Genetics and Cell Function Vocabulary

DNA Structure and Chemical Composition

• Nucleic acids, including deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are polymers constructed from repeating units called nucleotides.

• Most human cells contain 46 individual molecules of DNA.

• Physical dimensions of DNA molecules:

  • Uniform diameter: $2\,\text{nm}$

  • Average physical length: approximately 2 inches ($2\,\text{in.}$)

• Structure of a nucleotide unit:

  • A pentose sugar (deoxyribose in DNA)

  • A phosphate group

  • A nitrogenous base

Nitrogenous Bases

• Nitrogenous bases are categorized into two structural classes based on their ring system:

  • Purines: Double-ring structures

  • Adenine (A)

  • Guanine (G)

  • Pyrimidines: Single-ring structures

  • Cytosine (C)

  • Thymine (T) — present exclusively in DNA

  • Uracil (U) — present exclusively in RNA

The DNA Double Helix

• Structural architecture of DNA:

  • Formed as a double-stranded helix, structural analogue to a spiral staircase or a flexible, twisted ladder.

  • Sugar-phosphate backbone: The upright sidepieces consist of alternating sugar (deoxyribose) and phosphate groups bonded covalently.

  • Step-like rungs: Horizontally aligned pairs of nitrogenous bases spanning between the two backbones.

• Law of Complementary Base Pairing:

  • Hydrogen bonding selectively joins complementary base pairs across the two strands:

  • Adenine pairs with Thymine ($A-T$)

  • Cytosine pairs with Guanine ($C-G$)

DNA Function and Genome Organization

• Primary function: DNA stores genetic instructions required for the synthesis of polypeptides and proteins.

• Gene definition: A discrete segment of DNA containing chemical instructions to code for one polypeptide chain or closely related functional proteins.

  • Genes dictate specific cellular functions, species characteristics, and individual phenotypes.

• Genome organization:

  • Genome refers to the complete set of genetic information in an individual.

  • The human genome contains an estimated 25,000 to 35,000 genes.

  • Coding DNA constitutes only approximately $2\%$ of total nuclear DNA.

  • The remaining $98\%$ of human DNA is noncoding, functioning either as structural/organizational DNA or non-translated regulatory sequences.

Complementary Base Pairing Calculation Examples

• Example 1: Determining a complementary DNA sequence

  • Given sequence: ATTGACTCG

  • Applying complementary base pairing rules ($A \leftrightarrow T$, $C \leftrightarrow G$):

  • Complementary sequence: TAACTGAGC

• Example 2: Calculating nucleotide proportions

  • Given: A DNA molecule composed of $20\%$ Adenine ($A$).

  • Step 1: By complementary base pairing, Thymine ($T$) content equals Adenine content: $T = 20\%$.

  • Step 2: Combined $A + T$ proportion is $20\% + 20\% = 40\%$.

  • Step 3: Remaining percentage allocated to $C + G$ is $100\% - 40\% = 60\%$.

  • Step 4: Because Cytosine ($C$) equals Guanine ($G$), Cytosine content is $\frac{60\%}{2} = 30\%$.

Higher-Order Chromatin Packaging and Chromosomes

• Chromatin composition:

  • Complex composed of nuclear DNA molecules combined with structural proteins (histones).

  • Represents the extended filamentous state of the 46 chromosomes in non-dividing (interphase) cell nuclei.

• Levels of DNA packaging and structural hierarchy:

  1. Double Helix: Unwrapped DNA molecule measures $2\,\text{nm}$ in diameter.

  2. Nucleosome ("Beads-on-a-string"): DNA strand winds around spherical histone core particles (forming $11\,\text{nm}$ complex beads) linked together by segments of linker DNA.

  3. Chromatin Fiber: Nucleosomes fold continuously into a dense $30\,\text{nm}$ zigzag fiber structure.

4. Irregular Loops: The $30\,\text{nm}$ fiber forms expansive $300\,\text{nm}$ irregular loops.

5. Condensed Chromatid: During active cell division, chromatin loops coil tightly into a $700\,\text{nm}$ thick chromatid.

6. Metaphase Chromosome: Highly condensed structure visible under light microscopy during cell division midpoints.

• Structure of Metaphase Chromosomes:

  • Consists of two identical longitudinal strands called sister chromatids.

  • Joined together at a constricted central junction known as the centromere.

  • Kinetochores: Protein complexes bound to the centromere on each side that serve as attachment points for spindle fibers during cell division.

Ribonucleic Acid (RNA) Structure and Types

• Structural features of RNA:

  • Composed of a single polynucleotide chain (does not form a double helix).

  • Sugar backbone contains ribose in place of deoxyribose.

  • Pyrimidine base uracil (U) replaces thymine (T).

  • Unpaired nitrogenous bases.

  • Significantly smaller in length than genomic DNA.

• Major functional types of RNA:

  • Messenger RNA (mRNA): Transcribes gene codes; generally possesses greater than 10,000 bases.

  • Ribosomal RNA (rRNA): Structural and catalytic component of ribosomes.

  • Transfer RNA (tRNA): Soluble RNA molecules that deliver amino acids; smallest type containing 70 to 90 bases.

• Compartmentalization of Nucleic Acid Functions:

  • DNA remains localized inside the cell nucleus to preserve the genome.

  • RNA molecules are synthesized in the nucleus and exported to the cytoplasm to execute protein assembly.

Gene Expression and Control of Cellular Function

• Genetic control of cellular activities:

  • DNA provides master genetic instructions encoding all cellular proteins.

  • Encoded proteins include enzymes, which directly catalyze synthesis pathways for non-protein molecules (e.g., carbohydrates, lipids, steroids).

