protein synthesis

Introduction to Protein Synthesis

Protein synthesis, also referred to as gene expression, is a crucial biological process that occurs in both eukaryotic and prokaryotic cells. The primary function of this process is to synthesize proteins from genes, leading to the production of specific proteins needed for various cellular functions.

DNA and Protein Synthesis

The fundamental relationship between DNA, RNA, and proteins can be summarized by the flow of genetic information: DNA is transcribed into messenger RNA (mRNA), and mRNA is translated into proteins. This is a critical point that differs from DNA replication, which occurs only once before cell division, whereas protein synthesis occurs continuously throughout the cell's life cycle.

Key Differences: DNA Replication vs Protein Synthesis

1. DNA Replication:

   • Happens once before cell division.

   • Involves the duplication of the entire DNA sequence (genome).

2. Protein Synthesis:

   • Occurs repeatedly during the cell's life as needed.

   • Involves the synthesis of specific proteins from single genes as required by the cell.

Types of RNA Involved in Protein Synthesis

The protein synthesis process involves three main types of RNA:

1. Messenger RNA (mRNA): Carries the genetic information from DNA to ribosomes for protein synthesis.

2. Transfer RNA (tRNA): Transfers specific amino acids to the ribosome, corresponding to the mRNA codons.

3. Ribosomal RNA (rRNA): A structural component of ribosomes, facilitating the translation process.

Process of Transcription

Transcription is the first step in protein synthesis:

1. Initiation:

   • The specific gene that needs to be expressed is activated ("turned on") by a signal sent to the nucleus.

   • Each gene has a promoter sequence that initiates transcription, and a terminator sequence that signals when to stop.

2. RNA Polymerase:

   • The enzyme responsible for transcription is RNA polymerase. It binds to the promoter sequence and starts synthesizing mRNA by reading the DNA template strand.

3. Template Strand:

   • In the process, the two strands of DNA separate, with RNA polymerase specifically reading the template strand (the strand being transcribed). The complementary mRNA sequence is synthesized by matching RNA nucleotides to the DNA template.

4. Termination:

   • The RNA polymerase continues transcription until it reaches the terminator sequence, at which point transcription ceases, and the mRNA strand is complete.

Transcription Example

Given a DNA sequence of:

• $ext{DNA: TAC GGT CAC}$
  The corresponding mRNA sequence post-transcription is:

• $ext{mRNA: AUG CCA GUG}$
  This exercise illustrates how you should be able to derive the mRNA sequence from a given DNA strand.

Role of mRNA

Once transcribed, mRNA exits the nucleus and travels to the cytoplasm, where ribosomes facilitate protein synthesis. mRNA functions as the template that provides the instructions for building proteins through the ribosome's reading of codons.

Transfer RNA (tRNA) and its Function

tRNA is essential in the translation phase of protein synthesis:

1. Structure:

   • tRNA is characterized by its cloverleaf shape, occurring as a single strand that folds upon itself.

2. Amino Acid Binding:

   • At one end, tRNA carries a specific amino acid, functioning as a molecular taxi that brings the correct amino acid to the ribosome corresponding to the mRNA codon.

3. Anticodon:

   • At the other end, tRNA has a three-nucleotide anticodon which defines the specific amino acid that is carried. For instance, if the anticodon is "AAG", it might correspond to the amino acid lysine. If it changes to "UUG", tRNA could carry a different amino acid.

Process of Translation

Translation follows transcription and is critical for synthesizing proteins:

1. Initiation of Translation:

   • The ribosome binds to mRNA at the start codon (AUG), indicating where the translation should begin. This codon specifies the amino acid methionine (MET).

2. Reading mRNA:

   • The ribosome reads the mRNA sequence in triplet codons. For example, the first codon would be "AUG", the second "CCA", the third "GUG", and so forth.

3. tRNA Binding:

   • Each tRNA brings the appropriate amino acid, corresponding to each codon in the mRNA sequence based on codon-anticodon pairing.

4. Peptide Bonds Formation:

   • Amino acids are linked together by peptide bonds forming a growing polypeptide chain.

5. Ribosome Movement:

   • As translation progresses, the ribosome shifts to read new codons, while previously used tRNA molecules leave to collect more amino acids. This continues until a stop codon is reached, signaling the end of translation.

6. Finalizing Protein:

   • Upon reaching a stop codon, translation stops, and the newly synthesized protein is released. The protein then undergoes folding and post-translational modifications to achieve its final functional form.

The Codon Table and Its Importance

Understanding protein synthesis also requires familiarity with the codon table, which maps mRNA codons to their respective amino acids. For example:

• The codon "UUU" codes for the amino acid phenylalanine, while "CCU" codes for proline.

Gene Sequence Determination

The specific sequence of the gene being expressed ultimately determines the amino acid sequence of the resultant protein. Thus, the original gene sequence is paramount in determining the structure and function of the synthesized protein, influencing its primary, secondary, and tertiary structures, leading to the final functional protein product.

Reading Assignments

It is essential to complete the assigned readings to understand the differences between prokaryotic and eukaryotic protein synthesis thoroughly. Please refer to page 193 of the assigned textbook for further information on this topic.