The Cell Cycle Chapter 12

A. Overview Continuity of life based upon reproduction of cells, or cell division Both unicellular and multicellular organisms reproduce using cell division, but reasons differ

Unicellular Multicellular Reproduction Development Growth Repair (a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM). 20 µm 200 µm (b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM). (c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).

Cell Cycle G0 phase G1 phase S phase G2 phase Mitosis INTERPHASE Cell division (mitosis in red) is integral part of cell cycle Cell Cycle G0 phase G1 phase S phase G2 phase Mitosis INTERPHASE

B. DNA Organization Cell division results in genetically identical daughter cells Cells duplicate genetic material (DNA replication) before they divide, ensuring each daughter cell receives exact copy of DNA cell’s inherited genetic information is called its genome

DNA molecules in a cell packaged into chromosomes Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division 50 µm

In eukaryotes, two types of cells: Somatic (body) cells have two sets of chromosomes Gametes (sex cells) have one set of chromosomes

Chromosomes are packages of DNA wrapped with help of proteins called histones Composed of two identical sister chromatids attached at centromere

Chromosome levels organization (smallest to biggest): dsDNA Nucleosome Coiled solenoid/nucleosome Chromatin Supercoiled (condensed) chromatin Chromosome

Why are histones “sticky?” Five different interactions with DNA! Histones are positive, DNA phosphate groups are negative DNA backbone and amide group on histone form H bonds Nonpolar interactions between sugars on histone and DNA Salt bridges and H bonds between basic amino acids on histone and phosphate oxygens on DNA groove insertions histone tails fit into grooves on DNA molecule

C. Cell Division In preparation for cell division, DNA is replicated and chromosomes condense

Each duplicated chromosome has two sister chromatids, which separate during cell division Chromosome duplication (including DNA synthesis) Centromere Separation of sister chromatids Sister chromatids Centromeres A eukaryotic cell has multiple chromosomes, one of which is represented here. Before duplication, each chromosome has a single DNA molecule. Once duplicated, a chromosome consists of two sister chromatids connected at the centromere. Each chromatid contains a copy of the DNA molecule. Mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells.

Eukaryotic cell division consists of Mitosis, the division of the nucleus Cytokinesis, the division of the cytoplasm In meiosis, sex cells are produced after a reduction in chromosome number

cell cycle consists of mitotic phase Interphase INTERPHASE G1 S (DNA synthesis) G2 Cytokinesis Mitosis MITOTIC (M) PHASE

Interphase can be divided into subphases: G1 phase: cellular contents excluding chromosomes are duplicated S phase: DNA synthesis; each of 46 chromosomes is duplicated by cell G2 phase: cell double-checks duplicated chromosomes for errors, makes any needed repairs G0 phase: cell cycle arrested (halted)

Differs between plants and animals The mitotic phase is made up of mitosis and cytokinesis. Mitosis consists of five distinct phases (PPMAT) Prophase (early prophase) Prometaphase (late prophase) Metaphase Anaphase Telophase (and Cytokinesis) Differs between plants and animals

G2 OF INTERPHASE PROPHASE PROMETAPHASE Centrosomes (with centriole pairs) Chromatin (duplicated) Early mitotic spindle Aster Centromere Fragments of nuclear envelope Kinetochore Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Kinetochore microtubule Nonkinetochore microtubules

TELOPHASE AND CYTOKINESIS Centrosome at one spindle pole Daughter chromosomes METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS Spindle Metaphase plate Nucleolus forming Cleavage furrow Nuclear envelope forming

arises from centrosomes, includes spindle microtubules and asters mitotic spindle is an apparatus of microtubules that controls chromosome movement during mitosis arises from centrosomes, includes spindle microtubules and asters Some spindle microtubules attach to kinetochores of chromosomes and move chromosomes to metaphase (midline) plate

Kinetochores microtubules Centrosome Chromosomes Microtubules 0.5 µm 1 µm metaphase plate

Prophase Nuclear membrane breaks down Spindle fibers emerge from centrioles PROPHASE Early mitotic spindle Aster Centromere Chromosome, consisting of two sister chromatids

