Meiosis Notes 4 Cell division to form the gametes, sperm (male gamete) and egg (female gamete). 4 Normal cells are diploid: 2 copies of every gene. 4 Gametes are haploid: 1 copy of every gene

Number of Chromosomes

Overview of Meiosis 4 2 cell divisions. 4 Starts with 2 copies of each chromosome (homologous), each with 2 chromatids (copies of DNA). 4 In meiosis I, homologous chromosomes are separated into 2 cells. 4 In Meiosis II the chromatids are separated into 4 cells.

Meiosis I (PMAT I) 4 PROPHASE I - The homologous chromosomes pair together (Sometimes crossing over occurs). 4 METAPHASE I - The pairs of homologous chromosomes line up along the middle 4 ANAPHASE I - Homologous chromosomes are pulled apart. 4 TELOPHASE I - One cell becomes two cells with one chromosome of the pair (haploid)

Meiosis II (PMAT II) 4 PROPHASE II – Prepare to divide 4 METAPHASE II – Chromosomes line up in the middle 4 ANAPHASE II – Chromatids (copies of DNA) pull apart 4 TELOPHASE II – The end result is four cells with one copy of each gene.

Interphase Mother cell Stages Of Meiosis: Meiosis I Meiosis II Prophase I: Tetrad formation/ crossing over Prophase I: Tetrad formation/ crossing over Metaphase I Telophase I Prophase I: Condensing Chromosomes Prophase I: Condensing Chromosomes Anaphase I

Telophase I Stages Of Meiosis: Meiosis II Metaphase II Anaphase II Telophase II The products of meiosis are 4 haploid cells each with a unique set of chromosomes. Prophase II

Segregation 4 In humans meiosis starts with one cell containing 46 chromosomes (23 pairs) and results in four cells containing 23 chromosomes. 4 The copies of DNA are separated when gametes are formed.

Independent Assortment 4 Homologous chromosomes are positioned randomly so any copy can be passed to the gametes with any combination of other chromosomes 4 There are 2 n combinations possible during meiosis with n the haploid number of chromosomes for the organism

How many combinations are possible in human meiosis? 4 Possible combinations: 2 n 4 n=23 in humans =about 8,300,000 combinations

Crossing Over 4 During Prophase I, the exchange of genetic material between homologous chromosomes

Prophase I: Tetrad formation/ crossing over Prophase I: Tetrad formation/ crossing over Crossing Over Anaphase I Telophase II Metaphase I Telophase I Because of crossing over, every gamete receives a unique set of genetic information.

Fertilization 4 The combination of a sperm and an egg which forms a zygote. 4 1 sperm (1 of 8 million possible chromosome combinations) x 1 ovum (1 of 8 million different possibilities) = 64 trillion diploid combinations!

Egg Haploid nucleus Fertilization Results In A Diploid Zygote Sperm Haploid nucleus

Sperm Fertilization Results In A Diploid Zygote Egg Haploid nucleus Haploid nucleus

Sperm Fertilization Results In A Diploid Zygote Egg Haploid nucleus Haploid nucleus

Sperm Fertilization Results In A Diploid Zygote Egg Haploid nucleus Haploid nucleus

From Zygote to Embryo Zygote 2n Zygote Diploid

Mitosis From Zygote to Embryo

Mitosis From Zygote to Embryo

Mitosis From Zygote to Embryo

Mitosis From Zygote to Embryo

Twins

Monozygotic Twins (Identical) 4 1/3 of all twins are identical 4 Twins that form from one zygote (one egg fertilized by one sperm). 4 These twins have identical genes and must be the same sex. 4 Having identical twins is random, not genetic

Dizygotic Twins (fraternal) 4 2/3 of all twins are fraternal 4 Twins that form from two zygotes (two eggs fertilized by two sperm) 4 Can be the same sex or different sexes. 4 The ability to have fraternal twins is thought to be genetic.

Phenotype The way an ORGANISM LOOKS

Genotype THE GENETIC MAKEUP OF AN ORGANISM

Dominant and Recessive Traits

Dominant Traits Characteristics you can always see if you have the genes.

Symbol 4 capital letters

Recessive Traits Characteristics you cannot see if you have a dominant gene.

Symbol 4 lower case letters