Download The genetic code is a degenerate, non-overlapping set of

The genetic code is a degenerate, non-overlapping set of 64 codons
that encodes for 21 amino acids and 3 stop codons.
LEARNING OBJECTIVES [ edit ]
Discuss the origin of transcription on prokaryotic organisms
Describe the genetic code and how the nucleotide sequence prescribes the amino acid and the
protein sequence
KEY POINTS [ edit ]
The relationship between DNA base sequences and the amino acid sequence in proteins is called
the genetic code.
There are 61 codons that encode amino acids and 3 codons that code for chain termination for a
total of 64 codons.
Unlike, eukayrotes, a bacterial chromosome is a covalently-closed circle.
The DNA double helix must partially unwind for transcriptionto occur; this unwound region is
called a transcription bubble.
TERMS [ edit ]
redundancy
duplication of components, such as amino acid codons, to provide survival of the total system in
case of failure of single components
nucleotide
the monomer comprising DNA or RNA molecules; consists of a nitrogenous heterocyclic base that
can be a purine or pyrimidine, a five-carbon pentose sugar, and a phosphate group
amino acid
Any of 20 naturally occurring α-amino acids (having the amino, and carboxylic acid groups on the
same carbon atom), and a variety of side chains, that combine, via peptide bonds, to form
proteins.
Give us feedback on this content: FULL TEXT [edit ]
The Genetic
Code:Nucleotide sequences
prescribe the amino acids
The genetic code is the relationship
between DNA base sequences and the
amino acid sequence in proteins. Features
of the genetic code include:
Amino acids are encoded by three
nucleotides.
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It is non-overlapping.
It is degenerate.
There are 21 genetically-encoded amino acids universally found in the species from all
three domains of life. ( There is a 22nd genetically-encooded amino acid, Pyl, but so far it has
only been found in a handful of Archaea and Bacteria species.) Yet there are only four
different nucleotides in DNA or RNA, so a minimum of three nucleotides are needed to code
each of the 21 (or 22) amino acids . The set of three nucleotides that codes for a single amino
acid is known as a codon. There are 64 codons in total, 61 that encode amino acids and 3 that
code for chain termination. Two of the codons for chain termination can, under certain
circumstances, instead code for amino acids.
Genetic Code Table.
A codon is made of three nucleotides. Consequently there are 43 (=64) different codons. The 64 codons
encode 22 different amino acids and three termination codons, also called stop codons.
Degeneracy is the redundancy of the genetic code. The genetic code has redundancy, but no
ambiguity. For example, although codons GAA and GAG both specify glutamic acid
(redundancy), neither of them specifies any other amino acid (no ambiguity). The codons
encoding one amino acid may differ in any of their three positions. For example, the amino
acid glutamic acid is specified by GAA and GAG codons (difference in the third position); the
amino acid leucine is specified by UUA, UUG, CUU, CUC, CUA, CUG codons (difference in
the first or third position); while the amino acid serine is specified by UCA, UCG, UCC, UCU,
AGU, AGC (difference in the first, second or third position). These properties of the genetic
code make it more fault-tolerant for point mutations.
Origin of transcription on prokaryoticorganisms
Prokaryotes are mostly single-celled organisms that, by definition, lack membrane-bound
nuclei and other organelles. The central region of the cell in which prokaryotic DNA resides is
called the nucleoid region. Bacterial and Archaeal chromosomes are covalently-closed circles
that are not as extensively compacted as eukaryotic chromosomes, but are compacted
nonetheless as the diameter of a typical prokaryotic chromosome is larger than the diameter
of a typical prokaryotic cell. Additionally, prokaryotes often have abundant plasmids, which
are shorter, circular DNA molecules that may only contain one or a few genes and often carry
traits such asantibiotic resistance.
Transcription in prokaryotes (as in eukaryotes) requires the DNA double helix to partially
unwind in the region of RNA synthesis. The region of unwinding is called a transcription
bubble. Transcription always proceeds from the same DNA strand for each gene, which is
called the template strand. The RNA product is complementary to the template strand and is
almost identical to the other (non-template) DNA strand, called the sense or coding strand.
The only difference is that in RNA all of the T nucleotides are replaced with U nucleotides.
The nucleotide on the DNA template strand that corresponds to the site from which the first
5' RNA nucleotide is transcribed is called the +1 nucleotide, or the initiation site. Nucleotides
preceding, or 5' to, the template strand initiation site are given negative numbers and are
designated upstream. Conversely, nucleotides following, or 3' to, the template strand
initiation site are denoted with "+" numbering and are called downstream nucleotides.