Patterns of Chromosome Inheritance

Points to Ponder What is the structure of chromosomes? What is the cell cycle and what occurs during each of its stages? Explain what mitosis is used for, in what cells and the 4 stages. Explain the 2 divisions of meiosis. What is meiosis used for and in what cells? Compare and contrast mitosis and meiosis. Compare and contrast spermatogenesis and oogenesis. What are trisomy and monosomy? What most often causes these changes in chromosome number? What are the syndromes associated with changes in sex chromosomes? Explain the 4 changes in chromosome structure.

18.1 Chromosomes and cell cycle Chromosomes: A review Humans have 46 chromosomes that are in 23 pairs within a cell’s nucleus Pairs of chromosomes are called homologous chromosomes Autosomes are the 22 pairs of chromosomes that control traits that do not relate to gender of an individual Sex chromosomes are the 1 pair that contain the genes that do control gender Cells (body cells) that have 46 (2N) chromosomes are called diploid Cells (sex cells) that have only 23 (N) chromosomes not in pairs are called haploid

© CNRI/SPL/Photo Researchers 18.1 Chromosomes and cell cycle What is a karyotype? Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. sister chromatids centromere homologous autosome pair Sex chromosomes are not homologous in males. The 46 chromosomes of a male © CNRI/SPL/Photo Researchers

The cell cycle 2 parts: Interphase: 18.1 Chromosomes and cell cycle The cell cycle 2 parts: Interphase: G1 stage – cell doubles its organelles; cell grows in size S stage – DNA replication occurs G2 stage – proteins needed for division are synthesized Cell division (mitosis and cytokinesis): Mitosis – nuclear division Cytokinesis – cytoplasmic division

The cell cycle Interphase S growth and DNA replication G2 G1 18.1 Chromosomes and cell cycle The cell cycle Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Interphase S growth and DNA replication G2 G1 growth and final preparations for division growth M Mitosis Cytokinesis elophase Prophase Anaphase Metaphase T

Chromosome structure in mitosis Chromosomes contain both DNA and proteins (collectively called chromatin) Chromosomes that are dividing are made up of 2 identical parts called sister chromatids The sister chromatids are held together at a region called the centromere Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. duplication duplicated chromosome chromosome division

18.2 Mitosis The spindle in mitosis Centrosome – the microtubule organizing center of the cell Aster – an array of microtubules at the poles (ends of the cell) Centrioles – short cylinders of microtubules that assist in the formation of spindle fibers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. centrioles in centrosome spindle fibers chromosomes aster pole

18.2 Mitosis Overview of mitosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. A diploid cell makes and divides an exact copy of its nucleus Used in cell growth and cell repair Occurs in body cells 4 phases: Prophase Metaphase Anaphase Telophase centrosome centriole chromosome 2n = 4 DNA REPLICATION DURING INTERPHASE duplicated chromosome consisting of two sister chromatids centromere 2n = 4 MITOSIS 2n = 4 2n = 4

1. Mitosis: Prophase Chromosomes condense and become visible Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chromosomes condense and become visible Nuclear envelope fragments Nucleolus disappears Centrosomes move to opposite poles Spindle fibers appear and attach to the centromere aster 20 µm duplicated chromosomes 20 µm nuclear envelope fragments centromere chromatin condenses nucleolus disappears spindle fibers forming Early Prophase Prophase Centrosomes have duplicated. Chromatin is condensing into chromosomes, and the nuclear envelope is fragmenting. Nucleolus has disappeared, and duplicated chromosomes are visible. Centrosomes begin moving apart, and spindle is in process of forming.

18.2 Mitosis 2. Mitosis: Metaphase Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chromosomes line up at the middle of the cell (equator) Fully formed spindle spindle pole 20 µm chromosomes at equator 20 µm centromere centromeric spindle fiber centromeric spindle fiber polar spindle fiber Early Metaphase Metaphase Each chromatid is attached to a spindle fiber. Some spindle fibers stretch from each spindle pole and overlap. Centromeres of duplicated chromosomes are aligned at the equator (center of fully formed spindle). Spindle fibers attached to the sister chromatids come from opposite spindle poles.

18.2 Mitosis 3. Mitosis: Anaphase Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sister chromatids separate at the centromeres and move towards the poles daughter chromosome 20 µm Anaphase Sister chromatids part and become daughter chromosomes that move toward the spindle poles. In this way , each pole receives the same number and kinds of chromosomes as the parental cell.

4. Mitosis:Telophase and cytokinesis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chromosomes arrive at the poles Chromosomes become indistinct chromatin again Nucleoli reappear Spindle disappears Nuclear envelope reassembles Two daughter cells are formed by a ring of actin filaments (cleavage furrow) cleavage furrow 16µm nucleolus Telophase Daughter cells are forming as nuclear envelopes and nucleoli reappear . Chromosomes will become indistinct chromatin.

18.3 Meiosis Overview of meiosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Two nuclear divisions occur to make 4 haploid cells Used to make gametes (egg and sperm) Occurs in sex cells Has 8 phases (4 in each meiosis I & II) centromere homologous chromosome pair nucleolus centrioles homologous chromosome pair 2n = 4 CHROMOSOME DUPLICATION synapsis 2n = 4 sister chromatids MEIOSIS I Duplicated homologous pairs synapse and then separate. MEIOSIS II Sister chromatids separate, becoming daughter chromosomes. n = 2 n = 2

Meiosis I Prophase I Metaphase I Anaphase I Telophase I Homologous chromosomes pair (synapsis) and crossing-over occurs in which there is exchange of genetic information Metaphase I Homologous pairs lined at the equator Anaphase I Homologous chromosomes separate and move towards opposite poles Telophase I 2 daughter cells result each with 23 duplicated chromosomes

18.3 Meiosis What is crossing over? Crossing-over is the exchange of genetic information between nonhomologous sister chromatids during synapsis This occurs during prophase I of meiosis and increases genetic variation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. sister chromatids crossing-over between nonsister chromatids chromatids after exchange recombinant daughter chromosomes synapsis of homologues

Meiosis I 18.3 Meiosis 2n = 4 Animal Cell at Interphase Prophase I Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Animal Cell at Interphase 2n = 4 Prophase I Chromosomes have duplicated. Homologous chromosomes pair during synapsis and crossing-over occurs.

