12/26/2015 Sexual vs. Asexual Reproduction MEIOSIS

12/26/2015 Asexual Reproduction The new organism has a single parent, the purpose is to create genetically identical individuals rapidly. The new organism has a single parent, the purpose is to create genetically identical individuals rapidly. Mitosis is an example of asexual reproduction, specifically binary fission (dividing in half after replicating DNA). Mitosis is an example of asexual reproduction, specifically binary fission (dividing in half after replicating DNA). Other types are budding and vegetative reproduction. Other types are budding and vegetative reproduction.

12/26/2015 Sexual Reproduction Two cells from different parents unite to form the first cell of the new individual, increase genetic variability. Two cells from different parents unite to form the first cell of the new individual, increase genetic variability. Examples of organisms that reproduce sexually Examples of organisms that reproduce sexually Humans Humans Plants Plants Dolphins Dolphins Snakes Snakes So how does sexual reproduction occur? So how does sexual reproduction occur?

12/26/2015 MEIOSIS The second type of cell division The second type of cell division Takes place in the gonads (ovaries, testes) to produce gametes (sperm, eggs) Takes place in the gonads (ovaries, testes) to produce gametes (sperm, eggs) Gametes have half of the chromosomes of the all other body cells (called somatic cells) Gametes have half of the chromosomes of the all other body cells (called somatic cells) For humans: somatic cells – 46 chromosomes; gametes – 23 chromosomes For humans: somatic cells – 46 chromosomes; gametes – 23 chromosomes

12/26/2015 MEIOSIS Somatic cells are called diploid cells (2n); gametes are called haploid cells (n) Somatic cells are called diploid cells (2n); gametes are called haploid cells (n) Meiosis ensures that new organisms have the correct number of chromosomes Meiosis ensures that new organisms have the correct number of chromosomes Upon fertilization, the normal chromosome number (diploid) is restored Upon fertilization, the normal chromosome number (diploid) is restored The zygote formed by fertilization divides by mitosis and grows to be a new organism. The zygote formed by fertilization divides by mitosis and grows to be a new organism.

12/26/2015 LIFE CYCLE

12/26/2015 MEIOSIS Consists of two stages, meiosis I and meiosis II Consists of two stages, meiosis I and meiosis II In diploid cells chromosomes occur as homologous chromosomes In diploid cells chromosomes occur as homologous chromosomes Chromosomes that have similar genes Chromosomes that have similar genes Meiosis I begins with homologous chromosomes composed of 2 chromatids (tetrads) Why? Meiosis I begins with homologous chromosomes composed of 2 chromatids (tetrads) Why?

12/26/2015 HOMOLOGOUS CHROMOSOMES

12/26/2015 PROPHASE I What is happening?

12/26/2015 PROPHASE I Chromatin coils and thickens into chromosomes Chromatin coils and thickens into chromosomes Nucleoli disappear and the nuclear membrane is disassembled Nucleoli disappear and the nuclear membrane is disassembled Spindle fibers form from centrioles Spindle fibers form from centrioles Homologous chromosomes synapse (pair up) with one another forming a tetrad Homologous chromosomes synapse (pair up) with one another forming a tetrad Crossing-over may occur – exchange of DNA Crossing-over may occur – exchange of DNA

12/26/2015 METAPHASE I What is happening?

12/26/2015 METAPHASE I Tetrad moves to the equatorial plane (middle) Tetrad moves to the equatorial plane (middle) Chromosomes become attached to the spindle fibers at the centromere Chromosomes become attached to the spindle fibers at the centromere Homologous chromosomes line up randomly in a process known as independent assortment Homologous chromosomes line up randomly in a process known as independent assortment

12/26/2015 ANAPHASE I What is happening?

12/26/2015 ANAPHASE I Chromosome number is reduced from diploid to haploid when homologous chromosomes split and move to opposite poles – called segregation Chromosome number is reduced from diploid to haploid when homologous chromosomes split and move to opposite poles – called segregation Each chromosome is independently attached to a spindle at the centromere Each chromosome is independently attached to a spindle at the centromere The centromeres do not split – each chromosomes still consists of two chromatids The centromeres do not split – each chromosomes still consists of two chromatids

12/26/2015 MEIOSIS I What is happening?

12/26/2015 TELOPHASE I Chromosomes uncoil and become long, thin threads Chromosomes uncoil and become long, thin threads The nuclear membrane reforms around the chromosomes The nuclear membrane reforms around the chromosomes Nucleoli reappear Nucleoli reappear Cytokinesis divides the cell into two daughter cells. Cytokinesis divides the cell into two daughter cells. Each daughter cell has one member of the original homologous chromosome pair Each daughter cell has one member of the original homologous chromosome pair

12/26/2015 MEIOSIS II The two daughter cells formed in meiosis I go through another division The two daughter cells formed in meiosis I go through another division Events are similar to those in mitosis Events are similar to those in mitosis Chromosomes ARE NOT duplicated before this stage Chromosomes ARE NOT duplicated before this stage

12/26/2015 PROPHASE II What is happening?

12/26/2015 PROPHASE II Chromosomes recoil Chromosomes recoil Nuclear membrane disintegrates Nuclear membrane disintegrates Nucleoli disappear Nucleoli disappear Spindle fibers reform Spindle fibers reform Crossing over, segregation, and independent assortment do not occur during meiosis II Crossing over, segregation, and independent assortment do not occur during meiosis II

12/26/2015 METAPHASE II What is happening?

