Powerpoint Presentation: Gene Expression
Molecular Genetics Index
The Chemical Nature if the Gene
The Nucleic Acids
Ribonucleic Acid (RNA)
The Characteristics of the Genetic Code
The Genetic Code
The lac Operon
Topic Chapters Index
Editing the mRNA
prokaryotes the transcribed mRNA goes straight to the ribosomes in the cytoplasm.
In eukaryotes the freshly transcribed mRNA in the nucleus is about 5000 nucleotides long
When the same mRNA is used for translation at the ribosome it is only 1000 nucleotides long.
mRNA has been edited. The parts which are kept for gene expression are called
The parts which are edited out (by snRNP molecules) are called
The gene is a part of a chromosome which functions as a unit to produce a polypeptide.
Two steps are required
The synthesis of mRNA use the gene on the DNA molecule as a template. This happens in the nucleus of
The synthesis of a polypeptide chain using the genetic code on the mRNA molecule as its guide.
Transcription: The synthesis of a
strand of mRNA
Uses an enzyme RNA polymerase.
Proceeds in the same direction as replication (5' to 3')
Forms a complementary strand of mRNA
It begins at a promotor site which signals the beginning of gene
is not much further down the molecule (about 20 to 30
the end of the gene is reached there is a terminator sequence that tells RNA polymerase to stop transcribing.
Location: The ribosomes in the cytoplasm that provide the environment for translation.
The genetic code is brought by the mRNA molecule.
What is the genetic code?
The genetic code consists of the sequence of bases found along the mRNA molecule.
There are only four letters to this code (A, G, C and U).
The code needs to be complex enough to represent
20 different amino acids used to build proteins.
How many combinations of these four bases are needed to code for 20 different amino acids?
If one base represented one amino acid this would only be able to produce 4 different combinations. (A, C, G
of bases represented each amino acid this would only
be able to produce 4 x 4 = 16 combinations. (AA, AC, AG, AU, CA, CC, CG, CU etc).
If triplets of bases represented each amino acid, this would be able to produce 4 x 4 x 4 = 64 combinations.
This is enough combinations to code for the 20 amino acids but is the code actually made of triplets?
Nature is logical!
Over 10 years biochemists synthesised bits of mRNA with different combinations. Then they used them to
synthesise polypeptides. The results proved the logical answer was correct. The genetic code is made of
triplets of bases called codons.
tRNA molecules do the translating. At one end the have a
specific amino acid attached. At the other end they have an
is complementary to the mRNA codons.
The tRNA molecules with the correct anticodons are lined up with their bases
complementary to the mRNA codons.
The process of translation proceeds in the same direction
as replication and transcription (5' to 3').
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