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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.
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
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 cycle
1. Interphase: 20 hr, 90% of cell cycle
–
G1 stage
•
•
Cell doubles its organelles and grows in size
Accumulate material needed for DNA synthesis
–
–
S stage
•
–
Proteins needed to change chromatin into chromosomes
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
18.1 Chromosomes and cell cycle
The cell cycle
18.2 Mitosis
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.
2.
3.
4.
Prophase
Metaphase
Anaphase
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
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
–
•
Malfunctioning cell cycle result in
–
•
Growth and injury repair
Benign tumor or a cancerous tumor
Cell Cycle Control System
1. 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 Gene
4. 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)
18.3 Meiosis
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 meiosis
and increases genetic
variation
18.3 Meiosis
Meiosis I
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
18.3 Meiosis
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 II
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
18.4 Comparison of meiosis and mitosis
Mitosis
vs.
Meiosis
• Growth and repair of cells • Formation of gametes
• Occurs in body cells
• Occurs in sex cells
• 1 division
• 2 divisions
• Results in 2 diploid
genetically identical cells
• Results in 4 haploid
genetically different
cells
Comparing mitosis and meiosis I
Comparing mitosis and meiosis II
18.4 Comparison of meiosis and mitosis
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
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 chromosome
e.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)
18.5 Chromosome inheritance
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
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