13.2 Ribosomes and Protein Synthesis
What is the genetic code, and how is it read?- What role does the ribosome play in assembling proteins?
- What is the “central dogma“ of molecular biology?
polypeptide • genetic code • codon • translation • anticodon • gene expression
Outline Before you read, write down the green headings in this lesson. As you read, keep a list of the main points, and then write a summary for each heading.
THINK ABOUT IT How would you build a system to read the messages that are coded in genes and transcribed into RNA? Would you read the bases one at a time, as if the code were a language with just four words—one word per base? Perhaps you would read them, as we do in English, as individual letters that can be combined to spell longer words.
The Genetic Code
What is the genetic code, and how is it read?
The first step in decoding genetic messages is to transcribe a nucleotide base sequence from DNA to RNA. This transcribed information contains a code for making proteins. You learned in Chapter 2 that proteins are made by joining amino acids together into long chains, called polypeptides. As many as 20 different amino acids are commonly found in polypeptides.
The specific amino acids in a polypeptide, and the order in which they are joined, determine the properties of different proteins. The sequence of amino acids influences the shape of the protein, which in turn determines its function. How is the order of bases in DNA and RNA molecules translated into a particular order of amino acids in a polypeptide?
As you know from Lesson 13.1, RNA contains four different bases: adenine, cytosine, guanine, and uracil. In effect, these bases form a “language” with just four “letters”: A, C, G, and U. We call this language the genetic code. How can a code with just four letters carry instructions for 20 different amino acids? The genetic code is read three “letters” at a time, so that each “word” is three bases long and corresponds to a single amino acid. Each three-letter “word” in mRNA is known as a codon. As shown in Figure 13–5, a codon consists of three consecutive bases that specify a single amino acid to be added to the polypeptide chain.
FIGURE 13–5 Codons A codon is a group of three nucleotide bases in messenger RNA that specifies a particular amino acid. Observe What are the three-letter groups of the codons shown here?
Table of Contents
- Formulas and Equations
- Applying Formulas and Equations
- Mean, Median, and Mode
- Estimation
- Using Measurements in Calculations
- Effects of Measurement Errors
- Accuracy
- Precision
- Comparing Accuracy and Precision
- Significant Figures
- Calculating With Significant Figures
- Scientific Notation
- Calculating With Scientific Notation
- Dimensional Analysis
- Applying Dimensional Analysis