Prometaphase Centrioles move to opposite poles Spindle fibers attach to centromeres PROMETAPHASE Fragments of nuclear envelope Non-kinetochore microtubules Kinetochore Kinetochore microtubule

Metaphase Chromosomes Migrate to Midline METAPHASE Metaphase plate Centrosome at one spindle pole METAPHASE Spindle Metaphase plate

Anaphase Spindle fibers contract, pulling sister chromatids apart Chromatids move along kinetochore microtubules to opposite poles Anaphase Daughter chromosomes ANAPHASE

EXPERIMENT The microtubules of a cell in early anaphase were labeled with a fluorescent dye that glows in the microscope (yellow). Spindle pole Kinetochore

Telophase & Cytokinesis Non-kinetechore microtubules from opposite poles overlap and push against each other, elongating cell Genetically identical daughter nuclei form at opposite ends of the cell

In plant cells, during cytokinesis a cell plate forms Daughter cells 1 µm Vesicles forming cell plate Wall of patent cell Cell plate New cell wall (b) Cell plate formation in a plant cell (SEM) In plant cells, during cytokinesis a cell plate forms

In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow Contractile ring of microfilaments Daughter cells 100 µm (a) Cleavage of an animal cell (SEM)

Mitosis in a plant cell Chromatin condensing Nucleus Chromosome 1 Prophase. The chromatin is condensing. The nucleolus is beginning to disappear. Although not yet visible in the micrograph, the mitotic spindle is staring to from. Prometaphase. We now see discrete chromosomes; each consists of two identical sister chromatids. Later in prometaphase, the nuclear envelop will fragment. Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate. Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of cell as their kinetochore microtubles shorten. Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divided the cytoplasm in two, is growing toward the perimeter of the parent cell. 2 3 4 5 Nucleus Nucleolus Chromosome Chromatin condensing

D. Binary Fission Prokaryotes (bacteria) reproduce by a type of cell division called binary fission bacterial chromosome replicates two daughter chromosomes actively move apart

Replication continues. One copy of origin is now at each end of cell. Chromosome replication begins. Soon thereafter, one copy of origin moves rapidly toward other end of cell. Replication continues. One copy of origin is now at each end of cell. Replication finishes. Plasma membrane grows inward, and new cell wall is deposited. Two daughter cells result. Origin of replication E. coli cell Bacterial Chromosome Cell wall Plasma Membrane Two copies of origin Origin

E. Evolution of Cell Cycle Since prokaryotes preceded eukaryotes by billions of years, likely that mitosis evolved from bacterial cell division Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis carried out by most eukaryotic cells

A hypothetical sequence for evolution of mitosis: Prokaryotes. During binary fission, origins of daughter chromosomes move to opposite ends of cell. Proteins may anchor daughter chromosomes to specific sites on plasma membrane. Bacterial chromosome

Dinoflagellates. In these unicellular protists, nuclear envelope remains intact during cell division, chromosomes attach to nuclear envelope. Microtubules pass through nucleus inside cytoplasmic tunnels, reinforcing spatial orientation of nucleus, which then divides in fission process reminiscent of bacterial division. Chromosomes Microtubules Intact nuclear envelope

Diatoms. In this group of unicellular protists, nuclear envelope also remains intact during cell division. But microtubules form a spindle within nucleus. Microtubules separate chromosomes, and nucleus splits into two daughter nuclei. Kinetochore microtubules Intact nuclear envelope

Most eukaryotes. In most eukaryotes, including plants and animals, spindle forms outside nucleus, and nuclear envelope breaks down during mitosis. Microtubules separate chromosomes, and nuclear envelope then re-forms. Centrosome Kinetochore microtubules Fragments of nuclear envelope

F. Regulation of Cell Cycle cell cycle regulated by molecular control system frequency of cell division varies with type of cell cell cycle differences result from regulation at molecular level

Cytoplasmic signals: molecules present in cytoplasm regulate progress through cell cycle EXPERIMENTS In each experiment, cultured mammalian cells at two different phases of cell cycle were induced to fuse. RESULTS

Experiment 1 S G1 M G1 Experiment 2 RESULTS S S M M When a cell in S phase was fused with cell in G1, G1 cell immediately entered S phase—DNA was synthesized. When a cell in M phase was fused with cell in G1, G1 cell immediately began mitosis— a spindle formed and chromatin condensed, even though chromosome had not yet been duplicated.

