BL414 Genetics Spring 2006
Lecture 5 Outline
February 1, 2006
Chapter 4: Chromosomes history: chromosome behavior matched behavior of “genes”
 pairs
 segregation
 independent assortment
4.1
Chromosome complement: the complete set of chromosomes
Somatic cell: cell of the body – somatic cells have nuclei which contain chromosomes in pairs or 2 homologous sets, i.e. they are diploid
diploid: containing 2 homologous sets of chromosomes
Germ cell: gamete, or cell that unites with another gamete during fertilization – germ cells are haploid, i.e. they contain only one set from the pair of homologous chromosomes
4.2 Mitosis: the process of nuclear division in which replicated chromosomes divide; the daughter cells have the same chromosome number and genetic
composition as the parent nucleus
Cytokinesis: physical division of the cell into two daughter cells, usually accompanies mitosis

Each chromosome is a duplicated structure already at the beginning of nuclear division

Each chromosome divides longitudinally into 2 halves that separate from each other
 The separated halves, “chromatids,” move to opposite ends of the cell, one for each new daughter cell that will be formed
Interphase: stage during which DNA is not condensed, the extended strands are too thin to be seen under a light microscope

DNA replication occurs during this phase, during the S part of the cell cycle o Cell cycle: the “Life cycle” of cell, includes phases G1, S, G2
and M and transitions called checkpoints

o Cell cycle checkpoints: certain conditions must be present in order for the cell to pass through each checkpoint

Interphase is consists of 3 parts of the cell cycle: G1, S and G2, and the intervening checkpoints
Stages of Mitosis:
1) Prophase: a.
Condensation of the chromosomes from extended chromatin– they become visible under light microscope b.
Nucleoli disappear; nuclear membrane disintegrates
2) Metaphase: the mitotic spindle forms, a system of fibers extending from two focal point at each end of the cell called the spindle poles

Centromere: region at center of chromosome where chromatids are attached to each other, also attaches to spindle

Kinetochore: the specific portion of the centromere that attaches to the spindle a.
Spindle forms b.
Spindle fibers attach to chromosomes c.
Chromosomes line up along a central plane of the cell equidistant from the spindle poles, called the metaphase plate i.
The proper alignment of the chromosomes is monitored in the cell by a signal from kinetochore proteins
3) Anaphase a.
the centromeres divide b.
sister chromatids separate and migrate to opposite poles, pulled by shortening spindle fibers c.
each sister chromatid is now considered a chromosome
4) Telophase a.
Two nuclear envelopes form b.
Nucleoli form (nucleoli are small bodies in the nucleus where biosynthesis of ribosomal RNA and ribosomes takes place) c.
Spindle disappears d.
Chromosomes de-condense e.
Cell usually divides
4.3
Meiosis: cell division process that creates haploid daughter cells, involves 2
nuclear divisions
1) Pairs of homologous chromosomes become closely associated -
(exchange of genetic information will take place)

2) First nuclear division – different from mitosis in that the homologous chromosomes are separated, resulting in haploid daughter cells (based on the number of centromeres)
3) Second nuclear division – somewhat like mitosis, but no DNA replication occurs between the two nuclear divisions

All chromosomes align at the metaphase plate and are separated
Meiosis occurs in plants and animals to create gametes
In animals – meiosis occurs in special cell called meiocytes; more specifically
oocytes and spermatocytes: oocytes form egg cells, spermatocytes form sperm cells
In plants - more complicated – they go through alternation of generations
Stages of Meiosis
First Meiotic Division: Reductional Division: reduces the chromosome number by half
1) Prophase I a) Leptotene “thin thread” chromosomes begin to condense – dense granules, “chromomeres,” appear along length b) Zygotene “paired thread” homologous chromosomes are paired side-byside in zipper like fashion as chromomeres are lined up and joined together, this is called synapsis c) Pachytene “thick thread” chromosomes shorten and thicken; set of tetrads are together; two of the chromatids are paired tightly and undergo crossing over and genetic exchange d) Diplotene “double thread" synapsed chromosomes begin to separate;
chiasmata are visible – occur where physical exchange between homologous chromosomes has occurred – there is usually at least one chiasma per chromosome, in larger chromosomes there can be 3 or more chiasmata e) Diakinesis i) Chromosomes repel each other more, move apart more, chiasmata are still present ii) Spindle begins to form iii) Nuclear envelope breaks down
2) Metaphase I a) Spindles join centromeres of the synapsed chromosome sets (“bivalents”) b) Chromosome bivalents line up on metaphase plate

