Chapter 9: DNA Structure and Genetics Practice

Course Logistics, Schedule, and Exam Details

  • Genetics in the News Topic:

    • A rare autism-linked genetic disorder may be far more common than scientists previously estimated.
    • The study identifies specific genes that may lead to autism.
    • Possessing the associated genes indicates a genetic predisposition but does not guarantee that an individual is autistic.

  • Course Schedule and Progress:

    • Completion of the first third of the course material.
    • Transition to Chapter 9 covering DNA structure.
    • Exam Review Session: Scheduled for Wednesday, covering every exam question step-by-step. Genetics in the news will briefly run at the beginning, followed by a complete review period.
    • First Exam: Scheduled for Thursday during prescribed laboratory time, allowing a full 22 -hour period for completion.

  • Exam Format and Permitted Materials:

    • Question types: Multiple choice, true/false, matching, short answer, and genetics/probability math problems.
    • Required equipment: A standard, non-programmable calculator.
    • Prohibited items: Cell phones, laptops, and programmable calculators.

  • Laboratory Handouts and Materials:

    • Handouts provided for genetic traits, paper test strips (Cheveo / 22 test strips), coin toss lab, and the chi-square test.

  • Research Group Topic Requirements:

    • All research groups must confirm and narrow their topics.
    • Example: Projects focusing on CRISPR/Cas in plants are too broad and must be focused on a specific species (e.g., CRISPR application in bananas).

Discovery and Characteristics of Genetic Material

  • Historical Context Prior to 1953:

    • In 1953, Francis Crick and James Watson discovered the molecular structure of DNA.
    • Long before 1953, it was known that a chemical substance passed genetic information from generation to generation because offspring inherit structural characteristics from parents.
    • Genetic Information Definition: Information contained in genes that, when passed to a new generation, influences the form and physical characteristics of the offspring.

  • Four Key Properties of Genetic Material:

    • Replication: Must possess the capability to self-replicate precisely.
    • Storage: Must store complex biological information.
    • Expression: Must express stored information to dictate cellular function.
    • Variation/Mutation: Must have the capacity to undergo occasional structural changes, creating genetic variation (e.g., variations in the growth hormone gene that determine tall or short height).

  • The Chromosomal Component Controversy (Prior to 1944):

    • Prior to 1944, scientists did not know whether nuclear proteins or nucleic acids served as the physical genetic material.
    • Nuclear extraction yielded large amounts of protein and smaller amounts of DNA.
    • Early researchers incorrectly hypothesized that proteins (specifically histones) carried genetic information, believing DNA was merely a structural string holding histones together.
    • Research by Watson, Crick, and Rosalind Franklin ultimately proved that DNA, not protein, carries genetic information.

Central Dogma and Gene Expression Regulation

  • Cellular Uniformity:

    • Every somatic cell in an organism contains an identical nucleus and the exact same set of chromosomes.
    • Brain cells, eye cells, hair cells, skin cells, liver cells, and heart cells all contain identical genetic blueprints.

  • Differential Gene Regulation:

    • Cells selectively activate (turn on) or repress (turn off) specific genes based on their bodily location and functional tissue type.
    • Example: A skin cell activates skin color genes, freckle genes, and hair follicle genes while repressing eye color genes.

  • Pathway of Gene Expression:

    • Nuclear DNA Localization: The master genomic DNA permanently remains inside the cell nucleus.
    • Transcription: Occurs entirely within the nucleus. Double-stranded DNA is transcribed into a single-stranded RNA molecule.
    • Primary RNA Classes:
    • Messenger RNA (mRNA): Carries genetic coding information out to the cytoplasm.
    • Ribosomal RNA (rRNA): Structural and catalytic component of ribosomes.
    • Transfer RNA (tRNA): Delivers specific amino acids during translation.
    • Nuclear Export: MRNA exits the nucleus through nuclear pores into the cytoplasm.
    • Translation: Occurs in the cytoplasm. Ribosomes read the mRNA sequence to synthesize functional proteins.
    • Phenotypic Expression: Synthesized proteins produce the observed physical trait (e.g., brown or blue eye color). The original genomic gene never leaves the nucleus.

Chemical and Molecular Structure of Nucleotides

  • Chemical Definition of DNA:

    • Deoxyribonucleic Acid.

  • Nucleotide Subunits:

    • A nucleotide is the fundamental monomeric building block of DNA, consisting of three distinct chemical components:
    1. Pentose Sugar: 55-carbon deoxyribose ring.
    2. Phosphate Group: Attached to the sugar ring.
    3. Nitrogenous Base: Single or double ring containing nitrogen.

  • Carbon Numbering of the Pentose Sugar:

    • Sugar carbons are designated with prime symbols (1′1' through 5′5'):
    • 1′1' -carbon: Always attached to the nitrogenous base (A, G, C, or T).
    • 2′2' -carbon: Has an −H-H group (lacks an −OH-OH group, making it deoxyribose).
    • 3′3' -carbon: Always contains a free hydroxyl (−OH-OH) group.
    • 4′4' -carbon: Forms part of the ring structure.
    • 5′5' -carbon: Always covalently bonded to the phosphate group.

