Chapter 9 Cellular Reproduction PDF

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Chapter 9: Cellular Reproduction 9.1 What Are the Functions of Cell Division? Genetic Material is Replicated During Cell Division The genetic material is replicated during cell division, during which each daughter cell receives a complete set of hereditary information, identical to the parent cell. Each cell also receives about half the original cell’s cytoplasm. The hereditary information is contained in deoxyribonucleic acid (DNA). DNA: ● is a polymer composed of subunits called nucleotides ● is packaged into chromosomes, which has two long strands of nucleotides wrapped around each other, which is called a double helix ● has proteins that organize the 3 dimensional structure and regulate its use The units of inheritance are called genes, and they range from a few hundred to many thousands of nucleotides in length. The specific sequences spell out the instructions for making proteins in a cell. Nucleotides contain a phosphate, a sugar, and a base. The four possible bases are: Adenine (A) Thymine (T) Guanine (G) Cytosine (C)

Cell Division is Required for Growth, Development, and Repair of Multicellular Organisms A major part of a multicellular organism's growth is an extensive amount of mitotic cell division, where each division produces two daughter cells. This is an important process in repairing cells that have been killed off by everyday life. The daughter cells formed by cell division may grow and divide again, in a repeating pattern called the cell cycle. Many of these

daughter cells will differentiate and become specialized for specific functions (i.e contraction in muscle cells, or fighting infections in white blood cells). Most multicellular eukaryotic organisms have three categories of cells, based on their ability to divide and differentiate. These are: stem cells, differentiated cells capable of dividing, and permanently differentiated cells STEM CELLS ❖ most cells formed by the first few cell divisions of a fertilized egg ❖ they have two important characteristics: ➢ self renewal : maintaining the capacity to divide, in some cases for the entire life of the organism ➢ potency: dividing stem cells produce daughter cells that can differentiate into a variety of specialized cell types ❖ early embryo stem cells have the potential to divide into any of the specialized cell types in the body ❖ in adults they are usually more limited and produce daughter cells that can differentiate into only a few cell types ❖ Plants also contain stem cells called meristem cells, often found in the growing tips of roots, stems, and branches DIFFERENTIATED CELLS CAPABLE OF DIVIDING ❖ daughter cells typically differentiate into only one or two cell types ❖ animal cell example, liver cells can only divide into more liver cells to help replace lost or damaged cells ❖ plant cell example, damaged xylem tissue (conducts water and nutrients upward from a plant’s roots) can use these cells to repair the tissues PERMANENTLY DIFFERENTIATED CELLS ❖ Some cells will differentiate and never divide again ❖ These cells lose their nucleus ❖ examples are: red blood cells, heart cells, brain cells, and most cells on the outer layer of the skin

Sexual and Asexual Reproduction Cell division is required for sexual and asexual reprodcution. Sexual reproduction occurs when offspring are produced by a fusion of gametes. To produce these, cells in the adult reproductive system undergo a specialized type of cell division called meiotic cell division. Reproduction in

which offspring are formed from a single parent is called asexual reproduction. This method of reproduction produces genetically identical offspring, to both the parent and to each other. Bacteria and archaea reproduce asexually by a type of cell division called prokaryotic fission. Many single-celled eukaryotic organisms, such as Paramecium, reproduce asexually by mitotic cell division, which produces new, genetically identical miniature versions of the adult. An example of this is budding on a Hydra cell, by which a smaller version appears on its body and then eventually breaks off. Many plants and fungi can reproduce either asexually or sexually. For example, having a single parent tree where others are connected by a root system (asexual), or having a seed fall down and create another tree (sexual).

9.2 Prokaryotic Cell Cycle The prokaryotic cell cycle consists of a relatively long period of growth, during which the cell replicates its DNA, followed by a type of cell division called prokaryotic fission, which is often called binary fission-- this is often used to describe prokaryotes as well as single celled eukaryotes though. The cell cycle process follows these steps: 1. The prokaryotic chromosome, a circular DNA double helix, is attached to the plasma membrane at one point. 2. The DNA replicates and the two resulting chromosomes attach to the plasma membrane at nearby points. 3. New plasma membrane is added between the attachment points, pushing the two chromosomes farther apart. 4. The plasma membrane grows inward at the middle of the cell. 5. The parent cell divides into two daughter cells. In a prokaryotic cell, the DNA is not contained in a membrane-bound nucleus but rather a single circular chromosome which is usually attached to the plasma membrane of the cell.

