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Summary Genetics Chapter 13
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Chapter 13: translation of mRNATranslation = the process in which the sequences of codons within mRNA provides the information to
synthesize the sequence of amino acids that constitute a polypeptide.
The genetic basis for protein synthesis
Proteins are composed of one or more polypeptides, important as critically active participants in cell
structure and function. The primary role of DNA is to store the information needed for the synthesis of all
the proteins that an organism makes. Protein-encoding genes (=structural genes) = genes that encode the
amino acids sequence of a polypeptide. Messenger RNA (mRNA) = the RNA transcribed from a protein-
encoding gene.
Garrod proposed that some genes code for the production of enzymes
Metabolic pathway; consists of a series of metabolic conversions of one molecule to another, each step
catalyzed by a specific enzyme. Each enzyme is a distinctly different protein that catalyzes a particular
chemical reaction.
Alkaptonuria, abnormal levels of homogentisic acid. Due to defective enzyme; homogentisic acid oxidase.
Inborn error of metabolism (caused by a mutation in agene that causes an enzyme to be inactive).
Relationship between inheritance of the trait and the inheritance of a defective enzyme.
Beadle and tatum’s experiments with neurospora led them to propose the one gene/ one-enzyme
hypothesis
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
, Translation involves an interpretation of one language (the language of mRNA, an nucleotide sequence) into
the language of proteins (an amino acid sequence). The first step to access this information in genes, in
which mRNA is made, is called transcription.
The role of DNA in polypeptide synthesis is storage. It stores the information that specifies the amino acid
sequence of a polypeptide.
The ability of mRNA to be translated relies on the genetic code. The sequences of bases within an mRNA
molecule provides coded information that is read in groups of three nucleotides (codons).
- Sense codons; the sequence of three bases specifies a particular amino acid.
- Start codon; AUG; methionine; the first codon that begins a polypeptide sequence AUG can also be
used to specify additional methionines within the coding sequence.
- UAA, UAG, UGA; stop codons; are signals that end translation. Termination codons; nonsense
codons.
- 5’ untranslated region and 3’untranslated region; regions do not encode a polypeptide; precede the
start codon and follow the stop codon.
The codons in mRNA are recognized by anticodons in transfer RNA (tRNA) molecules. Anticodons are three-
nucleotide sequences that are complementary to codons in the mRNA. The tRNA carry the amino acids that
are specified by the codons on the mRNA.
Genetic code is composed of 64 different codons. Polypeptides are composed of 20 different kinds of amino
acids. The genetic code is said to contain degeneracy; this means that more than one codon can specify the
same amino acid. The codons that specify the same amino acid are termed synonymous codons.
The start codon (AUG) defines the reading frame (= a series of codons determined by reading the basis in
groups of three, beginning with the start codon as a frame of reference) of an mRNA.
Exceptions to the genetic code include the incorporation of selenocysteine and pyrrolysine into polypeptides
The genetic code is nearly universal. There are a few exceptions. Mitochondria (eukaryotic organelles) have
their own DNA, which includes a few protein-
encoding genes. The mitochondrial genetic
code differs from the nuclear genetic code.
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