Protein Synthesis: Transcription, Translation, and the Genetic Code CHAPTER. 5B

Protein Synthesis

Overview of Protein Synthesis

Protein synthesis, also known as gene expression, is the process by which genetic information encoded in DNA is used to produce proteins. This process is essential for cell survival and function, as proteins perform most of the structural and functional roles in cells. Protein synthesis involves two main stages: transcription and translation.

• Transcription: The process of copying a gene's DNA sequence into messenger RNA (mRNA).

• Translation: The process by which ribosomes decode the mRNA to build proteins.

Protein synthesis occurs in the nucleus of eukaryotes and the cytoplasm of prokaryotes.

Transcription

Steps of Transcription

Transcription is the first stage of protein synthesis, where the information in a DNA sequence is copied into a complementary RNA sequence.

1. Initiation: RNA polymerase binds to the promoter region of the gene.

2. Elongation: RNA polymerase unwinds the DNA and synthesizes a complementary RNA strand by adding ribonucleotides (A, U, G, C).

   • U pairs with A

   • G pairs with C

3. Termination: Transcription continues until a termination sequence is reached. RNA polymerase then falls off the DNA, and the newly made RNA transcript is released.

Reverse Transcription

While genetic information usually flows from DNA to RNA to proteins, some cells and viruses can perform reverse transcription. In this process, RNA is used as a template to build complementary DNA (cDNA), requiring the enzyme reverse transcriptase.

• Reverse transcription is a key step in the replication of retroviruses (e.g., HIV).

Types of RNA in Protein Synthesis

Three Main Types of RNA

• Messenger RNA (mRNA): Contains codons that code for either an amino acid or a stop signal.

• Transfer RNA (tRNA): Contains an anticodon loop complementary to the codon and carries the correct amino acid to the ribosome for protein synthesis.

• Ribosomal RNA (rRNA): Folds into 3D structures and combines with proteins to form ribosomes, the site of translation.

In prokaryotes, one mRNA molecule can carry codes for several different proteins (polycistronic). In eukaryotes, mRNA is usually monocistronic (codes for one protein).

mRNA Splicing in Eukaryotes

Splicing Mechanism

In eukaryotic cells, mRNA must be processed before translation. This involves RNA splicing, where certain sequences are removed and the remaining parts are joined by a spliceosome.

• Exons: Segments of mRNA that are kept and decoded to build a protein.

• Introns: Intervening sequences that are cut out and not used in protein synthesis.

Alternative splicing allows for different combinations of exons, increasing protein diversity.

Translation and Ribosomes

Structure and Function of Ribosomes

Ribosomes are composed of a large and a small subunit that combine to form an active ribosome. In eukaryotes, the complete ribosome is 80S (60S large subunit + 40S small subunit).

• Ribosomes perform translation, the process of synthesizing proteins from mRNA templates.

The Genetic Code and Amino Acids

Codons and Redundancy

The genetic code is made up of codons, which are sequences of three nucleotides in mRNA. There are four nucleotides in RNA (A, U, G, C), so there are 64 possible codons ($4 \times 4 \times 4 = 64$).

• 60 sense codons code for 22 amino acids

• 3 nonsense codons serve as stop signals

• 1 start signal (AUG, which codes for methionine)

The genetic code is redundant: a single amino acid can be coded by multiple codons (e.g., leucine has six codons). This redundancy helps protect cells from genetic mutations.

Table: The Genetic Code

Codon	Amino Acid
UUU, UUC	Phenylalanine
UUA, UUG, CUU, CUC, CUA, CUG	Leucine
AUG	Methionine (Start)
UAA, UAG, UGA	Stop (Nonsense codons)

Translation Process

Translation is a rapid and accurate process. For example, E. coli can add up to 20 amino acids per second and only misreads about one in 10,000 codons. Translation occurs in three main steps:

1. Initiation: The ribosome assembles around the target mRNA and the first tRNA is attached at the start codon.

2. Elongation: The ribosome moves along the mRNA, and amino acids are added one by one to the growing polypeptide chain.

3. Termination: When a stop codon is reached, the ribosome releases the completed protein.

Key Terms

• Codon: A sequence of three nucleotides in mRNA that specifies an amino acid or stop signal.

• Anticodon: A sequence of three nucleotides in tRNA that is complementary to a codon in mRNA.

• Spliceosome: A complex that removes introns from a transcribed pre-mRNA.

• Polycistronic: mRNA that codes for multiple proteins (common in prokaryotes).

• Monocistronic: mRNA that codes for a single protein (common in eukaryotes).

Example: Start and Stop Codons

• Start codon: AUG (methionine)

• Stop codons: UAA, UAG, UGA

Additional info: The notes above are based on textbook-style lecture slides and include expanded explanations and context for clarity and completeness.