Meiosis and Sexual Reproduction Chapter 7
7.1 Meiosis In some organisms haploid gametes join to form a diploid zygote If gametes weren’t haploid imagine how the number of chromosomes would increase of time Meiosis is the form of cell division that makes diploid cells haploid when forming gametes There are 2 divisions in Meiosis – Meiosis I and Meiosis II Before Meiosis begins DNA is replicated so cell has homologous chromosomes
Prophase I Chromosomes condense Nuclear envelope breaks down Homologous chromosomes pair up Crossing over occurs Chromatids exchange
Metaphase I Homologous pairs move to equator by spindle
Anaphase I Homologous pairs separate and pulled towards poles Chromatids do not separate; still connected by centromeres
Telophase I Individual chromosomes gather at poles Most organisms now have cytokinesis 2 new cells formed Chromosomes do not replicate again
Prophase II New spindle forms
Metaphase II Chromosomes line up along equator and are still attached at centromeres
Anaphase II Centromeres divide Chromatids separate; Now called chromosomes
Telophase II Nuclear envelope forms around chromosome sets
Meiosis in Action
Meiosis and Genetic Variation Three key ways meiosis creates new genetic variations Independent Assortment Crossing Over Random Fertilization
Independent Assortment Most organisms have more than 1 chromosome In humans the gamete gets 1 chromosome from the 23 pairs Which of the pair you get is chance This random distribution is called Independent Assortment Mathematically 223 = over 8.3 million combinations
Crossing Over Portion of 1 homologous chromosome is broken off and exchanged with the corresponding portion of its homologous partner
Random Fertilization What egg and sperm meet is also completely random Mathematically that means that the possible combinations of the sperm (about 8.3 million) can be multiplied by the combinations for the egg (about 8.3 million) giving 70 trillion possible combinations
Meiosis generates variation in a species very quickly Pace of evolution is sped up due to variation A + B = C, a new individual not identical to its parents
Gametogenesis – Formation of Gametes Spermatogenesis – sperm production occurs in testes Diploid cell increases in size and becomes immature germ cell Meiosis I = 2 immature sperm cells Meiosis II = 4 haploid cells Develop tails and become sperm
Oogenesis – egg production occurs in ovaries Diploid cell increases and becomes germ cell Meiosis I = after cytokinesis cytoplasm is not equally divided Smaller one called polar body Meiosis II = larger cell divides again, again cytoplasm not equally divided Forms another polar body Polar bodies will die Ovum (pl. = ova) is larger cell More cytoplasm means more stored energy
7.2 Sexual Reproduction Asexual Reproduction Single parent passes on copies to offspring called “clone” Identical to parent Prokaryotes use binary fission
Many eukaryotes use asexual reproduction Fission – parent into 2 equal sized offspring Fragmentation Body into several pieces Missing parts grow back Budding – new split off from old; may stay attached
Genetic Diversity Asexual reproduction is simplest method In stable environment can produce many offspring in short time’ Don’t need to waste energy making gametes or find a partner DNA varies very little In changing environment may not be able to adapt
Sexual Reproduction 2 parents form reproductive cells with half the number of chromosomes Diploid Mother makes haploid gamete Diploid Father makes haploid gamete Haploid gametes make diploid offspring different from parents
Evolution of Sexual Reproduction May have come as a way for protists to repair DNA Only diploid cells can repair Protists form diploid cells under stress Many enzymes that repair DNA are involved in meiosis
Sexual Life Cycle in Eukaryotes Life Cycle – span from 1 generation to next All sexually reproductive organisms have a diploid-haploid-diploid lifecycle Only 3 Types of Sexual Life Cycles Only difference is which phase becomes multicellular
Haploid – simplest and most common Zygote is the only diploid cell and it undergoes meiosis immediately Haploid cells in multicellular organism produce gametes through mitosis Fusion – gametes fuse to form zygote and meiosis follows Meiosis allows for damage repair 2 homologous chromosomes are lined up, special repair enzymes remove damaged sections of DNA and fill in gaps Protists, Fungi, Algae
Haploid Life Cycle
Diploid Adults are diploid, inherit from 2 parents Diploid cell then meiosis then haploid gamete Gametes join during fertilization forming diploid zygote Zygote divides by mitosis Gametes are the only haploid cells
Diploid Life Cycle
Parthenogenesis New individual forms from unfertilized egg Hypothesized that mothers chromosomes are copied instead of getting fathers chromosomes Produces clones Honeybees provide drones this way (haploid) Whiptail lizards are all females Doesn’t seem possible in mammals Only mammal clones are twins from split egg
Alternation of Generations Regularly alternates between haploid and diploid Plants – diploid phase called sporophyte produces spores Spore forming cells produce haploid spore that can form adult without another cell Gametophyte is haploid phase Produces gametes by mitosis Gametes fuse and produce diploid phase Diploid – Haploid – Diploid – Haploid – etc… Moss – haploid spores develop at top of sporophyte stalk