“He learned all about genetics at school today.”

Chromosomes Always come in pairs (homologous chromosomes) are present in pairs in all body cells except the sex cells

Chromosomes and Numbers Body Cells – Contain two whole sets – Are diploid (2n) – Reproduce by mitosis Sex Cells – Contain one whole set – Are haploid (n) – Are called gametes (sperm or egg)

Meiosis fertilization is the process in which two haploid cells come together to form a unique diploid This is the premise of sexual reproduction

Fertilization Zygote – Result of the fusion of two sex cells – Diploid (2n) – Cell division occurs by mitosis Sperm (n) + egg (n) = zygote (2n)

Why Meiosis Ensures variation Ensures the correct number of chromosomes to each new organism in species that reproduce sexually

meiosis animation

Compare & Identify

Meiosis vs. Mitosis Mitosis: period of nuclear division in which two daughter cells are formed, each identical to the parent cell

Meiosis vs. Mitosis Meiosis: type of division in which four cells are formed, each with half the number of chromosomes as the parent cell

Meiosis Interphase: stage in which chromosomes duplicate themselves (same as mitosis) Meiosis is then separated into two division stages Meiosis I and Meiosis II

Meiosis I Prophase I – Chromosomes coil and spindle forms – Each pair of homologous chromosomes and their sister chromatids come together to form a four- part tetrad joined together by a centromere

Meiosis I Prophase I – During this phase, an exchange of genetic material may occur between two non-sister chromatids (crossing over) – Crossing over allows for more variation in sexual reproduction

Genetic Recombination: the re-assortment of chromosomes and the genetic information they carry, providing a major source of genetic variation

Meiosis I Metaphase I – Chromosomes become attached to spindle – Tetrads are pulled to the mid-line of the cell

Meiosis I Anaphase I – Homologous chromosomes, along with their sister chromatids, separate

Meiosis I Telophase I – Spindle breaks down – Chromosomes uncoil – Cytoplasm divides to form two new cells * Each cell has half the genetic information as the original parent cell, BUT the chromosomes are still doubled so another division must take place

Meiosis II Prophase II – A spindle forms and the chromosomes in each cell attach – Sister chromatids are pulled to the center of the cell

Meiosis II Metaphase II – Sister chromatids line up on the mid-line of the cells

Meiosis II Anaphase II – The centromere breaks allowing the sister chromatids to migrate to opposite ends the cells

Meiosis II Telophase II – Nuclei reform – Spindles break down – Cytoplasm splits

How much variation? “n”: The number of homologous pairs in a cell Human body cell contains 46 chromosomes (23 homologous pairs) 2n=46 Human sperm cell contains 23 chromosomes (one representative from each homologous pair) n=23 Human egg cell (same as sperm) n=23

WOW!!! Possible different kinds of egg or sperm cells created by a person = 2²³ or 8 million Fertilization of one egg by one sperm results in 2²³ x 2²³ or 70 trillion possible zygote combinations * This does not take into consideration possible crossovers that occur during prophase I

End Result---We are ALL truly unique

Can our bodies make mistakes during this process? Nondisjunction The failure of chromosomes to separate properly during meiosis

Comparing Mitosis and Meiosis MitosisMeiosis Number of daughter cells produced? Daughter cells haploid or diploid? Daughter cells identical? Number of copies of same chromosome given to each gamete