Biology Unit 3 10 11 2013 Mitosis Nuclear division is divided into four phases based on the appearance and behavior of the chromosomes Prophase Metaphase Anaphase Telophase Prophase Early prophase poles Late prophase Cytokinesis cytoplasmic division usually occurs during Telophase o Chromosomes condense o Spindle microtubules for between centrioles o Centrioles move towards opposite sides of cell Opposite o Nuclear envelope disappears o Centrioles are at opposite poles o Spindle microtubules capture chromosomes at centromeres Metaphase the middle phase Microtubules from either side lengthen shorten Chromosomes line up at the equator of the cell Microtubules are attached to each sister chromatid at an area of the centromere call the kinetochore Anaphase Sister chromatids separate each becoming an independent daughter chromosome o Each pole gets one copy of each parental chromosome Polar microtubules attach to each other and push causing the cell to elongate Telophase the end phase Begins when chromosomes reach the poles Spindle microtubules disintegrate A nuclear envelope form around each group od chromosomes Cytokinesis Cytokinesis Microfilaments form along the equator of the dividing cell and constrict Like the drawstring of Sweatpants and squeezes the cell into two Both daughter cells get one nucleus and about half of the cytoplasm Stiff cell wall prevents the cells from being squeezed into two o Vesicles filled with carbohydrates bud off the Golgi o These vesicles line up at the equator and fuse to form the cell plate o Edges merge with plasma membrane o Carbohydrates become new cell wall The cell cycle must be controlled Some cells like skin cells divide frequently Some cells only divide in response to damage or infection Some cells like brain heart and skeletal muscle never divide in an adult The cell cycle must be controlled Most cells of your body are in G1 phase of the cell cycle o Cells usually will not divide unless they receive signals form things like growth factors Growth factors stimulate cell division o Growth factors stimulate the synthesis of cyclins o Cyclins bind to Cdks Cyclin dependent kinases Cyclin Cdk complexes stimulate synthesis of proteins required for DNA replication and the cell enters S phase Interesting fact Dog saliva contains large amount of epidermal growth factor So when dogs lick their wounds EGF speeds up synthesis of cyclins and helps to regenerate skin cells After DNA has been replicated the cell enters G2 phase More CDK s become activated in G2 to control the entry into cell During cell division even more CDK s control the transitions from division metaphase to anaphase Three major checkpoints regulate the cell cycle At each checkpoint protein complexes determine whether the cell has completed a specific phase Checkpoint proteins usually have regulated the production of cyclins or activity of CDK s Normal control of the G1 S checkpoint o Photo oncogenes any gene whose protein product tends to promote cell division receptors Ex Genes for growth fathers and growth factor o Tumor suppressor genes genes whose protein product prevents cell division if the something is wrong Like mutated DNA The G1 S checkpoint and cancer o Mutated proto oncogenes can become cancer causing oncogenes that are always turned on o Mutated tumor suppressor genes may create inactive proteins that cannot stop the cell from dividing o Cancer is so hard to control because cancer cells use the same machinery for cell division as normal cells Sexual reproduction Mutations in DNA are the ultimate source of variability o Alleles alternate forms of a gene that may produce differences in structure or function Sexual reproduction may combine different parental alleles in a single offspring Meiotic cell division produces haploid cells During meiosis a cell undergoes one round of DNA replication followed by two rounds of nuclear division Meiosis I o Separates homologous chromosomes Meiosis II o Separates sister chromatids Meiotic cell division produces haploid gametes This preserves the chromosome number of the species Cytokinesis follows each round of meiosis Chromosomes were replicated during interphase so each chromosome consists of two sister chromatids attached to one another at their centromeres Meiosis I Meiosis II o Prophase I o Metaphase I o Anaphase I o Telophase I o Prophase II o Metaphase II o Anaphase II o Telophase II Resulting in four haploid daughter cells Prophase I homologous chromosomes pair up and exchange DNA Chromosome condense Homologous chromosomes pair up DNA is exchanged between homologues in a process called crossing over The nuclear envelope disintegrates Spindle microtubules form Crossing over o Chiasmata are sites where chromatids of two homologous chromosomes exchange DNA o Can cause genetic recombination The formation of new combinations of alleles on a chromosome Metaphase I paired homologous chromosomes line up at the center of the cell The homologous pairs are held together by Chiasmata Which homologue lines up on which side is random Independent assortment Independent assortment and crossing over account for the genetic diversity o the haploid cells Anaphase I homologous chromosomes separate Chiasmata untangle and homologues move to opposite poles Each homologue still consists of two sister chromatids Telophase I two haploid clusters of duplicated chromosomes form at the poles Spindle microtubules disappear The forming nuclei each contain one member of each homologous The resulting cells are haploid with each chromosome consisting of chromosome pair two sister chromatids Meiosis II There is little or no interphase between Meiosis I and Meiosis II Meiosis II is virtually identical to mitosis Except it takes place in haploid cells Chapter 10 Patterns of Inheritance 10 11 2013 The punnett square method A test cross can be used to determine if an individual with a dominant phenotype is homozygous or heterozygous for that trait Example o You have a plant with purple flowers If the purple flower with the unknown genotype is Homozygous all flowers were purple Heterozygous half purple flowers half white flowers Multiple traits Once Mendel determined how single traits were inherited he move on to more complicated things Peas have many other traits besides flower color Inheritance of multiple traits To do this Mendel looked at two traits at once Cross a true breeding plant with smooth yellow seeds to a true breeding plant with wrinkled green seeds All
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