Meiosis

Chromosomes and Chromosome Number Sexual Reproduction and Genetics Meiosis Chromosomes and Chromosome Number Human body cells have 46 chromosomes Each parent contributes 23 chromosomes Homologous chromosomes—one of two paired chromosomes, one from each parent

Chromosomes and Chromosome Number Sexual Reproduction and Genetics Chromosomes and Chromosome Number Same length Same centromere position Carry genes that control the same inherited traits

Haploid and Diploid Cells Sexual Reproduction and Genetics Haploid and Diploid Cells An organism produces gametes to maintain the same number of chromosomes from generation to generation. Human gametes contain 23 chromosomes. A cell with n chromosomes is called a haploid cell. A cell that contains 2n chromosomes is called a diploid cell.

The sexual life cycle in animals involves meiosis. Sexual Reproduction and Genetics Meiosis I The sexual life cycle in animals involves meiosis. Meiosis produces gametes. When gametes combine in fertilization, the number of chromosomes is restored.

Sexual Reproduction and Genetics Stages of Meiosis Reduces the chromosome number by half through the separation of homologous chromosomes Involves two consecutive cell divisions called meiosis I and meiosis II

Chromosomes replicate. Sexual Reproduction and Genetics Meiosis I Interphase Chromosomes replicate. Chromatin condenses. Interphase

Pairing of homologous chromosomes occurs. Sexual Reproduction and Genetics Meiosis I Prophase I Pairing of homologous chromosomes occurs. Each chromosome consists of two chromatids. Prophase I The nuclear envelope breaks down. Spindles form.

Crossing over produces exchange of genetic information. Sexual Reproduction and Genetics Meiosis I Prophase I Crossing over produces exchange of genetic information. Crossing over—chromosomal segments are exchanged between a pair of homologous chromosomes.

Chromosome centromeres attach to spindle fibers. Sexual Reproduction and Genetics Meiosis I Metaphase I Chromosome centromeres attach to spindle fibers. Metaphase I Homologous chromosomes line up at the equator.

Homologous chromosomes separate and move Sexual Reproduction and Genetics Meiosis I Anaphase I Homologous chromosomes separate and move to opposite poles of the cell. Anaphase I

The spindles break down. Sexual Reproduction and Genetics Meiosis I Telophase I The spindles break down. Telophase I Chromosomes uncoil and form two nuclei. The cell divides.

A second set of phases begins Sexual Reproduction and Genetics Meiosis II Prophase II A second set of phases begins as the spindle apparatus forms and the chromosomes condense. Prophase II

A haploid number of chromosomes Sexual Reproduction and Genetics Meiosis II Metaphase II A haploid number of chromosomes line up at the equator. Metaphase II

The sister chromatids are Sexual Reproduction and Genetics Meiosis II Anaphase II The sister chromatids are pulled apart at the centromere by spindle fibers and move toward the opposite poles of the cell. Anaphase II

The chromosomes reach the poles, and Sexual Reproduction and Genetics Meiosis II Telophase II The chromosomes reach the poles, and the nuclear membrane and nuclei reform. Telophase II

Sexual Reproduction and Genetics Meiosis II Cytokinesis results in four haploid cells, each with n number of chromosomes. Cytokinesis

Meiosis Provides Variation Sexual Reproduction and Genetics Meiosis Provides Variation Depending on how the chromosomes line up at the equator, four gametes with four different combinations of chromosomes can result. Genetic variation also is produced during crossing over and during fertilization, when gametes randomly combine.

Meiosis the process of cellular division where the number of the chromosomes is halved. produces four daughter cells all daughter cells are haploid chromosomes are shuffled in the process, so that each daughter cell has a unique combination used to create gametes (sperm and egg)

Beginning Stages Prophase 1 homologous chromosomes pair up (tetrad) crossing-over occurs Metaphase & Anaphase tetrads line up along the equator, homologs separate

Crossing Over Crossing Over - chromosomes exchange sections (increases variability)

Independent Assortment In addition to Crossing-Over, the process of meiosis ensures that chromosomes are randomly assorted. The following images show three separate possibility for a single cell that has undergone meiosis. Look at all the different combinations.

Sexual Reproduction v. Asexual Reproduction Sexual Reproduction and Genetics Sexual Reproduction v. Asexual Reproduction Asexual reproduction The organism inherits all of its chromosomes from a single parent. The new individual is genetically identical to its parent. Sexual reproduction Beneficial genes multiply faster over time.

Sexual Reproduction Involves the fusion of gametes, creates unique offspring Eggs are much larger than sperm Image of egg and sperm

Sexual Reproduction

Asexual Reproduction --Does not involve the fusion of gametes --Offspring are identical to parents --Budding, Fragmentation (Regeneration), Fission