Yasmine Abugrin
Group 4A
Even though deoxyribonucleic acid(DNA) and ribonucleic acid(RNA) have
different molecular structures, they both carry genetic information. Briefly describe
the structures of both nucleic acids and explain in detail the characteristics of the
genetic code.
Synopsis:
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DNA structure.
RNA structure.
Characteristics of genetic code: triple code, unambiguous code, degeneracy, unpunctuated code,
universal code, non-overlapping code.
DNA is made up of molecules called nucleotides. Each nucleotide contains a phosphate group, a sugar
group (deoxyribose sugar) and a nitrogen base. The types of bases are adenine, thymine, guanine and
cytosine. Two poly nucleotide chains are held together by complementary nitrogenous bases with
hydrogen bonds between them. Adenine pairs with thymine via two hydrogen bonds while guanine
pairs thymine via three hydrogen bonds. This means that the base pairs are specific and are called
complementary base pairs. The two strands of DNA run in opposite directions enabling the bases to
face each other and thus be able to pair. Therefore, the strands are described as being antiparallel. The
nitrogenous bases of the two strands of DNA are bound together, according to the base pairing rules,
to make double stranded DNA called a double helix.
RNA on the other hand is typically single stranded and made up of ribonucleotides which are linked
together by phosphodiester bonds. A ribonucleotide in RNA contains one of the four nitrogenous bases
which are adenine, uracil, guanine and cytosine, ribose the pentose sugar and a phosphate group. RNA
nucleotides have a uracil base instead of a thymine. Uracil will base pair with adenine the same way
thymine base pairs with adenine. Single stranded RNA can form many secondary structures where a
single RNA molecule folds over and forms loops and a clover-leaf structure.
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Yasmine Abugrin
Group 4A
The nucleotide base sequence on DNA and on mRNA which will be translated into a sequence of
amino acids of the protein to be synthesized is called the genetic code. The collection of codons is
called the genetic code. There are four different bases and 20 amino acids, so the bases are read in
triplets thus giving 64 combinations.
A group of three adjacent bases, in mRNA or DNA, coding for an amino acid is called a codon. The
codons are formed using the bases present in mRNA. A sequence of three nucleotides code for one
amino acid in protein meaning that the code is triplet. The four nucleotide bases adenine, guanine,
uracil and cytosine in mRNA are used to produce the three base codons. There are 64 codons in total
in which three of them are stop codons.
To begin with, the genetic code is said to be unambiguous meaning specific. This means that a
particular codon will only code for a particular amino acid. For example, UUU codes for Phenyl
Alanine and it cannot code for any other amino acid. Generally, the same codon should never code for
two different amino acids but in some cases of an ambiguous code, the same codon could code for two
or more than two amino acids. For example, the codon GGA may code for two amino acids which are
glycine and glutamic acid.
More than one codon may code for the same amino acid, this is called degeneracy of the code. For
example, except for methionine and tryptophan, all other 18 amino acids have more than one codon.
Thus, nine amino acids have two codons each, an example is glutamine. Isoleucine has three codons.
Five amino acids have four codons each, an example is valine. Finally, three amino acids have six
codons each, an example is leucine. Degeneracy of the genetic code has certain advantages. For
example, it provides a mechanism of minimizing mutations.
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Yasmine Abugrin
Group 4A
The genetic code is unpunctuated. There are no spaces between individual codons. It means that after
an amino acid is coded, the second amino acid will be immediately coded without any wasted
nucleotides in between. However, there is one codon which means "start" which means the start of the
gene sequence.The codon is AUG which codes for the amino acid methionine. Methionyl specifically
binds initiation site of the mRNA containing the AUG initiation codon.
There are three stop codons which are UAG, UAA and UGA. They do not code for any amino acid.
These codons are read by some specific proteins called release factors but are not read by any tRNA
molecules via their anticodons.
For all organisms ranging from bacteria to man, the genetic code is valid. However, there is an
exception in mitochondrial codons where UGA codes for tryptophan and AUA codes for methionine,
instead of stop codon respectively of cytoplasmic protein synthesis machinery. AGG and AGA code
for Arginine in the cytoplasm but in mitochondria they are stop codons.
The code as said before is read in groups of three. A nucleotide that forms part of a triplet never forms
part of the next triplet. Each triplet is read from the 5' to the 3'. When translating mRNA, the codons
do not overlap. Thus, this means that a base in mRNA is not used for different codons. Therefore, the
genetic code is said to be non-overlapping.
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