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UI BIOL 1411 - Meiosis
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BIOL 1411 1st Edition Lecture 16Outline of Last Lecture I. The Cell Cyclea. Cell Divisionb. Check PointsOutline of Current Lecture II. The Cell CycleIII. Reproduction IV. Meiosis ErrorsV. MendelLectureCell Cycle- Cyclins are transient in the cell cycle- Cyclins are synthesized only at certain times in the cell cycle- Check pointso 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 DNAo 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: CANCERo 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 tissueso Cell cycle regulation involves 2 types of molecules Normal positive regulators such as growth factors or their receptors (HER-2) that stimulate the cell cycle Normal negative regulatory proteins are abnormalThese 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- Sexo Asexual reproduction- can occur in one organism aloneo Sexual Reproduction- eukaryotes, join gametes to produce a new individual - Types of Cell Divisiono 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 ½ and each products is unique - Sexual Reproductiono Systematic joining of gametes to produce a diploid phase of the life cycle, coupled with meiosis that reduces chromosome number in the haploid phaseo 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 cellso Shuffles genetic variation Offspring are not identical to parents or to each other Exception: identical twinso Why is having the chromosome number necessary? Going to double every time if not halvedo 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 Meiosiso 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 Io Events Unique events of meiosis I- Duplicated homologous pairs of chromosomes come together andpair along their entire lengtho Pairing occurs during prophase Io Pairing is called synapsiso The four chromatids of each homologous pair form a tetrad, of bivalento 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 poleso Cells at end of meiosis I are haploid, but with each chromosome still containing 2 chromatids- ChiasmataL genetic exchange between chromatidso Exchange of genetic material occurs during prophase I- crossing over- Events of Meiosis IIo Duplicated cells at end of meiosis I are haploid, but each chromosome still consists of 2 chromatidso Critical event of meiosis II is the separation of the sister chromatidso Similar to mitosis, because in both cases sister chromatids segregate to opposite poles o Products are genetically distinct- Timing of Eventso 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 pubertyMeiosis: Errors- Nondisjunctiono Homologous pairs fail to separate at anaphase Io Or sister chromatids fail to separate at anaphase IIo Either results in aneuploidy- chromosomes missing or present in excess- Potential causeso Aneuploidy is sometimes caused by lack of cohesions, both homologous pairs together. Without cohesions, both homologs segregate at randomo Failure to undergo crossing overo Frequency of nondisjunction increases as a women ageso Nondisjunction leads to aneuploidy- Consequences of errors in meiosis for humanso 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 syndromeo 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 moleculeso 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


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UI BIOL 1411 - Meiosis

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