9.3 DNA Organization in Eukaryotic Cells Eukaryotic chromosomes differ from prokaryotic chromosomes in several respects. Eukaryotic chromosomes are separated from the cytoplasm within a membrane-bound nucleus, and are linear instead of circular. They also contain more proteins, and have different kinds. The amount of DNA in eukaryotic organisms is also far greater in these chromosomes; humans have 10 to 50 times more DNA than a typical prokaryote. The number of chromosomes also vary tremendously. The complex events of eukaryotic cell division are largely an evolutionary solution to the problem of duplicating and parceling out a large number of long chromosomes.

The Eukaryotic Chromosome Consists of a Linear DNA Double Helix Bound to Proteins DNA resides in a nucleus that is only a few ten-thousandths of an inch in diameter, which is no easy task. This is solved by wrapping the DNA around protein supports, greatly reducing its length. These proteins are called histones. Other proteins coil up the DNA/histone beads, much like a spring or Slinky toy. These coils are attached in loops to protein “scaffolding” to complete chromosome packaging as it occurs during most of the life of a cell. This winding and looping condenses the DNA to about 1/1000th of its extended length. This still leaves the proteins much too long to be sorted for cellular division, which is resolved by the proteins folding up the chromosomes again, yielding another 10 fold condensation. The final chromosome is now less than 2 ten-thousandths of an inch long (about 4 micrometres). Every chromosome has specialized regions that are crucial to its structure and function: two telomeres and one centromere. Telomeres are protective caps at the end of each chromosome, which prevent genes located at the end of the chromosome being lost during replication. The centromere has two principal functions: (1) it temporarily holds two daughter DNA double helices together after DNA replication, and (2) it is the attachment site for microtubules that move the chromosomes during cell division.

9.4 Eukaryotic Cell Cycle In the eukaryotic cell cycle, new cells usually acquire nutrients from their environment, synthesize more cytoplasm and organelles, and grow larger. Some cells differentiate and never divide again, while others divide, create daughter cells, and divide again. Generally, cells only divide if they receive chemical signals that tell them to enter into a new cell cycle.

The Eukaryotic Cell Cycle Consists of Interphase and Mitotic Cell Division

During Interphase, a Cell Grows in Size, Replicates Its DNA, and Often Differentiates Most eukaryotic cells spend the majority of their time in interphase, the period between cell divisions. Interphase contains three subphases: ● G1 (the first growth phase and the first gap in DNA synthesis) ● S (when DNA synthesis occurs) ● G2 (the second growth phase and the second gap in DNA synthesis) A newly formed daughter cell in the G1 phase often undergoes at least  one of three processes. Firstly, it almost always grows in size. Secondly, it often differentiates, developing the structures and biochemical pathways that allow it to perform a specialized function (ex. Nerve cells growing long strands called axons that allow them to connect to other cells, liver cells producing fluid to aid in digestion, proteins to aid blood clotting, enzymes to detoxify poisonous material). Thirdly, the cell responds to internal and external signals that determine whether or not it will divide. The S phase occurs when the signals tell the cell to divide, and it begins duplicating chromosomes, and making exact copies of the DNA in each. The cell then proceeds to the G2 phase, where it may grow more in size, and synthesizes proteins needed for cell division.

Mitotic Cell Division Consists of Nuclear Division and Cytoplasmic Division Mitotic cell division consists of two processes: mitosis and cytokinesis. Mitosis is the division of the nucleus, which is derived from the Greek word “thread” because the chromosomes condense and become visible in a light microscope. Mitosis produces two daughter nuclei, both containing a copy of the parent chromosomes. Cytokinesis is the division of the cytoplasm, which places about half the cytoplasm, half the organelles, and one newly formed nuclei in each of the daughter cells. Therefore, mitotic cell division typically produces daughter cells that are physically similar and genetically identical to each other and to the parent cell.

