Nucleic Acids

Nucleic Acids - Lecture 4

• Genome: A complete set of DNA strands holding all the instructions for an organism, inheritable. Formed by sequences of bases (A, T, G, C) – these sequences are GENES. Each gene codes for one protein. The central dogma is DNA -> RNA -> Protein .

• DNA Structure: Double-stranded, anti-parallel helix. Bases pair specifically: A with T (2 hydrogen bonds), G with C (3 hydrogen bonds). Purines (A, G) and Pyrimidines (T, C) pair in a consistent manner. The numbers of A and T bases are equal to the numbers of G and C bases (Chargaff's rule). DNA strands have 5' and 3' ends.

• RNA Structures: Nucleic acid, single-stranded but can form complex 3D shapes. Three main types:

• mRNA: Copies the DNA code and carries it to ribosomes for protein synthesis.

• tRNA: Carries amino acids to ribosomes during protein synthesis (translation).

• rRNA: Component of ribosomes; site of protein synthesis.

• Nucleotide Structure: Nucleotides consist of a phosphate group, a pentose sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base. Common nucleotides include ATP (energy), cAMP (second messenger), GTP (energy and information transfer).

• DNA Packing (Chromatin):

• DNA is neatly organized within cells by histones (proteins). Histones form a core around which DNA wraps (nucleosome).

• Nucleosomes are like beads on a string; they coil and stack, creating chromatin fibers.

• Humans have 25,000 genes, but most of the DNA is non-coding (“junk DNA”). Non-coding DNA plays important regulatory roles. Enhancers and repressors are types of regulatory elements in non-coding DNA.

• Chromatin remodeling is crucial for controlling gene expression and cell differentiation.

• Gene Expression & Cell Specialization:

• Cells have the potential to become any cell type; however a cell becomes specialized by only expressing a subset of its genes. This means that cells only express 20% of their total genes.

• DNA Replication:

• Semi-conservative process: Each new DNA molecule contains one old strand and one newly synthesized strand.

• DNA polymerase is the enzyme that builds new DNA strands using an existing strand as a template.

• Mutations & Repair:

• Mutations are changes to DNA, sometimes leading to incorrect protein production.

• Mechanisms to detect and repair errors exist.

• Genetic Code (Translation):

• Instructions for assembling proteins are stored as codons (three-nucleotide sequences).

• 61 codons code for amino acids, 3 are stop signals.

• The mRNA code directs tRNA to bring the appropriate amino acids to ribosomes, creating a polypeptide chain.

• Transcription:

• Copying DNA to mRNA.

• Catalyzed by RNA polymerase.

• RNA nucleotides are added following base-pairing rules (U replaces T).

• Translation:

• Protein synthesis using mRNA as a template.

• Ribosomes synthesize protein using tRNA to deliver amino acids.