Fundamentals of Anatomy and Physiology

48

Ribosomes

Ribosomes (RYE-boh-sohmz) are tiny granules distrib-
uted throughout the cytoplasm and are attached to the
rough or granular ER. They are not surrounded by a
membrane. Ribosomes are composed of ribosomal RNA
and proteins. Messenger RNA attaches to ribosomes dur-
ing protein synthesis. There are many, many ribosomes in
the cell because they are so essential to cell function. They
are the sites of protein synthesis.

Protein Synthesis
Proteins are essential to cellular function and structure.
Proteins are part of membrane structures (proteins are
embedded in the double phospholipid layer). Enzymes are
protein catalysts (all chemical reactions in the cell require
enzymes), and our immune system functions

Chapter 3

through the production of antibodies (large proteins) that

attack foreign proteins (antigens).

The code to make a particular protein lies on a DNA

molecule in the nucleus. Genes on the DNA molecule

constitute the code. However, proteins are made at the ri-

bosomes. Therefore, this code must be copied and taken to

the ribosomes. A special molecule called messenger-

RNA (mRNA) copies the code from the DNA molecule in

the nucleus. This process is called -transcription and

-occurs with the assistance of an enzyme called RNA

polymerase. The mRNA then leaves the nucleus through a

nuclear pore and goes into the cytoplasm to a ribo-some or

group of ribosomes. The ribosome will now as-sist in the

assemblage of the protein because it now has the code or

recipe to produce the protein. To make the protein, the

ribosome now needs the ingredients, which are amino

acids.

Amino acid^

OH attaches here

Loop^

Loop

Anticodonloop

A G U Anticodon

U C A Codon

mRNA

Figure 3- 10 The structure of a transfer RNA molecule.

(^) ®
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