12/26/2015 METAPHASE II Chromosomes are attached to the spindle by their centromere Chromosomes are attached to the spindle by their centromere The chromosomes move to the equatorial plane (middle) The chromosomes move to the equatorial plane (middle) Pairs of chromosomes are not attached, therefore, each chromosome moves as a separate unit Pairs of chromosomes are not attached, therefore, each chromosome moves as a separate unit

12/26/2015 ANAPHASE II What is happening?

12/26/2015 ANAPHASE II The centromere of each chromosome divides as the chromatids (daughter chromosomes) move to the opposite poles (like mitosis) The centromere of each chromosome divides as the chromatids (daughter chromosomes) move to the opposite poles (like mitosis) No segregation or independent assortment at this stage – very similar to mitosis No segregation or independent assortment at this stage – very similar to mitosis

12/26/2015 MEIOSIS What is happening?

12/26/2015 TELOPHASE II Cytokinesis occurs Cytokinesis occurs Nuclear membrane reappears and spindle fibers disappear Nuclear membrane reappears and spindle fibers disappear Chromosomes uncoil, nuclei re-form Chromosomes uncoil, nuclei re-form Four gametes are formed (sperm or eggs) Four gametes are formed (sperm or eggs) In humans and other organisms only one functional egg is produced. The other three (polar bodies) disintegrate. In humans and other organisms only one functional egg is produced. The other three (polar bodies) disintegrate.

12/26/2015 SOURCES OF VARIATION Genetic variation is influenced by: Genetic variation is influenced by: Mutations Mutations Crossing-over Crossing-over Segregation Segregation Independent assortment Independent assortment Fertilization Fertilization Mutations increase variation by the production of different proteins. Mutations increase variation by the production of different proteins. May or may not be beneficial May or may not be beneficial

12/26/2015 GENETIC VARIATION Crossing-over, which occurs during prophase I, causes genetic information to be exchanged between chromosomes Crossing-over, which occurs during prophase I, causes genetic information to be exchanged between chromosomes Can occur at any point on a chromosome, resulting in new chromosomal combinations Can occur at any point on a chromosome, resulting in new chromosomal combinations Can explain why a child can show a mixture of family characteristics Can explain why a child can show a mixture of family characteristics

12/26/2015 CROSSING OVER

12/26/2015 GENETIC VARIATION Segregation involves the separation and movement of homologous chromosomes to the poles Segregation involves the separation and movement of homologous chromosomes to the poles Occurs during anaphase I Occurs during anaphase I Causes genes to be separated from each other so that they have an equal chance of being transmitted to the next generation. Causes genes to be separated from each other so that they have an equal chance of being transmitted to the next generation. Independent assortment refers to how homologous pairs line up during metaphase I Independent assortment refers to how homologous pairs line up during metaphase I Effects of this arrangement increases as the number of chromosome pairs increase Effects of this arrangement increases as the number of chromosome pairs increase

12/26/2015 GENETIC VARIATION Effects of independent assortment can be explained using a mathematical equation: Effects of independent assortment can be explained using a mathematical equation: 2n, where n is the number of pairs of chromosomes. 2n, where n is the number of pairs of chromosomes. If n=3, 23=8 possibilities If n=3, 23=8 possibilities If n=23, 223 = 8,388,608 possibilities! If n=23, 223 = 8,388,608 possibilities! More than 8 million kinds of sperm or egg cells are possible resulting from independent assortment alone! More than 8 million kinds of sperm or egg cells are possible resulting from independent assortment alone!

12/26/2015 FERTILIZATION As a result of all genetic variation mechanisms, the number of different types of possible offspring is large. As a result of all genetic variation mechanisms, the number of different types of possible offspring is large. Because of these, every human being is genetically unique. Because of these, every human being is genetically unique. Except for identical twins (identical DNA) Except for identical twins (identical DNA)

12/26/2015 NONDISJUNCTION Normally during meiosis, diploid cells separate and create haploid cells Normally during meiosis, diploid cells separate and create haploid cells Occasionally, a pair of homologous chromosomes remain attached and end up on the same gamete Occasionally, a pair of homologous chromosomes remain attached and end up on the same gamete Known as nondisjunction Known as nondisjunction Usually causes cell death Usually causes cell death

12/26/2015 NONDISJUNCTION

NONDISJUNCTION Some cells can survive Some cells can survive If a sperm or egg carrying the wrong number of chromosomes fertilizes another gamete, the offspring will show abnormalities If a sperm or egg carrying the wrong number of chromosomes fertilizes another gamete, the offspring will show abnormalities One example is Down syndrome One example is Down syndrome Affected individuals carry an extra chromosome 21. Affected individuals carry an extra chromosome 21.

12/26/2015 SEX DETERMINATION In many organisms sex-determining genes are located on specific chromosomes called sex chromosomes. In many organisms sex-determining genes are located on specific chromosomes called sex chromosomes. In humans, males have XY and females have XX In humans, males have XY and females have XX All other chromosomes are known as autosomal chromosomes. All other chromosomes are known as autosomal chromosomes. Genes that determine male characteristics are located on the Y chromosome. Genes that determine male characteristics are located on the Y chromosome. X chromosomes have genes necessary for the survival of both males and females X chromosomes have genes necessary for the survival of both males and females

12/26/2015 Mitosis vs. Meiosis Two divisions of nucleusOne division of nucleus Produces 4 daughter cellsProduces 2 daughter cells Haploid cells – nDiploid cells – 2n Occurs in GametesOccurs in body cells MeiosisMeiosis MitosisMitosis