Meiosis

Definition Cell division producing gametes (sex cells) 2 sets of divisions to produce 4 cells with half the number of chromosomes as the parent cell Male= sperm cells 23 chromosomes Female= egg **large

Meiosis does 2 things: 1)Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid). 2)In meiosis, one diploid cells produces four haploid cells

Ensures that humans have the same number of chromosomes in each generation.

Meiosis also allows genetic variation through a process of DNA shuffling while the cells are dividing.

Meiosis TWO divisions ONE DNA replication

The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis

Meiosis I Reduction of Chromosome Number in the First Division of Meiosis –Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

Meiosis II Sister chromatids Separate Result = 4 haploid cells

Crossing Over Where a section of one chromosome switches places with the same section from the other chromosome of the pair. Recombination

Crossing over

Mitosis vs meiosis

Oogenesis –production of egg

Spermatogenesis –production of sperm

Fertilization = combination of egg and sperm to produce the zygote 46 chromosomes

After fertilization the cell undergoes mitosis!

Human development 1) Egg + sperm = zygote ** fertilization=occurs in the oviduct 2) Zygote divides repeatedly by mitosis to form a BLASTOCYST 3) The inner mass of the BLASTOCYST later becomes the EMBRYO. 4) Blastocyst divides and germ layers form called Gastrula- GERM LAYERS a) Ectoderm– outer layer; develops into the epidermis and the nervous system b) Mesoderm– middle; develops into the body systems c) Endoderm– inner; develops into the lining of organs

PARTS OF AN EMBRYO 1) Amnion– fluid filled sac that bathes the embryo and protects it 2) Allantois– where gases are exchanged between embryo and mother 3) Umbilical cord- extension of embryo digestive system that connects to the mother’s placenta for exchange of nutrients 4) Placenta- tissue that connects to uterus 5) Yolk sac– in other vertebrate used as food source 6) blastocyst– ball of cells that is implanted in the uterus which later develops inot the embryo 7) Chorionic villi– connects embryo to uterus 8) Chorion– encloses the amnion ; later becomes the placenta

Embryo development=First 2 months of development Fetal development = months 3 to 9 1 ST TRIMESTER = embryo becomes a fetus » organ systems develop * gender determined 2 ND trimester= fetal growth and movement 3 Rd trimester= end fetus weighs 3000g and head oriented down

Embryo development 1 st week= a)fertilization occurs in oviduct b) cleavage occurs ( mitotic divisions producing an 8 cell mass) c) morula forms ( larger mass of cells) D) Blastocyst forms ( fluid filled cavity with layer of outer cells and inner cell mass that will become embryo) and is implanted in the uterine lining.

2 nd week A) Gastrulation occurs– inner cell mass becomes embryo ; germ layers form B) 2 new membranes form 1) yolk sac= 1 st site of blood cell formation 2) amnion cavity = acts as insulator ; absorbs shock C) outer cell layer becomes chorion ( placenta)

Embryo development

Weeks after umbilical cord forms to exchange materials from mother to embryo and visa versa Embryo develops organ systems and grows

Meiosis allows the creation of unique individuals through sexual reproduction.

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