Genes, Genetic Code and DNA Replication

Genes, Genetic Code and DNA Replication

1. Genes

• Definition: A gene is a segment of a DNA molecule that carries genetic information encoding for a polypeptide chain or an RNA molecule.

• Structure of a structural gene:

• Consists of 3 regions:

• Regulatory region: Located at the 3′ end of the template strand, containing specific nucleotide sequences that enable RNA polymerase to recognize and bind for initiating transcription.

• Coding region: Carries the information encoding for amino acids. There are two types:

• SVNS: Continuous coding region (non-fragmented gene).

• SVNT: Discontinuous coding region (fragmented gene) because it has exons (coding segments for aa) alternating with introns (non-coding segments for aa).

• Terminator region: Located at the 5′ end of the template strand, carrying the transcription termination signal.

• Number of strands: A gene has 2 strands:

• Template strand: Serves as the template for transcription.

• Complementary strand: Complementary to the template strand.

• Strand direction:

• Template strand: 3′?5′

• Complementary strand: 5′?3′

2. Genetic Code

• Codon: A triplet code for aa on mRNA.

• Anti-codon: A triplet code complementary to tRNA.

• Triplet: A triplet code for aa on DNA.

• Stop codon: UAG, UGA, UAA (on mRNA). Specifies the termination signal for transcription.

• Start codon: AUG. Initiates translation, encoding for methionine (NT) and formylmethionine (NS).

Characteristics of the genetic code:

• Continuity: The genetic code is read from a specific point in triplets, without overlapping.

• Specificity: Each triplet codes for a unique amino acid.

• Universality: All species share the same genetic code.

• Degeneracy: Multiple different triplets can specify the same amino acid (except for AUG and UGG).

3. DNA Replication

• Stage in the cell cycle: S phase of interphase.

• Application: Creating countless copies in a short time for research and practical applications.

• Process:

• Step 1: Unwinding the DNA molecule:

• The unwinding enzyme separates the two single strands of DNA to create a Y-shaped fork, exposing the template strand.

• Step 2: Synthesizing new DNA strands:

• DNA polymerase uses one strand as a template to synthesize the new strand according to the complementary principle (A=T, G?X).

• DNA polymerase synthesizes the new strand in the 5′-3′ direction. Therefore:

• Template strand 3′-5′: The new strand is synthesized continuously.

• Template strand 5′-3′: The new strand is synthesized discontinuously, creating short segments (Okazaki fragments), which are then joined together by ligase.

• Step 3: Two DNA molecules are formed:

• Each DNA molecule formed consists of one newly synthesized strand and one old strand from the original DNA molecule (semi-conservative principle).

Purpose of DNA replication:

• Transmit genetic information to the next generation.

• Underlies cell duplication.

Location of replication:

• Prokaryotes: Nucleoid region.

• Eukaryotes:

• Nucleus: DNAs with the same replication frequency.

• Organelles (mitochondria, chloroplasts): Different replication frequencies and different from nuclear DNA.

Direction of new strand synthesis: 5′-3′.

Capabilities of DNA polymerase:

• Unable to unwind DNA.

• Unable to self-synthesize nucleotides/starting sequences.

Ligase (joining enzyme):

• Joins Okazaki fragments together, forming phosphodiester bonds.

• Works on both new single strands (because both have Okazaki fragments).