Have Prokaryotes a Simple Cell Cycle
Cell division in prokaryotes takes place in two stages
The DNA is replicated
The cell elongates, then splits into two daughter cells The process is called binary fission
Cell division in prokaryotes
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
Eukaryotes Have a Complex Cell Cycle
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
The complex cell cycle of eukaryotic cell is composed of several stages
G1 phase
Primary growth phase Interphase
S phase
DNA replication
G2 phase
Microtubule synthesis Mitosis
M phase
Chromosomes pull apart
Cytokinesis
C phase
Cytoplasm divides
How the cell cycle works
Chromosomes
Chromosomes were first observed by the German embryologist Walther Fleming in 1882 The number of chromosomes varies enormously from species to species
The Australian ant Myrmecia spp. has only 1 pair Some ferns have more than 500 pairs
Chromosomes exist in somatic cells as pairs Homologous chromosomes or homologues
Diploid cells have two copies of each chromosomes Replicated chromosomes consist of two sister chromatids These are held together at the centromere
Chromosomes
Humans have 46 chromosomes The 23 pairs of homologous chromosomes can be organized by size
This display is termed a karyotype
Chromosomes
Chromosomes are composed of chromatin Complex of DNA (~ 40%) and proteins (~ 60%) A typical human chromosome contains about 140 million nucleotides in its DNA This is equivalent to
About 5 cm in stretched length 2,000 printed books of 1,000 pages each!
In the cell, however, the DNA is coiled
Chromosomes
The DNA helix is wrapped around positively-charged proteins, called histones 200 nucleotides of DNA coil around a core of eight histones, forming a nucleosome
The nucleosomes coil into solenoids
Solenoids are then organized into looped domains The looped domains appear to form rosettes on scaffolds
Levels of eukaryotic chromosome organization
Cell Division
The eukaryotic cell cycle consists of the following stages Interphase Mitosis Division of the nucleus
Also termed karyokinesis
Subdivided into Prophase, metaphase, anaphase, telophase Cytokinesis Division of the cytoplasm
Interphase
Chromosomes replicate and begin to condense
Mitosis Prophase
Nuclear envelope breaks down Chromosomes condense further Spindle apparatus is formed
Metaphase
Chromosomes align along the equatorial plane Spindle fibers attach at the kinetochores
On opposite sides of the centromeres
Mitosis Anaphase
Sister chromatids separate They are drawn to opposite poles by shortening of the microtubules attached to them
Telophase
envelope Nuclear reappears Chromosomes decondense Spindle apparatus is disassembled
Cytokinesis
Two diploid daughter cells form
Cytokinesis
Animal cells Cleavage furrow forms, pinching the cell in two
Plant cells Cell plate forms, dividing the cell in two
Cell Death
During fetal development, many cells are programmed to die
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 can divide endlessly
Programmed cell death
7.5 Controlling the Cell Cycle
The eukaryotic cell cycle is controlled by feedback at three checkpoints
Controlling the Cell Cycle
1. Cell growth is assessed at the G1 checkpoint
G0 is an extended rest period
2. DNA replication is assessed at the G2 checkpoint 3. Mitosis is assessed at the M checkpoint
What is Cancer?
Cancer is unrestrained cell growth and division The result is a cluster of cells termed a tumor Benign tumors
Encapsulated and noninvasive
Malignant tumors
Not encapsulated and invasive
Can undergo metastasis Leave the tumor and spread throughout the body
What is Cancer?
Most cancers result from mutations in growthregulating genes There are two general classes of these genes 1. Proto-oncogenes Encode proteins that stimulate cell division If mutated, they become oncogenes 2. Tumor-suppressor genes Encode proteins that inhibit cell division
Cancer can be caused by chemicals, radiation or even some viruses
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
Cell division and p53 protein
Curing Cancer
Potential cancer therapies are being developed to target seven different stages in the cancer process Stages 1-6 Prevent the start of cancer within cells Focus on the decision-making process to divide
Stage 7 Act outside cancer cells Prevents tumors from growing and spreading
New molecular therapies for cancer
Receiving the signal to divide
Stopping tumor growth
Passing the signal via a relay switch
Amplifying the signal
Stepping on the gas
Releasing the "brake"
Checking that everything is ready
Review of cell cycle
The cell cycle is the orderly series of events by which a cell duplicates its contents and divides. Control of the cell cycle is fundamentally the same in all eukaryotes. In multicellular organisms, cell cycle control accounts for proportional growth of tissues and organs and for maintenance of the organism. Deregulation of the cell cycle control is the root of cancer. The 2000 Noble Prize went to pioneers of molecular and genetic investigations of the cell cycle: Leland Hartwell, Tim Hunt and Paul Nurse.