Chapter 18
Patterns of Chromosome Inheritance
Mader, Sylvia S. Human Biology. 13th Edition. McGraw-Hill, 2014.
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.
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
18.1 Chromosomes and cell cycle
What is a karyotype?Computer arrangement of chromosomes in pairs
Karyotype
• In dividing cells– Chromosomes are duplicated– Each chromosome is composed of two identical
parts, called sister chromatids– Each sister chromatid contain the same genes
• Genes of inheritance
– Chromatids are held together at a region called centromere
– Centromeres hold sister chromatids together until certain phase of mitosis
The cell cycle1. Interphase: 20 hr, 90% of cell cycle
– G1 stage• Cell doubles its organelles and grows in size• Accumulate material needed for DNA synthesis
– Proteins needed to change chromatin into chromosomes
– S stage • DNA replication occurs; duplicated chromosomes
– G2 stage • proteins needed for division are synthesized
– Proteins found in microtubules
2. Cell division (mitosis and cytokinesis): 4 hrs– Mitosis – nuclear division
• Each new nucleus contains the same number and kind of chromosomes as the former cell
– Cytokinesis – cytoplasmic division
The cell cycle
18.1 Chromosomes and cell cycle
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
18.2 Mitosis
The spindle in mitosis• Centrosome
– microtubule organizing center of the cell
• Aster – array of microtubules at the poles (ends of the cell)
• Centrioles – short cylinders of microtubules that assist in the formation
of spindle fibers
18.2 Mitosis
Overview of Mitosis
• 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:
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
18.2 Mitosis
1. Mitosis: Prophase• Chromosomes condense and become visible
• Nuclear envelope fragments
• Nucleolus disappears
• Centrosomes move to opposite poles
• Spindle fibers appear and attach to the centromere
18.2 Mitosis
2. Mitosis: Metaphase• Chromosomes line up at the middle of the cell
(equator)• Fully formed spindle
18.2 Mitosis
3. Mitosis: Anaphase
• Sister chromatids separate at the centromeres and move towards the poles
18.2 Mitosis
4. Mitosis:Telophase and cytokinesis
• 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)
Importance of Cell Cycle and Mitosis• Cell cycle important for well-being and involved in
– Growth and injury repair
• Malfunctioning cell cycle result in– Benign tumor or a cancerous tumor
• Cell Cycle Control System1. Reception:
- External signal delivers message to receptor in plasma membrane of receiving cell
2. Transduction: - receptor relays signal to protein inside cell cytoplasm
3. Activates Gene4. Response:
- protein production that stimulate or inhibit the cell cycle- proto-oncogenes stimulate cell cycle- Tumor suppressor genes inhibit cell cycle
Overview of Meiosis
• 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)
Meiosis I
• Prophase I– Homologous chromosomes pair (synapsis) and
crossing-over occurs in which there is exchange of genetic information• Homologous chromosomes
– chromosomes look alike and carry genes for the same traits
• Synapsis – line up of homologous chromosomes side by side
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 meiosisand increases genetic variation
18.3 Meiosis
Meiosis I18.3 Meiosis
Meiosis I• Metaphase I
– Homologous pairs lined at the equator– Number of possible chromosomal combinations in the
gametes is 8,388,608 without accounting for cross-over
• Anaphase I– Homologous chromosomes separate and move towards
opposite poles
• Telophase I– 2 daughter cells result each with 23 duplicated
chromosomes
Meiosis II
• Prophase 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
18.3 Meiosis
Meiosis II18.3 Meiosis
Significance of Meiosis• Keep the chromosome number constant from
generation to generation• Asexual reproduction by binary fussion
– Bacteria, protozoans, and yeasts– Bacteria can increase to over one million cells in about 7
hours
• Sexual reproduction, requires more energy– Humans and other animals– Results in genetic recombination
• Ensures offspring are genetically different compared to each other and their parents
• Recombination is due to crossing-over and independent alignment of chromosomes
– Benefit generates diversity needed so that a few will be suited to new and different environmental circumstances
Mitosis vs. Meiosis
• Growth and repair of cells
• Occurs in body cells
• 1 division
• Results in 2 diploid genetically identical cells
• Formation of gametes
• Occurs in sex cells
• 2 divisions
• Results in 4 haploid genetically different cells
18.4 Comparison of meiosis and mitosis
Comparing mitosis and meiosis I
Comparing mitosis and meiosis II
The production of sperm and eggs
• Spermatogenesis– Process of making
sperm in males– A continual process
after puberty – About 400 million
sperm are produced per day
• 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 ~400 during the entire reproductive cycle
18.4 Comparison of meiosis and mitosis
Changes in chromosome number• Nondisjunction
– when both members of a homologous pair go into the same daughter cell during meiosis I
– 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 chromosomee.g. Down syndrome (3 copies of chromosome 21)** Too many chromosomes is tolerated better than a deficiency of
chromosomes
– Survival is greater when monosomy and trisomy involves sex chromosomes• Because in female XX, one X chromosomes is a Barr body is
inactive
Down Syndrome• Most common autosomal trisomy
– Three copies of chromosome 21
• Characteristics:– Short stature, an eyelid fold, flat face, fissured
tongue, round head, mental retardation
• Genes that carry Down Syndrome– Located on bottom third of chromosome 21– Responsible for increased tendency toward
leukemia, cataracts, accelerated rate of aging and mental retardation (Gart gene)
Changes in 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
18.5 Chromosome inheritance
Changes in sex chromosome number
Changes in chromosome structure• Deletions: causes abnormalities
– loss of a piece of the chromosome
• Translocations– movement of chromosome segments from one
chromosome to another nonhomologous chromosome
• Duplications:– presence of a chromosome segment more than
once in the same chromosome
• Inversions: lead to altered gene activities, deletions and duplications– a segment of a chromosome is inverted 180
degrees resulting in a reversed chromosome
Changes in chromosome structure