Transcript Chromosomes and Cell Reproduction

Chromosomes and Cell
Reproduction
Chapter 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 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:
 Over producing growth molecules and
accelerating the cell cycle.
 Inactivating control enzymes that stop or
slow the cell cycle.
Interphase
• Resting, growing cell
Prophase
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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
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Chromatids begin to unwind into DNA
Spindle fibers disappear
Nucleolus reappears
Nuclear membranes reform