Chapter 12 The Cell Cycle

Chapter 12:The cell cycle Overview: The Key Roles of Cell Division The continuity of life; is based on the reproduction of cells or cell division. Cell division is defined as the distribution of identical genetic material (DNA) to 2 daughter cells. Figure 1: Chromosomes in a dividing cell

Genome; is the complete complement (makes up the whole ) of organisms genetic material. prokaryotic genmoe; single long DNA molecule Eukaryotic genome; Many DNA molecules A human cell contains a DNA that is 3 m long i.e 300,000 the cell diameter.

Cell division functions in: reproduction, in unicellular organisms the division of one cell to form two reproduces the entire organism (e.g. Amoeba). 100 µm (a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM). Figure 12.2 A

In multi-cellular organisms, cell division allows: growth and development from fertilized egg. replacement of damaged or dead cells (repair). 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). Figure 12.2 B, C

The Cell division process is a complex process that passes the genome from one generation of cells to another. A dividing cell: precisely replicates its DNA. Equally distributes the DNA to opposite ends of the cell. Separates into two identical cells. Has high fidelity i.e DNA is passed from one generation to another generation without dilution.

Cell division results in genetically identical daughter cells Cells duplicate their genetic material Before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA.

Cellular Organization of the Genetic Material Chromosomes: Thread-like structures in eukaryotic nuclei that are composed of DNA and proteins. i.e the DNA is packaged in the chromosomes. each species has its characteristic chromosome number (e.g. human somatic cells have 46 chromosomes) Gametes (sperms and ova) contain half the number of chromosomes of somatic cells (human gametes have 23 chromosomes). 50 µm Figure 12.3

Eukaryotic chromosomes Consist of chromatin, a complex of DNA and protein that condenses during cell division Chromatin is a DNA-protein complex organized into a thin long fiber that is folded and coiled to form the chromosome. It maintains chromosome structure Helps control activity of the genes Long thin fibers On duplication of genome, chromatin condenses in preparation for division

In animals Somatic cells have two sets of chromosomes Gametes have one set of chromosomes

Distribution of Chromosomes During Cell Division In preparation for cell division DNA is replicated and the chromosomes condense to form Chromatids Each duplicate chromosome has two sister chromatids that contains the identical copies of the chromosome’s DNA molecule They are attached along their length They have a narrow waist that is called the centromer Figure 12-3 Later in cell division, sister chromatids separate and each gets packaged as complete chromosome.

Separation of sister chromatids Each duplicated chromosome Has two sister chromatids, which separate during cell division 0.5 µm 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. Figure 12.4

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

Mitosis Nuclear division during which duplicated chromosomes are evenly distributed into two daughter nuclei. Before mitosis, the cell copies its genome by duplicating every chromosome, each of which forms two identical sister chromatids which are attached to each other in a region called centromere. During mitosis, sister chromatids are pulled apart forming two complete chromosome sets, one at each end of the cell. Mitosis is followed by the Cytokinesis which is the division of the cytoplasm.

Meiosis; is the division of gonads (ovaries and testes) and thus reduces the chromosome’s numbers from 46 to 23 in humans.

The cell cycle Cell Cycle: A sequence of events between the time a cell divides to form two daughter cells and the time those daughter cells divide. Cell Cycle alternates between: Mitotic, M phase: dividing phase (shortest part of the cycle) includes; Mitosis: division of the nucleus Cytokinesis: division of the cytoplasm. Interphase: the none-dividing phase which includes most of cell’s growth and metabolic activities. (Longest part of the cycle) is a bout 90% of the cell cycle. Is a period of intense biochemical activity during which the cell grows and copies its chromosomes A labeled probe can reveal patterns of gene expression in different kinds of cells

Interphase Consists of three periods or sub-phases: - G1 phase: first growth phase (first gap) S phase: period of chromosomes duplication. G2 phase : second growth phase. Figure 12-4

Phases of the Cell Cycle The cell cycle consists of The mitotic phase Interphase INTERPHASE G1 S (DNA synthesis) G2 Cytokinesis Mitosis MITOTIC (M) PHASE Figure 12.5

Sub-phases of Mitotic Cell Division Interphase: during this phase the nucleus is well defined and contains. One or more nucleoli Two centrosomes adjacent to the nucleus. In animals, each centrosome features a pair of cenrioles Duplicated chromosomes that can not be distinguished individually. Prophase: in this phase nucleoli disappear chromatin fibers condense into discrete chromosomes composed of two identical sister chromatids. Mitotic spindle forms from microtubules and proteins Spindle is constructed and elongates Centrosomes move apart.

