Cell Division, Cell Cycle, Interphase

• The reasons cells divide is for growth, to replace old or damaged cells, and reproduction
• Cell division differs in prokaryotes and eukaryotes
• The surface area of a 1cm x 2cm x 2cm cell is 4cm3
• Asexual reproduction involves one organism reproducing by itself
• Sexual reproduction involves two organisms which create a new life
• Chromatin is the material that chromosomes are composed of in eukaryotes
• Chromatin consists of protein, RNA, and DNA
• Chromosomes are threadlike structures made of protein and a single molecule of DNA that carry genomic information

The Cell Cycle

• The cell cycle consists of interphase (G1, S, and G2 phases), followed by the mitotic phase (mitosis and cytokinesis), and the G0 phase

Interphase

• Interphase is the resting phase between successive mitotic divisions of a cell, or between the first and second divisions of meiosis
• Interphase is composed of G1 phase (cell growth), S phase (DNA synthesis), and G2 phase (cell growth)
• The mitotic phase, which consists of mitosis and cytokinesis, occurs at the end of interphase and leads to the formation of two daughter cells

Stages of Interphase

• During the G1 phase, the cell grows
• During the S phase, DNA is synthesized
• During the G2 phase, the cell continues to grow

Cell Division

• The two types of cell division are mitosis and meiosis

Mitosis

• Mitosis has four stages: prophase, metaphase, anaphase, and telophase
• In early prophase, the chromatin in the nucleus condenses to form visible chromosomes, and the mitotic spindle forms from fibers in the cytoskeleton or centrioles (in animal cells)
• In late prophase, the nuclear membrane and nucleolus break down, chromosomes continue condensing and are clearly visible, spindle fibers (kinetochores) attach to the centromere of each chromosome, and the spindle finishes forming between the poles of the cell
• Sister chromatids are held together at the centromere
• A chromatid is each of the two threadlike strands into which a chromosome divides longitudinally during cell division, and each contains a double helix of DNA
• Sister chromatids are two identical copies of the same chromosome formed by DNA replication, attached to each other by the centromere, which are separated during cell division so that each daughter cell receives one copy of the chromosome

Cytokinesis

• Cytokinesis is the cytoplasmic division of a cell at the end of mitosis or meiosis, resulting in the separation into two daughter cells
• Cytokinesis in plant and animal cells differs in that plant cells form a new cell wall surrounding the daughter cells by forming a cell plate between them, while animal cells form a cleavage furrow between the two daughter cells
• In mitotic division, daughter cells enter interphase after the completion of cytokinesis
• Cyclins are regulatory proteins that control cell division, and are among the most important core cell cycle regulators
• The four basic types of cyclins found in humans and most other eukaryotes are G1 cyclins, G1/S cyclins, S cyclins, and M cyclins

Cell Cycle regulators

• Cdks, cyclins, and the APC/C are direct regulators of cell cycle transitions that respond to internal and external cues to influence the activity of core regulators to determine whether the cell moves forward in the cell cycle
• A checkpoint is a stage in the eukaryotic cell cycle where the cell examines internal and external cues and "decides" whether or not to move forward with division
• Apoptosis is the death of cells, which occurs as a normal and controlled part of an organism's growth or development
• Cancer is a disease caused when cells divide uncontrollably and spread into surrounding tissues
• Oncogenes are special proteins that increase the chance that a normal cell develops into a tumor cell
• Tumors can be benign (noncancerous) or malignant (cancerous)
• Benign tumors tend to grow slowly and do not spread
• Malignant tumors can grow rapidly, invade and destroy nearby normal tissues, and spread throughout the body
• Meiosis and the activation of meiotic genes in somatic cells can cause cancer, and the re-expression of meiotic genes in somatic cancer cells is responsible for driving some of the hallmarks of cancers
• Meiosis is a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes and plant spores
• Synapsis is the fusion of chromosome pairs at the start of meiosis

Mitosis vs. Meiosis

• Mitosis functions in cell growth and repair
• Meiosis functions in reproduction
• Mitosis occurs in somatic cells
• Meiosis occurs in the testes and ovaries
• Meiosis is a special version of cell division that occurs only in the organs that produce the male and female reproductive cells: the sperm and eggs
• Haploid refers to the presence of a single set of chromosomes in an organism's cells
• Sexually reproducing organisms are diploid (having two sets of chromosomes, one from each parent)
• In humans, only egg and sperm cells are haploid