Chromosomes and Cell Chromosomes and Cell ReproductionReproduction

Chapter SixChapter Six

About two trillion new cells are produced by the human body each day. They function in

growth, healing, and maintainance.

Cell division (or Cell Reproduction) is also important in asexual reproduction and the formation of gametes in sexual

reproduction.

Regardless of the type and purpose of cell reproduction,

each new cell must receive an accurate set of DNA so that it

will know how to function in the organism. Thus it is important in cell reproduction to copy and

distribute DNA.

Prokaryotes simply divide to reproduce. This reproduction is called binary fission. The DNA

is copied and then the cell membrane begins to constrict between the single DNA loops

and a new cell wall forms.

During cell division, the long coiling DNA must be packed into compact pellets that can be efficiently moved through the cell. These pellets are called

chromosomes.

Each chromosome is a single DNA molecule associated with certain proteins. Genes are

segments of the DNA that are responsible for the formation of the correct proteins at the proper time.

Since a chromosome is located in a dividing cell, it consists of two identical halves called chromatids.

At the center of each, there is a centromere that holds the chromatids together until the cell divides.

At the time of division, each new cell gets a complete set of DNA.

Each human cell has 23 pairs of similar (homologous)

chromosomes. One is received from each parent.

All the cells in the body other than the gametes are considered

somatic cells. They have a full number of chromosomes and are considered diploid (2n). Gametes have only one set of chromosomes and are considered haploid (1n). The zygote is a fertilized egg cell

and is the first cell of an organism.

Each organism has a different number of chromosomes.

Although the number is constant for a species, the number in

different species varies widely.

Of the 23 pairs of chromosomes in the human, 22 pairs are not involved in determining gender but control other life activities. These are called autosomes.

The sex chromosomes, usually labeled X and Y determine the gender of the organism. The X

chromosome is usually considered female and the Y, male. In humans

and most other organisms, XX indicates a female and XY, a male.

In some species such as the grasshopper, the Y chromosome is lacking. In these species, XX is a

female and XO is a male.

Since each chromosome contains thousands of genes, all 46

chromosomes are a must for the normal development of a person.

Sometimes, chromosomes fail to separate properly (dysjunction) and a

gamete ends up with both chromosomes and the other gamete recieves none. This nondysjunction causes the gamete to be 3N or show

trisomy.

Trisomy of the 21st chromosome causes a condition called Down syndrome. Down

syndrome is characterized by short stature, round face, distinct upper eyelids, and varying degrees of mental retardation.

In mothers younger than 30, Down syndrome occurs in less than one

of 1500 births. In mothers over 37, it occurs in one of 290 births. In

mothers over 45, it occurs in one of 46 births. This would imply that a

women’s egg cells become damaged throughout her lifetime.

Changes in an individuals chromosome structure are called mutations. There are four basic types of mutations

Deletion

Duplication

Inversion

Translocation

The Cell CycleThe Cell Cycle

The lifetime of a cell is called the cell cycle. If the cell goes through the division process, it occurs in a

definite set of steps. The cell spends 90 percent of its lifetime in the first three phases, or interphase.

How is the cell cycle controlled? There are checkpoints built into the DNA of the cell that moniters its growth and development. The production of enzymes can cause a change in the cell or delay it. There are three main checkpoints.

• Cell growth checkpoint – Enzymes produced by the nucleus check the health and volume of the cell. If the feedback is correct, the cell proceeds to DNA synthesis.

• DNA synthesis checkpoint – The cell checks the DNA with repair enzymes and if it is in proper order, mitosis proceeds.

• Mitosis checkpoint – This checkpoint triggers mitosis.

If the genes that control the production of checkpoint

enzymes, are mutated, cell growth and development may

be disrupted. Cancer, the uncontrolled growth of cells,

may result.

Cancer may affect the cell cycle by:Cancer may affect the cell cycle by:

Over producing growth molecules and Over producing growth molecules and accelerating the cell cycle.accelerating the cell cycle.

Inactivating control enzymes that stop or Inactivating control enzymes that stop or slow the cell cycle.slow the cell cycle.

Interphase

• Resting, growing cell

Prophase

• Nucleur membrane disintergrates

• Nucleolus disappears

• Spindle fibers form

• Centrioles migrate in animal cells

• Chromosomes form

Metaphase

• Chromosomes are lined up along the equator of the cell.

Anaphase

• Spindle fibers separate the chromosomes and pull the chromatids toward the ends of the cell.

Telophase

• Chromatids begin to unwind into DNA

• Spindle fibers disappear

• Nucleolus reappears

• Nuclear membranes reform