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Summary Chapter 13 - Translation of mRNA
Chapter 13 of Genetics by Brooker.
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Chapter 13 Translation of mRNATranslation is the process in which the sequence of codons within mRNA provides the
information to synthesize the sequence of amino acids that constitute a polypeptide.
13.1 The genetic basis for protein synthesis
The primary role of DNA is to store the information needed for the synthesis of all the
proteins that an organism makes. Genes that encode the amino acid sequence of a
polypeptide are known as protein-encoding genes (also called structural genes). The RNA
transcribed from a protein-encoding gene is called messenger RNA (mRNA). The main
function of the genetic material is to encode the production of cellular proteins in the correct
cell, at the proper time, and in suitable amounts. This is an extremely complicated task
because living cells make thousands of different proteins.
Garrod proposed that some genes code for the production enzymes. Garrod studied
patients who had defects in their ability to metabolize certain compounds. He was
particularly interested in the inherited disease known as alkaptonuria. In this disorder, the
patient's body accumulates abnormal levels of homogentisic acid, which is excreted in the
urine, causing it to appear black on exposure to air. In addition, the disease is characterized
by bluish-black discoloration of cartilage and skin. Garrod proposed that the accumulation of
homogentisic acid in these patients is due to a missing enzyme, namely, homogentisic acid
oxidase. Garrod described alkaptonuria as an inborn error of metabolism.
Beadle and Tatum's experiments with neurospora led them to propose the
one-gene/one-enzyme hypothesis. Taken together, the analysis of these mutants allowed
Beadle and Tatum to conclude that a single gene controlled the synthesis of a single
enzyme. This was referred to as the one-gene/one-enzyme hypothesis. In later decades, this
hypothesis had to be modified in four ways:
1. Enzymes are only one category of cellular proteins.
2. Some proteins are composed of two or more different polypeptides.
3. Many genes do not encode polypeptides
4. One gene can encode multiple polypeptides due to alternative splicing and RNA
editing.
13.2 The relationship between the genetic code and protein
synthesis
During translation, the codons in mRNA provide the information to make a
polypeptide with a specific amino acid sequence. The ability of mRNA to be translated
into a specific sequence of amino acids relies on the genetic code. The sequence of three
bases in most codons specifies a particular amino acid. These codons are termed sense
codons.
- Start codon = AUG
- Stop/ termination/ nonsense codon = UAA, UAG and UGA
The codons in mRNA are recognized by the anticodons in transfer RNA. Anticodons are
three-nucleotide sequences that are complementary to codons in mRNA. The tRNA
molecules carry the amino acids that are specified by the codons in the mRNA. In this way,
the order of codons in mRNA dictates the order of amino acids within a polypeptide.
Degeneracy = more than one codon can specify the same amino acid. These codons are
, termed synonymous codons.
Exceptions to the genetic code include the incorporation of selenocysteine and
pyrrolysine into polypeptides. The genetic code is nearly universal. However, a few
exceptions to the genetic code have been noted.
A polypeptide has directionality from its amino-terminus to its carboxyl-terminus. As a
polypeptide is made, a peptide bond is formed between the carboxyl group in the last amino
acid of the polypeptide and the amino group in the amino acid being added. The first amino
acid is said to be at the N-terminus, or amino terminus, of the polypeptide. An amino group is
found at this site. The term N-terminus refers to the presence of a nitrogen atom at this end.
By comparison, the amino acid is in a completed polypeptide located at the C-terminus, or
carboxyl-terminus. A carboxyl-group is always found at this site in the polypeptide.
The amino acid sequence of polypeptide determine the structure and function of
proteins. Figure 13.5 shows the 20 different amino acids that are most commonly found
within polypeptides. Each amino acid contains a unique side chain, or R group, that has its
own particular chemical properties. Following gene transcription and mRNA translation, the
end result is a polypeptide within a defined amino acid sequence. This sequence is the
primary structure of a polypeptide. The primary structure of a typical polypeptide may be a
few hundred or even a couple thousand amino acids in length.
This folding of polypeptides is governed by their primary structure and occurs in multiple
stages (Figure 13.6). The first stage involves the formation of a regular, repeating shape
known as a secondary structure. The two main types of secondary structures are the α helix
and the β sheet (Figure 13.6b). A single polypeptide may have some regions that fold into an
α helix and other regions that fold into a β sheet. Secondary structures within polypeptides
are primarily stabilized by the formation of hydrogen bonds between atoms that are located
in the polypeptide backbone. In addition, some regions do not form a repeating secondary
structure. Such regions have shapes that look very irregular in their structure because they
do not follow a repeating folding pattern.The short regions of secondary structure within a
polypeptide are folded relative to each other to make the tertiary structure.
A protein is a functional unit that can be composed of one or more polypeptides. Some
proteins are composed of a single polypeptide. Many proteins, however, are composed of
two or more polypeptides that associate with each other to make a functional protein with a
quaternary structure. The individual polypeptides are called subunits of the protein, each of
which has its own tertiary structure. The association of multiple subunits is the quaternary
structure of a protein.
Proteins are primarily responsible for the characteristics of living cells and an
organism's traits. Why is the genetic material largely devoted to storing the information to
make proteins? To a great extent, the characteristics of a cell depend on the types of
proteins that it makes. In turn, the traits of multicellular organisms are determined by the
properties of their cells. Proteins perform a variety of roles that are critical to the life of cells
and to the morphology and function of organisms. Table 13.3 describes several examples of
how proteins function.
13.3 Experimental determination of the genetic code
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