• Steroidogenesis regulatory pathway example (Testosterone production):

  1. Luteinizing Hormone (LH) secreted from the pituitary binds receptors on interstitial cells of the testis.

  2. Second messenger signaling pathways are activated inside the cell.

  3. Gene transcription is stimulated, producing specific mRNA.

  4. mRNA is translated into metabolic enzymes.

  5. Activated enzymes process cholesterol precursors into testosterone for cellular secretion.

• Cellular Differentiation and Gene Activation:

  • All somatic cell genomes possess identical DNA sequences.

  • Specialized cell phenotypes arise from selective gene activation, determining which specific proteins are synthesized by a given cell type.

Overview of Protein Synthesis

• Central Dogma Sequence:   $\text{DNA} \rightarrow \text{mRNA} \rightarrow \text{Protein}$

• Transcription:

  • Process occurring within the nucleus where an mRNA copy is generated from an activated DNA gene strand.

  • Newly formed mRNA exits through nuclear pores into the cytoplasm.

• Translation:

  • Cytoplasmic process where mRNA base sequences are decoded by ribosomes.

  • Transfer RNA (tRNA) molecules transport specific amino acids to the active ribosomal sites.

  • Ribosomal complexes join amino acids via peptide bonds in the exact order commanded by the mRNA transcript.

The Genetic Code

• Conceptual basis: System that translates sequences of 4 nucleotide bases ($A, T, G, C$) into 20 distinct amino acids.

• Base Triplet:

  • A sequence of 3 consecutive DNA nucleotides representing one specific amino acid (e.g., DNA sequence TAC transcribes to mRNA codon AUG).

• Codons:

  • A complementary 3-nucleotide sequence present on mRNA molecules.

  • Total possible codon combinations: $4^3 = 64$ unique codons.

  • Redundancy/Degeneracy: Multiple distinct codons (often 2 to 3) code for the same amino acid.

  • Start Codon: AUG (codes for methionine and signals the initiation of translation).

  • Stop Codons: UAG, UGA, and UAA (terminate translation; do not code for amino acids).

Genetic Code Correspondence Table

• Examples of DNA base triplets, corresponding mRNA codons, and translated amino acids:

  • DNA CCT $\rightarrow$ mRNA GGA $ ightarrow$ Glycine (Gly)

  • DNA CCA $ ightarrow$ mRNA GGU $ ightarrow$ Glycine (Gly)

  • DNA CCC $ ightarrow$ mRNA GGG $ ightarrow$ Glycine (Gly)

  • DNA CTC $ ightarrow$ mRNA GAG $ ightarrow$ Glutamic acid (Glu)

  • DNA CGC $ ightarrow$ mRNA GCG $ ightarrow$ Alanine (Ala)

  • DNA CGT $ ightarrow$ mRNA GCA $ ightarrow$ Alanine (Ala)

  • DNA TGG $ ightarrow$ mRNA ACC $ ightarrow$ Threonine (Thr)

  • DNA TGC $ ightarrow$ mRNA ACG $ ightarrow$ Threonine (Thr)

  • DNA GTA $ ightarrow$ mRNA CAU $ ightarrow$ Histidine (His)

  • DNA TAC $ ightarrow$ mRNA AUG $ ightarrow$ Methionine (Met)

• Detailed Multi-Step Sequence Translation Example:

  • DNA Coding Triplets: TAC - CGC - CCT - TGC - GTA - CTC - ACT

  • Transcribed mRNA Codons: AUG - GCG - GGA - ACG - CAU - GAG - UGA (AUG = Start; UGA = Stop)

  • Corresponding tRNA Anticodons: UAC - CGC - CCU - UGC - GUA - CUC

  • Resulting Polypeptide Sequence: Methionine (Met) - Alanine (Ala) - Glycine (Gly) - Threonine (Thr) - Valine (Val) - Glutamic acid (Glu)

Detailed Mechanism of Translation

• Functional Sites on the Ribosome Large Subunit:

  • P Site (Peptidyl site): Holds the tRNA attached to the growing polypeptide chain.

  • A Site (Acceptor/Aminoacyl site): Accommodates incoming aminoacyl-tRNA complexes carrying the next amino acid.

  • E Site (Exit site): Uncharged tRNA transitions here prior to release from the ribosome.

• Step-by-Step Translation Process:

  1. Initiation Phase (tRNA Binding):

  • The small ribosomal subunit binds mRNA at the mRNA binding site.

  • Initiator tRNA loaded with methionine pairs its anticodon (UAC) to the start codon (AUG) on the mRNA strand.

2. Ribosomal Assembly:

   • The large ribosomal subunit unites with the small subunit, creating a fully functional ribosome.

   • The initiator tRNA seats directly into the P site.

3. Elongation (Codon Recognition):

   • The anticodon of an incoming tRNA pairs specifically with the next codon of mRNA exposed in the vacant A site.

4. Peptide Bond Formation:

   • Ribosomal peptidyl transferase catalyzes a peptide bond between the amino acid detached from the P-site tRNA and the amino acid attached to the A-site tRNA.

5. Translocation:

   • The uncharged tRNA at the P site exits the ribosome.

   • The ribosome shifts along the mRNA strand by exactly one codon ($5' \rightarrow 3'$ direction).

   • The tRNA carrying the peptide chain shifts from the A site to the P site, opening the A site for the next incoming tRNA.

6. Termination Phase:

   • Translation terminates when the ribosome encounters a stop codon (UAG, UGA, or UAA) at the A site.

   • Release factors cleave the completed polypeptide chain, releasing it into the cell, and the ribosomal complex dissociates.