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DNA Structure, Chromosomes, and Chromatin: Comprehensive Notes

DNA Structure, Chromosomes, and Chromatin

Part 1: DNA Structure

  • Nucleotides as Subunits: Nucleotides are the fundamental building blocks of nucleic acids, specifically DNA.

    • Each nucleotide consists of three main components: a sugar (deoxyribose in DNA), a phosphate group, and a nitrogenous base.

    • Examples of nitrogenous bases include Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).

  • Formation of DNA Polymers: Nucleotides are joined together to form long strands, which are polymers of DNA.

    • A DNA strand has a 5' end and a 3' end, owing to the orientation of the phosphodiester bonds forming the sugar-phosphate backbone.

  • Double-Stranded DNA: DNA typically exists as a double-stranded molecule.

    • The two strands run in opposite directions, a characteristic known as being antiparallel.

    • The bases pair specifically: Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C).

    • A and G are purines (double-ring structures), while C and T are pyrimidines (single-ring structures).

  • Complementary Strands and Hydrogen Bonds:

    • The specificity of base pairing means the two DNA strands are complementary.

    • Hydrogen bonds form between these complementary base pairs: two hydrogen bonds between A and T, and three hydrogen bonds between G and C.

    • These hydrogen bonds contribute to the stability of the double helix.

  • DNA Double Helix Structure:

    • A DNA molecule consists of two chains of DNA coiled around a common axis, forming a double helix.

    • The chains are antiparallel: one runs 5' to 3', and the other runs 3' to 5'.

    • The sugar-phosphate backbone forms the outer part of the helix.

    • The nitrogenous bases stack on the inside of the helix.

    • The DNA helix has a fixed diameter of 20 ext{ Å}.

  • Genes and Proteins (Gene Expression):

    • Most genes contain the instructions, encoded in the DNA sequence, to make specific proteins.

    • Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product, such as a protein or RNA molecule.

Chromosomes and Chromatin

  • Eukaryotic DNA Packaging:

    • Eukaryotic DNA is housed within the cell's nucleus and is organized into multiple chromosomes.

    • The human genome contains approximately 3.2 imes 10^9 nucleotides, which are packaged into 24 distinct chromosomes.

    • Chromatin is the complex of DNA and proteins that forms chromosomes within the nucleus of eukaryotic cells.

    • Chromosomes are inherited from parents, with homologous chromosomes being pairs of chromosomes that have the same genes at the same loci but may have different alleles.

  • Abnormal Chromosomes: Deviations in chromosome number or structure can lead to inherited genetic diseases.

  • Genes and Genome:

    • A gene is defined as a segment of DNA that contains instructions for making a particular protein or RNA molecule.

    • A genome refers to the total genetic information carried by all chromosomes in a cell or an entire organism.

  • Correlation of Genome Size and Organism Complexity: While there is some correlation, it's not always direct.

    • Examples of Genome Size and Gene Count:

      • Amoeba dubia: ~6.7 imes 10^{11} base pairs (genes not listed)

      • Human: ~2.9 imes 10^9 base pairs, ~20,000-25,000 genes

      • Grape: ~4.7 imes 10^8 base pairs, ~30,000 genes

      • Fruit fly (Drosophila melanogaster): ~1.2 imes 10^8 base pairs, ~13,600 genes

      • Baker's yeast (Saccharomyces cerevisiae): ~1.2 imes 10^7 base pairs, ~6,300 genes

      • E. coli: ~4.1 imes 10^6 base pairs, ~4,800 genes

  • Genome Size vs. Chromosome Number: Two closely related species can have similar genome sizes but vastly different numbers of chromosomes.

    • Example: The Chinese muntjac and Indian muntjac have vastly different chromosome numbers despite being closely related.

  • Chromosome Forms Throughout Cell Life:

    • Interphase chromosomes: During interphase (the period of growth and DNA replication before mitosis), chromosomes are less condensed, allowing for gene expression and DNA replication.

    • Mitotic chromosomes: During M phase (mitosis), chromosomes become highly condensed and visible, facilitating cell division.

Part 2: Discovery of DNA

  • Early Understanding of Genetic Material:

    • Gregor Mendel postulated the existence of