c) Orientation of each bivalent can be with paired chromosomes facing either pole of the cell – this determines which cell will receive the maternal or paternal chromosome – it is a 50-50 chance for each chromosome
“Genes on different chromosomes undergo independent assortment because nonhomologous chromosomes align at random on the metaphase plate in meiosis I”
3) Anaphase I a) homologous chromosomes separate and migrate to opposite poles – the centromeres of each chromosome remain intact b) “The physical separation of homologous chromosomes in anaphase is the physical basis of Mendel’s principle of segregation”
4) Telophase I a) Spindle breaks down b) Nuclear envelope may form around each set of chromosomes
The Second Meiotic Division: Equational Division – called this because the chromosome number remains the same before and after this division
 cells will not undergo a DNA replication between telophase I and prophase II; in some cases chromosomes may uncoil to some extent
1) Prophase II: short because nuclear envelope may already be gone and chromosomes should already be somewhat condensed
2) Metaphase II: a) spindles form and attach to centromeres b) chromosomes align at metaphase plate
3) Anaphase II: sister chromatids separate and migrate to opposite poles
4) Telophase II: return to interphase a) Spindle breakdown b) Nuclear envelope forms c) Cytokinesis
Final result of Meiosis  4 haploid daughter cells
*because of crossing-over, sister chromatids may not be genetically identical
4.4
Sex-Chromosome Inheritance
Sex chromosomes: X and Y, involved in determining the sex of an organism
XX – female, XY – male

X and Y associate during meiosis as a homologous chromosome pair, although they contain different genes. X and Y are different in appearance, unlike other homologous chromosomes. Y is smaller than X and contains many fewer genes. females are homogametic because their gametes contain the same sex chromosome and males are heterogametic because their gametes contain different sex chromosomes
XX-XY sex determination occurs in humans, other animals, some insects and plants
autosomes are the rest of the chromosomes that are not X or Y
An autosomal genetic disease indicates that the gene is not located on the
X or Y chromosome, i.e. it is not sex-linked
X-linked inheritance: refers to the inheritance of a gene that is physically located on the X chromosome. Early studies of this provided key evidence that genes were physically located on chromosomes
In Mendel’s studies, reciprocal crosses (with either male or female having a trait) provided the same ratios of phenotypes

An exception to this was found in the lab of Thomas Hunt Morgan at
Columbia, in 1910.

A white-eyed male mutant was crossed to wild-type females (P1).

All the offspring (F1) were red-eyed, indicating that the white-eyed gene is recessive.

When the (F1) generation were crossed, the results were unusual:
 2459 red-eyed females
 1011 red-eyed males
 782 white-eyed males
 We see the expected 3:1 ratio of red:white, but the unexpected result is that all of the homozygous recessive offspring are males!

A testcross of a red-eyed F1 female to a parental white-eyed male gave a
1:1:1:1 ratio of red-eyed female: white-eyed female: red-eyed male: whiteeyed male

A reciprocal cross of the original cross, mating a white-eyed female to a red-eyed male gives the result of all red-eyed females and all white males.
The explanation for the above results is that this white-eyed gene is located on the X chromosome. The Y chromosome doesn’t have the genetic locus for the

white-eyed gene. So a male who has an X chromosome with the white-eyed mutation is essentially recessive because he has no copy on the Y chromosome.
Nondisjunction: During Meiosis (Anaphase I) some chromosomes do not separate from each other properly, resulting in gametes with too many or too few chromosomes. This process can result in three copies of a chromosome in the zygote, which happens in the case of trisomy 21, or Down syndrome.
Nondisjunction of X chromosome would result in some eggs with XX and some with no X
 fertilization of these eggs could result in zygotes that were:
XXX
XXY
X
Y
Calvin Bridges took some rare results that were contradictory to what was expected, and figured out that non-disjunction was causing the results by examining the chromosomes of some of the fruit flies in question. In this way he actually provided further strong evidence for genes being physically present on chromosomes.
“Chromosome theory of heredity: Genes are physically located within chromosomes.”
Sections on heterogametic females and sex determination on Drosophila will not be covered on the test. They are interesting though and you are free to read them on your own.