Transcription and Translation – From Gene to Protein

Transcription and Translation – From Gene to Protein

1. Transcription

a. Definition: Transcription is the process of synthesizing RNA from a DNA template.

b. Structure and function of RNA types:

• mRNA (messenger RNA):

• Function: Serves as a template for translation at the ribosome, determining the amino acid sequence in the polypeptide chain.

• Structure:

• The 5′ end has a specific nucleotide sequence (not translated) that helps the ribosome recognize and bind to it.

• Straight chain.

• Usually degraded after the translation process is complete.

• tRNA (transfer RNA):

• Function:

• Transports amino acids to the ribosome.

• Participates in translation, converting genetic information from mRNA into the amino acid sequence on the polypeptide chain.

• Structure:

• Has a specific anticodon triplet to recognize and base-pair with the corresponding codon on mRNA.

• There are many different types of tRNA, each carrying a specific amino acid.

• rRNA (ribosomal RNA):

• Function: Combines with proteins to form ribosomes, the sites where protein synthesis occurs.

• Structure:

• Consists of two subunits that exist separately in the cytoplasm.

• They combine to form a ribosome during translation.

c. Purpose: RNA synthesis.

d. Principle: Using one strand of DNA as a template to copy a strand of mRNA with a 5′-3′ direction, following the complementary base pairing rules A-T, G-X.

e. Location: Nucleus of the cell.

f. Process:

• Step 1: Initiation of transcription:

• RNA polymerase enzyme binds to the regulatory region (DH) of the gene, causing the gene to unwind, revealing the template strand (3′-5′).

• RNA polymerase begins synthesizing mRNA at a specific location (transcription start).

• Step 2: mRNA synthesis:

• RNA polymerase moves along the template strand of the gene, synthesizing new mRNA with a 5′-3′ direction according to the complementary base pairing rules (A-T, G-X).

• When it encounters a transcription termination signal at the end of the gene, RNA polymerase stops and releases the mRNA molecule.

• The region of the gene that has just been transcribed immediately winds back up.

• Step 3: Synthesized mRNA performs its function:

• In prokaryotes (SVNS): mRNA after transcription is directly used as a template for protein synthesis.

• In eukaryotes (SVNT): mRNA after transcription must have its introns removed, exons spliced together to form mature mRNA, and then pass through the nuclear membrane into the cytoplasm to serve as a template for protein synthesis.

2. Translation

a. Definition: Translation is the process of synthesizing protein from an mRNA template.

b. Stages: Translation is divided into two main stages:

• Amino acid activation: In the cytoplasm, each amino acid is activated and attached to its corresponding tRNA, forming an aminoacyl-tRNA complex (aa-tRNA) by a specific enzyme and ATP energy.

• Polypeptide chain synthesis:

• Initiation:

• The small subunit of the ribosome binds to mRNA at a specific recognition site (near the start codon AUG).

• The initiator complex Met-tRNA (UAX) base-pairs correctly with the start codon AUG on mRNA.

• The large subunit combines with the small subunit to form a complete ribosome, ready to synthesize the polypeptide chain.

• Elongation of the polypeptide chain:

• The second codon on mRNA binds complementarily to the anticodon of the aa2-tRNA complex.

• The ribosome acts as a scaffold holding mRNA and the aa-tRNA complex together, allowing Met and aa2 to form a peptide bond between them.

• Subsequent steps occur similarly.

• Termination:

• When the ribosome encounters a stop codon on mRNA, translation is complete.

• The amino acid at the beginning of the chain (Met) is usually removed by a specific enzyme.

c. Result: The polypeptide chain formed continues to form higher-order structures, becoming an active biological protein.

d. Note: In practice, mRNA often simultaneously binds to a group of ribosomes called a polyribosome (or polysome), which increases protein synthesis efficiency.

3. Genetic information and trait expression:

• Genetic information is expressed as traits:

• Transcription: DNA -> mRNA

• Translation: mRNA -> protein

• Protein is expressed as a trait.

• Genetic material is transmitted to offspring through: DNA replication.

This translation provides a comprehensive explanation of transcription and translation in English, including the essential steps and details. Please let me know if you have any further questions.