Kinetochore microtubule Mitosis consists of five distinct phases Prophase Prometaphase 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 Figure 12.6 Nonkinetochore microtubules

Pro-metaphase: in this phase Nuclear envelop fragments. Spindle fibers (microtubules) extends from each pole towards cell’s equator. Each chromatid has now a specialized structure called kinetochore located at the centromere region. Kinetochore microtubules are now attached to kinetochores. Metaphase: Centrosomes are positioned at opposite poles of the cell. Chromosomes move to metaphase plate. Centromeres of all chromosomes are aligned on the metaphase plate. Kinetochores of sister chromatids face opposite poles, so identical chromatids are attached to kinetochore fibers.

Kinetochore microtubule Mitosis consists of five distinct phases Prophase Prometaphase 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 Figure 12.6 Nonkinetochore microtubules

TELOPHASE AND CYTOKINESIS Metaphase Anaphase Telophase Centrosome at one spindle pole Daughter chromosomes METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS Spindle Metaphase plate Nucleolus forming Cleavage furrow Nuclear envelope forming Figure 12.6

Telophase and Cytokinesis: Anaphase: Sister chromatids split apart into separate chromosomes and move towards opposite poles of the cell The poles of the cell move farther apart, elongating the cell. Telophase and Cytokinesis: Nonkinetochore microtubules further elongate the cell. Daughter nuclei begin to form at the two poles. Nuclear envelop form around the chromosomes. Nucleoli reappear. Chromosomes coil and become less distinct. Cytokinesis begins and the appearance of two daughter cells occur shortly after mitosis is complete.

TELOPHASE AND CYTOKINESIS Metaphase Anaphase Telophase Centrosome at one spindle pole Daughter chromosomes METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS Spindle Metaphase plate Nucleolus forming Cleavage furrow Nuclear envelope forming Figure 12.6

The Mitotic Spindle: A Closer Look Is an apparatus of microtubules that controls chromosome movement during mitosis The spindle arises from the centrosomes And includes spindle microtubules and asters (A star-shaped structure formed in the cytoplasm of a cell and having raylike fibers that surround the centrosome during mitosis)

Some spindle microtubules Attach to the kinetochores of chromosomes and move the chromosomes to the metaphase plate Centrosome Aster Sister chromatids Metaphase Plate Kinetochores Overlapping nonkinetochore microtubules Kinetochores microtubules Chromosomes Microtubules 0.5 µm 1 µm Figure 12.7

In anaphase, sister chromatids separate And move along the kinetochore microtubules toward opposite ends of the cell 1 Spindle pole Kinetochore Figure 12.8

Nonkinetechore microtubules from opposite poles Overlap and push against each other, elongating the cell In telophase Genetically identical daughter nuclei form at opposite ends of the cell

Cytokinesis: A Closer Look In animal cells: - A cleavage furrow forms as a shallow groove in the cell surface near the old metaphase plate. A contractile ring of actin microfilaments associated with myosin forms on the cytoplasmic side of the furrow, this ring contractile until it pinches the parent cell into two. Finally the remaining mitotic spindle breaks and the two cells become completely separate. Cleavage furrow Contractile ring of microfilaments Daughter cells 100 µm (a) Cleavage of an animal cell (SEM) Figure 12.9 A

Cytokinesis In plant cells: occurs by cell plate formation across the parent’s cell midline (no cleavage furrow) Vesicles from Golgi move to the cell’s center where they fuse into a disk-like cell plate. Additional vesicles fuse around the edge of the plate expanding it laterally until its membranes touch and fuse with the existing cell’s plasma membrane. A new cell wall forms as cellulose deposited between the two membranes of the cell plate.

