Chapter 9
Chromosomes get their name from the Greek words chroma, color and soma, body.
A chromosome is made of highly coiled and condensed chromatin.
Chromatin is made of DNA and associated proteins.
In non-dividing cells, the chromatin is extended and uncoiled.
Chromosomes are or organized into informational units called genes .
Every individual of a species has the characteristic number of chromosomes of the species, e.g. all humans have 46 chromosomes.
The number varies from species to species.
It is possible for an individual to have an abnormal number of chromosomes.
The information in the chromosomes is what makes the uniqueness of the species and not the number of chromosomes.
Chromosomes exist in pairs, half of which was been contributed by each parent of the individual.
Body cells have the full complement of chromosomes and they are called diploid cells.
Gametes have only half of the species chromosomes and they are called haploid .
The cell cycle is the period of time from the beginning of one cell division to the beginning of the next division.
The time it takes for the cycle to be completed is called the generation time .
Generation time can vary widely.
Cell division involves two processes,
1. Mitosis : it involves the nucleus and it insures that each daughter cell will receive the same number and type of chromosome as were present in the original nucleus.
2. Cytokinesis : refers to the division of the cytoplasm of the cell to form two daughter cells.
When mitosis is not followed by cytokinesis, the cell becomes multinucleated or coenocytic.
A. Interphase : it can be divided into the first gap phase (G
1
) , chromosomal synthesis phase
(S) , and the second gap phase (G
2
) .


First gap phase (G
1
): the cell grows and prepares itself for the S phase.

Chromosomal synthesis phase (S): DNA and chromosomal proteins are synthesized.

Second gap phase (G
2
) : protein synthesis increases in preparation for cell division.
Cells that are not dividing become arrested at the G
1
stage, which is not part of the cell cycle.
Cells at this state are called to be at the G o
state.
B. Mitosis ensures orderly distribution of chromosomes.
1. Prophase : duplicated chromosomes are made of two sister chromatids held together at the centromere; the nucleolus disappear; nuclear envelope breaks down; spindle of microtubules begins to form.

Dividing cells have a microtubule-organizing center at each pole, from which the microtubules grow outward.

In animal cells, the MTOC has a pair of centrioles in its center and which the pericentriolar material surrounds.

The spindle microtubules end in the pericentriolar material.

The MTOC of higher plants lack centrioles and is made of dense fibrillar material.

Centrioles become duplicated during the interphase.

Centrioles are needed in the formation of the basal body of flagella and cilia. Flowering plants and gymnosperms lack ciliated cells and also lack centrioles.

Additional microtubules extend outward from the MTOC in all directions forming the aster .

The function of the aster is not understood.
2. Metaphase : chromosomes align at the cell's equatorial plane; mitotic spindle is finished; kinetochores of sister chromatids are attached to microtubules to opposite poles of the cell.

The mitotic spindle is made of two types of microtubules, the polar microtubules that overlap at the equator, and the kinetochore microtubules that are attached at the kinetochore of the centromere.
3. Anaphase : sister chromatids become separated and move to opposite poles of the cell; each free chromatid is referred now as a chromosome.

The overall movement of chromosomes to their respective poles is poorly understood.

It seems that tubulin units are disassembled at the poles and the microtubules shorten pulling the chromosomes.


Polar microtubules are not connected to kinetochores but are part of the spindle.

As they elongate, the poles are pushed apart which also pushed the chromosomes apart since they are attached to the poles by the kinetochore microtubules.
4. Telophase : a nuclear envelope re-forms around each set of chromosomes; nucleoli are also formed; spindle disappears; chromosomes uncoil.
C. Cytokinesis normally begins telophase and overlaps with mitosis.
1. The cytoplasm divides to form two individual cells.
2. In animal cells a ring of microfilaments contracts, producing a furrow that divides the cytoplasm.
3. In plant cells, the cell plate provides materials for new plasma membranes and cell walls.
The cell plate is formed by the fusion of vesicles that originated in the Golgi bodies. The vesicle membranes fuse to form the plasma membrane.
Most cytoplasmic organelles are distributed randomly to the daughter cells.
Mitochondria and chloroplasts have their own DNA and are formed from the division of preexisting organelles usually during the interphase.
Under optimal conditions, prokaryotes can divide every 20 minutes.
The generation time of eukaryotic cells is usually much longer.
There seems to be a fundamental regulatory mechanism common to all eukaryotes.
1. Mitosis-promoting factor (MPF) is required for the cell to make the transition from G
2 to mitosis.
2. Active MPF is produced when the protein called cyclin B binds to the protein kinase enzyme called Cdk.
3. Active MPF phosphorylates other enzymes thus activating those needed for mitosis or inactivating those that prevent mitosis.
4. Later on, as cyclin B is degraded, MPF becomes inactive until Cdk joins again to another cyclin B molecule.
Certain drugs can stop the cell cycle by preventing DNA synthesis, inhibiting regulatory proteins or proteins involved in the formation of the mitotic spindle.

