Chapter 11: Mitosis, Meiosis, Cell Cycle
Cells and multicellular organisms replicate
Asexual reproduction: 1 cell divides to make 2; one organism divides to make 2
Sexual reproduction: cell fusion followed by cell division
Replication requires different types of cell divison
Mitosis
Meiosis
Cell division consists of 2 independent, coordinated processes
nuclear division: replication and separation of DNA strands
cytokinesis: cytoplasmic divison
Cell division in bacteria
attachement of genome to cell membrane
replication: cell elongates/expands as replication fork moves around cell
complete replication of division
binary fission: wall grows in from 2 different sides --> 2 daughter cells
The genome of eukaryotes is divided into pieces (chromosomes)
DNA is organized with histones --> chromatin --> chromosomes
histones are positively charged and associate with DNA because it's negatively charged
Structure of a chromosome
2 strands (chromatids) that contain DNA
the chromatids are held together by a centromere
the kinetochore at the centromere is where the microtubules attach when chromosomes separate during mitosis
Mitosis: 2 stages - interphase and mitosis (chromosomes are clearly visible and separate)
Interphase
G1: I DNA strand (1 chromatid)
S phase (DNA synthesis): DNA inside of the chromosome replicates
G2: 2 DNA stands (2 identical chromatids, i.e. sister chromatids)
Mitosis: separation of sister chromatids
1) Prophase: DNA/chromosomes condense, nuclear membrane breaks down, centrosome divides into 2 parts at opposite ends
2) Prometaphase: microtubules of the mitotic spindle attach to each kinetochore of the chromosomes
3) Metaphase: chromosomes align in cell center
4) Anaphase: centromere degrades, sister chromatids separate, chromatids migrate to 2 opposite poles
5) Telophase: nuclear envelope reforms, cytokinesis (new cell wall forms between the 2 reforming nuclei)
CYTOKINESIS IS DIFFERENT IN ANIMALS AND PLANTS
Animal: formation of the contractile ring (made up of microfilaments) in center of cell
contractile ring pulls against each other, divides into 2 cells
Plants: cell builds a new cell wall in the middle of cell --> formation of a new "cell plate"
Meiosis
sex: uniting of 2 genomes
haploid set of chromosomes = the human genome : contains 1 copy of every chromosome
Homologous chromosomes: chromosomes that have same size same set of genes
23 PAIRS of chromosomes (1 from dad, 1 from mom) = 46 total chromosomes
Somatic (non-gamete) cells are DIPLOID: TWO copies of each chromosome
Meiosis solves the chromosome number problem
organisms that reproduce sexually must find a way to reduce chromosme number in their gametes by half to prevent an exponential increase in chromosome number
accomplished by meiosis
Cycle
Haploid Gametes are the product of meiosis
Fertilized: produces daughter cell called diploid zygote (chromosomes from mom+dad)
mitosis: produces organism
Meiosis: In gonads: produces gametes
Meiosis has 2 divisions
INTERPHASE: homologous pair of chromosomes in diploid parent cell REPLICATE --> Homologues pair
MEIOSIS I: Homologous chromosomes separate --> haploid cells with replicated chromosomes (pair of sister chromatids)
MEIOSIS II: sister chromatids separate
PRODUCT: 4 haploid gametes
Meiosis produces genetic novelty
Independent assortment
crossing over
depending on how the homologous chromosomes separate in meiosis I --> various combinations when sister chromatids split in meiosis II
non-sister chromatids break and then join in such a way that maternal and paternal genetic material is exhanged
Regulation of Cell Cycle
Cell cycle is driven by cyclin dependent kinase (Cdk)
Cdk regulates other proteins' activity by phosphorylation
levels of Cdk stays the same
Cdk activity is monitored by cyclin --> produces MPF complex when connected
Cdk by itself DOES NOT have activity
MPF phosphorylates proteins
Different points in cell cycle where cdk/cyclin is important: restriction points
Start point: at end of G1
G2-M transition
influenced by growth factors, nutrients, cell size, DNA damage
influenced by cell size, DNA damage, DNA replication
Metaphase-anaphase transition
influenced by chromosome attachements to spindle