Meiosis Reproduction of Sex Cells

Meiosis Introduction Meiosis is part of the life-cycle of every organism that reproduces sexually. Meiosis is cell division that produces sex cells ( sperm and eggs), also called gametes

Gametes Each gamete contains half the number of chromosomes of the parent. Because they only have half the chromosomes of non-sex cells we call gametes haploid.

Gametes During the fertilization stage of sexual reproduction, the male gamete (sperm) fuses with the female gamete (egg) to produce a zygote with two sets of chromosomes.

Why is this important? The chromosomes in the gametes contain genes from the parents that will determine the characteristics of the zygote (baby).

Why is the important? Genes are the units of heredity Genes are segments of DNA Each gene has a specific location on a certain chromosome

Why is the important? One set of chromosomes is inherited from each parent Reproductive cells called gametes (sperm and eggs) unite, passing genes to the next generation

Sexual vs. Asexual Reproduction In asexual reproduction, one parent produces genetically identical offspring by mitosis In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents

Haploid vs. Diploid Non-sex cells (most cells) contain two sets of each type of chromosome. A cell that has two full sets of chromosomes is called diploid.

5 µm Pair of homologous chromosomes Sister chromatids Centromere

Homologous Chromosomes In diploid cells, each pair of chromosome has the same genes (example = gene for eye color).

Homologous Chromosomes However, they do NOT usually have the same alleles (variation of gene = gene for blue eyes). The pairs of NOT identical, but they are homologous.

Human Chromosomes Sex Chromosomes The sex chromosomes are called X and Y

Human Chromosomes Human females have a homologous pair of X chromosomes (XX) Human males have one X and one Y chromosome

Human Chromosomes Each pair of homologous chromosomes includes one chromosome from each parent The 46 chromosomes in non-sex human cell are two sets of 23: one from the mother and one from the father

Human Chromosomes The number of chromosomes in a single set is represented by n A cell with two sets is called diploid (2n) For humans, the diploid number is 46 (2n = 46)

Human Sex Cells Gametes are haploid cells, containing only one set of chromosomes For humans, the haploid number is 23 (n = 23) Each set of 23 consists of 22 autosomes and a single sex chromosome

Human Sex Cells In an unfertilized egg (ovum), the sex chromosome is X In a sperm cell, the sex chromosome may be either X or Y – father cells determine gender.

Sexual Reproduction -- Fertilization At sexual maturity, the ovaries and testes produce haploid gametes Gametes are the only types of human cells produced by meiosis, rather than mitosis Meiosis results in one set of chromosomes in each gamete Fertilization, the fusing of gametes, restores the diploid condition, forming a zygote The diploid zygote develops into an adult

Key Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm cell (n) Testis Ovary Mitosis and development Multicellular diploid adults (2n = 46) FERTILIZATIONMEIOSIS Diploid zygote (2n = 46)

Process of Meiosis. The number of chromosomes are reduced from diploid to haploid in the process of meiosis.

Overview of Meiosis By the end of meiosis, the diploid cell that entered meiosis has become 4 haploid cells.

Phases of Meiosis Meiosis II Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II Telophase I and Cytokinesis I

Formation of sperm by Meiosis

Formation of Egg Cells by Meiosis In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction

Variation The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation Two mechanisms contribute to genetic variation: 1.Independent assortment of chromosomes 2.Random fertilization

Independent Assortment of Chromosomes. In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs The number of combinations possible when chromosomes assort independently into gametes is 2 n, where n is the haploid number For humans (n = 23), there are more than 8 million (2 23 ) possible combinations of chromosomes

Independent Assortment

Random Fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) The fusion of gametes produces a zygote with any of about 64 trillion diploid combinations Each zygote has a unique genetic identity

Dragon Meiosis Step 1: Cut out ONE set of 14 chromosomes

Dragon Meiosis Step 2: Flip the 14 chromosomes over so that you canNOT see the letters

Dragon Meiosis Step 3: Randomly pick 7 chromosomes ONE OF EACH SIZE. After you have picked 7, throw other 7 chromosomes in the recycling bin.

Dragon Meiosis Step 4: Flip the Chromosomes over and write down the letters on your worksheet. BLUE is for MALES & PINK IS FOR FEMALES. You have just simulated Random Assortment of Chromosomes Male Gene (1 st color)Female Gene (2 nd color)

Dragon Meiosis Step 5: Flip the Chromosomes over and write down the letters on your worksheet. BLUE is for MALES & PINK IS FOR FEMALES. You have just simulated Random Assortment of Chromosomes Male Gene (1 st color)Female Gene (2 nd color)

Dragon Meiosis Step 6: Now write down the letters of your partners chromosomes. These 2 sets of chromosomes will fuse in the nucleus of the new baby dragon, and form its genetic make-up and determine its characteristics. You are simulating Random FERTILIZATION. Male Gene (1 st color)Female Gene (2 nd color)

Dragon Meiosis Step 7: Now write the genotype (the letters) of your new baby dragon in the other table in your worksheet. Notice that your baby dragon get one chromosome from mom and one chromosome from dad. Because your baby dragon has two sets of chromosomes it is diploid. TraitGenotypePhenotype Fire/No fire (A/a ’ s) Toes (M/m ’ s) Spikes on tail (Q/q ’ s) Tail color (T/t ’ s) Body color (E/e ’ s) Wing color (L/l ’ s) Horn/no horn (D/d ’ s) (D/d ’ s)

Dragon Meiosis Step 9: Determine the phenotype of your baby dragon. Phenotype is its physical appearance. Use the chart on the other worksheet TraitGenotypePhenotype Fire/No fire (A/a ’ s) Toes (M/m ’ s) Spikes on tail (Q/q ’ s) Tail color (T/t ’ s) Body color (E/e ’ s) Wing color (L/l ’ s) Horn/no horn (D/d ’ s) (D/d ’ s)

Dragon Key

Dragon Meiosis Step 10: Draw your dragon & answer the question. Turn in by the end of the period. Pick up Meiosis Webquest and Reading Question worksheet

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