Cancer cells divide much rapidly than normal cell. These drugs can affect by these drugs.

Colchicine affects the spindle formation by combining with tubulin monomers. Sister chromatids cannot move to the poles and one daughter cell ends with extra sets of chromosomes, a condition knows a s polyploidy .

Citokinins stimulate cell division in plant cells.

Proteins known as growth factors stimulate cell division in animal cells.
A. Ploidy, the number of sets of chromosomes present in a cell.
In asexual reproduction a single parent produces the offspring(s) by splitting, budding, or fragmenting.

Parents and offsprings are genetically identical and they are known as clones .
Sexual reproduction involves the union of two specialized cells, known as gametes , to form a single cell, the zygote .

Each chromosome found in somatic cells of higher plants and animals has a partner chromosome.

The two partners of the pair are known as homologous chromosomes .

The members of a homologous pair are commonly referred to as maternal and paternal chromosome, because a different parent provided each.

Homologous chromosomes carry similar but not identical genetic material.

Chromosomes formed by the separation of sister chromatids are, however, identical.

If a cell contains two sets of chromosomes, it is called diploid (2n) .

If the cell has only one set of chromosomes, then it is called haploid (n) .

Sometimes cells may have more than one set of chromosomes and are called polyploid
(3n, 4n, etc.) .

The somatic cells of animals are diploid. Only reproductive cells, gametes, are haploid.

The process of meiosis produces haploid cells.

Reduction in the number of chromosomes occurs before the formation of gametes.
B. Gamete formation is called gametogenesis.


Male gametogenesis results in the formation of four sperm cells and it called spermatogenesis .

Female gametogenesis produces a single egg or ovum , and it is called oogenesis .

Gamete formation usually follows meiosis.

Many eukaryotes (e.g. fungi) remain haploid throughout most of their life. The only diploid stage is the zygote that undergoes meiosis after fertilization has occurred.

They become multicellular by mitosis of the haploid cells.
Plants and many algae have alternation of generations .

A multicellular diploid sporophyte divides mitotically to produce a multicellular haploid gametophyte ., which produces gametes by mitosis.

Two haploid gametes fuse to form a diploid zygote, which divides and produces a diploid multicellular sporophyte.
B. Process of meiosis.
The phases of meiosis are similar to those mitosis but with the following differences:
1. Meiosis involves two successive nuclear and cytoplasmic divisions, producing up to four cells.
2. DNA and other chromosomal components duplicate only once.
3. Each cell produced by meiosis has only one set of chromosomes.
4. During meiosis, the genetic information of both parents is shuffled, so each resulting haploid cell has a unique combination of genes.
The two divisions are known as meiosis I and meiosis II .
1. Prophase I: the nuclear envelope disintegrates; the chromosome members of a pair become to lie lengthwise side by side, in what is called synapsis .

The four chromatids lying side by side are called tetrads , a term preferred by geneticists.

The term tetrad refers to the four chromatids, two forming each homologous chromosomes

Bivalent is another term synonym of tetrad and preferred by cytologists .

During synapsis the chromosomes undergo crossing over.


Crossing-over is a process of genetic recombination in which homologous chromosomes exchange sections of DNA: chromatids break, exchange sections of
DNA, and rejoin.

There is no crossing-over between sister chromatids.

The synaptonemal complex , made of proteins, is formed between synapsed homologous chromosomes and holds the chromosomes together.
2. Metaphase I: tetrads line up in the equatorial plane and are held together by chiasmata ,
(sing. chiasma).

In late prophase I, homologous chromosomes are held together only at chiasmata, the regions where crossing over occurred.

Both kinetochores of sister chromatids are attached to the same pole.

In mitosis, sister kinetochores are attached to opposite pole.
3. Anaphase I: the members of each homologous pair separate and move to opposite poles.

Each pole receives a mixture of maternal and paternal chromosomes but only one member of the homologous pair.

The sister chromatids are still united at their centromere region.
4. Telophase I: nuclear envelope may reorganize; chromatids decondense somewhat; cytokinesis may take place.

Each nucleus contains only a haploid number of chromosomes.

Each chromosome, however, has two chromatids.
5. Interkinesis is the stage that separates meiosis I from meiosis II.

It is brief in most organisms or absent in some.
Meiosis II is similar to mitosis and results in the separation of the sister chromatids.

Since two cells were formed in meiosis I and each enter meiosis II, the final yield is four daughter cells.
Homologous chromosomes separate in meiosis I.
Sister chromatids separate in meiosis II.