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?