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DNA and RNA Structure and Molecular Components

DNA (Deoxyribonucleic acid) Overview: - Most human cells contain a total of $46$ molecules of DNA. - Physical Dimensions: DNA has a uniform diameter of $2\,nm$ and an average length of approximately $2\text{-in.}$ per molecule.
Molecular Level Composition: - Nucleic acids, which include both DNA and RNA, are polymers composed of repeating units called nucleotides. - A single nucleotide consists of three specific components: 1. A sugar ( $Deoxyribose$ in DNA; $Ribose$ in RNA). 2. A phosphate group. 3. A nitrogenous base. - Overall Shape: DNA is structured as a double helix, often compared to the shape of a spiral staircase.
Nitrogenous Bases in Detail: - There are five nitrogenous bases in total, divided into two categories: - Purines (double-ring structure): Adenine (A) and Guanine (G). - Pyrimidines (single-ring structure): Cytosine (C), Uracil (U), and Thymine (T). - Specificity to Nucleic Acids: - Thymine (T): Only found in DNA. - Uracil (U): Only found in RNA.

DNA Structural Organization and Base Pairing

The "Twisted Ladder" Model: - DNA is often visualized as a "soft rubber ladder" that can be twisted into a helix. - Backbone: The sidepieces of the ladder are formed by alternating phosphate groups and the sugar deoxyribose. - Step-like Connections: The "steps" of the ladder are formed by pairs of nitrogenous bases.
Law of Complementary Base Pairing: - Nitrogenous bases from opposite strands form hydrogen bonds to connect the double helix. - Specific base pairs occur as follows: - Adenine (A) always pairs with Thymine (T). - Cytosine (C) always pairs with Guanine (G).

DNA Function and the Human Genome

Primary Role: DNA carries the essential instructions provided by genes for the process of protein synthesis.
Definitions: - Gene: A specific segment of DNA that codes for the production of one polypeptide or closely related proteins. Genes are the fundamental determinants of the characteristics of a species and the distinct traits of each individual. - Genome: The comprehensive collection of all the genes belonging to one person.
Genomic Data and Statistics: - Humans possess an estimated $25,000$ to $35,000$ genes. - Coding vs. Noncoding DNA: - Only approximately $2\%$ of DNA consists of actual genes (coding sequences). - The other $98\%$ of DNA is noncoding, referred to as either "junk" DNA or organizational DNA.

Chromatin and Chromosome Folding Hierarchy

Chromatin Structure: - Chromatin is the filamentous material composed of both DNA and proteins that make up the $46$ chromosomes in the interphase nucleus. - Visual Appearance: Often described as "beads on a string."
Nucleosomes: - The "beaded" string is divided into segments known as nucleosomes. - Each nucleosome consists of histones (core particles) and linker DNA.
Compaction Stages in Dividing Cells: 1. DNA double helix: $2\,nm$ diameter. 2. DNA winds around core particles: Forms nucleosomes and linker DNA with an $11\,nm$ diameter. 3. Zigzag fiber: Nucleosomes fold into a zigzag structure measuring $30\,nm$ in diameter. 4. Irregular loops: The $30\,nm$ fiber is organized into irregular loops, resulting in a $300\,nm$ fiber. 5. Chromatid formation: In dividing cells, the looped chromatin coils further to form a chromatid with a diameter of $700\,nm$ . 6. Metaphase Chromosome: The chromosome at the midpoint of cell division consists of two sister chromatids joined at a centromere (visible under a light microscope). - Associated structures include the kinetochore located at the centromere region.

RNA: Ribonucleic Acid Structure and Types

Comparison to DNA: - Size: RNA is significantly smaller than DNA, containing fewer bases. - Architecture: Consists of only a single nucleotide chain (single-stranded) rather than a double helix. - Chemical Differences: Ribose sugar replaces deoxyribose, and Uracil (U) replaces Thymine (T) as a nitrogenous base.
Types of RNA: - Messenger RNA (mRNA): Typically contains over $10,000$ bases. - Ribosomal RNA (rRNA). - Transfer RNA (tRNA): The smallest variety, consisting of $70$ to $90$ bases.
Core Functions: - RNA's essential role is to interpret the DNA code and directly manage protein synthesis in the cytoplasm.
Cellular Logistics: - DNA remains safely inside the nucleus, while RNA performs its primary work in the cytoplasm.

