Cell Cycles

• There are permanent cells (go to G0 permanently like neurons and osteocytes to never divide), stable cells (liver hepatocytes have a low level of replication), and labile cells (epithelium in the mouth continuously divides and replicates)
• Interphase is G1, normal function. G1 checkpoint (favourable environmental factors). S (chromosomes duplicated to produce two sister chromatids on each chromosome). G2 (spindle pole duplication, two centrosomes). G2 checkpoint (cell size, DNA errors). Start mitosis. M checkpoint clarifies correct spindle attachment.
• A chromosome is DNA carrying a linear array of genes, wrapped around histones
• Homologous chromosomes are similar in shape and size, one from each parent. They have the same array of genes, but aren't identical.
• Centromere: region of the chromosome to which the microtubules of the spindle attach, via the kinetochore, during cell division. Where the two sister chromatids are attached
• 3 types of centromeres: metacentric (middle), arocentric (toward one end) and telocentric (at end).
• Chromatids are one of two identical strands that result from self-duplication of a chromosome
• A karyotype is all the chromosomes in a cell. In humans, there are 23 pairs of homologous chromosomes.
• A somatic cell is a component of the body but not a reproductive cell
• Germ cells are reproductive cells capable at maturity of being fertilised and reproducing an entire organism
• A gamete is a mature reproductive cell
• Mitosis is the separation of duplicated chromsomes and division of the cytoplasm to produce two genetically identical diploid daughter cells.
• 4 phases of Mitosis: prophase - chromosomes condense. metaphase - nuclear envelope breaks down, chromosomes align in the middle of the cell. anaphase - chromosomes pulled to opposite ends of the cell by microtubules. telophase - spindles disassemble, nuclear envelope reforms, chromosomes decondense. Then the membrane is cleaved through cytokinesis. A cleavage furrow forms in the membrane, then a contractile ring of actin pinches it off.
• Meiosis occurs only in germ cells, and is the process by which chromosome number of a reproductive cell is halved to produce non-identical gametes (sperm/ovum) called haploid cells. This is an important source of variability through recombination,
• In meiosis, prophase I can be divided into: leptotene (duplicated chromosomes condense), zygotene (synapsis begins), pachytene (synapsis complete, crossing over occurs), diplotene (synaptonemal complex disappearing, chiasma visible), diakinesis (NE fragmenting, bivalent ready for metaphase)
• In metaphase I, homologous chromosomes are separated, NOT SISTER CHROMATIDS
• Meiosis involves two successive divisions; 1. reductional division, where chromosome number is halved, and 2. equational division, where sister chromatids separate.
• In Anaphase I homologous chromosomes move to opposite poles. CHROMATIDS DO NOT DIVIDE.
• In Telophase I the nuclear envelope reforms, which is followed by cytokinesis
• In prophase II, the chromosomes condense again
• Metaphase II separates sister chromatids. Anaphase II involves the centromeres dividing, daughter chromosomes moving to opposite poles. Telophase II; the nuclear envelope reforms. Cytokinesis; cytoplasm divides to produce 4 haploid cells, each containing one copy of one of the two homologous chromosomes.
• Oogenesis creates only one viable gamete through asymmetric cytokinesis, in order to preserve the cell membrane. At birth, all primary oocytes are present, created from the oogonia. At ovulation, Meiosis I occurs to create a secondary oocyte that is then released from an ovary. Meiosis II happens with sperm cells at fertilisation. The first and second polar bodies undergo apoptosis (cell death).
• Spermatogenesis creates 4 viable gametes through meiosis. These are called spermatids, and they mature into sperm cells.