Meiosis and Sexual Reproduction

Parents can produce many types of offspring Families have resemblances, but no two are alike

How to Make Offspring When sperm and egg combine, they form a zygote (offspring). The zygote gets half of its chromosomes from Mom and half from Dad. Sperm & Egg Zygote

Chromosomes Every human cell has 23 pairs of chromosomes. 1 set from Mom and 1 set from Dad

Homologous Chromosomes Chromosome pairs, one from each parent, that pair up during meiosis. These pairs have the same genes in the same location, or loci.

Diploid (2n) vs. Haploid (1n) Diploid - Cells with 2 copies of each chromosome. In humans, all cells except sex cells (gametes) Haploid - Cells with only 1 copy of each chromosome. In humans, only sex cells (gametes)

Meiosis Making Sperm and Eggs Meiosis occurs in sexual reproduction when a diploid (2n) germ cell produces 4 haploid (1n) daughter cells that can mature to become gametes (sperm or egg)

Goal of Meiosis Goal: Reduce genetic material by 1/2. from mom from dad child meiosis reduces genetic content too much! Just right! Why? 1n (Mom) + 1n (Dad) = 2n (offspring)

2 Phases of Meiosis Meiosis I - Homologous chromosomes separate Meiosis II - sister chromatids separate

Chromatids vs. Homologous Chromosomes Sister chromatids are 2 duplicates of a chromosome made during DNA replication. Homologous chromosomes are a pair of the same chromosome, one from Mom, one from Dad. Homologous chromosomes

Difference Between Mitosis and Meiosis YouTube Video Difference Between Mitosis and Meiosis

Meiosis I - Prophase I Chromatids pair and condense Crossing over occurs Spindle forms Nuclear envelope fragments

Crossing Over Crossing over occurs during Prophase I when parts of homologous chromosomes overlap. Crossing over increases genetic variation.

Meiosis I - Metaphase I Chromosome pairs align along the equator of the cell.

Meiosis I - Anaphase I Chromosomes separate and move to opposite poles. Sister chromatids remain attached at their centromeres.

Meiosis I - Telophase I Nuclear envelope reassembles Spindle disappears Cytokinesis divides cell into two

Meiosis II Sister chromatids carry identical genetic information. Meiosis II produces gametes with 1 copy of each chromosome and thus one copy of each gene.

Meiosis II - Prophase II Nuclear envelope fragments Spindle forms

Meiosis II - Metaphase II Chromosomes align along equator of cell

Meiosis II - Anaphase II Sister chromatids separate and move to opposite poles.

Meiosis II - Telophase II Nuclear envelope assembles Chromosomes unravel Spindles disappear Cytokinesis divides cell into 2

Results of Meiosis 4 haploid cells One copy of each chromosome

Meiosis in Males and Females X X Y Y X

Fertilization Meiosis allows the creation of unique individuals through sexual reproduction.

Comparison of Mitosis and Meiosis # of divisions 1 2 # of daughter cells 4 Genetically identical yes no Chromosome # Same as parent Half of parent Where Somatic cells Sex cells When Throughout life At sexual maturity Role Growth and repair Sexual reproduction

YouTube Video Meiosis

Mitosis Meiosis Occurs in somatic (body) cells Occurs only in gonads (sex organs): Ovary/Testes Produces cells for repair, maintenance, growth, asexual reproduction Only produces gametes (sex cells: egg & sperm) Results in identical diploid (2n) daughter cells Reduction division results in haploid (n) cells

Mistakes in Meiosis Non-disjunction = The failure of homologous chromosomes to separate properly during meiosis.

Mistakes in Meiosis Trisomy: When a gamete with extra chromosomes is fertilized by a normal gamete, the zygote will have an extra chromosome. Monosomy = When a gamete that is missing a chromosome fuses with a normal gamete during fertilization, the resulting zygote lacks a chromosome. Polyploidy = organisms with more than the usual number of chromosome sets.

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Chromosomes Chromosomes are made of two identical sister chromatids, which are joined at the centromere.