Clear and orderly summary of Chapter 13 of the book "Genetics: Analysis and Principles, 6th Edition by Robert Brooker". Together with all my other summaries of Genetics I got an 8,5 for this course.
Genetics Su mmary Chapter 13 – Translation of mRNA
Translation; mRNA provides the information to synthesize the sequence of amino
acids that constitute polypeptide
13.1 The genetic basis for protein synthesis
The role of some genes is to encode proteins that function as enzymes
Garrod experiment
Garrod proposed that some genes encode for the production of enzyme.
Each step of a metabolic pathway in living cells is catalyzed by a specific enzyme.
Defect in the metabolized compounds?
A disease is the result. Alkaptonuria for example missing enzyme namely
homogentisic acid oxidase
Inborn error of metabolism; genetic disease that involves a defect in metabolic
enzyme. It is caused by a mutation in a gene that causes an enzyme to be inactive.
Beadle and Tatum’s experiment
A strain mutant 1,2,3,4
1; missing enzyme 1. Non-conversion of homoserine into 0-acetylhomoserine.
2; missing enzyme 2. Non-conversion of 0-acetylhomoserine into cystathionine.
3; missing enzyme 3. Non-conversion of cystathionine into homocysteine.
4; missing enzyme 4. Non-conversion of homocysteine into methionine.
Conclusion; one-gene/one-enzyme hypothesis one gene encodes one enzyme.
Hypothesis had to be modified in four ways:
1. Many proteins do not function as enzymes
2. Some proteins are composed of two or more different polypeptides
a. Polypeptide refers to a structure – sequence of amino acids
b. Protein refers to the function – unit of one or more polypeptides
3. Genes do not encode polypeptides
4. Gene can encode multiple polypeptides due to mRNA is alternatively spliced
or edited
13.2 The relationship between the genetic code and protein synthesis
During translation
Translation; information
within mRNA is used to
make a polypeptide.
Codons in mRNA is
recognized by
anticodons in tRNA.
DNA storage the
information that
specifies the amino acid
sequence of polypeptide
, Genetic code; sequence of three bases in mRNA molecule. Consist of 64 different
codons. 20 different amino acids. Genetic code = relationship between three base
codon and an amino acid or the end of translation
Degeneracy; more than one codon specifies the same amino acid.
Synonymous codons; two or more codons specify the same amino acid.
Sense codons; codon that encodes a specific amino acid. Vb. AUG = methionine =
start codon
Nonsense codons/termination codons/stop codons;
codons that signals the end of translation of a polypeptide.
Exceptions to the genetic code
Genetic code is nearly universal (overal hetzelfde)
Few exceptions has been notice.
How do UGA and UAG codons specific for selenocysteine
and pyrrolysine instead of a stop codon?
- UGA is followed by a sequence (selenocysteine
insertion sequence SECIS) which forms a stem-
loop. SECIS is recognized by proteins that favor the
binding of a UGA codon to a tRNA carrying
selenocysteine instead of factors that needed for
polypeptide termination.
In bacteria; SECIS is located immediately
following the UGA codon
In eukaryotes/archaea; SECIS located downstream 3’ of the mRNA
- UAG works like the same as UGA
Direction of polypeptide
Polypeptide is made peptide bond (C=O N-H) is formed
between carboxyl (zuurgroep, COOH) and the amine group
(NH2).
This occurs via condensation reaction that releases (vrijlaat)
water.
Amino terminus/ N-terminus; free amino
group that is not attached to the second
amino acid. At the 5’- side
Carboxyl terminus/ C-terminus; free
carboxyl group that is not attached to
another amino acid. At the 3’-side
Amino acid sequence determine the
structure and function of proteins
20 different amino acids
Side chain; the chemical structure that is attached to the carbon atom located
between amino and carboxyl groups. R-group. Dus dat stuk wat voor iedere
aminozuur anders.
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