Molecular+Genetics

Nucleic Acids

  • Biomolecules: Nucleic acids are part of the major group of biomolecules.

  • Types of Nucleic Acids: DNA and RNA.

  • Function: Responsible for inheritance and transmission of specific characteristics across generations, focusing on DNA in this unit.

Structure of DNA

  • DNA is a double-stranded molecule composed of nucleotides.

  • Each nucleotide consists of:

    • Sugar

    • Phosphate groups

    • Nitrogen bases

Nitrogenous Bases

  • Components: DNA has four nitrogen bases:

    • Adenine (A)

    • Cytosine (C)

    • Guanine (G)

    • Thymine (T)

  • Base Pairs: A pairs with T, and C pairs with G.

Amino Acids and Genetic Code

  • Nitrogenous bases often occur in groups of three, forming a code for an amino acid.

  • Example: CAT codes for histidine.

  • Note: Understanding of all 20 amino acids is not required at this stage.

DNA Replication

  • Location: DNA replication occurs in the nucleus.

  • Chromosomes coil up, with each chromosome containing one DNA molecule.

  • Humans have 23 pairs of chromosomes. DNA is crucial in cell division.

DNA Packaging and Protection

  • Function: DNA is packaged to protect it during replication and gene expression.

  • Nucleosomes: Composed of DNA wrapped around proteins (histones), forming structures that compact DNA.

Steps of DNA Replication

Initiation

  • Occurs in the cytoplasm (prokaryotes) or nucleus (eukaryotes during S-phase).

  • Helicase: Unzips DNA by breaking hydrogen bonds at the replication fork.

  • Topoisomerase: Prevents DNA from recoiling during unzipping.

Elongation

  • Leading and Lagging Strands:

    • Leading strand is continuous, synthesized in the 5' to 3' direction by DNA polymerase III.

    • Lagging strand synthesized in small segments (Okazaki fragments), beginning at multiple primers.

Termination

  • Exonucleases remove RNA primers and replace them with DNA bases.

  • DNA ligase joins Okazaki fragments into a continuous strand.

  • Telomeres: Protect chromosome ends; synthesized by telomerase.

  • Final product: Two DNA molecules, each with one parent and one new strand.