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