Making gametes for reproduction MEIOSIS Making gametes for reproduction Diploid – two sets of chromosomes (humans = 46 chromosomes) Haploid – one set of chromosomes (humans = 23 chromosomes) Homologous chromosomes = chromosomes that are similar in size, shape and genetic content (one of a pair) we get one homologue from each parent Briefly review mitosis stages (I Promised My Aunt Teresa Cookies) using the teacher-directed coloring sheet Ask: If a human sperm and egg each had 46 chromosomes, like other human body cells, how many chromosomes would a fertilized egg have? 92 Why might increasing the number of chromosomes in a human cell cause problems? Answers vary, may include that it would change the genetic makeup of humans Meiosis is the process of cell division that results in gametes (sperm and egg) that have half of the number of chromosomes so that the joining of sperm and egg results in a zygote with the correct number of chromosomes

Meiosis Type of cell division where ONE body cell produces FOUR gametes Each gamete* contains half the number of chromosomes as a body cell (body cell: 46gamete: 23) *GAMETE- Male and female sex cells; sperm and egg Any organism that reproduces sexually (male and female produce gametes that join to produce new individual) must undergo meiosis to produce those gametes

Why does Meiosis happen? To reduce the chromosome number in the gametes to half of the original. When sperm and egg join to form a zygote, there will be the correct number of chromosomes 23 (egg) + 23 (sperm) = 46 (zygote) Meiosis has TWO divisions of the nucleus (meiosis I and meiosis II)

Meiosis Video Germ cells undergo meiosis (not all cells) Haploid – chromosomes not paired (humans – 23)

Phase I of Meiosis 0) Interphase Chromosomes invisible; cell growing Chromosomes replicate (DNA Synthesis) Centrioles replicate (other organelles, too) Interphase starts same as mitosis

Phase I of Meiosis Prophase I Centrioles migrate to the poles / spindle fibers form Chromosomes coil (now visible) Nuclear Membrane disappears Chromosomes (sister chromatids) pair up with another Chromosome to form a TETRAD First three steps same as mitosis Difference – Homologous chromosomes pair up with each other to form a Tetrad Crossing over can occur were portions of a chromatid on one homologous chromosome are broken and exchanged with the corresponding chromatid portions of the other homologous chromosome Demo crossing over with the pop beads on the board

Tetrad- consists of two HOMOLOGOUS chromosomes, each made up of two sister chromatids Homologous chromosomes- paired chromosomes with genes for the same traits arranged in the same order Crossing over occurs within the tetrads Homologous chromosomes – one of the pair comes from father and other of the pair comes from the mother

Sometimes crossing over occurs – exchange of genetic material between non-sister chromatids to create new allele combination * Allele – alternate forms of a gene for each variation of a trait ex. Alleles for flower color – red, white, pink

Phase I of Meiosis 2) Metaphase I Spindle fibers attach to the centromere of each chromosome (tetrads) Spindle fibers pull tetrads to middle of the cell (In mitosis each pair of sister chromatids line up alone, but in meiosis they line up in tetrads) Lining up along the middle, but as tetrads rather than individual chromosomes Homologous chromosomes remain together in meiosis, while in mitosis they line up independent of each other

Phase I of Meiosis 3) Anaphase I Spindle fibers shorten Chromosomes (each with 2 sister chromatids) separate and move to opposite ends (Each new cell will get only ONE chromosome from each homologous pair or tetrad) Homologous chromosomes are pulled apart, but the centromere remains intact each daughter cell will have a single chromosome, but the genetic material is still duplicated (two chromatids attached at a centromere) No pattern to which of the two chromosomes ends up in each cell / random distribution of homologous chromosomes (independent assortment)

Phase I of Meiosis 4) Telophase I Spindle fibers disappear Chromosomes uncoilchromatin Cytoplasm divides (Cytokinesis) (Each cell has HALF of the genetic information, but chromosomes still made up of sister chromatids) Process same as in mitosis, but result is different due to previous phase differences

Prophase I Metaphase I Telophase I Anaphase I

Phase II of Meiosis 0) Interphase Cell grows a bit Centrioles replicate, but chromosomes DO NOT replicate In some cases, interphase between meiosis I and meiosis II does not exist NO replication of chromosomes at this point

Phase II of Meiosis 5) Prophase II Centrioles move apart Spindle fibers form and attach to chromosomes Meiosis II is same process as mitosis, except that there are only single chromosomes (instead of pairs of chromosomes) involved

Phase II of Meiosis 6) Metaphase II Spindle fibers pull the chromosomes to the middle of the cell (equator)

Phase II of Meiosis 7) Anaphase II Spindle fibers shorten Chromosomes split (sister chromatids are pulled apart) and move to opposite poles

Phase II of Meiosis 8) Telophase II Nuclear membrane reforms Spindle fibers disappear Chromosomes uncoil (chromatin) CYTOKINESIS occurs End Result: 4 Haploid cells (each cell has 23 chromosomes- not 46)

Prophase II Metaphase II Anaphase II Telophase II

Importance of Meiosis Meiosis is important because 1. allows sexual reproduction to occur while maintaining the same number of chromosomes in the offspring 2. DNA of parents is resorted and recombined to produce unique individuals

Meiosis Video Quiz As a results of meiosis, sex cells are formed that have ________ the number of chromosomes of body cells. Half T/F Pairs of identical chromosomes are also called homologous chromosomes. True T/F Normal body cells are diploid because they possess one of each different chromosome. False, two T/F During meiosis, the DNA is replicated twice. False, once T/F In plant and animal cells, meiosis occurs only in germ cells. True

References Mitosis and Meiosis. Ancient Lights. 1994. Discovery Education. 28 November 2009 http://streaming.discoveryeducation.com/ Meiosis pictures from http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/page3.html