1 A duplicated or replicated chromosome is two sister chromatids joined at a centromere by cohesion proteins 2 The cell cycle refers to two components cell growth and mitotic cell division Events or stages involved are interphase which includes G1 S DNA synthesis and G2 phases and the mitotic phase includes mitosis and cytokinesis 3 Cells and organisms need to regulate or control the cell cycle so that damaged cells cannot be allowed to replicate and cause tumors or cancer and so cell growth is not over stimulated also causing cancer 4 Cells regulate or control the cell cycle through molecular signals signal molecules and regulatory proteins in the cytoplasm Checkpoints are meant to regulate the cell at various times in its life cycle They pause the cycle and receive signals from the environment on what to do In the G2 checkpoint signals assess preparations and if it s ok they direct the cell to enter the mitotic phase The MPF signal puts the cell through the G2 checkpoint and initiates mitosis MPF is a cyclin Cdk complex A particular cyclin is synthesized during S and G2 and when this cyclin level becomes high it gets bound to and activated by a particular Cdk this complex is now called MPF 5 The general purposes of mitotic cell division are development and tissue growth and repair Mitotic cell division begins with prophase where centrosomes move to opposite poles spindles start to form the nuclear envelope starts to dissolve the nucleolus disintegrates and chromatin begins to condense into distinct chromosomes Secondly is pro metaphase where the nuclear envelope is now disassembled chromosomes are clearly distinct a kinetochore appears at each centromere and the spindle apparatus is complete Next is metaphase where the spindle microtubules move chromosomes and the chromosomes like up single file at the metaphase plate midline cell equator Next is anaphase where the centromere splits spindle microtubules pull apart sister chromatids toward opposite poles and each separated chromatid is now a distinct chromosome The last step is telophase where the spindle disassembles the nuclear membrane reforms nucleolus reappears and mitosis is now complete Cytokinesis happens last and the cell membrane pinches in at the cleavage furrow and two daughter cells result The number of chromosomes in the beginning of mitosis is the same as the number of chromosomes in each daughter cell at the end 6 Cancer and the cell cycle have a lot to do with each other Cancer results when there are errors in the cell cycle It is caused by mutations in genes Gene regulation also has a lot to do with cancer If genes are not regulated properly cancer can occur Normal tumor suppressing genes restrict cell division but if there is an error it can result in over stimulation of cell division And normal proto oncogenes stimulate cell division but if there is an error they will over stimulate a cell to divide when it shouldn t 7 Somatic cells are diploid and have two sets of chromosomes maternal and paternal sets They are body cells in places such as skin muscle and nerves Their function is development and the growth and repair of the body They are formed through mitosis and in humans the chromosome number is 46 Germ cells are sex cell or gametes They are haploid and contain only one set of chromosomes In humans they have 23 chromosomes They are found in oocytes or egg cells and sperm cells and are formed through meiosis 8 A karyotype is an orderly way of displaying the number and types of chromosomes in a cell It displays visible condensed already replicated chromosomes ones undergoing mitosis A karyotype reveals how many chromosomes are in that organism and detect the gender They are also used to detect abnormalities in chromosome number size etc 9 When we say that in eukaryotes chromosomes exist in pairs it means that there is a chromosome from the father that matches a chromosome from the mother The two chromosomes that make up a pair are called homologous chromosomes They are similar to each other in that they have the same number and kinds of genes They are possibly different from each other in their alleles which are different versions of the same kind of gene 10 A diploid has twice the number of chromosomes as a haploid cell Diploid cells are somatic cells and haploid cells are germ cells or gametes 11 The general purpose of meiotic cell division is to produce four haploid sex cells There are 2 cell divisions in meiosis meiosis 1 and meiosis 2 The first division is a reduction division you go from 2n to n Chromosomes duplicate in a diploid cell homologous chromosomes pair up and form a tetrad and nonsister chromatids exchange segments crossing over tetrads line up at the metaphase plate and homologous pairs separate Two haploid cells are formed but the sister chromatids are still attached at centromeres Meiosis two then occurs which is alike a regular mitosis but in a haploid cell We get 4 haploid cells with just one set of chromosomes Compared to the beginning of meiosis after meiosis 1 there are half the number of chromosomes And after meiosis two there is still half the number of chromosomes 12 a In meiosis tetrads are formed by the synapsis of homologous chromosomes Chromosomes line up at the metaphase plate in mitosis but tetrads line up at the plate in meiosis In mitosis sister chromatids separate during anaphase but homologous chromosomes are separate in anaphase 1 in meiosis Sister chromatids don t separate until anaphase II in meiosis Mitosis produces two diploid daughter cells identical to each other and the parent but meiosis produces four haploid daughter cells that are genetically different b c 13 Unicellular organisms reproduce asexually through mitosis Meiosis can contribute to genetic variation among diploid organisms by independent assortment of chromosome pairs and crossing over Independent assortment is when the two chromosomes in a pair separate independently of how other chromosome pairs separate So a gamete can receive a mix of both maternal and paternal genes Crossing over is when non sister chromatids in a tetrad break and exchange sections at the chiasma This results in new allele combinations This is important so for evolutionary reasons 14 Mendel s purpose in his experiments was to study inheritance patterns His experimental system involved removing stamens male organs that make pollen and transferring the sperm bearing pollen to egg bearing flowers and planted pollinated seeds He
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