Cell Cycle, Mitosis, and Meiosis Biology is the only subject in which multiplication is the same thing as division… 2007-2008 1
The Cell cycle “Life cycle” of a cell cell is formed from a mitotic division cell grows & matures to divide again cell grows & matures to never divide again G1, S, G2, M liver cells G1G0 epithelial cells, blood cells, stem cells brain / nerve cells muscle cells 2
Cell Cycle Interphase: Divided into 3 phases: G0 Mitosis Phase G1 = 1st Gap cell doing its “everyday job” cell grows S = DNA Synthesis copies DNA (chromosomes) G2 = 2nd Gap prepares for division cell grows (more) Mitosis Phase cell division occurs G0 signal to divide 3
Replicated Chromosome 2 sister chromatids narrow at centromeres contain identical copies of original DNA Centromeres are segments of DNA which have long series of tandem repeats = 100,000s of bases long. The sequence of the repeated bases is quite variable. It has proven difficult to sequence. Why copy the DNA?
What if it gets out of control? How is this process regulated/controlled? What if it gets out of control?
Overview of Cell Cycle Control There’s no turning back, now! Overview of Cell Cycle Control Two irreversible points in cell cycle replication of genetic material separation of sister chromatids Checkpoints regulate cycle process is assessed & possibly halted centromere sister chromatids single-stranded chromosomes double-stranded
Checkpoint control system 3 major checkpoints: G1 can DNA synthesis begin? G2 has DNA synthesis been completed correctly? commitment to mitosis M are all chromosomes attached to spindle? can sister chromatids separate correctly? Which checkpoint will start prep for cell division?
Checkpoint Consequences: If checkpoint is not met: Exits cycle to G0 phase (only if at G1) otherwise… Apoptosis – cell death
How Do Cells Know When to Divide? Chemical signals in cytosol CYCLINS: Proteins that buildup in cytosol CDK’s – “Cyclin Dependent KINASE” enzymes always present in cytosol bind with cyclins to phosphorylate other proteins
CDK’s and Cyclins More cyclins = more bonding with CDK More CDK/cyclin complexes = cell division more likely to happen or passing checkpoints Genes for CDK and cyclins are highly conserved through evolution.
Checkpoint Failure: CANCER! Uncontrolled cell division Loss of checkpoint control Oncogenes turned on (growth promoter genes) Tumor suppressor genes off p53 major protein in checkpoint G1 Escapes apoptosis (no cell suicide) Touch-sensor genes off no density-dependent inhib Blood vessel growth on What are carcinogens?
Draw and label stages in notebook! What about Mitosis? Evolved from Binary Fission of prokaryotes Cells start and end diploid (2N) Two sets of chromosomes; somatic cells Starts with 1 cell and ends with 2 cells Stages: Prophase, Metaphase, Anaphase, Telophase with Cytokinesis Draw and label stages in notebook!
Comparing Cytokinesis
What about Meiosis? Only when making sex cells (gametes) Cells start 2N, but finish haploid (N) Start with 1 cell, end with 4 cells Increases variability by: genetic recombination (sperm/egg) crossing over by chromosomes Goes through 2 cycles: Meiosis 1 and Meiosis 2 P1M1A1T1 and P2M2A2T2
Meiosis 1 and Meiosis 2 How many chromosomes in the cell? At what point are the cells haploid? (Having 1/2 the number of chromosomes?)
Human male karyotype 46 chromosomes 23 pairs 2 sets
Homologous chromosomes Paired chromosomes both chromosomes of a pair carry “matching” genes homologous = same information single stranded homologous chromosomes diploid 2n 2n = 4 TETRAD : 4 chromatids double stranded homologous chromosomes
Crossing Over A.K.A Synapsis During Prophase 1 homologous pairs (tetrads) swap genetic material Increases genetic variations
Meiosis 1 1st division crossing over occurs single stranded Meiosis 1 1st division crossing over occurs separation of homologous pairs 2N to N (diploid to haploid) 2n = 4 double stranded prophase 1 2n = 4 double stranded metaphase 1 reduction 1n = 2 double stranded telophase 1
4 Meiosis 2 separates sister chromatids stays haploid 4 cells at end double stranded prophase 2 1n = 2 double stranded metaphase 2 1n = 2 single stranded telophase 2 4
Putting it all together… meiosis fertilization mitosis + development gametes 46 23 46 23 46 46 46 46 46 23 meiosis 46 46 egg 46 46 23 zygote fertilization mitosis sperm development