The mRNA (blueprint) leaves the nucleus through the
nuclear pores and goes to the ER (home site).
Ribosomes (the scaffolding) attach to the mRNA at
a specific start sequence (AUG) and move along the
Sets of RNA molecules called transfer RNA (tRNA)
(the workers) bring specific amino acids (building
materials) to the ribosomes:
• There are 20 different types of tRNA molecules, one
type for each amino acid.
• As a project manager may tell workers what to do, the
sequence of codons in mRNA helps direct the tRNA.
Each type of tRNA has a three-nucleotide sequence
on one end (anticodon) that is complementary to the
codon on mRNA for that specific amino acid. Only
the tRNA with the complementary anticodon can
bind to the codon in mRNA.
The tRNA molecules (workers) assemble the protein
(house) according to the instructions in the mRNA
(blueprint). The process of assembling the protein
from the instructions in mRNA (steps 2-5) is called
translation (Figure 3.19):
tRNA molecules bound to specific amino acids enter
• The anticodons of the appropriate tRNA pair up
with the appropriate codons in the mRNA.
• A peptide bond forms between amino acids within
• The empty tRNA leaves and the process repeats
until the protein (home) gets assembled completely.
• Eventually, the ribosome reaches a stop sequence in
the mRNA (the last part of the blueprint). This stop
sequence is called a stop codon (UGA, UAG, UAA).
Upon reaching the stop codon, the ribosome falls
apart and releases the newly made protein (house).
Approximately 15 amino acids are translated every
second, and proteins can have hundreds to thousands of
amino acids. It can take as long as 90 minutes to make
new proteins from the start of transcription to the end of
translation. Therefore, many ribosomes can be involved
in making multiple copies of a protein. As one ribosome
moves along the mRNA, a second one can attach and be-
gin to make another protein. This process repeats, pro-
(one mRNA with many ribosomes
and growing protein chains). The polyribosome activity
allows many proteins to be made from a single molecule
Cells Divide by Mitosis or by Meiosis
Cells transport materials and make proteins as part of
their normal functions. They grow and, at some point,
divide to produce new cells. Cell division is the way your
body grows and how it replaces worn-out cells and cells
damaged by disease or injury. Most of the cells in your
body are somatic cells (so-MAT-ik) and divide through a
process called mitosis (mi-TO-sis). Somatic cells are cells
other than sex cells (sperm and egg). During mitosis, one
starting cell divides into two identical cells. Each cell has
exactly the same genetic makeup as the parent cell (two
sets of chromosomes).
Specialized cells called gametes undergo a different
process of cell division called meiosis. During meiosis
(me-O-sis), a starting cell under-
goes two rounds of cell division to
produce four cells. Each cell has
one-half the genetic material of
the starting cell (only one set of chromosomes). Let’s look
at mitosis first.
Mitosis is one part of the cell’s normal life cycle, called
the cell cycle. The cell is continually changing from the time
it forms until it divides. Although the cell cycle is continuous,
it is commonly divided into interphase (IN-ter-faz) and mi-
tosis. During interphase, the cells goes through three stages:
G1—a growth phase in which proteins are synthesized, S—
when DNA is replicated, and G2—another growth phase in
which proteins are made. Interphase, which may take 20 to
22 hours, is followed by mitosis. Mitosis itself consists of four
anaphase (AN-a-faz), and telophase (TEL-o-faz) (Figure
3.20 on the next page). As a result
of mitosis, each cell contains 23
pairs of chromosomes. This make-
up, which is identical to that of the
starting cell, is called diploid.
sex cell, such as an
egg or a sperm.
a cell or an organism
that has two sets of
Cells Carry Out Many Processes 67