Cell Biology

DNA Structure

• DNA (deoxyribonucleic acid) composition

  • Repeating units known as nucleotides

  • Components of nucleotides:

  • Sugar:

    • Deoxyribose (lacks an oxygen atom on the 2’ carbon compared to ribose in RNA)

  • Phosphate Group

  • Nitrogenous Base

    • Basic structure: nitrogenous base attached to the 1’ carbon of the sugar; phosphate connected to the 5’ carbon

Nucleotide Classification

• Types of nitrogenous bases:

  • Purines:

  • Double-ring structures

  • Includes adenine (A) and guanine (G)

  • Pyrimidines:

  • Single-ring structures

  • Includes cytosine (C) and thymine (T)

• Base Pairing Rules:

  • Adenine pairs with thymine through two hydrogen bonds

  • Guanine pairs with cytosine through three hydrogen bonds

  • Ensures consistent width of the DNA strands

Structure of a DNA Strand

• Double Helix Formation:

  • Two antiparallel strands twist around each other

  • One strand: 5’ to 3’ end; other strand: 3’ to 5’

• Sugar-Phosphate Backbone:

  • Phosphate group of one nucleotide joins to the 3’ carbon of the next deoxyribose

  • Crucial for structural integrity of DNA

DNA Packing

• DNA organization within the cell nucleus

• Histones:

  • Positively charged proteins that facilitate DNA packing

• Formation of Chromatin:

  • Wrapping of DNA around histones forming nucleosomes

• Further coiling into Solenoids:

  • Condensation into Chromosomes

• Human Genome:

  • Consists of 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes)

Gene Expression

• DNA as the blueprint for cellular function

• Instructions for Protein Synthesis:

  • Proteins carry out cellular activities

• Role of Messenger RNA (mRNA):

  • Translates genetic information from DNA to proteins

• Central Dogma of Molecular Biology:

  • Flow of information: DNA → RNA → Protein

  • Illustrates sequential transfer of genetic information within a biological system