Cell Division. Why divide? Characteristic of life Continuity Growth (zygote → multicellular org)...
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Transcript of Cell Division. Why divide? Characteristic of life Continuity Growth (zygote → multicellular org)...
Cell Division
Why divide?
Characteristic of life Continuity Growth (zygote → multicellular org) Repair, renewal, replacement
Requirements
Distribution of identical DNA to daughter cells
Figure 12.0 Mitosis
DNA – a closer look
Genome – cell’s entire genetic info Prok – often a single long DNA molecule Euk – several DNA molecules
Human cells must copy ~ 3 m of DNA before division
Packaging DNA
Chromosomes Contain DNA and Protein Called chromosomes because
they can be stained with certain dyes
Eukaryotic Chromosomes
Composed of CHROMATIN protein DNA
Chromosomes become visible as distinct structures when the cell divides
When not dividing, the chromosomes decondense
Chromosome Structure
Duplicated chromosome = 2 sister chromatids
Chromatids → identical copies of the DNA As they condense, the area where strands
connect shrinks → centromere
Genes
Organization of DNA informational units
Chromosomes contain hundreds to thousands of genes
Humans: 35,000 - 45,000 genes
Number of Chromosomes
Differ by species Humans - 46 chrom (somatic cells) The number is not indicative of complexity
What is this called?
KARYOTYPE
Gametes
Gametes contain half the # of chromosomes present in somatic cells
Human gametes – 23 chromosomes WHY?
Cell Cycle
A sequence of cell growth and division
Numerous factors control when cells divide
Cell Division Mitosis- division of chromosomes Cytokinesis- division of cytoplasm
Cell Cycle
Chrom duplicate during INTERPHASE (90% of cell’s life) G1 phase - cells grow and synthesize
biological molecules S phase - DNA replication G2 phase - gap of time between S phase and
mitosis (preparation for division)
Mitosis
Purpose is to ensure the orderly distribution of chromosomes
Four Stages: Prophase Metaphase Anaphase Telophase
Late Interphase
Chrom duplicated, still loosely packed Centrosomes duplicated, organization of
microtubules into an “aster”
Mitosis – Prophase (early) Duplicated chromosomes visible Chromatin condenses Sister chromatids are bound at the
centromere Centromeres have kinetochores
(proteins) to which microtubules will bind
Mitosis – Prophase (early)
The mitotic spindle, composed of microtubules, forms between the poles
The MTOC (microtubule organizing center) surrounds a pair of centrioles in animal cells and some plant cells
Centrioles are surrounded by pericentriolar material
Mitosis – Prophase (middle)
Asters extend from the MTOCs at the poles (in cells that have centrioles)
The nucleolus disappears
ASTER
Mitosis – Prophase (late)
The nuclear envelope disappears
Mitosis - Metaphase
Duplicated chromosomes line up at midplane
Chromatids are highly condensed Polar microtubules extend from the pole to
the equator, typically overlap Kinetochore microtubules extend from the
pole to the kinetochores
Mitosis – Anaphase (early)
Chromosomes move toward the poles
Chromatids separate at the centromeres and are now referred to as chromosomes
Mitosis – Anaphase (late)
The chromosomes are pulled by the kinetochore microtubules to the poles and form a “V” shape
The movement mechanism by which the microtubules and other mitotic spindle components move the chromosomes is largely unknown
Mitosis- Telophase
Two separate nuclei form Cell returns to conditions similar to
interphase Nuclear envelope reforms; nucleoli
reappear Cytokinesis occurs
Cytokinesis
Formation of two separate daughter cells Begins during telophase In animals cells, a furrow develops
caused by contractile actin filaments that encircle the equatorial region
In plant cells, a cell plate forms originating from the Golgi complex
How do chromosomes move?
A chromosome’s kinetochore is “captured” by microtubules, & it moves toward the pole connected to microtubules
Microtubules attach to the other pole, tug-of-war ensues
Other microtubules from opposite poles interact as well, elongating the cell
Chromosome movement
Hypothesis for chromosome movement is that motor proteins at kinetochore “walk” attached chromosome along microtubule toward opposite pole
Excess microtubule sections depolymerize
Experiments support hypothesis that spindle fibers shorten from end attached to chromosome, not centrosome
Mitosis - General Notes
Mitosis (generally) produces two daughter cells genetically identical to the parent cell
Most cytoplasmic organelles are distributed randomly to the daughter cells
Mitochondria and chloroplasts divide on their own during interphase
Mitosis- General Notes
Eukaryotic cells typically divide less frequently than prokaryotes
The Cell Cycle
Frequency of cell division varies with cell type
Driven by specific chemical signals in the cytoplasm
Experiment
Fusion of S phase cell and G1 phase cell induces G1 nucleus to start S phase
Fusion of cell in mitosis with one in interphase induces 2nd cell to enter mitosis
Control of the Cell Cycle
Protein kinases are active when complexed with cyclins (regulatory proteins)
When Cdk complexes with a certain cyclin, it activates specific enzymes and can inactivate others
Colchicine (one of a number of drugs that block cell division) can block cell division in eukaryotes by interfering with spindle formation
More about CdKs
Protein kinases activate/deactivate other proteins by phosphorylating them
Levels of kinases are constant, but they require cyclin, to become activated Levels of cyclin fluctuate (cyclically) Form cyclin-dependent kinases (Cdks)
Cyclin levels rise sharply in interphase, fall abruptly during mitosis
Peaks in the activity of cyclin-Cdk complex, MPF, correspond to M phase
MPF
“Maturation-promoting factor”; cyclin-Cdk complex
Phosporylates kinases Fragmentation of nuclear envelope Breakdown of cyclin
External Control
Growth factors (mammalian cells), proteins released by one group of cells - stimulate other cells to divide platelet-derived growth factors (PDGF)
PDGF Fibroblasts in culture only divide in a
medium that contains PDGF
Density-dependent inhibition
Cultured cells divide until they form single layer on inner surface of dish
If gap is created, cells will grow to fill gap At high densities, amt of growth factors &
nutrients insufficient to allow continued cell growth
Anchorage Dependence
To divide, cells must be anchored to something, typically extracellular matrix of a tissue
Control mediated by connections between extracellular matrix, plasma membrane proteins, & parts of cytoskeleton
Cancer cells – HOW DO THEY WORK?