9.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? All of a cell's chromosomes are copied during the S phase of interphase. Each resulting duplicated chromosome consists of two identical DNA double helices (and their associated proteins) called sister chromatids, which are attached at the centromere. During mitotic cell division, the chromatids separate, becoming an independent chromosome in one of the new daughter cells. Mitosis is divided into four stages: prophase, metaphase, anaphase, telophase. These phases are a continuum, not four different events.

Prophase This is the first stage of mitosis, and during this stage, four major events occur. 1) The duplicated chromosomes condense and the nucleolus disappears 2) Microtubules, which are just called spindle microtubules, form the spindle. In animal cells, the spindle microtubules originate from the centriole. In plant, fungi and many algae cells, the spindle still forms without centrioles. In animal cells, a new pair of centrioles forms during interphase, then they each migrate to opposite sides of the nucleus in prophase. The area of cytoplasm around each centriole is called the spindle pole. These microtubules radiate inward toward the nucleus and outward toward the plasma membrane. 3) The nuclear envelope disintegrates, releasing the duplicated chromosomes. Each sister chromatid in the duplicated chromosome has a kinetochore, which are arranged back to back, facing away from one another. 4) The kinetochore of one sister chromatid binds to the ends of spindle microtubules leading to one pole of the cell, while the kinetochore of the other sister chromatid binds to spindle microtubules leading to the opposite pole. The spindle microtubules that attach to the kinetochore that lead to the poles are called kinetochore microtubules.

Metaphase During metaphase, each chromosome lines up along the equator of the cell, with one kinetochore facing each pole. This is a result of a molecular tug of war, in which microtubules lengthen or shorten, causing the two kinetochores on a duplicated chromosome to face either pole.

Anaphase At the beginning of anaphase, the sister chromatids separate, becoming independent daughter chromosomes. After this separation, each kinetochore moves its chromosome poleward, while simultaneously disassembling the end of the attached microtubule, thereby shortening it. Each of the daughter chromosomes moves to a different pole of the cell, therefore each pole contains one identical copy of every chromosome from the parent cell. At the same time, the free ends of the microtubules attach to one another where they overlap at the equator. They simultaneously lengthen and push on each other, forcing the poles farther apart.

Telophase When the chromosomes reach the poles, telophase begins. The spindle microtubules disintegrate, and a nuclear envelope forms around each group of chromosomes. The chromosomes revert to their extended state, and nucleoli begin to re-form. Cytokinesis occurs along with telophase, isolating each daughter nucleus in its own daughter cell.

Cytokinesis In animal cells, microfilaments attached to the plasma membrane assemble into a ring around the equator of the cell, usually late in anaphase or early in telophase. The ring contracts and restricts the cell’s equator, eventually completely constricting the equator and separating the two new daughter cells. In plant cells, carbohydrate filled sacs called vesicles bud off the golgi apparatus and line up along the equator of the cell between the two nuclei. The vesicles fuse, creating a structure called the cell plate. The edges of the cell plate fuse with the plasma membrane of the cell. The carbohydrates from the vesicles remain, forming a new cell wall.

9.6 How is the Cell Cycle Controlled? The Activities of Specific Proteins Drive the Cell Cycle When cell division is needed, such as during development, after an injury, or to compensate for normal wear and tear, many cells in the body release molecules called growth factors. Most growth factors stimulate cell division by controlling the synthesis of proteins called cyclins, which regulate the activity of enzymes called cyclin-dependent kinases (Cdks). Particular cyclins and their associated Cdks stimulate different steps in the cell cycle.

Checkpoints Regulate Progress Through the Cell Cycle Uncontrolled cell division can be dangerous. If a cell has a mutation that disrupts regulation of the cell cycle, its daughter cells may become cancerous. To prevent this, the eukaryotic cell cycle has three major checkpoints, where proteins in the cell determine whether the cell has successfully completed a specific phase of the cycle:

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G1 to S: Is the cell’s DNA intact and suitable for replication? G2 to Mitosis: Has the DNA been completely and accurately replicated? Metaphase to Anaphase: Are all the chromosomes attached to the spindle and aligned properly at the equator of the cell? The checkpoint proteins usually regulate the production of cyclins and/or the activity of Cdks, thereby regulating progression from one phase of the cell cycle to the next. In most cases, if the checkpoint processes are activated by mutated DNA or misaligned chromosomes, the cell cycle is stopped until the problem is repaired. If the defect is not repaired, the defective cells usually either destroy themselves or are killed by the immune system. When checkpoint control malfunctions, the result may be cancer.

