CHAPTER 9

Why do cells divide?

DNA Overload: the genetic material is only capable of “managing” a limited amount of cellular activity

Materials Exchange: As a cell get bigger, the volume increases faster than the surface area, making it difficult to get material in and wastes out at a pace fast enough to allow the cell to survive.

Surface Area to Volume Ratio

How does cell division help cells?

• Keeps surface area to volume ratio high.

• Before cell becomes too large it splits, maintaining nuclear control and efficient transport of materials

Chromosomes

What are they?• Segments of

condensed DNA.

• DNA is coiled around proteins for organization.

• Only visible during division.

Structure:

The Cell Cycle – Events a cell goes through between one

division and the next

– During this time a cell:1. Grows & Develops2. Prepares for Division3. Divides to form two daughter cells

Phases of the Cell Cycle (visual)

G1

S

G2

Phases of the Cell Cycle (words)

Stage (in order)

Description

G1 Initial growthIncrease in size & make new organelles etc.

S DNA replication

G2 Prepare for division

M Nuclear (mitosis) & Cell division (cytokinesis)

*G0 Resting stage following G1“Waits” here until it is ready to divide againSome cells (nerve cells) cannot come out of G0

Interphase

• Non-dividing stage• Cell engaged in

metabolic activity & preparing for mitosis

• DNA is not condensed (chromatin)

• Nucleolus may be visible

Prophasea. Nuclear membrane

breaks down

a. Chromosomes condense and become visible

a. Centrioles move to opposite ends and begin to form spindle

Metaphase

a. Chromosomes line up in the middle

b. Microtubules connect the centromere of each chromosome to the poles of the spindle

Anaphasea. Chromatids split at

centromere (each is now an individual chromosome)

a. The new, single-stranded, chromosomes begin to move to opposite ends

Telophase

a. Chromosomes reach opposite ends and begin to de-coil.

a. 2 new nuclear membranes begin to form.

Cytokinesisa. Division of the cell

b. Often occurs during or directly after telophase

c. Mitosis can occur independently of cytokinesis, this results in a multinucleated cell, such as muscle cells.

Cytokinesis: Plants v. Animals

1. Plants: Cell plate forms and gives rise to a cell wall. No centrioles involved.

2. Animals: membrane pinches in and cell splits.

Controls on Cell Division Some cells stop dividing when mature (stay in G0)

ex) many neurons (nerve) & muscle cells

Others divide for growth & repair

Normally, cells will stop dividing when they come in contact with other cells

Cell Cycle Regulators:

• Proteins called cyclins start/stop the cell cycle.

• The cycle is regulated by various factors:

1. Intracellular factors: - All chromosomes are

replicated- All spindles are attached

2. Extra-cellular factors: - Growth factors- Cell to cell contact

Uncontrolled Cell Growth: CANCER

• Tumor Suppressor Genes: (p53) - genes that normally halt the cell

cycle - stuck OFF (cells keep dividing)

• Oncogenes: - genes that tell cells to divide - “stuck ON” (cells keep dividing)

Stem Cells & Development• Cells take on specialized roles through the

process of differentiation

• Differentiated cells develop from undifferentiated cells called stem cells

• Fully undifferentiated stem cells are totitpotent– (ex. Fetilized egg through first few divisions)

• Mostly undifferentiated are pluripotent – (ex. Early embryonic cells)

• Partially differentiated are multipotent – (ex. Adult marrow cells)

• When/Why do organisms rely on mitosis/cytokinesis?

• How do the daughter cells formed as a result of mitosis/cytokinesis compare to the original parent cell?

• Can you think of a time when new cells need to be formed that do not compare to the parent cell in this way?

Gamete ProductionGamete Production• Sexual reproduction

begins with the fusion of egg and sperm to produce a zygote.

• Each organism must inherit a single copy of each gene from each parent.

• Therefore, when an organism produces its gametes (egg or sperm) their gene pairs must be separated.

Chromosome NumberChromosome Number

1. The fruit fly (drosophila) has 88 chromosomes

2.2. 4 4 come from the mommom and 44 from the daddad

2. These two sets of chromosomes are called homologous chromosomes.

2. A cell with both sets is called diploid (2n).

5. Gametes (egg/sperm) only have one set of chromosomes and are referred to as haploid (1N)

6. The process of forming haploid (N) cells, {gametesgametes}, from diploid (2N) cells {body/somatic cellsbody/somatic cells} is called meiosismeiosis.

5. Meiosis involves two distinct stages meiosis Imeiosis I and meiosis IImeiosis II

Chromosome Number Chromosome Number (con’t)(con’t)

Crossing OverCrossing Over• Genetic material is

exchanged between homologous chromosomes during Prophase 1

• This is referred to as genetic recombination.

• During “synapsis” the crossed over homologous chromosomes are referred to as a tetrad.

What is the benefit of crossing over? HINT: Think from an evolutionary perspective.

Meiosis I v. Meiosis II

• During Meiosis I, homologus pairs are separated.

• During Meiosis II, sister chromatids of double-stranded chromosomes are separated to form single-stranded chromosomes.

Gamete FormationGamete Formation

MalesMales“spermatogenesis”

• All 4 haploid cells produced become sperm

FemalesFemales“oogenesis”

• Only one of the cells becomes an egg

• Egg is larger, needs the cytoplasm

Male v. Female Gamete Male v. Female Gamete FormationFormation

Mitosis v. MeiosisMitosis v. Meiosis• MitosisMitosis:

– 2 genetically identical diploid cells– produces somatic (regular body) cells– Method of asexual reproduction

• MeiosisMeiosis:– 4 genetically different haploid cells– Reduction division– Produces gametes (reproductive cells)

for sexual reproduction