Meiosis is the process by which diploid cells in organisms undergo two cell divisions to produce four haploid gametes. In meiosis I, homologous chromosomes pair up and undergo crossing over, then separate so each daughter cell has one of each type of chromosome. Meiosis II then separates the sister chromatids, resulting in four haploid cells each containing one set of chromosomes. This allows for genetic variation between gametes and offspring through independent assortment of chromosomes and new combinations from crossing over during meiosis I.

1. MEIOSIS Pp275-278

2. What Mendel Knew… Each organism must inherit a single copy of every gene from each of its “parents” Each of the organisms gametes must contain just one set genes When gametes are formed, there must be a process that separates the 2 sets of genes so each gamete gets one set

3. Chromosome Number Homologous chromosomes Chromosome that has a corresponding chromosome from the opposite-sex parent Fruit fly has 8 chromosomes 4 from mom 4 from dad

4. Diploid Di= two sets Cell that contains both sets of homologus chromosomes Cell contains 2 complete sets of chromosome 2 complete sets of genes Number of chrms in diploid cell represented by 2N For Drosophilia (fruit fly) 2N=8 Mendel said: Each adult cell contains two copies of each gene

5. Haploid Means “one set” Refers to cells that contain only one set of chromosomes Gametes (sex cells) Represented by N Drosophilia fruit fly N=4

6. How are haploid (N) gametes made from diploid (2N) cells?

7. Meiosis Process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell

8. Meiosis 2 distinct stages Meiosis I A diploid cell enters here Meiosis II At the end of this, the diploid cell that entered meiosis has become 4 haploid cells

11. Meiosis I Before meiosis 1, each chromosome is replicate Then they divide like in mitosis What happened in mitosis? PMAT Tetrad STRUCTURE MADE WHEN EACH CHROMOSOME PAIRS UP WITH ITS HOMOLOGOUS CHROMOSOME 4 CHROMATIDS IN A TETRAD

12. Prophase 1 Each chromosome pairs with its homologous chromosome making a tetrad As they pair up in tetrads, chromosomes exchange portions of their chromatids in the process …. CROSSING OVER

13. Crossing Over

15. Metaphase1 Spindle fibers attach to chromosomes Anaphase 1 The spindles pull homologous chromosomes apart to opposite poles/ends Telophase 1 Nuclear membranes form and cell separates into two new cells

17. Now what do we have? 2 new daughter cells Are they identical to the parents? No The parent has 4 chromosomes Each daughter cell only has 2 chromosomes (4 chromatids, but they are different because of crossing-over) Each daughter cell has a set of chromosomes and alleles different from each other and different from the parent diploid cell

18. Meiosis II Unlike Mitosis, Neither cell goes through a round of chromosome replication Each cell’s chromosome has 2 chromatids

19. Prophase II Meiosis resulted in 2 haploid (N) cells, each with ½ the number of chromosomes in the original cell

20. Metaphase 2 Chromosomes line up in middle Anaphase 2 Sister chromatids separate and move to opposite poles Telophase 2 Meiosis II results in 4 haploid (N) daughter cells 4 daughter cells contain haploid number of chromosomes, just 2 each

23. Gamete Formation Male Haploid gametes produced by meiosis are called sperm Female Haploid gamete produced by meiosis is called an egg Cell divisions at the end of meiosis one and two are uneven so one cell gets most of the cytoplasm (the EGG) and the other three are called polar bodies (don’t participate in reproduction)

29. Mitosis vs. Meiosis Mitosis Results in the production of two genetically identical DIPLOID cells Daughter cells have sets of chromosomes identical to each other and to parent cell MITOSIS allows body to grow and replace other cells Asexual reproduction Meiosis Results in four genetically different HAPLOID cells MEIOSIS is how sexually reproducing organisms make gametes

32. Genes

33. Gene Linkage Would genes on the same chromosome be inherited together? Thomas Hunt Morgan gave us the answer 1910 PRINCIPLE of LINKAGE 50 Drosophilia genes Seemed to contradict Principle of Independent Assortment b/c certain genes were always inherited together He grouped the fly’s genes into linkage groups

34. Linkage groups are made up of genes that seem to be inherited together Linkage groups assort independently but all genes in one group are inherited together Drosophilia 4 linkage groups 4 chromosomes What can be concluded?

36. Conclusions Each chromosome is actually a group of linked genes Mendel’s Principle of Independent Assortment holds true IT IS THE CHROMOSOMES THAT ASSORT INDEPENDENTLY, NOT THE INDIVIDUAL GENES

37. How Mendel missed it… What 7 genes did he study? 6 of the 7 genes were on different chromosomes Two genes were on the same chromosome but they were so far apart on the same chromosome that they assorted independently

38. If two genes are on the same chromosome, does that mean they are linked for ever? No, chromosomes cross over during meiosis so they can separate Crossing over produces new combinations of alleles Important for genetic diversity

39. Lucky Student Alfred Sturtevant 1911 Columbia University Worked in Morgan’s Lab Hypothesis If two genes are farther apart on a chromosome, the they are more likely to be separated during meiosis Experiment Measured the rate at which linked genes were separated and recombined to make a “map” of distances between genes Conclusion Recombination rates could be used to make gene maps Gene maps showed the location of a gene on a chromosome