CHROMOSOMES AND MEIOSIS Chromosomes DNA coils around histones

CHROMOSOMES AND MEIOSIS

Chromosomes ◦ DNA coils around histones to form nucleosomes, which coil to form chromatin fibers ◦ The chromatin fibers supercoil to form chromosomes that are visible in the metaphase stage of mitosis. ◦ Formed during prophase

Chromosomes and Chromosome Numbers ◦ Human body cells have 46 chromosomes ◦ Each parent contributes 23 chromosomes ◦ Homologous chromosomes—one of two paired chromosomes, one from each parent ◦ Same length ◦ Same centromere position ◦ Carry genes that control the same inherited traits

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 2 n chromosomes is called a diploid cell.

Meiosis ◦ The sexual life cycle in animals involves meiosis. ◦ Meiosis produces gametes. ◦ When gametes combine in fertilization, the number of chromosomes is restored.

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

Stages of Meiosis ◦ Interphase ◦ Chromosomes replicate ◦ Chromatin condenses ◦ Prophase I ◦ Metaphase I ◦ Anaphase I ◦ Telophase I ◦ Prophase II ◦ Metaphase II ◦ Anaphase II ◦ Telophase II ◦ Cytokinesis

Interphase ◦ Chromosomes replicate ◦ Chromatin condenses Prophase I o Pairing of homologous chromosomes occurs o Each chromosome consists of two chromatids o The nuclear envelope breaks down. o Spindles form o Crossing over occurs

Crossing Over in Prophase I ◦ Crossing over produces exchange of genetic information ◦ Crossing over—chromosomal segments (Genes) are exchanged between a pair of homologous chromosomes

Metaphase I ◦ Chromosome centromeres attach to spindle fibers ◦ A diploid number (46) of homologous chromosomes line up at the equator (center of the cell) Anaphase I o Homologous chromosomes separate and move to opposite poles (ends) of the cell

Telophase I ◦ The spindles break down ◦ Nuclear envelope reforms ◦ Chromosomes uncoil and form two nuclei ◦ The cell divides

Prophase II ◦ A second set of phases begins as the spindle apparatus forms and the chromosomes condense ◦ Nuclear envelope breaks down Metaphase II o Spindles attach to centromere of each sister chromatid o A haploid (23) number of chromosomes line up at the equator

Anaphase II o. The sister chromatids are pulled apart at the centromere by spindle fibers o. Sister chromatids move toward the opposite poles of the cell Telophase II o The chromosomes reach the poles o The nuclear envelope reform o 4 nuclei are created o Spindle fibers break down

Cytokinesis ◦ Cytokinesis results in four haploid cells, each with n number of chromosomes ◦ Cells divide into 4 haploid daughter cells ◦ They are not identical ◦ Can be sperm or egg cells, depending on the gender of the diploid original cell

Meiosis Summary ◦ Meiosis consists of two sets of divisions ◦ Produces four haploid daughter cells that are not identical ◦ Results in genetic variation