Molecular Mechanisms of Gene Expression and DNA Replication

Protein Synthesis and Protein Structure

Definition of Proteins

Proteins are large molecules formed by the union of amino acids. There are 20 unique amino acids, of which 8 are considered essential because they cannot be synthesized by living organisms. Proteins differ from each other based on the specific order (sequence) of these amino acids.

Functions of Proteins

• They form part of cellular structures.
• They are involved in cellular metabolism.

The Synthesis Process (Gene Expression)

Protein synthesis occurs in two main phases:

1. Transcription: Information transfer from DNA to mRNA (occurs in the nucleus).
2. Translation: Information transfer from mRNA to Protein (occurs in the cytoplasm, involving ribosomes).

Transcription: DNA to mRNA

Genetic information passes from DNA to messenger RNA (mRNA) within the nucleus:

1. The two DNA strands unwind and separate.
2. The information base sequence stored in the template strand is transferred to mRNA by the enzyme RNA polymerase.

The resulting mRNA is constituted by nitrogenous bases complementary to the template strand, but thymine (T) is replaced by uracil (U).

Translation: mRNA to Protein

mRNA passes from the nucleus to the cytoplasm where it binds to the ribosomes. Simultaneously, free amino acids in the cytoplasm bind to transfer RNA (tRNA), which transports them to the mRNA.

The process involves the following recognition mechanism:

• Each amino acid bound to its tRNA recognizes a specific sequence of three mRNA bases, known as a codon.
• tRNA facilitates this recognition because it possesses an area of three complementary bases for each mRNA codon, called the anticodon.

The tRNA links to the mRNA, creating a growing protein chain. Finally, the completed protein separates from the ribosomes.

The Genetic Code

The genetic code is the correspondence between mRNA codons and the amino acids that form the protein. There are 64 possible correspondences (derived from 4 mRNA nucleotides combined in groups of 3 bases, where each triplet, or codon, determines one amino acid).

Characteristics of the Genetic Code

1. Specificity: Each codon (triplet) codes for a specific amino acid.
2. Degeneracy: Amino acids can be determined by more than one codon or triplet.
3. Non-ambiguity: Each codon only encodes one type of amino acid.
4. Universality: The code is universal, apart from a few small exceptions.
5. Start and Stop Signals:
   • The beginning of translation (Start Codon) is AUG (which codes for the amino acid methionine, Met).
   • The end of translation (Termination or Stop Codons) are UAA, UAG, and UGA (these do not code for any amino acids).

Transmission of Genetic Information: DNA Replication

Definition of DNA Replication

DNA replication is the process by which an identical copy of DNA is made.

Stages of DNA Replication

1. The DNA unwinds, and the two chains separate.
2. Each original DNA strand is duplicated independently, serving as a template for a new complementary DNA chain. This process is catalyzed by the enzyme DNA polymerase (also known as replicase).
3. Two identical double helices are obtained, each of which is made up of one new chain and one old chain. This mechanism is known as semiconservative replication.