Objectives for Exam 1PCB 3063, Spring 2012Mitosis 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 in 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:•First Division•homologous chromosomes separate into different daughter cells•crossing over (chromosome recombination) occurs•Meiosis II:•Second division•sister chromatids are separated into different daughter cells•in both processes, DNA is first 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 fibers 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 donʼt 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 shuffles the genes•ʻcrossing overʼ--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 first 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:•Bivalents line up along the metaphase plate•Anaphase I:•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•ʻcrossing overʼ- exchange of genetic material between homologous chromosomes, one of the final phases of recombination•Recombination- a
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