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Knex 78780 - Education DNA Replication Teachers Guide Manual

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into the protein needed by the cell. Ribosomes exist in two places in the cytoplasm. Some
ribosomes are in the cytosol unattached to any other organelle, and some ribosomes are attached
to the endoplasmic reticulum (termed rough endoplasmic reticulum due to the ribosomes attached).
Ribosomes themselves are made up of two sub-units each consisting of rRNA and associated proteins.
One sub-unit is larger than the other and they are only functional when they are attached to each other
and to the mRNA molecule.
A transfer RNA molecule is a piece of single-stranded RNA that loops back over
itself in a rather distinguishable, cloverleaf-like shape. The middle “loop” of the
tRNA molecule (analogous to the middle cloverleaf) possesses a triplet sequence
of nitrogen-containing bases called an anticodon, which pairs with the codon
on the mRNA. At the opposite end of the molecule (analogous to the stem of
the cloverleaf) an amino acid is bound to the tRNA molecule. As we saw in the
genetic code in Table 1 (shown again below), each codon and therefore each
anticodon is specific for one and only one amino acid. This distinguishes each
type of amino acid-carrying tRNA molecule from others.
MODULE III: CODING, TRANSLATION, AND MUTATIONS
LESSON 6
Table 1 – The Genetic Code: the three-letter codes representing the possible sequences of nitrogen-containing
bases found in mRNA to represent the 20 amino acids and stop codons.
Translation is very systematic, takes place on a ribosome, and is assisted by specific proteins.
Three steps: initiation, elongation, and termination describe the translation process.
1. The initiation step is the first step in translation. This step combines the sub-units of the
ribosome, the mRNA initiation codon, and the initiator tRNA. The mRNA initiation codon is
AUG which complementary base-pairs with the tRNA anticodon UAC. The UAC tRNA anticodon
is specific for the amino acid methionine. Therefore, as stated in Lesson 4, the code AUG,
methionine, is the start signal for the synthesis of a polypeptide chain. This first step is assisted
by proteins called initiation factors.
2. The second step in translation is elongation. This step is assisted by proteins called elongation
factors. Elongation of the polypeptide chain occurs as the ribosome sub-units move down the
mRNA molecule in the 5’ to 3’ direction. As the ribosome moves down the mRNA, two major
events occur. First, each mRNA codon is paired with its complementary anticodon on a tRNA.
[Source for Table 1: Fig. 5.6, p.143 from BIOCHEMISTRY, 3rd ed. by Christopher K. Mathews, K.E. van Holde and Kevin G. Ahern. Copyright
©
2000
by Addison Wesley Longman, Inc. Reprinted by permission of Pearson Education, Inc.]
www.knexeducation.com
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