Cell Division and Protein Synthesis in One Page
All of the functions of a cell are controlled by small sections of DNA called genes.
The genes are also responsible for the organism’s individual characteristics called
traits, such as eye color and baldness. These genes are located on large
molecules of DNA called chromosomes. Chromosomes come in pairs and are
located in the cell’s nucleus. Having all of your chromosomes in pairs is known
diploid or 2n. DNA contains all of the essential information about cell function in
coded messages. Three letters of code stand for one of 20 different amino acids.
Genes control cell functions by making chains of amino acids called proteins.
Some proteins are structural like muscle and skin; others are chemical like
enzymes and hormones. To make a protein (protein synthesis), a gene makes a
copy of its DNA code. The copy is called messenger RNA, (m-RNA). Other RNA
called transfer RNA (t-RNA) brings amino acids to the m-RNA. The amino acids
are then strung together into chains of protein by little structures called ribosomes.
When a cell gets too big, it must divide. This process is called mitosis. In mitosis,
each chromosome makes a copy of itself for the new cell. Therefore, each cell,
which had all of its pairs of chromosomes (2n), makes a new cell that is also 2n.
All of the genes in the new cell are identical to those in the old. This is the way we
make new skin cells muscle cells, nerve cells etc.
Some organisms like bacteria and protozoa actually reproduce by mitosis. This
method is sometimes called asexual (without sex) reproduction. The problem
with this method is that it doesn’t allow for variety in the offspring. A better way,
which will give some variety, is called sexual reproduction. In sexual reproduction,
cells from two organisms fuse together to make a new individual. This way the
new cell receives genes from both parent cells. To keep the new cell from getting
too many chromosomes (complete pairs from both parent cells), the parent cells
separate the pairs and only send one chromosome from each pair to the new cell.
(Having your chromosomes not paired is called haploid or n)
The process of separating the pairs is called meiosis. In meiosis, each parent
creates a special cell called a gamete, or sex cell, to receive this n number of
chromosomes. When the sex cells unite, in a process called fertilization, the new
cell, called a zygote is n + n or 2n.
The possibility exists that a gene can become randomly and accidentally damaged
or altered. This occurrence, known as mutation, often leads to the death of the
organism. However, sometimes the organism survives a mutation and even finds
the changed gene to be better than the old one. In this case the mutation can be
passed on to the new cell during either mitosis or meiosis if the mutation
happened in a sex cell.