Definitions Section 9.1 cell division: splitting of one cell into two; the process of cellular reproduction daughter cell: one of the two cells formed by cell division deoxyribonucleic acid (DNA): a molecule composed of deoxyribose nucleotides; contains the genetic information of all living cells nucleotides: a subunit of which nucleic acids are composed; a phosphate group bonded to a sugar (deoxyribose in DNA), which is in turn bonded to a nitrogen-containing base (adenine, guanine, cytosine or thymine in DNA). Nucleotides are linked together, forming a strand of nucleic acid, by bonds between the phosphate of one nucleotide and the sugar of the next nucleotide chromosomes: a DNA double helix and associated proteins that help to organize and regulate the use of DNA

genes: the unit of heredity; a segment of DNA located at a particular place on a chromosome that usually encodes the information for the amino acid sequence of a protein, and hence, a particular trait mitotic cell division: mitosis followed by cytokinesis cell cycle: the sequence of events in the life of a cell, from one cell division to the next differentiate: the process whereby a cell becomes specialized in structure and function stem cells: an undifferentiated cell that is capable of dividing an giving rise to one or more distinct types of differentiated cells sexual reproduction: a form of reproduction in which genetic material from two parent organisms is combined in the offspring; usually, two haploid gametes fuse to form a diploid zygote gametes: a haploid sex cell, usually a sperm or an egg, formed in sexually reproducing organisms asexual reproduction: reproduction that does not involve the fusion of haploid gametes

Section 9.2 prokaryotic fission: the process by which a single bacterium divides in half, producing two identical offspring

Section 9.3 telomere: the nucleotides at the end of a chromosome that protect the chromosome from damage during condensation, and prevent the end of one chromosome from attaching to the end of another chromosome centromere: the region of a replicated chromosome at which the sister chromatids are held together until they separate during cell division

Section 9.4 interphase: the stage of the cell cycle between cell divisions in which chromosomes are duplicated and other cell functions occur, such as growth, movement, and acquisition of nutrients mitosis: a type of nuclear division, used by eukaryotic cells, in which one copy of each chromosome (already duplicated during interphase before mitosis) moves into each of two daughter nuclei; the daughter nuclei are therefore genetically identical to each other cytokinesis: the division of the cytoplasm and organelles into two daughter cells during cell division; normally occurs during telophase of mitotic and meiotic cell division

Section 9.5 duplicated chromosome: a eukaryotic chromosome following DNA replication; consists of two sister chromatids joined at the centromeres chromatids: one of the two identical strands of DNA and protein that forms a duplicated chromosome. The two sister chromatids of a duplicated chromosome are joined at the centromere

prophase: the first stage of mitosis, in which the chromosomes first become visible in the light microscope as thickened, condensed threads, and the spindle begins to form; as the spindle is completed, the nuclear envelope breaks apart spindle: an array of microtubules that moves the chromosomes to opposite poles of a cell during mitotic and meiotic cell division spindle microtubule: one of the microtubules organized in a spindle shape that separate chromosomes during meiotic and mitotic cell division centriole: in animal cells, a short barrel-shaped ring consisting of nine microtubule triplets; a pair of centrioles is found near the nucleus and may play a role in the organization of the spindle; centrioles also give rise to the basal bodies at the base of each cilium and flagellum that give rise to the microtubules of cilia and flagella kinetochore: a protein structure that forms at the centromere regions of chromosomes; attaches chromosomes to the spindle metaphase: in mitosis, the stage in which the chromosomes attached to the spindle fibres at kinetochores, are lined up along the equator of the cell; also the approximately comparable stages in meiosis I and meiosis II. anaphase: in mitosis, the stage in which the sister chromatids of each chromosome separate from one another and are moved to opposite poles of the cell telophase: in mitosis and both divisions of meiosis, the final stage in which the spindle fibres usually disappear, nuclear envelopes reform, and cytokinesis generally occurs. In mitosis and meiosis II, the chromosomes also relax in their condensed form
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