Week 4 Notes, Chapter 17
Week 4 Notes, Chapter 17 2203
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Date Created: 10/07/15
Grace Hansen 1 Chapter 17 Cell Reproduction and Differentiation Should we allow human reproductive cloning In 1997 adult sheep named Dolly was an exact clone of her mother A clone is an exact copy of a molecule a gene a cell or even an entire organism Human erythropoietintreats anemia Insulinfor diabetes Growth hormone for endocrine disorders The excitement about Dolly was that she was the first exact copy of a whole organism raising the possibility for the first time of making copies of humans Therapeutic cloningcloning cells or tissues for the purpose of treating disease 0 Looked upon with favor because it offers the possibility of real progress in treating certain chronic diseases and conditions Reproductive cloning making copies of adult organisms 0 Questions personal identity and human dignity In 2008 a company in California cloned skin cells of the company s CEO which may have developed exact copies of the CEO In 2009 and Italian doctor claimed to have helped three couples back in 2000 using human reproductive cloning techniques Biological Issues With cloning by somatic cell nuclear transfer any defects in the cell used for cloning would be passed on to the clone There is a very real possibility that the first human clones would be born with birth defects Scientists are unsure whether such problems are due to accumulated mutations unstable donor cells or errors in the cloning procedure itself The Politics of Cloning President Bush placed a ban on the use of federal funds for the creation of human embryos for research purposes The rules have softened somewhat under the Obama administration The US still prohibits the use of federal funds for human cloning research under most circumstances effectively preventing cloning research on a large scale in government institutions and most universities Ethical Concerns Grace Hansen 2 Ethical objections to reproductive cloning generally fall into three categories 0 Reproductive cloning may cause harm to the child 0 Reproductive cloning may have adverse consequences for society I It might transform the normal process of having children into a process more akin to manufacturing 0 Reproductive cloning is contrary to what it means to be human 171 The cell cycle creates new cells The creation of new cells from existing cells involves a repetitive sequence of events known as the cell cycle The cell cycle consists of two periods 0 Interphase a long growth period where the cell grows and DNA is duplicated in preparation for the next division I G1 phase first gapquot Period between the last cell division and DNA synthesis The cell is at its smallest size but still a period of very active cell growth I S phase synthesis of DNA The cell s chromosomes are duplicated Growth continues but at a slower pace I G2 phase second gapquot Grows slowly and prepares for cell division 0 Mitotic phase much shorter time period the nucleus and then the cytoplasm divide It consist of two steps I Mitosis the duplicated DNA is divided into two sets and then the nucleus divides I Cytokinesis the cytoplasm divides and two new cells called daughter cellsquot are formed This phase repeats over and over In mammals the cell cycle takes about 1824 hours Most of the cells in my body enter a nongrowing nondividing states called GO 0 In tissues that don t need a supply of new cells G0 is a healthy condition 0 Neurons and osteocytes remain in GO forever after adolescence 172 Replication transcription and translation an overview DNA deoxyribonucleic acid is a doublestranded string of nucleotides intertwined into a helical shape Grace Hansen 0 3000000000 pairs of DNA are packed into 46 separate chromosomes 0 It represents all instructions for life every cell must contain the same set of DNA Chromosomes organize and arrange the DNA within the nucleus 0 They contain proteins called histones that confer a certain structure to the chromosome molecule Humans have 46 chromosomes in their cell nuclei Not easily visible because they are long and thin Moved around a lot during nuclear division but they are so compact so it helps them not break 0 Two identical sister chromatids are held together by a centromere Just before the nucleus divides the duplicated chromosomes condense brie y into a shorter compact shape that can be seen under the microscope Mitochondria also contain DNA A single mitochondrial DNA contains 37 genes and 5 of the genes needed for mitochondrial functions DNA replication is the process of copying the DNA prior to cell division A gene is a short segment of DNA that contains the code or recipe for one or more proteins 0 Smallest functional unit of DNA 0 20000 genes on the 46 chromosomes 0 For a gene to be useful its code must be converted to a form that can leave the nucleus and enter the cytoplasm I Function of a singlestranded ribonucleic acid RNA more specifically a certain form of RNA called messenger RNA mRNA Transcription is the process by which the DNA code of a single gene is converted into a complementary single strand of mRNA mRNA is small enough to pass through nuclear pores and enter the cytoplasm The process of converting the mRNA template code into one or more proteins is called translation Replication Copying DNA before cell division Replication begins with uncoiling the DNA helices In each doublehelical the DNA molecule consists of a sugar group a phosphate group and one of the four bases adenine thymine guanine or cytosine The base pairing of the nucleotides ensures that the new complementary strand is exactly like the original complementary strand Grace Hansen 4 The new nucleotides are positioned and linked together by enzymes called DNA polymerases The unzipping of the DNA strand does not proceed linearly it becomes detached from one another at multiple locations More on replication enzymes bind at various points along the DNA molecules and gently unwind and unzip a portion of the DNA creating