Presentation on theme: "SEXUAL REPRODUCTION Sex cells formed by meiosis Requires 2 parents Offspring not identical The process that results in the formation of gametes Sex cells,"— Presentation transcript:
SEXUAL REPRODUCTION Sex cells formed by meiosis Requires 2 parents Offspring not identical The process that results in the formation of gametes Sex cells, egg or sperm, which have haploid chromosomes Haploid means HALF, as in the number of chromosomes, written as n Meiosis Gametes Haploid
Fertilization The union of an egg and a sperm The cell that is formed when the egg becomes fertilized by the sperm. This results in a full set of chromosomes. All body cells except for sex cells that have diploid chromosomes Diploid means they have chromosome found in pairs, written as 2n n n 2n Zygote Somatic Cells Diploid
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 KKKK KKKK 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 1 st division - pairs split 2 nd division – produces 4 gamete cells with ½ the original no. of chromosomes
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 2 n distinct gametes, where n = the number of unique chromosomes. That’s a lot of diversity by this mechanism alone. In humans, n = 23 and 2 23 = 6,000,0000.
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.
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
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 21 st pair – producing a child with Downs Syndrome
21 trisomy – Downs Syndrome Can you see the extra 21 st chromosome? Is this person male or female?