Meiosis Animation

Meiosis A second type of cell division designed to make haploid cells called gametes Gametes include sperm and egg Meiosis occurs in 2 parts Meiosis 1 and Meiosis 2Meiosis 1 Meiosis 1 is a reductive division where homologous chromosomes line up together on a spindle fiberMeiosis 1 Therefore, # spindle fibers required = half of diploid number = haploid number Meiosis Tutorial: The Biology Project!

Structure formed when chromosomes come together is called a tetrad A tetrad consists of 2 replicated chromosomes or 4 chromatids Chromatids are chromosomes that are still attached to their sister strand via a centromere When centromere eventually splits and chromatids are free of their sister chromatid, both are called chromosomes…. this does not happen until anaphase 2 Excellent Narrated Movie of Meiosis

Main Difference between Mitosis and MeiosisDifferenceMeiosis During prophase 1, homologous (similar) chromosomes come together This process is known as synapsis During this time the chromosomes are so close together that crossing over can occur This leads to genetic variation since sections of the DNA are exchanged mixing maternal and paternal genes

Prophase 1 Nuclear membrane disintegrates Spindle fibers form Chromosomes become distinct Homologous (similar) chromosomes come together This process is called synapsis

Stages of Meiosis Always ask yourself “What’s going on here?”

Stages of Meiosis

Metaphase 1 Homologous replicated chromosomes line up on the same spindle fiber Individual chromatids in the replicated chromosomes do not separate yet Not until Anaphase 2 In human cell, since 2n = pairs line up Therefore 23 spindle fibers are needed If 2n = 10, 5 spindle fibers are required for this stage

Stages of Meiosis

Anaphase 1 Homologous replicated chromosomes separate to opposite poles Centromeres do not split yet

Stages of Meiosis

Telophase 1 Chromosomes arrive at opposite poles Cytoplasm pinches in Cytokinesis occurs in animal cells Equatorial plate forms in plant cells 2 haploid cells form Chromosomes may become chromatin and enter an interphase 2 in some cells

Stages of Meiosis

Prophase 2 New spindle fibers form Nuclear membrane dissolves Chromosomes become visible If 2n = 10, then only five spindle fibers needed…why? Half of the chromosomes are in the other cell

Stages of Meiosis

Metaphase 2 Chromosome line up on new spindle fibers Chromosomes here are made up of 2 chromatids If 2n = 46, 23 chromosomes are in each cell produced from telophase 1 This means that 23 spindle fibers must be present If 2n = 8, 4 spindle fibers needed now

Stages of Meiosis

Anaphase 2 Centromere splits Chromatids (now called chromosomes) migrate to opposite poles of the cell

Stages of Meiosis

Telophase 2 Chromatids (now called chromosomes) arrive at poles Cytoplasm starts to pinch in Cytokinesis occurs End result is 4 haploid cells 2 from each of the 2 cells produced at the end of telophase 1

Stages of Meiosis

Notice the difference

Stages of Meiosis End result of

Overview of Meiosis 1

Overview of Meiosis II

Meiosis in males and females In males called Spermatogenesis Involves production of 4 equal sized cells each with same amount of cytoplasmic organelles In females called Oogenesis First division unequal One cell is a polar body and smaller Second division produces another smaller cell called a polar body and then the Egg Egg contains the majority of cytoplasmic organelles

Oogenesis

Spermatogenesis

Meiosis Produces Sperm Cells

Structure of Human Sperm

Abnormal Meiosis When homologous chromosomes fail to separate during anaphase of meiosis… Abnormal gametes form with abnormal numbers of chromosomes (trisomy 21 etc..) Process called non-disjunction non-disjunction Karyotyping used to detect disorders Chromosomes are arranged according to 3 characteristics –Centromere position –Banding pattern –Size Non-disjunction animation

Sources of Variation 1. Gamete success 2. Crossing overCrossing over depends on distance between genes and length of chromosome 3. Independent AssortmentIndependent Assortment the way one homologous pair lines up has nothing to do with the way another homologous pair lines up!

Animation of Crossing Over Another great animation!!

Homologous pair of chromosomes

Independent Assortment

Detecting genetic Disorders Ultrasound locates position of fetusUltrasound Amniocentesis uses a needle to withdraw some amniotic fluid from around fetusAmniocentesis Fluid is in uterus Fetal cells can be analysed Chorionic Villus Sampling (CVS) draws cells from outer membrane of embryoChorionic Villus Sampling CVS can be used as early as 8 weeks of pregnancyCVS

Animal Life Cycle

Plant Life Cycle Alternation of Generations Sexual Stage Alternates with Asexual

Plant and Animal Life Cycle

Spermato-genesis Oogenesis SpermEgg (Ovum) Zygote Embryo Fetus Child Meiosis Mitosis

Moss Life Cycle Demonstrates Alternation Of Generations

Alternation of Generations Moss Plant Sexual stage alternates with an asexual stage Sexual stage involves fertilization by union of 2 gametes (needs water) Asexual stage involves spore production One cannot exist without the other Mitosis & Meiosis occurs Sexual & Asexual stage separate Fern Plant Same Sexual & Asexual stage on same plant

Fern Life Cycle Alternation of generations

Alternation of Generations in Fern Plants

Cell Division Tutorials/Questions