(b) Cell plate formation in a plant cell (SEM) 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) Figure 12.9 B

Mitosis in a plant cell Nucleus Chromatine condensing 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 Chromatine condensing Figure 12.10

Binary Fission Prokaryotes (bacteria) Reproduce by binary fission (division in half), Most of its genes are located in one chromosome Once DNA chromosome begins to replicate, the copies of the replication region ( origin of replication) move apart rapidly. E. coli chromosome is 500 times longer than the cell

Binary Fission In binary fission The bacterial chromosome replicates The two daughter chromosomes actively move apart Origin of replication E. coli cell Bacterial Chromosome Cell wall Plasma Membrane Two copies of origin Origin Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. 1 Replication continues. One copy of the origin is now at each end of the cell. 2 Replication finishes. The plasma membrane grows inward, and new cell wall is deposited. 3 Figure 12.11 Two daughter cells result. 4

The Evolution of Mitosis Since prokaryotes preceded eukaryotes by billions of years It is 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 the evolution of mitosis Most eukaryotes. In most other eukaryotes, including plants and animals, the spindle forms outside the nucleus, and the nuclear envelope breaks down during mitosis. Microtubules separate the chromosomes, and the nuclear envelope then re-forms. Dinoflagellates. In unicellular protists called dinoflagellates, the nuclear envelope remains intact during cell division, and the chromosomes attach to the nuclear envelope. Microtubules pass through the nucleus inside cytoplasmic tunnels, reinforcing the spatial orientation of the nucleus, which then divides in a fission process reminiscent of bacterial division. Diatoms. In another group of unicellular protists, the diatoms, the nuclear envelope also remains intact during cell division. But in these organisms, the microtubules form a spindle within the nucleus. Microtubules separate the chromosomes, and the nucleus splits into two daughter nuclei. Prokaryotes. During binary fission, the origins of the daughter chromosomes move to opposite ends of the cell. The mechanism is not fully understood, but proteins may anchor the daughter chromosomes to specific sites on the plasma membrane. (a) (b) (c) (d) Bacterial chromosome Microtubules Intact nuclear envelope Chromosomes Kinetochore microtubules Fragments of nuclear envelope Centrosome Figure 12.12 A-D

Regulation of the Cell Cycle There was certrain theories about the regulation of the cell cycle. For example in 1970 there was a theory stating that the cell cycle is driven by specific chemical signals present in the cytoplasm. Various cell types differ in their pattern of cell divisions (The frequency of cell division) with the type of cell for example: human skin cells divide frequently. liver cells only divide when necessary, e.g. wound repair. Nerve, muscle and other specialized cells do not divide in adult human. The cell cycle is regulated by a molecular control system with differences in cell cycles that results from regulation at the molecular level.

Evidence for Cytoplasmic Signals Molecules present in the cytoplasm Regulate progress through the cell cycle In each experiment, cultured mammalian cells at two different phases of the cell cycle were induced to fuse. When a cell in the M phase was fused with a cell in G1, the G1 cell immediately began mitosis— a spindle formed and chromatin condensed, even though the chromosome had not been duplicated. EXPERIMENTS RESULTS CONCLUSION The results of fusing cells at two different phases of the cell cycle suggest that molecules present in the cytoplasm of cells in the S or M phase control the progression of phases. When a cell in the S phase was fused with a cell in G1, the G1 cell immediately entered the S phase—DNA was synthesized. S M G1 Experiment 1 Experiment 2 Figure 12.13 A, B

The Cell Cycle Control System The sequential events of the cell cycle Are directed by a distinct cell cycle control system, which is similar to a clock Figure 12.14 Control system G2 checkpoint M checkpoint G1 checkpoint G1 S G2 M

These signals come from the cell surveillance mechanisms. The cell cycle is coordinated by the cell cycle control system which has checkpoints in the G1, G2 and M phases. For many cells, the G1 checkpoint (also called restriction point) is the most important. Critical points where stop or a go-ahead signal usually regulate the cell cycle Animal cell; signals stop cylces at check points until overridden by a a go-ahead signal These signals come from the cell surveillance mechanisms. There are three major checkpoints G1 checkpoint, this is the most important one G2 checkpoint M checkpoint