CONCLUSION Results of fusing cells at two different phases of cell cycle suggest that molecules present in cytoplasm of cells in S or M phase control progression of phases.

sequential events of cell cycle are directed by a distinct cell cycle control system, which is similar to a clock Control system G2 checkpoint M checkpoint G1 checkpoint G1 S G2 M

clock has specific checkpoints where cell cycle stops until a go-ahead signal is received G1 checkpoint G1 G0 (a) If a cell receives a go-ahead signal at the G1 checkpoint, cell continues on in cell cycle. (b) If cell does not receive a go-ahead signal at G1checkpoint, cell exits cell cycle and goes into G0, a non-dividing state.

Two types of regulatory proteins are involved in cell cycle control: Cyclins and cyclin-dependent kinases (Cdks)

activity of cyclins and Cdks fluctuates during cell cycle During G1, conditions in the cell favor degradation of cyclin, and the Cdk component of MPF is recycled. 5 During anaphase, the cyclin component of MPF is degraded, terminating the M phase. The cell enters the G1 phase. 4 Accumulated cyclin molecules combine with recycled Cdk mol ecules, producing enough molecules of MPF to pass the G2 checkpoint and initiate the events of mitosis. 2 Synthesis of cyclin begins in late S phase and continues through G2. Because cyclin is protected from degradation during this stage, it accumulates. 1 Cdk G2 checkpoint Cyclin MPF Cyclin is degraded Degraded Cyclin G1 G2 S M MPF activity Time (a) Fluctuation of MPF activity and cyclin concentration during the cell cycle (b) Molecular mechanisms that help regulate the cell cycle MPF promotes mitosis by phosphorylating various proteins. MPF‘s activity peaks during metaphase. 3

Both internal and external signals control the cell cycle checkpoints Growth factors stimulate other cells to divide

A sample of connective tissue was cut up into small pieces. Scalpels EXPERIMENT A sample of connective tissue was cut up into small pieces. Enzymes were used to digest the extracellular matrix, resulting in a suspension of free fibroblast cells. Cells were transferred to sterile culture vessels containing a basic growth medium consisting of glucose, amino acids, salts, and antibiotics (as a precaution against bacterial growth). PDGF (growth factor)was added to half the vessels. The culture vessels were incubated at 37°C. 3 2 1 Petri plate Without PDGF With PDGF

In density-dependent inhibition, crowded cells stop dividing Most animal cells exhibit anchorage dependence in which they must be attached to a substratum to divide Cells anchor to dish surface and divide (anchorage dependence). When cells have formed a complete single layer, they stop dividing (density-dependent inhibition). If some cells are scraped away, remaining cells divide to fill gap and then stop (density-dependent inhibition). Normal mammalian cells. The availability of nutrients, growth factors, and a substratum for attachment limits cell density to a single layer. (a) 25 µm

Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence 25 µm Cancer cells do not exhibit anchorage dependence or density-dependent inhibition. Cancer cells. Cancer cells usually continue to divide well beyond a single layer, forming a clump of overlapping cells. (b)

Cancer cells Do not respond normally to body’s control mechanisms Form tumors After the AP test, we will read “The Immortal Life of Henrietta Lacks” which discusses growing cancer cells in a petri dish

Malignant tumors invade surrounding tissues and can metastasize Exporting cancer cells to other parts of the body where they may form secondary tumors Tumor Glandular tissue Cancer cell Blood vessel Lymph vessel Metastatic Tumor A tumor grows from a single cancer cell. 1 Cancer cells invade neighboring tissue. 2 Cancer cells spread through lymph and blood vessels to other parts of the body. 3 A small percentage of cancer cells may survive and establish a new tumor in another part of the body. 4

metastasis