed.
Challenges in cloning include low success rates and age-associated diseases due to improper genomic imprinting.

Early pattern formation establishes two axes:
- Anterior/posterior (head-to-tail) axis.
- Dorsal/ventral (back-to-front) axis.
Environmental cues inform gene activity, directing appropriate cellular fates based on location.
Hox gene– Homeobox-containing gene that specifies the identity of a body part
Function as transcription factors that bind DNA using their homeobox domain
Ultimate targets of Hox gene function must be genes that control cell behaviors associated with organ morphogenesis

Drosophila (fruit fly) undergoes metamorphosis from larval form to adult form.
Initial development influenced by maternal mRNAs and their gradients, affecting the anterior/posterior and dorsal/ventral formation.
Morphogens are signaling molecules that carry information from one group of cells to another, which helps regulate the fate of nearby cells based on their concentration

Morphogenesis encompasses form and structure development through:
- Cell division, shape and size, death, and migration (plants lack cell migration).
Environmental influences affect development:
- Examples include germination conditions and temperature-dependent sex determination in reptiles.

Cell migration involves adhesion and loss of adhesion mediated by cadherins and integrins interacting with extracellular matrix (ECM).
Apoptosis (programmed cell death) is vital for normal development, while necrosis refers to accidental cell death.