Meiosis

Definition Cell division producing gametes (sex cells) Male= sperm cells 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.

It is a two-step process that reduces the chromosome number by half—from 46 to 23—to form sperm and egg cells. This is a reduction in genetic material.

When the sperm and egg cells join at fertilization, each contributes 23 chromosomes so the resulting embryo will have the usual 46.

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

Meiosis TWO divisions ONE DNA replication

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

Meiosis Interphase: Before meiosis begins, genetic material is duplicated. First division of meiosis –Prophase 1: Duplicated chromatin condenses. Each chromosome consists of two, closely associated sister chromatids. Crossing over can occur during the latter part of this stage. –Metaphase 1: Homologous chromosomes align at the equatorial plate. –Anaphase 1: Homologous pairs separate with sister chromatids remaining together. –Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

Meiosis Second division of meiosis: Gamete formation –Prophase 2: DNA does not replicate. –Metaphase 2: Chromosomes align at the equatorial plate. –Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole. –Telophase 2: Cell division is complete. Four haploid daughter cells are obtained. One parent cell produces four daughter cells. Daughter cells have half the number of chromosomes found in the original parent cell and with crossing over, are genetically different.

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

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

Oogenesis –production of egg

Spermatogenesis –production of sperm

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

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 4) Embryo is surrounded by fluid sac called the AMNION

5) Embryo connected to the wall of the uterus by its CHORIONIC VILLI 6) The amniotic sac is enclosed by the CHORION. Which later forms the PLACENTA 7) Nutrients and oxygen from the mother and wastes from the embryo are exchanged in the UMBILICAL CORD.

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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