Meiosis II Prophase II Metaphase II Anaphase II Telophase II Chromosomes condense again Metaphase II Chromosomes align at the equator Anaphase II Sister chromatids separate to opposite poles Telophase II 4 daughter cells result each with 23 unduplicated chromosomes

Meiosis II 18.3 Meiosis n = 2 Prophase II Cells have one chromosome Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. n = 2 Prophase II Cells have one chromosome from each homologous pair .

Mitosis vs. Meiosis Formation of gametes Growth and repair of cells 18.4 Comparison of meiosis and mitosis Mitosis vs. Meiosis Formation of gametes Occurs in sex cells 2 divisions Results in 4 haploid genetically different cells Growth and repair of cells Occurs in body cells 1 division Results in 2 diploid genetically identical cells

Comparing mitosis and meiosis 18.4 Comparison of meiosis and mitosis Comparing mitosis and meiosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2n = 4 Prophase I Synapsis and crossing-over occur. Metaphase I Homologous pairs align independently at the equator. Anaphase I Homologous chromosomes separate and move towards the poles. MEIOSIS I 2n = 4 Prophase Metaphase Chromosomes align at the equator. Anaphase Sister chromatids separate and become daughter chromosomes. MITOSIS

Comparing mitosis and meiosis 18.4 Comparison of meiosis and mitosis Comparing mitosis and meiosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Telophase I Daughter cells are forming and will go on to divide again. n = 2 Daughter cells Sister chromatids separate and become daughter chromosomes. Four haploid daughter cells. Their nuclei are genetically different from the parent cell. n = 2 n = 2 MEIOSIS I cont'd MEIOSIS II Daughter cells Telophase Daughter cells are forming. Two diploid daughter cells. Their nuclei are genetically identical to the parent cell. MITOSIS cont'd

The production of sperm and eggs 18.4 Comparison of meiosis and mitosis The production of sperm and eggs Oogenesis Process of making eggs in females During meiosis 1 egg and 3 polar bodies are formed Polar bodies act to hold discarded chromosomes and thus disintegrate Normally 1 egg per month is produced and 500 during the entire reproductive cycle Spermatogenesis Process of making sperm in males A continual process after puberty About 400 million sperm are produced per day

The production of sperm and eggs 18.4 Comparison of meiosis and mitosis The production of sperm and eggs Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. SPERMATOGENESIS primary sper matocyte 2n Meiosis I secondary spermatocytes Meiosis II n spermatids n Metamorphosis and maturation sperm n OOGENESIS primary oocyte 2n Meiosis I first polar body n secondary oocyte n Meiosis II Meiosis II is completed after entry of sperm second polar body n Fertilization cont'd egg sperm nucleus n n fusion of sperm nucleus and egg nucleus zygote 2n

Changes in chromosome number 18.5 Chromosome inheritance Changes in chromosome number Nondisjunction occurs when both members of a homologous pair go into the same daughter cell during meiosis I or when sister chromatids fails to separate in meiosis II. Results of nondisjunction: Monosomy: cell has only 1 copy of a chromosome e.g. Turner syndrome (only one X chromosome) Trisomy: cell has 3 copies of a chromosome e.g. Down syndrome (3 copies of chromosome 21)

Changes in chromosome number 18.5 Chromosome inheritance Changes in chromosome number Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. extra chromosome 21 21 Gart gene a. b. a: Courtesy Chris Burke; b: © CNRI/SPL/Photo Researchers, Inc.

Changes in sex chromosome number 18.5 Chromosome inheritance Changes in sex chromosome number Turner syndrome (X) – short stature, broad shouldered with folds of skin on the neck, underdeveloped sex organs, no breasts Klinefelter syndrome (XXY) – underdeveloped sex organs, breast development, large hands and long arms and legs Poly-X female (XXX, XXXX) XXX tends to be tall and thin but not usually retarded XXXX are severely retarded Jacobs syndrome (XYY) – tall, persistent acne, speech and reading problems

Changes in chromosome structure 18.5 Chromosome inheritance Changes in chromosome structure Deletions – loss of a piece of the chromosome (e.g. Williams syndrome) Translocations – movement of chromosome segments from one chromosome to another nonhomologous chromosome (Alagille syndrome) Duplications – presence of a chromosome segment more than once in the same chromosome Inversions – a segment of a chromosome is inverted 180 degrees

Changes in chromosome structure 18.5 Chromosome inheritance Changes in chromosome structure Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a a a a a a a b b b b b b b b l l c c c c c m c m + a c d d n d n d d d d c e e d e o e e e o e e f d f p f p f f e f f g g q q g g g f g g h r r h g a. Deletion b. Duplication c. Inversion d. Translocation

Changes in chromosome structure 18.5 Chromosome inheritance Changes in chromosome structure Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a s s a a a b b t b t b h + c c deletion lost u u c c translocation d v d v d d e e w e w e f x x f f f g g y g y g h z h z h