makes identical cells

• During fetal development, many cells are programmed to die, apoptosis

Control of Cell Division and Cell Death • Human cells appear to be programmed to

undergo only so many cell divisions– About 50 in cell cultures

Fingers and toes form from these paddlelike hands and feet

Only cancer cells and stem cells can divide endlessly

Apoptosis happens if cells of the human myeloid cell line are deprived of growth factors. SEM 10,000.

Controlling the Cell Cycle• The eukaryotic cell cycle is controlled by feedback at three checkpoints

– 1. Cell growth is assessed at the G1 checkpoint– 2. DNA replication is assessed at the G2 checkpoint– 3. Mitosis is assessed at the M checkpoint

• Cancer is characterized by cells that have lost the ability to control their division

G0 is an extended rest period

see Figure 8.9

• Cancer is unrestrained cell growth and division • Most cancers result from mutations in growth-regulating genes• There are two general classes of these genes

– 1. Proto-oncogenes• Encode proteins that simulate cell division• If mutated, they become oncogenes

– 2. Tumor-suppressor genes• Encode proteins that inhibit cell division

What is Cancer?

– Can undergo metastasis

– Leave the tumor and spread throughout the body

• Cancer can be caused by chemicals, radiation or viruses• Rous Sarcoma Virus (RSV) chicken cancer from cell-free extracts

- Peyton Rous 1911 Nobel Prize 1966- Oncogenes - cancer causing genes

- Protooncogenes- normal genes which stimulate cell growth- growth factors, receptors, signal transduction proteins, etc

Cancer and Control of the Cell Cycle • The p53 gene plays a key role in the G1 checkpoint

of cell division

• The p53 protein (the gene’s product), monitors the integrity of DNA– If DNA is damaged, the protein halts cell division

and stimulates repair enzymes

• If the p53 gene is mutated– Cancerous cells repeatedly divide – No stopping at the G1 checkpoint

New molecular therapies for cancer

Receiving the signal to divide

Passing the signal via a relay switch

Amplifying the signal

Releasing the “brake”

Checking that everything is ready

Stepping on the gas

Stopping tumor growth

Prokaryotes Have a Simple Cell Cycle

• Prokaryotic cells divide asexually– These cells possess a single circular chromosome, containing

about 1000 genes– The chromosome is replicated– The cell then divides into two cells, a process called binary fission– 17 million daughter cells in 8-hours

Eukaryotes Have a Complex Cell Cycle

• Cell division in eukaryotes is more complex than in prokaryotes because– 1. Eukaryotic contain far more DNA– 2. Eukaryotic DNA is packaged differently

• It is in linear chromosomes compacted with proteins

• Some eukaryotes also make exact copies of themselves via asexual reproduction

amoeba

– G1 phase• Primary

growth phase– S phase

• DNA replication

– G2 phase• Microtubule

synthesis

– M phase (MITOSIS)

• Chromosomes pull apart

– C phase (CYTOKINESIS)

• Cytoplasm divides

The Cell Cycle

Figure 8.5

Interphase

• A typical human chromosome contains about 1000 genes~ 140 million nucleotides in its DNA, ~5 cm in stretched length

• The DNA helix is wrapped around positively-charged proteins, called histones

~200 nucleotides of DNA + histones= nucleosome

• During interphase, the DNA is coiled into chromatin but not visible chromosomes

• A chromosome is a complex of one molecule of DNA (~ 40%)

and proteins (~ 60%)

^Figure 8.2

Chromosomes

• Diploid cells have two SIMILAR* chromosomes

called homologous chromosomes or homologs

• Humans have 46 chromosomes or 23 pairs of homologs (similar chromosomes, one from this person’s Mom and one from their Dad)

Homologous Chromosomes

The chromosomes can be organized by size into a karyotype

Figure 8.4

* Important

• Replicated chromosomes consist of two IDENTICAL* sister chromatids held together at the centromere

• DNA replication occurs in S phase (Sister)

• Homologs are SIMILAR* chromosomes

Sister Chromatids

* Important see Tables 8.1 & 8.2 for terms

• Before a cell starts dividing, the chromosomes duplicate

• Because the strands of the double helix are complementary

A+TC+G

an exact copy of the DNA can be made

• An emergent property

• Chromosomes duplication (DNA replication or Synthesis) produces IDENTICALsister chromatids

• When the cell divides, the sister chromatids separate

• Each of the two daughter cells has a complete and identical set of chromosomes

MITOSIS makes identical diploid (2n) cellsMEIOSIS makes haploid (1 n) gametes

Eukaryotic cells divide in one of two ways– Mitosis

• Occurs in somatic (non-reproductive) cells

– Meiosis• Occurs in germ

(reproductive) cells

• Results in the production of gametes

MEIOSIS FERTILIZATION

Haploid gametes (n = 23)

Egg cell

Sperm cell

Diploidzygote

(2n = 46)Multicellular

diploid adults (2n = 46)

Mitosis anddevelopment

– M phase MITOSIS• Chromosomes pull apart

– Prophase– Metaphase– Anaphase– Telophase

– C phase CYTOKENESIS• Cytoplasm divides

INTERPHASE– G1 phase

• Primary Growth phase– S phase

• DNA Synthesis– G2 phase

• Growth & Microtubule synthesis

Mitotic Cell Cycle

Chromosomes aredecondensed during interphase

chromatin

Figure 8.5

Prophase• Chromosomes coil

• Nuclear membrane breaks down

• Spindle fibers formsee Figure 8.7

Metaphase• Chromosomes

line up on the midline

• Spindle fibers attach to centromeres

see Figure 8.7

Anaphase

• Centromeres divide

• Spindle fibers shorten

• Sister chromatids separate and move to opposite poles

see Figure 8.7

Telophase

• Cell elongates

• Nuclear membrane reforms• • Chromosomes uncoil

• Spindle disappears

see Figure 8.7

Cytokinesis

• Division of the cytoplasm

• Cleave furrow forms at equator of the cell

• Constriction tightens by contraction of filaments

• Cell is divided into two identical cells

see Figure 8.7

• Cytokinesis• Animal cells • Plant cells

– Cleavage furrow forms, pinching the cell in two

– Cell plate forms, dividing the cell in two

see Figure 8.8

- Asexual reproduction, development, growth and cell replacement are mitotic divisions

Deadcells

Dividingcells

Epidermis, the outer layer of the skin

Dermis

skin