Interphase.
Meiosis I.
- Generates the most diversity.
- Starts diploid.
- Prophase 1. DNA has already been replicated prior to Meiosis I. Homologous chromosomes pair (bivalents) to form synapses. Chromosomes condense.
- Bivalent. Has 2 chromosomes and 4 chromatids. A pair of homologous chromosomes.
- Homologous chromosomes. One maternal chromosome and one paternal chromosome.
- Synapses. Crossover “mutations” can occur when homologous chromosomes pair up.
- Synaptonemal complex. Protein that holds homologuess together.
- Prometaphase 1. Nuclear membrane disappears. One kinetochore per one chromosome (instead of per chromatid). Chromosomes attach to spindle fibers.
- Metaphase 1. Bivalents (2 chromosomes, 4 chromatids) align at metaphasic plate.
- Anaphase 1. Chiasmata separate. Chromosomes (each with 2 chromatids) move to opposite poles. Each daughter is now haploid (n).
- Telephase 1. Nuclear envelope may reform or it may just go into Meiosis 2.
- Chiasmata. The point where paired chromosomes remain in contact together during meiosis 1. X-shape area. Help keep homologues together after synaptonemal complex breaks down.
- Ends haploid.
Meiosis 2.
- There is no copying of DNA when going from Meiosis I to Meiosis II.
- “Meiosis for haploid cells”.
- Starts haploid.
- Meiosis 2 is shorter and simpler.
- Cells entering are haploid. They have one chromosome from each homologous pair. Each chromosome still has 2 sister chromatids.
- Prophase 2. Nuclear envelope (if it had reformed prior) breaks. Chromosome condense.
- Prometaphase 2. Spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to spindle.
- Metaphase 2. Sister chromatids line up at metaphasic plate.
- Anaphase 2. Sister chromatids split up and move to opposite poles. Pulled apart by kinetochore microtubules.
- Telophase 2. Chromosomes decondense. Nuclear envelope reforms. Cytokinesis. Results with 4 haploid cells.
- Ends haploid.
Resources.
The Cell Cycle
- Two main stages: Interphase and Mitosis.
- G0 Phase. The phase after mitosis and it can be a “dormant” type of phase before interphase actively begins again.
Interphase.
- G1 (1st Gap) Phase. Physically grows larger and proliferates its organelles. Make a ton of shit to get ready to divide it up. Synthesize RNA and proteins. G1 Checkpoint. Make sure things are ready for DNA synthesis.
- S (Synthesis) Phase. Duplicate DNA. Duplicate centrosomes.
- G2 (2nd Gap) Phase. Grows more and makes more stuff to get ready for mitosis. G2 ends when mitosis begins. G2 Checkpoint.
Cell Cycle Regulation.
- R, Restriction Point. Major checkpoint at end of G1 prior to S.
- S phase checkpoint: CDK2 and Cyclin A.
- G2 Checkpoint: CDK1 and Cyclin B.
- M Checkpoint.
- G1 Checkpoint: CDK4 and Cyclin D; CDK2 and Cyclin E.
- Cdk. Cyclin dependent kinase. Adds phosphate to protein. Major checkpoint. If all is good, allows cell to move from G1 to S to G2 to M.
- MPF. Maturation Promotion Factor. Includes Cdk and cyclins. Allows cells to move between phases.
- p53. Protein that blocks cell cycle if DNA is damaged. If damage is real bad, causes cell apoptosis. p53 levels increased if DNA damaged; a “hold” is put and allows time for fixing DNA. p53 mutation is big factor leading to cancer.
- p27. Binds to cyclin and Cdk, blocks entry into S phase.
Mitotic Phase (Mitosis).
- Goal: divide the already duplicated DNA and divide the organelles to make two completely identical copies of the original cell.
- Mitosis is nuclear division plus cytokinesis (division of cytoplasm, organelles, and cleaving/pinching off the cell membrane to make 2 distinct and identical cells).
- Interphase. High metabolic activity. Chromatin are not distinct. A pair of centrioles have not yet been duplicated.
- Prophase. Chromatin condense and are visible = chromosomes. Nucleolus disappears. Centrioles move to opposite poles. Mitotic spindle fibers begin forming.
- Prometaphase. *Nuclear membrane dissolves. Proteins attach to centromeres (region where microtubules of spindle will attach), creating kinetochores (protein complex assoc. with centromeres).
- Metaphase. Paired chromosomes line up at the metaphasic plate.
- Anaphase. Paired chromosomes separate at kinetochores and move to opposite poles.
- Telophase. Chromatids are at opposite poles, nuclear membrane reforms, chromosomes get all loose again = chromatin. Spindle fibers disassemble/disperse.
- Cytokinesis. An actin ring forms and divides the cell.
- Chromatin = DNA + structural proteins, histones. The loose, uncondensed form of DNA.
- Chromosome = condensed form of DNA.
- Chromatid = each of a sister chromatin. One of two identical halves of a replicated chromosome. The two identical copies are joined at the centromere.
Resources.
