Objectives for Exam 1 PCB 3063 Spring 2012 Mitosis and Meiosis Lectures 1 2 Chapter 2 Of basic importance in this material is to know the essential differences between mitosis and meiosis what homologous chromosomes are and how they behave in mitosis and meiosis Mitosis mitosis produces somatic cells all cells except for reproductive cells Mitosis produces 2 genetically identical daughter cells each daughter cell gets 1 chromatid from each chromosome new sister chromatid made for each chromosome to count the number of chromosomes basically just count centromeres Mitosis does not change chromosome number a chromosome can have 1 or 2 arms Homologous chromosomes pair synapse during meiosis the cell division that occurs as part of the creation of gametes Sections of the DNA can sometimes cross over between homologous pairs Meiosis produces the reproductive cells sperm and egg formation of gametes one diploid cell divides twice and forms 4 haploid cells each daughter cell gets one chromatid from each pair of homologous chromosomes If 2n 4 then during meiosis n 2 Meiosis I Meiosis II First Division homologous chromosomes separate into different daughter cells crossing over chromosome recombination occurs Second division sister chromatids are separated into different daughter cells in both processes DNA is rst replicated and then different methods are used for separation of the replicated genetic information chromosomes before replication consist of a single DNA double helix bound by histone proteins chromatin 2n 2 copies of each chromosome 2n 2 pairs of homologous chromosomes diploid of 4 2n 4 chromosomes Understand the concept of the cell cycle and know what happens during S phase Be able to name the stages of mitosis and know what events take place in each G0 cell getting ready for DNA replication resting phase where the cell has left the cycle and stopped dividing Interphase the chromosomes are un condensed cell is growing during interphase proceeds in the following 3 stages G1 phase checkpoint makes sure everything is ready for DNA synthesis replication synthesis of enzymes and lots of growth S synthesis phase chromosome duplication occurs this phase is complete when each chromosome has 2 sister chromatids the amount of DNA in the cell has technically doubled but the ploidy remains the same G2 phase cell is preparing structures like microtubules necessary for mitosis checkpoint makes sure the cell is ready to enter cell division phase M phase Mitosis cell growth stops here division into 2 daughter cells Prophase chromatin begins to condense and become visible centrioles start moving to opposite ends start of spindle formation coil Metaphase spindle bers align the chromosomes along the metaphase plate align Anaphase the paired chromosomes separate at the kinetochore and move to opposite sides of the cell separate Telophase new membranes form around daughter cells chromosomes un condense uncoil Cytokinesis actin ring pinches the cell into two daughter cells in animals in plants a cell plate Know what sister chromatids are sister chromatids are exact duplicates a condensed chromosome ready for division mitosis has 2 arms the sister chromatids Understand the term diploid and haploid and know what homologous chromosomes are how they are similar to each other and how they differ from non homologous chromosomes The haploid number n is the number of chromosomes in a gamete A somatic cell has twice that many chromosomes 2n Humans are diploid A human somatic cell contains 46 chromosomes 2 complete haploid sets which make up 23 homologous chromosomes pairs Homologous Chromosomes carry the same genes but may have different alleles have same centromere position same length non homologous chromosomes would carry completely different genes exception sex chromosomes are homologous chromosomes but don t carry the same genes Know the important events that occur in meiosis and understand how meiosis differs from mitosis Differences between mitosis and meiosis chromosomes in meiosis undergo a recombination which shuf es the genes crossing over results in different genetic combinations in each gamete outcome of meiosis is 4 haploid cells all genetically unique compared with the two genetically identical diploid cells produced from mitosis In the rst meiotic division the number of cells is doubled but the number of chromosomes is not results in 1 2 as many chromosomes per cell The second meiotic division is like mitosis the number of chromosomes does not get reduced At the end of division 1 each of 2 daughter cells has 2 chromosomes each with 2 chromatids and no pairs Second division 4 daughter cells each with 2 chromosomes each of these chromosomes only have 1 chromatid Meiosis I Prophase 1 homologous chromosomes pair up and form synapses these paired chromosomes are called bivalents tetrads bivalents synapsis joining of homologous chromosomes results in tetrads bivalent has 2 chromosomes and 4 chromatids Metaphase 1 Anaphase I Bivalents line up along the metaphase plate chiasmata separate chromosomes each with 2 chromatids move to separate poles each of the daughter cells is now haploid 23 chromosomes but each chromosome has 2 chromatids Telophase I Cytokinesis I nuclear envelopes may reform or cell may go right into meiosis II 2 complete daughter cells form Meiosis II in metaphase II the chromatids separate and become independent chromosomes No DNA replication before meiosis II everything else is pretty similar to Meiosis I except the chromosome number is not reduced Be able state how many chromosomes and pairs of homologous chromosomes are present in cells of a given diploid number that are undergoing meiosis Understand the terms tetrad chiasmata crossing over and recombination tetrad bivalents paired and replicated chromosomes 2 chromosomes 4 chromatids with 1 chromosome coming from each parent chiasma the point where two homologous non sister chromatids exchange genetic material during chromosomal cross over during meiosis crossing over exchange of genetic material between homologous chromosomes one of the nal phases of recombination Recombination a process in which a molecule of DNA is broken and then joined to a different one tetrads pairs Skill be able to solve genetics problems similar to those in the rst homework assignment related to chapter 2 Review of Mendel s Principles Lectures 3 5 Chapter 3 Be able to state Mendel s postulates Random Segregation and Independent Assortment and
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