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Powerpoint Presentation: Gene Expression

Molecular Genetics Index

The Chemical Nature if the Gene
The Nucleic Acids
The Gene
Ribonucleic Acid (RNA)
The Characteristics of the Genetic Code
The Genetic Code
Translation
The lac Operon
Gene Mutations
Genetic Engineering
Electrophoresis

Topic Chapters Index

 

Editing the mRNA

In 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.

The mRNA has been edited. The parts which are kept for gene expression are called EXONS.
The parts which are edited out (by snRNP molecules) are called INTRONS.

MOLECULAR GENETICS

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Gene Expression

The gene is a part of a chromosome which functions as a unit to produce a polypeptide.

Two steps are required

  1. Transcription.

    The synthesis of mRNA use the gene on the DNA molecule as a template. This happens in the nucleus of eukaryotes.

  2. Translation.

    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 nucleotides).

  • After the end of the gene is reached there is a terminator sequence that tells RNA polymerase to stop transcribing.

 

Translation

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 and U)

  • If pairs 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 anticodon which 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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