a replication bubblequot in the two strands New strands form bubbles Once replication is complete the 2 identical sister chromatids remain attached at a single point called the centromere The centromere holds the chromatids together until they are physically pulled apart during mitosis Mutations are alterations in DNA Alterations in DNA are called mutations They may result from mistakes made during DNA replication When the DNA of a somatic cell a cell of the body is severely damaged it may not be possible to repair all of the errors It can cause changes in the cell function and even lifethreatening cancers may result ex skin cancer Are not passed to offspring Mutations that occur in the gametes sperm or egg may be passed to future generations Mechanisms of DNA repair These mechanisms are quire efficient unless they are overwhelmed by massive damage DNA repair recognizes the errors cuts out and replaces the damaged sections or incorrect nucleotide base and reconnect the DNA backbone This uses DNA repair enzymes Repair prior to cell division ensures that the best possible copy of DNA is passed on to each daughter cell The genetic code plays a vital role in repairing itself Mutations of certain genes that direct DNA repairs are associated with an increased risk of sever cancers colon and breast Transcription Converting a gene s code into mRNA Transcription is similar to the process of DNA replication with the following exceptions 0 Only the segment of DNA representing a single gene unwinds as opposed to the entire molecule Grace Hansen 5 0 RNA is singlestranded so that only one of the two strands of DNA actually carries the genetic code specifying the synthesis of RNA 0 1 of 4 complementary base pairs of RNA is different from those of DNA uracil replaces thymine o The sugar group of RNA is ribose rather than deoxyribose Once it is transcribed the RNA molecule is released from the DNA strand and the two DNA strands entwine around each other again Promoter a unique base sequence that marks the beginning of every gene DNA Polymerase an enzyme that attaches to the promoter it starts the DNA unwinding process and assists in attaching the appropriate RNA molecule to the growing chain Primary transcript the first RNA molecule transcribed from DNA and is not yet functional because most of the DNA base sequences of a gene do not code for anything 0 The primary transcript is edited by enzyme that snip out the sections that do not carry any useful genetic information introns and leaving only sequences that do carry genetic information exons 0 The RNA molecule produced from the primary transcript is called messenger RNA 0 The message is encrypted as a triplet code so called because three successive bases of mRNA called a codon each code for one of the 20 amino acids I 60 different possible codons I Several different codons can specify the same amino acid because there are more possible codons than there are amino acids I The codon AUG specifies a start code and three others specify stop codes These are needed to specify where to begin and end the protein Translation Making a protein from RNA Only mRNA carries the recipe for a new protein Transfer RNA tRNA molecules are small molecules that carry the code for just one amino acid 0 Carry an anticodon a base triplet that is the complementary sequence to a codon of mRNA 0 tRNA is to capture single amino acids and then bring them to the appropriate spot on the mRNA chain 0 A ribosome consists of two subunits composed of ribosomal RNA rRNA and proteins Grace Hansen 6 Translation occurs in 3 steps 0 Initiation I A tRNA carrying the start anticodon binds to the smaller ribosomal subunit and to the start codon of mRNA The larger ribosomal subunit joins them 0 Elongation I tRNA molecules capture free amino acids and bring them to the appropriate codon of the mRNA I A mRNA passes between the two ribosomal subunits the ribosome binds to the tRNA and catalyzes the formation of the bind between successive amino acids I The tRNA is then released to find another amino acid 0 Termination I When a stop codon is reached the ribosomal subunits and the newly formed protein detach from the mRNA 0 A cooking analogy I The mRNA brings the recipe I The tRNA find and deliver the ingredients I The ribosome is the cook who creates the final product 173 Cell reproduction one cell becomes two The division of the nucleus is called mitosis The division of the cytoplasm is called cytokinesis Mitosis daughter cells are identical to the parent cell Prophase Begins when the duplicated chromosomes first become visible The tubular elements of the cytoskeleton comes apart and are reassembled between the pairs of centrioles This new parallel arrangement of microtubules is called mitotic spindle The pairs of centrioles drift apart and the mitotic spindle lengthens between them Metaphase The duplicated chromosomes align on one plane at the center of the cell Anaphase The duplicate DNA molecules separate and move toward opposite sides of the cell Grace Hansen 7 The centromere abruptly comes apart and the nowseparated DNA molecules are pulled in opposite directions by the micro tubules to which they are attached Telophase Begins when the 2 sets of chromosomes arrive at the opposite poles The mitotic spindle comes apart and new nuclear membranes form around the chromosome The chromosomes uncoil and revert to form in which they are no long visible under a microscope Cytokinesis divides one cell into two identical cells A contractile ring of protein filaments forms just inside the cell membrane The ring tightens forming a cleavage furrow and then pinching the cell in twoproducing two daughter cells Mitosis produces diploid cells and meiosis produces haploid cells Human cells with 46 chromosomes are called diploid cells because the 46 chromosomes actually represent 23 pairs of chromosomes 0 They reproduce by replicating the 46 chromosomes and then undergo mitosis There are 22 pairs of autosomes chromosomes other than the sex chromosomes There are 22 pairs of