Meiosis and Sexual Reproduction

SEXUAL REPRODUCTION Sex cells formed by meiosis Requires 2 parents Offspring not identical Meiosis The process that results in the formation of gametes Gametes Sex cells, egg or sperm, which have haploid chromosomes Haploid Haploid means HALF, as in the number of chromosomes, written as n

The union of an egg and a sperm Fertilization n 2n The union of an egg and a sperm Zygote The cell that is formed when the egg becomes fertilized by the sperm. This results in a full set of chromosomes. Somatic Cells All body cells except for sex cells that have diploid chromosomes Diploid Diploid means they have chromosome found in pairs, written as 2n

Homologous Chromosomes Similar chromosomes that are found in pairs. The paired chromosomes come from the mother and father. * Human body cells have 46 chromosomes each * Human body cells have 23 homologous pairs Meiosis and Fertilization Important for survival of many species, because these processes result in genetic variation of offspring. Meiosis A kind of cell division that results in gametes (sex cells) with half the number of chromosomes.

A. Gametes do not have homologous pairs, but instead they have only half the chromosomes from each pair. Gametes are referred to as haploid (n). B. When fertilization occurs, the zygote will have a normal number of chromosomes (2n). C. In meiosis, specialized cells divide 2 times producing 4 daughter cells.

Phases of Meiosis (2 Cell divisions – Meiosis I & Meiosis II) MEIOSIS I Prophase I Homologous chromosomes pair together to form tetrads During Prophase I, crossing over occurs between homologous chromosomes and genes are exchanged causing genetic variation

Metaphase I Spindle fibers attach to chromosomes and line them up in the middle of the cell. Anaphase I The fibers pull the homologous chromosomes apart. This is called disjunction. Chromosomes move toward opposite sides of the cell.

Telophase I During Telophase I and cytokinesis, the nuclear membrane forms and the cell splits into two new cells. **** The Chromosome number is reduced to half during Meiosis I forming haploid (n) daughter cells

MEIOSIS II Prophase II Spindle fibers form and attach to double stranded chromosomes Metaphase II Chromosomes line up in the middle of the cell

Anaphase II Sister chromatids separate and move towards opposite sides of the cell Telophase II Cells divide, resulting in 4 haploid cells – gametes (sex cells)

Males form 4 haploid cells which develop into 4 sperm K Females form 4 haploid cells which develop into 1 egg and 3 small polar bodies Egg Polar Bodies Small polar bodies are not used and break down.

Meiosis Parent cell – chromosome pair Chromosomes copied 1st division - pairs split 2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

Meiosis – mouse testes Parent cell 1st division 2nd division 4 gametes

Figure 13.7 The stages of meiotic cell division: Meiosis I

Figure 13.7 The stages of meiotic cell division: Meiosis II

One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes. In humans, n = 23 and 223 = 6,000,0000. That’s a lot of diversity by this mechanism alone.

Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

Mitosis vs. Meiosis

The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis Mitosis The first (and distinguishing) division of meiosis

Mitosis vs. Meiosis

Boy or Girl? The Y Chromosome “Decides” X chromosome Y chromosome

Boy or Girl? The Y Chromosome “Decides”

Meiosis – division error Chromosome pair

Meiosis error - fertilization Should the gamete with the chromosome pair be fertilized then the offspring will not be ‘normal’. In humans this often occurs with the 21st pair – producing a child with Downs Syndrome

21 trisomy – Downs Syndrome Can you see the extra 21st chromosome? Is this person male or female?