Understanding DNA Replication, Gene Expression, and Mutations

Nucleotides: The Building Blocks of Nucleic Acids

Nucleotides are the fundamental structures of nucleic acids. They consist of a phosphate group, a pentose sugar (either ribose or deoxyribose), and a nitrogenous base.

DNA Replication: The Semiconservative Process

DNA replication is the process by which DNA copies itself to ensure daughter cells receive identical genetic information. The resulting molecules consist of two strands: one original strand and one newly synthesized strand, which is why this process is termed semiconservative.

The Replication Process

• The DNA double helix opens and the strands separate.
• Free nucleotides attach themselves to the two separated DNA strands.
• Each nucleotide joins to a complementary nitrogenous base.
• The attached nucleotides join together, resulting in two molecules identical to the original DNA.

Gene Expression and Protein Synthesis

Gene expression is the process by which an organism transforms information from nucleic acids into proteins. A gene is defined as a segment of DNA that codes for protein synthesis.

Evolution of the Central Dogma

Francis Crick's original theory has undergone several modifications:

• Reverse Transcription: The discovery of AIDS-causing viruses revealed that some viruses synthesize DNA from RNA.
• Prions: These are proteins capable of propagating without DNA.
• Ribozymes: These are RNA molecules that can replicate themselves.

Transcription: DNA to mRNA

Transcription is the process where genetic information is transferred from DNA to mRNA:

1. The DNA double helix opens.
2. Complementary nucleotides align with the template strand.
3. Only one strand is copied to create a complementary mRNA strand.
4. The new mRNA molecule exits the nucleus.

Translation: From mRNA to Proteins

Translation occurs through the following steps:

• mRNA exits the nucleus and attaches to ribosomes.
• tRNA carries amino acids to the ribosomes.
• Ribosomes read the mRNA sequence and join amino acids in the correct order based on the nitrogenous bases.
• The resulting protein chain separates from the ribosome.

The Genetic Code

The genetic code refers to the relationship between specific sequences of nitrogenous bases in mRNA and the amino acids that form proteins.

Genetic Mutations

Mutations alter the chemical structure of genes, which can lead to changes in the amino acid sequence and affect protein function.

Types of Mutations

• Chromosomal Mutations: Affect the structure of chromosomes by destroying, duplicating, or rearranging fragments.
• Genomic Mutations: Affect the number of chromosomes:
  • Aneuploidy: An excess or deficiency in the number of chromosomes.
  • Euploidy: An alteration in the diploid number of chromosomes.

Causes of Mutations

Mutations can be spontaneous (involuntary) or induced (provoked):

• Physical Mutagens: Radiation that can damage DNA.
• Chemical Mutagens: Compounds that alter the sequence of nucleotides in DNA.