Lesson 1: Genes, Genetic Code, and Replication Process

1. Basic Concepts:

a. Gene:

• Location: A gene is a segment of a DNA molecule that carries genetic information encoding for a specific trait.

• Structure: A gene is composed of nucleotides linked together in a specific sequence.

• Function: Genes contain genetic information, determining the synthesis of products (polypeptide chains or RNA) for the cell.

b. Nucleotide:

• Definition: A nucleotide is the monomer that makes up nucleic acids (DNA and RNA).

• Structure: Each nucleotide consists of 3 components:

• A pentose sugar molecule (deoxyribose in DNA and ribose in RNA)

• A phosphate group

• A nitrogenous base (adenine – A, guanine – G, cytosine – C, thymine – T in DNA and uracil – U in RNA)

c. Genetic Code:

• Definition: The genetic code is a triplet code, where each three-nucleotide sequence on the mRNA molecule encodes for an amino acid in the polypeptide chain.

• Characteristics:

• Specificity: Each codon encodes for a unique amino acid.

• Degeneracy: One amino acid can be encoded by multiple different codons.

• Universality: The genetic code is almost universal for all living organisms, with a few exceptions.

• Continuity: The genetic code is read continuously in triplets, without overlapping.

2. DNA Replication Process:

a. Definition:

• DNA replication is the process of copying an exact copy of the original DNA molecule, producing two identical daughter DNA molecules.

b. Mechanism:

• Stage 1: Unwinding and Separation of Strands: The helicase enzyme unwinds and separates the two single strands of the parental DNA molecule.

• Stage 2: Synthesis of New Strands:

• Leading Strand: Synthesized continuously in the 5′ → 3′ direction by the DNA polymerase enzyme.

• Lagging Strand: Synthesized discontinuously in the 5′ → 3′ direction, forming Okazaki fragments, which are later joined by the ligase enzyme.

• Stage 3: Checking and Repairing: Enzymes check and repair errors during replication to ensure the accuracy of genetic information.

c. Significance:

• DNA replication helps maintain genetic information stability across generations of cells.

• Provides genetic material for cell division and sexual reproduction.

Note:

• DNA replication is a complex process that requires the participation of many different enzymes.

• Replication occurs in the nucleus of eukaryotic cells and the nucleoid region of prokaryotic cells.

3. Transcription Process:

a. Definition:

• Transcription is the process of synthesizing RNA from a DNA template strand.

b. Mechanism:

• Step 1: Initiation: RNA polymerase binds to the promoter region of the gene, unwinds, and separates the two DNA strands.

• Step 2: Elongation: RNA polymerase moves along the DNA template strand, using free nucleotides in the cell to synthesize the new RNA strand according to the principle of complementarity (A-U, G-C, T-A).

• Step 3: Termination: RNA polymerase encounters a termination signal on the DNA, releasing the newly synthesized RNA molecule.

c. Products:

• mRNA: Carries genetic information from genes to ribosomes for protein synthesis.

• tRNA: Transports amino acids to ribosomes to participate in translation.

• rRNA: Combines with proteins to form ribosomes, where translation occurs.

Note:

• Transcription occurs in the nucleus of eukaryotic cells and the nucleoid region of prokaryotic cells.

• The template strand of DNA is used as a template for RNA synthesis.

• The complementary strand of DNA is called the coding strand.

4. Translation Process:

a. Definition:

• Translation is the process of synthesizing proteins from genetic information encoded on the mRNA molecule.

b. Mechanism:

• Step 1: Initiation: The ribosome binds to the mRNA, the tRNA carrying the start amino acid (methionine – Met) binds to the start codon of the mRNA.

• Step 2: Elongation: The ribosome moves along the mRNA, each codon is read, the corresponding tRNA carrying the appropriate amino acid binds to the ribosome, and the amino acids are linked together by peptide bonds.

• Step 3: Termination: The ribosome encounters a stop codon on the mRNA, the polypeptide chain is released from the ribosome.

c. Location:

• Translation occurs in the cytoplasm, at the ribosomes.

Note:

• Ribosomes are composed of rRNA and proteins.

• A polyribosome (polysome) is multiple ribosomes acting simultaneously on one mRNA molecule, increasing the efficiency of protein synthesis.

5. Regulation of Gene Activity:

a. Definition:

• Regulation of gene activity is the process of controlling gene expression, determining which genes are transcribed and translated, and at what levels.

b. Mechanism:

• Transcriptional Regulation: Control of gene transcription by regulating the binding of RNA polymerase to the promoter region of the gene.

• Translational Regulation: Control of mRNA translation by regulating the binding of ribosomes to mRNA.

• Post-translational Control: Control of protein activity after it is synthesized.

c. Significance:

• Regulation of gene activity helps cells adapt to changing environments, maintaining balance in life processes.

• It brings diversity in morphology and function to living organisms.

Note:

• The regulation of gene activity is a complex process, controlled by various factors, including regulatory proteins, chemical signals, and environmental factors.

6. Types of Genes:

a. Structural Genes:

• Definition: Structural genes are genes that encode protein products, involved in building structures and performing functions of the cell.

• Examples: Genes encoding enzymes, structural protein genes, antigen genes.

b. Regulatory Genes:

• Definition: Regulatory genes are genes that encode regulatory proteins, which play a role in controlling the activity of other genes.

• Examples: Genes encoding repressor proteins, genes encoding activator proteins.

Note:

• Regulatory genes are often located near or far from structural genes but have functional relationships.

• The coordinated activity of structural genes and regulatory genes creates gene expression regulation, enabling efficient cell operation.

7. Stop Codons and Start Codons:

a. Stop Codons:

• Definition: Codons that encode termination signals for the translation process.

• Stop Codons: UAA, UAG, UGA.

• Function: When the ribosome encounters a stop codon, the polypeptide chain is released from the ribosome, and the translation process ends.

b. Start Codons:

• Definition: Codons that encode the start amino acid (methionine – Met) in the polypeptide chain.

• Start Codon: AUG.

• Function: The start codon identifies the translation start point on the mRNA, helping the ribosome bind to the mRNA and begin protein synthesis.

Note:

• Start and stop codons are special codons that do not encode any amino acids.

• Start and stop codons play a critical role in ensuring the accuracy and efficiency of the translation process.

8. Frequently Asked Questions:

• What is the difference between transcription and translation?

Transcription is the process of synthesizing RNA from a DNA template strand, while translation is the process of synthesizing proteins from genetic information on the mRNA molecule. Transcription occurs in the cell nucleus, while translation occurs in the cytoplasm.

• What is the role of RNA polymerase in transcription?

RNA polymerase is the enzyme that catalyzes the transcription process, it binds to the promoter region of the gene, unwinds and separates the two DNA strands, using free nucleotides in the cell to synthesize the new RNA strand.

• What is the role of ribosomes in translation?

Ribosomes are organelles where translation occurs, they bind to mRNA, read each codon on the mRNA, the corresponding tRNA carrying the appropriate amino acid binds to the ribosome, and the amino acids are linked together by peptide bonds.

• What is the difference between a structural gene and a regulatory gene?

Structural genes encode protein products, involved in building structures and performing functions of the cell, while regulatory genes encode regulatory proteins, which play a role in controlling the activity of other genes.

Note:

• The above article only introduces basic concepts of genes, genetic code, and replication, transcription, and translation processes.

• To better understand these processes, you need to learn more information from specialized materials.

• Feel free to refer to other articles on genetics to enhance your knowledge.