BIOL 1411 1st Edition Lecture 16 Outline of Last Lecture I The Cell Cycle a Cell Division b Check Points Outline of Current Lecture II The Cell Cycle III Reproduction IV Meiosis Errors V Mendel Lecture Cell Cycle Cyclins are transient in the cell cycle Cyclins are synthesized only at certain times in the cell cycle Check points o G1 s Cdk phosphorylate RB protein Unphosphorylated RB inhibits the cell cycle at restriction point cell does not enter Sphase When RB is phosphorylated by G1 S cyclin Cdk RB is inactivated and no longer blocks the cell cycle cell becomes committed to replicated DNA o But if there is DNA damage in G1 p21 protein prevents activation of G1 S Cdk DNA damage triggers synthesis of p21 protein P21 binds to G1 S Cdk preventing activation by cyclin Cell cycle stops while DNA is repaired Unregulated Cell Division CANCER o Cancer calls differ from normal cells in 2 way Cell cycle is not regulated by extracellular signals or internal check points Cells migrate to other tissues o Cell cycle regulation involves 2 types of molecules Normal positive regulators such as growth factors or their receptors HER2 that stimulate the cell cycle Normal negative regulatory proteins are abnormal These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Oncogene proteins are overactive or excess positive regulators of cancer cells proliferation Tumor suppressors are negative regulators in both cancer and normal cells but in cancer cells they are inactive Sex o Asexual reproduction can occur in one organism alone o Sexual Reproduction eukaryotes join gametes to produce a new individual Types of Cell Division o Binary Fission and Mitosis DNA copied and a complete copy segregated to each daughter cell Products identical to the mother cell o Meiosis DNA copied follow by 2 rounds of nuclear segregation DNA content reduced by and each products is unique Sexual Reproduction o Systematic joining of gametes to produce a diploid phase of the life cycle coupled with meiosis that reduces chromosome number in the haploid phase o Meiosis is a specialized cell division where a single round of DNA synthesis is followed by two stages of chromosome segregation Diploid mother cell pairs of chromosomes haploid daughter cells o Shuffles genetic variation Offspring are not identical to parents or to each other Exception identical twins o Why is having the chromosome number necessary Going to double every time if not halved o Three main variations in this cycle Diplontic Haplontic Alteration of generations Diplontic Life Cycle gametes are the only haploid cells in the life cycle somatic body cells are diploid humans Haplontic life cycle diploid zygote 2n forms and undergoes meiosis to produce haploid spores Spores n divide by mitosis to produce the mature organism yeast Alteration of generations a multicellular haploid stage and a multicellular diploid stage plants Human Karyotype 23 pairs of homologous chromosomes 2n 46 numbered 1 22 for autosomes approximately by size and a pair of sex chromosomes Summary of Meiosis o Functions Reduce chromosome number from diploid to haploid Ensure that each haploid cell has a complete set of chromosomes Generate diversity among the daughter cells gametes or spores o Key Features Two nuclear divisions but DNA is replicated only once Begins in a diploid cell meiocyte with all chromosomes in pairs Ends with haploid products Homologous chromosomes pair and exchange genetic information then segregate from each other in meiosis I o Events Unique events of meiosis I Duplicated homologous pairs of chromosomes come together and pair along their entire length o Pairing occurs during prophase I o Pairing is called synapsis o The four chromatids of each homologous pair form a tetrad of bivalent o Can lead to crossing over between non sister chromatids After metaphase I the homologous pairs separate o Maternal and paternal centromeres of each pair segregate to opposite poles o Cells at end of meiosis I are haploid but with each chromosome still containing 2 chromatids ChiasmataL genetic exchange between chromatids o Exchange of genetic material occurs during prophase I crossing over Events of Meiosis II o Duplicated cells at end of meiosis I are haploid but each chromosome still consists of 2 chromatids o Critical event of meiosis II is the separation of the sister chromatids o Similar to mitosis because in both cases sister chromatids segregate to opposite poles o Products are genetically distinct Timing of Events o Prophase I may last a long time Human males prophase I lasts about 1 week and 1 month for entire meiotic cycle Human females prophase I begins in utero pauses then resumes at puberty Meiosis Errors Nondisjunction o Homologous pairs fail to separate at anaphase I o Or sister chromatids fail to separate at anaphase II o Either results in aneuploidy chromosomes missing or present in excess Potential causes o Aneuploidy is sometimes caused by lack of cohesions both homologous pairs together Without cohesions both homologs segregate at random o Failure to undergo crossing over o Frequency of nondisjunction increases as a women ages o Nondisjunction leads to aneuploidy Consequences of errors in meiosis for humans o If both homologs go to the same pole and the resulting egg is fertilized it will be trisomic for that chromosome Trisomy 21 down syndrome Trisomy 18 Edwards syndrome o A fertilized egg that does not receive a copy of a particular chromosome will be monosomic Lethal except for one situation turner syndrome o Aneuploidy is common in human zygotes Survival of the aneuploidy is very uncommon Most aneuploidies result in lethal in utero o As women age the change of nondisjunction is greater What happens during meiosis o Crossing Over Exchange between non sister chromatids produces recombinant DNA molecules o Independent assortment Haploid sets of chromosomes inherited from parents mixed during segregation of homologous pairs in meiosis I Possible combinations 2 n n number of homologous pairs What are the Mendelian laws of inheritance o First Law The Law of Segregation 2 alleles of a gene separate and are transmitted individually and equally to gametes 50 50 chance o Second Law Law of independent assortment alleles of different genes assort independently during gamete formation