Chromosomal Inheritance Chromosomal Division for Sperm and Egg Production Chromosomal Abnormalities

Human Life Cycle Diploid number of chromosomes, two full sets Nuclear division retaining the original chromosome number Human Life Cycle Nuclear division reducing the chromosome number, leading to sperm or eggs Haploid number of chromosomes, one full set

Cell Cycle With Meiosis Interphase includes G1 = growth phase 1 S = synthesis phase G2 = growth phase 2 Meiosis cytokinesis G2 synthesis of proteins and assembly of components for division Meiosis I Meiosis II cytokinesis Meiosis includes two chromosomal divisions Meiosis I Meiosis II Cytokinesis = division of cytoplasmic contents S chromosomes duplicate G1 cytoplasm doubles, including organelles Interphase

Applying Your Knowledge G1 phase S phase G2 phase Meiosis I and II When does chromosome duplication occur? When do the cytoplasmic contents double? When does chromosomal division occur? 2 1 4

Homologous Chromosome Pairs Diploid cells have pairs of chromosomes Each homologous chromosome pair is genetically matched so that the genes are arranged in the same order Meiosis distributes members of each pair to separate nuclei Figure: 11.UN06b Title: Formation of daughter chromosomes Caption: Humans have 1 pair of sex chromosomes and 22 pairs of autosomes

Outcome of Meiotic Divisions Diploid Human Cell with 46 chromosomes (23 pairs) Duplication of Cell Contents Meiosis I: Reduction Division Separation of Homologous Chromosomes into Different Nuclei 2 Genetically-Different Haploid Human Cells with 23 duplicated chromosomes Meiosis II: Equational Division Separation of Duplicated Chromosomes into Different Nuclei 4 Genetically-Different Haploid Human Cells with 23 chromosomes

Duplicated Chromosomes in Early Meiosis Homologous Chromosome Pair Duplicated chromosomes joined at their centromeres Nonsister Chromatids are joined to different centromeres Centromere = constricted region Association of four chromatids of a duplicated homologous chromosome pair

Prophase I of Meiosis I Chromosomes thicken Genetic exchange occurs between nonsister chromatids through crossing-over

Metaphase I of Meiosis I Spindle fiber Homologous pairs line up at the cell equator Spindle fibers attached to centromeres cause chromosomal movement

Anaphase I and Telophase I of Meiosis I Members of homologous pair separate and are pulled to opposite poles of the cell Telophase I Chromosomes reach opposite poles + cytokinesis Cytokinesis Cytoplasm divides into two separate cells Daughter cells are haploid with duplicated chromosomes

Prophase II and Metaphase II of Meiosis II Chromosomes thicken Metaphase II Chromosomes line up at cell equator

Anaphase II and Telophase II of Meiosis II Sister Chromatids separate and move to opposite poles Telophase II Chromosomes have reached opposite poles + cytokinesis Cytokinesis Cytoplasm divides into separate cells Daughter cells are haploid and have genetic differences

Applying Your Knowledge Prophase I Metaphase I Anaphase I Metaphase II Anaphase II When do sister chromatids separate? When does crossing-over occur? When do homologous pairs line up at the cell equator? 5 1 2

Fertilization occurs after Meiosis I and stimulates Meiosis II Sperm formation Equal divisions of cytoplasm Four functional products per meiosis Egg formation Unequal divisions of cytoplasm Small polar bodies formed One functional product per meiosis Fertilization occurs after Meiosis I and stimulates Meiosis II

Sex Determination Sex Chromosomes: homologous chromosomes that differ in size and genetic composition between males and females Human Chromosome X Y Size Larger Smaller Genetic Composition Multiple genes unrelated to gender Very few genes TDF region determines maleness

Chromosomal Number Abnormalities Arise by Nondisjunction = Failure of homologous chromosomes or chromatids to separate during meiosis Nondisjunction during Meiosis I Nondisjunction during Meiosis II

Human Autosomal Abnormality Down Syndrome Trisomy 21 Three copies of chromosome 21 How can Down Syndrome occur? Eg. Egg with 2 copies of #21 (24 chromosomes) + Sperm with 1 copy of #21 (23 chromosomes) = Embryo with 3 copies of #21 (47 chromosomes)

Karyotype for Down Syndrome Physical Features Eye fold Palm Crease

Human Sex Chromosome Abnormality Turner Syndrome XO One copy of X No second sex chromosome How can Turner Syndrome occur? Eg. Egg with 0 copies of X (22 chromosomes) +Sperm with 1 copy of X (23 chromosomes) = Embryo with 1 copy of X (45 chromosomes)

Karyotype for Turner’s Syndrome Non-functional Ovaries From Adult Female with Turner’s Syndrome Normal uterus, tubes and ovaries

Other Human Sex Chromosome Number Abnormalities Klinefelter Syndrome XXY Poly-X Females XXX Jacobs Syndrome XYY Sterile Male Fertile Female Fertile Male

Chromosome Structure Abnormalities Williams syndrome is caused by a deletion = loss of a chromosomal segment.

Chromosome Structure Abnormalities Alagille syndrome is caused by a translocation = movement of a chromosomal segment to another non-homologous chromosome.

Applying Your Knowledge Crossing-over Nondisjunction Autosomes Sex chromosomes Translocation What is the name for the X and Y chromosomes? Which one describes the failure of chromatids to separate in Anaphase II? Which one involves exchange of material between nonsister chromatids? 4 2 1