There are two types of proteins that regulate the cell cycle; If there is a go- ahead signal at the G1, then the cell completes cycle and divide. In the absence of a go-ahead signal, the cell may exit the cell cycle to the none-dividing state, called G0 phase where many cells of the human body are in G0 phase. The ordered sequence of cell cycle events is controlled by the activity of a certain Protein kinases. There are two types of proteins that regulate the cell cycle; Protein kinases which are enzymes that activate or inactivate other proteins by phosphorylating them. Other type of proteins might give the go ahead signal at the G1 & G2 checkpoints.

The clock has specific checkpoints Where the 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, the cell continues      on in the cell cycle. (b) If a cell does not receive a go-ahead signal at the G1checkpoint, the cell exits the cell cycle and goes into G0, a nondividing state. Figure 12.15 A, B

The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases Kinases that drive the cell cycle Present at constant concentration in growing cell but they are in the inactive form for much of the time. to become active, they must be linked to a cyclin Cyclin is a protein that changes in concentration cyclically Because of the need for the cyclin, these kinases are called cyclin-dependent kinases or CdKs The activity of CdK rises and falls with changes in the concentration of its partner (maturation promoting factor) MPF which controls the cell’s progress through the G2 checkpoint to mitosis or the M phase by phosphorylating several proteins.

These phosphorylated proteins participate in mitosis and initiate the following processes: chromosome condensation during prophase nuclear envelop fragmenting during prometaphase. Activate proteolytic enzymes which break down cyclin by the end of mitosis.

The activity of cyclins and Cdks Fluctuates during the 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 Figure 12.16 A, B

Stop and Go Signs: Internal and External Signals at the Checkpoints A number of internal and external signals play role in the control of cell cycle. An example of internal signals is the APC (anaphase promoting complex) which check the status of the kinetochores chromosomes interaction. all chromosomes must be attached to spindle microtubules before the M phase check point allows the cycle to proceed to anaphase. This step occurs to ensure that daughter cells will not have extran or missing chromosomes. Kinetochores not attached to spindle triggers signals that keeps the APC inactive.

Once all kinetochores are attached, the wait signal stops and the APC complex becomes active. The APC complex contains proteolytic enzymes which break down cyclin inactivating of proteins holding the sister chromatids. An example of external signals is the PDGF (platelets derived growth factors). These binds to cell membrane receptors to stimulate division of fibroblasts, a response that helps heal wounds.

External factors that affect cell growth Growth factors Stimulate other cells to divide 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 was added to half the vessels. The culture vessels were incubated at 37°C. 3 2 1 Petri plate Without PDGF With PDGF Scalpels Figure 12.17

Results In basic growth medium without platelet derived growth factor (PDGF, the control), cells failed to derive. In a basic growth medium plus PDGF, cells proliferated (Figure 12-17) Conclusion This experiment confirmed that PDGF stimulates the division of human fibroblast cells in culture.

Density-dependent inhibition of cell division 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, the remaining cells divide to fill the 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 Figure 12.18 A

Cancer cells Exhibit neither density-dependent inhibition nor anchorage dependence 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) 25 µm Figure 12.18 B

Loss of Cell Cycle Controls in Cancer Cells Do not respond normally to the body’s control mechanisms They divide progressively. The problem begins when a single cell in a tissue undergoes transformation to cancer cell. the do not stop growing in response to density dependant inhibition. May have abnormal number of chromosomes They have growth factors themselves. May have abnormal growth factor signaling system. Continuous division of cancer cells form tumors. There are two types of tumors:

Benign tumors: cells remain in its original site Benign tumors: cells remain in its original site. This type can be removed by surgery. malignant tumor: invades sites other than its original one can not be removed completely by surgery. the spread of cancer cells to another sites is called: metastasis. Metabolism is deranged; stop functioning in constructive way

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 Figure 12.19

End of the chapter