MEIOSIS, Chromosomes, & Fertilization Objective: D4 - Recognize the significance of meiosis to sexual reproduction

Homologous Chromosomes Term used to refer to chromosomes that each have a corresponding chromosome from the opposite-sex parent. chromosomesmaternal paternal In other words: A pair of chromosomes (maternal and paternal) that are similar in shape and size. homologous chromosomes. Humans have 23 pairs of homologous chromosomes. autosomes 22 pairs of autosomes sex chromosomes 1 pair of sex chromosomes

Organisms that reproduce Sexually are made up of two different types of cells. 1. Somatic Cells are “body” cells and contain the normal number of chromosomes ….called the “Diploid” number (the symbol is 2n). Examples would be … skin cells, brain cells, etc. 2. Gametes are the “sex” cells and contain only ½ the normal number of chromosomes…. called the “Haploid” number (the symbol is n)….. Sperm cells and ova (eggs) are gametes.

Meiosis Meiosis is describe as reduction division because the number of chromosomes per cell are cut in half through the separation of homologous chromosomes. Meiosis During Meiosis diploid cells are reduced to haploid cells Diploid (2n)  Haploid (n) Diploid (2n)  Haploid (n) If Meiosis did not occur the chromosome number in each new generation would double…. The offspring would die.

Meiosis Has 2 cell divisions Meiosis I Meiosis II (called Meiosis I and Meiosis II) with only one duplication of chromosomes. So, 1 diploid cell becomes 4 haploid cells.

In prophase of meiosis I, each chromosome pairs with its corresponding homologous chromosome to form a Tetrad In prophase of meiosis I, each chromosome pairs with its corresponding homologous chromosome to form a Tetrad (shown below) PaternalMaternal eye color locus eye color locus hair color locus hair color locus

Crossing Over - Process in which homologous chromosomes exchange portions of their chromatids during Prophase I of meiosis - Creates variation (diversity) in the offspring’s traits nonsister chromatids chiasmata: site of crossing over variation Tetrad

Phases of Meiosis Interphase I mitosis Similar to mitosis interphase. Chromosomes(S phase). Chromosomes replicate (S phase). chromosome chromatids centromeres Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres. Centriole Centriole pairs also replicate.

Meiosis I (four phases) Cell division chromosome one-half. Cell division that reduces the chromosome number by one-half. four phases four phases: a.prophase I b.metaphase I c.anaphase I d.telophase I

Prophase I Chromosomes Chromosomes condense. Homologous chromosomes tetrad Homologous chromosomes come together to form a tetrad. Reminder: Tetradchromosomes chromatids Reminder: Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad

During Prophase I “Crossing Over” occurs. During Crossing over chromatids chromatidChiasmata (chiasma) crossing over During Crossing over segments of nonsister chromatids break and reattach to the other chromatid. The Chiasmata (chiasma) are the sites of crossing over. Crossing Over is one of the Two major occurrences of Meiosis (The other is Non-disjunction)

Prophase I centrioles spindle fiber aster fibers

Metaphase I metaphase plate OR metaphase plate Tetradsmetaphase plate Tetrads align on the metaphase plate. INDEPENDENT ASSORTMENT OCCURS: INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation

Anaphase I Homologous chromosomes Homologous chromosomes separate and move towards the poles. Sister chromatids centromeres Sister chromatids remain attached at their centromeres.

Telophase I haploid chromosomes Each pole now has haploid set of chromosomes. Cytokinesis Cytokinesis occurs and two haploid daughter cells are formed.

Telophase I

Meiosis II No interphase II No interphase II DNA replication (or very short - no more DNA replication) Meiosis II mitosis Meiosis II is similar to mitosis

Prophase II prophasemitosis same as prophase in mitosis

Metaphase II metaphasemitosis same as metaphase in mitosis metaphase plate

Anaphase II anaphasemitosis same as anaphase in mitosis sister chromatids separate sister chromatids separate

Telophase II telophasemitosis Same as telophase in mitosis. Nuclei form. Cytokinesis Cytokinesis occurs. Remember:four haploid daughter cells produced. Remember:four haploid daughter cells produced. gametes = sperm or egg

Telophase II

Compare Mitosis & Meiosis Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells

Meiosis in Males = Spermatogenesis 2n=46 human sex cell diploid (2n) n=23 meiosis I n=23 sperm haploid (n) meiosis II 4 hapliod gametes or sperm cells are produced from each primary spermatocyte. Primary Spermatocyte Secondary Spermatocyte

Meiosis in Females = Oogenesis *** The polar bodies (do not participate in reproduction) die… only one haploid gamete or ovum (egg) is produced from each primary oocyte.

During Ovulation the ovum is released from the ovary and transported to an area where fertilization, the joining of the sperm and ovum, can occur…… fertilization, in Humans, occurs in the Fallopian tube. Fertilization results in the formation of the Zygote. (fertilized egg) Sperm + Ovum (egg) Zygote fertilization Fertilization

Fertilization Fertilization (continued) spermegg zygote The fusion of a sperm and egg to form a zygote. A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote

Question: 20 chromosomes (diploid) chromosomes A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

Answer: 10 chromosomes (haploid) 10 chromosomes (haploid)

Etymology Haploid: haplo- (Gk. HAPLOS, single) + - oid (shape or form) Diploid: Diplo- (Gk. DIPLOOS, double) + -oid (shape or form)