DNA & RNA

Genetic Material

  • Genes are fundamental units of heredity, located on chromosomes.

  • Chromosomes are composed of DNA tightly coiled around proteins (histones) to form chromatin when uncondensed in the nucleus.

  • Genes are specific sequences of DNA that encode for particular traits or functional products.

DNA (Deoxyribonucleic Acid)

  • Overview: The primary genetic material in most organisms, responsible for long-term storage of genetic information.

  • Structure:

    • A polymer composed of repeating monomer units called nucleotides.

    • Each nucleotide consists of three components:

    • A five-carbon sugar: deoxyribose (lacks a hydroxyl group at the 2' carbon).

    • A phosphate group (PO4PO_4).

    • A nitrogenous base: adenine (A), guanine (G), cytosine (C), or thymine (T).

      • Purines (double-ring structure): Adenine (A) and Guanine (G).

      • Pyrimidines (single-ring structure): Cytosine (C) and Thymine (T).

    • Forms a double-stranded alpha-helix (right-handed geometry).

    • The two strands are antiparallel: one runs in the 5' to 3' direction, and the other runs in the 3' to 5' direction.

    • The sugar-phosphate backbone forms the outer 'rails' of the helix, linked by phosphodiester bonds.

    • The nitrogenous bases face inwards, forming the 'rungs' and are held together by hydrogen bonds:

    • Adenine (A) always pairs with Thymine (T) via two hydrogen bonds.

    • Cytosine (C) always pairs with Guanine (G) via three hydrogen bonds.

    • This complementary base pairing is consistent with Chargaff's rules (A=TA=T and C=GC=G).

    • Primarily located in the nucleus in eukaryotes, and the nucleoid region in prokaryotes; also found in mitochondria and chloroplasts.

  • Functions:

    • Heredity Storage: Carries the genetic instructions for the development, functioning, growth, and reproduction of all known organisms and many viruses.

    • Protein Synthesis Blueprint: Contains the code for making messenger RNA (mRNA), which then serves as a template for protein synthesis. This central concept is often summarized by the Central Dogma of molecular biology: DNA <br>ightarrow<br>ightarrow RNA <br>ightarrow<br>ightarrow Protein.

    • Coordination of Cell Activities: By coding for enzymes and other proteins, DNA indirectly coordinates virtually all cellular activities, leading to the expression of specific traits.

  • Discovery:

    • James Watson and Francis Crick, utilizing X-ray diffraction data primarily from Rosalind Franklin and Maurice Wilkins, proposed the double-helix structure of DNA in 1953.

    • Watson and Crick, along with Wilkins, were awarded the Nobel Prize in Physiology or Medicine in 1962. Rosalind Franklin had passed away earlier and Nobel Prizes are not awarded posthumously.

RNA (Ribonucleic Acid)

  • Overview: A nucleic acid present in all living cells, essential in various biological roles in coding, decoding, regulation, and expression of genes.

  • Structure:

    • A polymer of nucleotides, similar to DNA.

    • Each nucleotide consists of three components:

    • A five-carbon sugar: ribose (contains a hydroxyl group at the 2' carbon).

    • A phosphate group (PO4PO_4).

    • A nitrogenous base: adenine (A), guanine (G), cytosine (C), or uracil (U). (Uracil replaces thymine found in DNA).

    • Typically single-stranded, although it can fold into complex three-dimensional structures with regions of complementary base pairing (e.g., hairpin loops).

    • Found in both the nucleus and cytoplasm (including ribosomes) in eukaryotes; in the cytoplasm (and associated with ribosomes) in prokaryotes.

  • Types of RNA:

    • mRNA (Messenger RNA):

    • Function: Carries the genetic information copied from DNA in the nucleus to the ribosomes in the cytoplasm. It dictates the sequence of amino acids to be bonded together to form a specific protein.

    • Structure: A single, relatively straight chain.

    • tRNA (Transfer RNA):

    • Function: Acts as a molecular 'adaptor' that translates the mRNA sequence into amino acids during protein synthesis. Each tRNA molecule picks up a specific amino acid and brings it to the ribosome, matching it to the corresponding codon on the mRNA.

    • Structure: Characteristically forms a clover-leaf shape due to intramolecular base pairing, with an anticodon loop that recognizes mRNA codons and an amino acid attachment site.

    • rRNA (Ribosomal RNA):

    • Function: A major component of ribosomes, the cellular machinery responsible for protein synthesis (translation). rRNA plays a structural role and also possesses catalytic activity (ribozyme) in forming peptide bonds between amino acids.

    • Structure: Highly coiled and folded, forming crucial parts of the ribosomal subunits.