Protein Synthesis and Genetic Control

Genetic Influence on Cell Action: - DNA provides the code for synthesizing all cellular proteins, including the enzymes responsible for creating nonprotein substances. - Example: The production of Testosterone. 1. Luteinizing Hormone (LH) from the pituitary travels to the interstitial cell of the testis. 2. LH triggers a second messenger system inside the cell. 3. Transcription occurs: DNA is used to produce mRNA. 4. Translation occurs: mRNA is used to create specific enzymes. 5. The activated enzymes facilitate the conversion of cholesterol into testosterone, which is then secreted.
Gene Activation: Different cells synthesize different proteins because different genes are activated in different cell types.
The Central Dogma Summary: DNA $\rightarrow$ mRNA $\rightarrow$ Protein.

The Genetic Code and Codon Logic

The Genetic Code: A system that permits the $4$ nucleotides (A, T, G, C) to represent the $20$ different amino acids.
Base Triplet: A sequence of $3$ nucleotides found on the DNA molecule that stands for one specific amino acid (e.g., DNA triplet $TAC$ ).
Codon: The "mirror-image" sequence of nucleotides located on the mRNA molecule. - There are $64$ possible codons ( $4^3$ ). - Redundancy: Often, $2$ to $3$ codons code for the same amino acid. - Start Codon: $AUG$ (the code for the amino acid Methionine). - Stop Codons: $UAG$ , $UGA$ , and $UAA$ (signify the end of synthesis).
Table 4.2 Examples (DNA Triplet $\rightarrow$ mRNA Codon $\rightarrow$ Amino Acid): - $CCT \rightarrow GGA \rightarrow$ Glycine ( $Gly$ ) - $CCA \rightarrow GGU \rightarrow$ Glycine ( $Gly$ ) - $CCC \rightarrow GGG \rightarrow$ Glycine ( $Gly$ ) - $CTC \rightarrow GAG \rightarrow$ Glutamic acid ( $Glu$ ) - $CGC \rightarrow GCG \rightarrow$ Alanine ( $Ala$ ) - $CGT \rightarrow GCA \rightarrow$ Alanine ( $Ala$ ) - $TGG \rightarrow ACC \rightarrow$ Threonine ( $Thr$ ) - $TGC \rightarrow ACG \rightarrow$ Threonine ( $Thr$ ) - $GTA \rightarrow CAU \rightarrow$ Histidine ( $His$ ) - $TAC \rightarrow AUG \rightarrow$ Methionine ( $Met$ )

The Mechanics of Translation

Ribosomal Binding Sites (Large Subunit): - P (peptidyl) site. - A (acceptor) site. - E (exit) site.
The Six-Step Process of Translation: 1. Initiator tRNA (carrying the amino acid methionine) attaches to the start codon ( $AUG$ ) on the messenger RNA. 2. The large and small ribosomal subunits join to create a functional ribosome; the initiator tRNA is positioned in the P site. 3. An incoming tRNA anticodon pairs with the next available mRNA codon at the A site. 4. The amino acid attached to the tRNA at the P site forms a peptide bond with the amino acid at the A site. 5. The ribosome moves by one codon: The tRNA at the P site exits, and the tRNA originally at the A site shifts into the P site. 6. Protein synthesis terminates when the ribosome reaches a stop codon on the mRNA, releasing the completed protein.

Questions & Discussion

Base Sequence Prediction: - Question: What would be the base sequence of the DNA strand across from $ATTGACTCG$ ? - Answer: Using complementary base pairing rules ( $A$ with $T$ , $C$ with $G$ ), the complementary strand is $TAACTGAGC$ .
Quantitative Composition Prediction: - Question: If a DNA molecule were known to be $20\%$ adenine, predict its percentage of cytosine. - Answer: - If $A = 20\%$ , then $T = 20\%$ (since $A = T$ ). - The sum of $A + T = 40\%$ . - The remaining amount for $C + G$ is $100\% - 40\% = 60\%$ . - Since $C = G$ , the percentage of cytosine is $\frac{60\%}{2} = 30\%$ .