Lesson 1: Genes, Genetic Code, DNA Replication

Lesson 1: Genes, Genetic Code, DNA Replication

1. Genes:

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

• Structure: A gene is composed of three main regions:

• Regulatory region: Located at the 3′ end of the coding strand of the gene, it has a specific nucleotide sequence for RNA polymerase to recognize, bind, and initiate the transcription process. This region also plays a role in regulating transcription activity.

• Coding region: Carries the information encoding amino acids.

• In prokaryotes, the coding region is characterized by continuous coding.

• In eukaryotes, the coding region is characterized by discontinuous coding.

• Termination region: Located at the 5′ end of the coding strand, it carries the termination signal for transcription.

2. Genetic Code:

• Definition: The genetic code is a triplet code, meaning that each triplet of nucleotides on DNA (or RNA) encodes for an amino acid.

• Stop codons: There are three stop codons: 3′ UAA, UAG, UGA 5′.

• Start codon: AUG.

• Universality: All species share the same genetic code, with a few exceptions.

• Reading frame: Read from a specific point in triplets, without overlapping.

• Specificity: One codon encodes only one amino acid.

• Degeneracy: Multiple codons can encode for the same amino acid.

3. DNA Replication:

• Time and location: Occurs during interphase, in the nucleus of the cell.

• Principles:

• Base pairing: A pairs with T, G pairs with C.

• Semi-conservative principle: Each daughter DNA molecule is composed of one strand from the parent DNA and one newly synthesized strand.

• Steps:

1. Unwinding the parent DNA: The two single strands of DNA separate from each other.

2. Synthesizing the new strand: Free nucleotides from the cell’s cytoplasm bind to the template strand according to base pairing rules, forming a new strand. The synthesis process occurs in the 5′ to 3′ direction.

3. Result: Two daughter DNA molecules are created, each identical to the parent DNA.

• Features of new strand synthesis:

• Template strand: Synthesized continuously, in the same direction as unwinding.

• Complementary strand: Synthesized discontinuously, in the opposite direction of unwinding, in short fragments.