autosomes plus the two sex chromosomes XX in the female are called homologous chromosomes because they look identical under the microscope and they have copies of the same genes in the same location Sperm and eggs are haploid cells meaning that they have only one set of 23 chromosomes and are created by meiosis Meiosis Preparing for sexual reproduction The two successive nuclear and cell divisions of meiosis are Meiosis I 0 Before it begins the precursor cell undergoes a typical S phase where the DNA is replicated and the 46 chromosomes are duplicated 0 During prophase of meiosis I the duplicated homologous chromosomes pair up and swap sections of DNA genes by a process called crossingover o The chromosomes now contain a combo of both of the parent s genes Meiosis II Grace Hansen 8 o 23 duplicated chromosomes line up and the sister chromatids are separated from each other 0 None of the 4 haploid daughter cells are exactly alike Sex di ferences in meiosis 4 sperm versus one egg In males there is only a slim chance that any one sperm will ever reach an egg Meiosis in males produces 4 equalsized but genetically different sperm from every precursor cell In females each egg is precious and needs energy 0 A large egg with plenty of cytoplasm and lot of organelles has a better chance of surviving the early stages of development 0 The smaller cell produced at each division is called a polar body I Polar bodies eventually degenerate In females meiosis II is not completed until a sperm penetrates the egg 0 Once the penetration occurs the secondary oocyte quickly completes meiosis II and the sperm and egg nuclei then unite 174 How cell reproduction is regulated What causes different and sometimes changing rates of cell division Cells have an internal control mechanism that undergoes cyclic changes and that the control mechanism can be stopped at certain checkpoint by signals from inside or outside the cell Progression through the G1 S and G1 phases is controlled by cyclic uctuations in the concentrations of certain proteins called cyclins Cyclins activate certain proteins that initiate events such as DNA replication or the formation of the mitotic spindle Cells also have internal surveillance and control mechanisms to ensure that the cell is ready for the next phase before it proceeds The cell cycle may stop at a checkpoint if Certain nutrients are not available If a particular hormone is not present If certain growth factors are not supplied by other cells 175 Environmental factors in uence cell differentiation Differentiation is the process by which a cell becomes different from its parent or sister cell 0 Cells differentiate because they begin to express different genes 0 The study of how gene expression is altered by factors other than the cell s underlying DNA sequence is called epigenetics Grace Hansen 9 Di ferentiation during early development Soon after fertilization the egg begins to divide becoming two cells then four and then eight During this early division up to about 16 or 32 cells the cells do not grow The environment that surround a cell at the center of the ball becomes different from the environment of a cell at the surface Cell differentiation begins long before homeostatic feedback mechanisms ever develop in the organism Clonecopies Di ferentiation later in development Each cell is shaped by two factors 0 The developmental history of the cells that came before it 0 Local environment Alteration of a gene that plays a critical role in early development can be so disruptive to embryonic differentiation that the embryo may not survive Substances that can harm a fetus Cigarette smoke Alcohol Prescription and overthecounter medications Illegal drugs Chemicals in air water and soil Radiation 0000 176 Reproductive cloning require an undifferentiated cell Making copies of cloning entire organisms called reproductive cloning has proven to be much more difficult than cloning specific molecules by genetic engineering techniques Embryo splitting produces identical o fspring In embryo splitting the undifferentiated cell is a fertilized egg The egg is fertilized in vitro and allows the divide to the eightcell stage The cells are the separated and each is implanted into a different surrogate mother Each of the cells will develop normally producing an organism that is an exact genetic copy of all the others Somatic cell nuclear transfer produces a clone of an adult Grace Hansen 10 A somatic cell is any cell in the body except a germ cell They have the full set of instructions for creating a complete unique individual Cloning an organism by somatic cell nuclear transfer yields a true clone of an adult organism The technique involves combining a single somatic cell taken from an adult animal with an enucleated fertilized egg no nucleus How it is done in practice 1 2 An egg is removed from a donor animal and the nucleus is carefully removed A somatic cell is removed from the animal to be clones and inserted into the enucleated egg Electric current is used to fuse the two cells into one The egg is then inserted into a surrogate mother and allowed to develop normally 177 Therapeutic cloning creating tissues and organs Therapeutic cloning is the cloning of human cells specifically for treating patients The ultimate goal to be able to take a single cell from a patient coax it along a path of differentiation and cell division until it produced just the right cells tissue or even a whole new organism The very best cells for use in therapeutic stem cell research are undifferentiated embryonic stem cells ES cells harvested from earlystage human embryos o The embryos are destroyed in the process of harvesting the stem cells and for many people the destruction of a human embryo represent an insurmountable ethical problem Induced pluripotent stem cells iPS cells 0 The idea is to put fully differentiated cells back into a state from which they can once again differentiate into one of many cell types given the right conditions
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