Download Chromosomes and Cell Reproduction

Chromosomes
Formation of New Cells by Cell Division
Cell Division, also called cell reproduction, occurs in humans
and other organisms at different times in their life.
Bacterial cells undergoing reproduction divide by one type of
cell division. Eukaryotic organisms undergoing growth,
development, repair, or asexual reproduction, divide by a
different type of cell division. And, the formation of
gametes (an organism’s reproductive cells, such as sperm and
eggs) involves a third type of cell division.
Prokaryotes reproduce by a type of cell division called binary
fission (a form of asexual reproduction that produces identical
offspring).
Eukaryotic Cell Reproduction
The vast majority of information encoded in DNA is organized into
units called genes. A gene is a segment of DNA that codes for a
protein or RNA molecule. Genes play an important role in
determining how a person’s body develops and functions.
As a eukaryotic cell prepare to divide, the DNA and the proteins
associated with the DNA coil into a structure called
chromosomes.
Before the DNA coils up; however, the DNA is copied. The two
exact copies of DNA that make up each chromosome are called
chromatids.
The two chromatids of a chromosome are attached at a point called
the centromere.
Each human somatic cell (any cell other than a sperm or egg cell)
normally has two copies of 23 different chromosomes, for a total
of 46 chromosomes. The 23 chromosomes differ in size, shape,
and set of genes.
Each of the 23 pairs of chromosomes consists of two homologous
chromosomes (similar in size, shape, and genetic content).
When a cell, such as somatic cell, contain 2 sets of chromosomes, it
is said to be diploid. When a cell, such as a gamete, contains one
set of chromosomes, it is said to be haploid. A haploid number is
a human gamete can be written as n = 23. The diploid number in a
somatic cell can be written as n = 46. A zygote is a fertilized
egg cell, the first of a new individual.
Of the 23 pairs of chromosomes in human somatic cells, 22 pairs
are called autosomes. Autosomes are chromosomes that are not
directly involved in determining the sex (gender) of an individual.
The sex chromosomes, one of the 23 pairs of chromosomes in
humans, contain genes that will determine the sex of the
individual.
In humans and many other organisms, the two sex chromosomes are
referred to as X and Y chromosomes. Any individual with a Y
chromosome is male, and an individual without a Y chromosome is
female. In human males, the sex chromosomes are made up of
one X and one Y chromosome (XY). In human females consist of
two X chromosomes (XX).
The presence of all 46 chromosomes is essential for normal
development and function. Humans who are missing even one of
the 46 chromosomes do not survive. Humans with more than two
copies of a chromosome, a condition called trisomy will occur.
Below are two pictures of an infant with trisomy 13.
Abnormalities in the chromosome number can be detected by
analyzing a Karyotype, a photo of the chromosomes in a dividing
cell that shows the chromosome arrangement by size.
A portion of a karyotype from an individiual with an extra copy of
chromosome 21 is called Down Syndrome, or trisomy 21.
When a sperm and egg cell form, each chromosome and its homologue
separate in an event called disjunction. If one or more
chromosomes fail to separate properly, an event called
nondisjunction – one new gamete ends up receiving both
chromosomes and the other gamete receives none.
The cell cycle is a repeating sequence of cellular growth and
division during the life of an organism. A cell spends 90% of its
time in the first 3 phases of the cycle, which are collectively
called interphase.
1.
First Growth (G1) phase. A cell grows rapidly and carries out its
2.
Synthesis (S) phase. A cell’s DNA is copied during this phase. At the
end of this phase, each chromosome consists of two chromatids
attached at the centromere.
routine functions. Cell’s that are not dividing remain in the G1 phase.
Second growth (G2) phase. Hollow protein fibers called
microtubules are assembled. The microtubules are used to
move the chromosomes during mitosis.
4. Mitosis. The process during cell division in which the nucleus of
a cell is divided into two nuclei. Each nucleus ends up with same
number and kinds of chromosomes as the original (cloning).
5. Cytokinesis. The process during cell division in which the
cytoplasm divides.
3.
Step 1: Prophase. Chromosomes coil up and become visible during
prophase. The nuclear envelope dissolves and a spindle forms.
Step 2: Metaphase. During metaphase, the chromosomes move to
the center of the cell and line up along the equator. Spindle
fibers link the chromatids to opposite poles.
Step 3: Anaphase. Centromeres divide during anaphase. The two
chromatids (now called chromosomes) move toward opposite poles
as the spindle fibers attached to them shorten.
Step 4: Telophase. A nuclear envelope forms around the
chromosomes at each pole. Chromosome, now at opposite poles,
uncoil and the spindle dissolves. The spindle fibers break down
and disappear.