  • Phosphodiester Bonds:

    • Phosphodiester bonds covalently link individual nucleotides into a single-stranded nucleic acid polymer.
    • Chemical Reaction: The 5′5' phosphate group of an incoming nucleotide reacts with the 3′3' −OH-OH group of the preceding sugar, knocking off a hydrogen atom and forming a phosphodiester linkage.
    • Directionality: DNA strands possess structural polarity, reading from a 5′5' end (free phosphate) to a 3′3' end (free hydroxyl).

  • Nitrogenous Base Categorization:

    • Purines:
    • Structure: Double-ring molecules composed of one 66-carbon ring fused to one 55-carbon ring.
    • Examples: Adenine (AA) and Guanine (GG).
    • Pyrimidines:
    • Structure: Single 66-carbon ring molecules.
    • Examples: Cytosine (CC), Thymine (TT), and Uracil (UU in RNA).
    • Mnemonic: Pyrimidine contains the letter 'y', as do Cytosine and Thymine.

  • Nucleotide vs. Nucleoside Terminology:

    • Nucleotide: Contains all 33 components (Sugar + Phosphate + Nitrogenous Base).
    • Nucleoside: Contains only 22 components (Sugar + Nitrogenous Base); lacks the phosphate group.

Structural Helical Geometry and Base Pairing

  • Chargaff's Rules:

    • In double-stranded DNA, the abundance of Adenine equals Thymine, and the abundance of Guanine equals Cytosine.
    • Purines always pair exclusively with Pyrimidines:
    • Adenine (AA) pairs with Thymine (TT).
    • Guanine (GG) pairs with Cytosine (CC).

  • Hydrogen Bonding Between Strands:

    • Non-covalent hydrogen bonds hold complementary nitrogenous base pairs together across the central axis:
    • A−TA-T Base Pairing: Held together by exactly 22 hydrogen bonds.
    • G−CG-C Base Pairing: Held together by exactly 33 hydrogen bonds.

  • Double Helix Geometry (Watson and Crick Model):

    • X-ray Crystallography: Rosalind Franklin and Maurice Wilkins generated key X-ray diffraction patterns looking axially down the twisted DNA double helix.
    • Right-Handed Helix: DNA strands wind around a central axis in a right-handed double-helical spatial configuration.
    • Antiparallel Strands: The two complementary backbones run in opposite directions (5′→3′5' \rightarrow 3' opposite 3′→5′3' \rightarrow 5').
    • Base Orientation: Nitrogenous bases lie flat, stacking horizontally perpendicular to the outer sugar-phosphate backbones.
    • Structural Grooves: Alternating major and minor grooves twist along the outer surface.

  • Dimensional Measurements of DNA:

    • Helical Width: Exactly 20$\,\text{\AA} (2020 Angstroms).
    • Distance Between Adjacent Base Pairs: Exactly 3.4$\,\text{\AA}.
    • Complete Turn Repeat: Exactly 1010 base pairs per 360∘360^\circ turn.
    • Length of One Complete Helical Turn: 34$\,\text{\AA} (10×3.410 \times 3.4\,\text{\AA} = 34 A˚\,\text{\AA}).

  • Distance Calculations for Genes:

    • Distance across 1010 base pairs: 10×3.410 \times 3.4\,\text{\AA} = 34 A˚\,\text{\AA}.
    • Distance across a 100100 -nucleotide gene segment: 100×3.4100 \times 3.4\,\text{\AA} = 3400 A˚\,\text{\AA}.

Replication Dynamics and Repair Mechanisms

  • High-Speed DNA Replication:

    • Somatic cell division requires copying the full human genome containing 3.6×1093.6 \times 10^9 (3,600,000,0003,600,000,000) base pairs.
    • Chromosomes condense, replicate, and separate during mitosis in under 2020 seconds in fast-dividing contexts.

  • UV Radiation Damage and the UVR Repair Gene:

    • Mechanism of Sunlight Damage: Solar ultraviolet (UV) radiation bombards exposed skin cells, inducing structural mutations and mismatches (such as incorrect T−GT-G pairing or bulky pyrimidine bulges).
    • UVR Gene Function: During REM sleep, the UVR repair gene activates. Its protein product forms a donut-shaped molecular complex with an internal diameter matching DNA's 20$\,\text{\AA} (20$\,\text{nm}) width.
    • Repair Process: The repair ring pulls the DNA strand through its center. Any bulge or distortion caused by mispaired bases halts the complex, triggering enzymatic excision and correct repair.
    • Xeroderma Pigmentosum Pathology ("Children of the Night"): Individuals with a mutated, non-functional UVR gene cannot repair UV-induced mutations during REM sleep. Accumulated DNA damage forces them to avoid daytime sunlight exposure completely.

Mathematical Probability and Statistical Rules in Genetics

  • Laboratory Concepts for Exam Review:

    • Coin Toss Lab: Demonstration of basic binomial probability.
    • Chi-Square Test (χ2\chi^2): Quantitative test for goodness-of-fit in genetic crosses.
    • Population Genetics (Frogs in the Pond Lab): Review Charles Darwin's fundamental tenets of natural selection and population genetics.

  • Fundamental Rules of Dice Probability:

    • Standard Die Specifications: 66 sides, numbered 11 through 66.
    • Probability of rolling a specific number (e.g., a 11 or a 66):     P=16P = \frac{1}{6}
    • Product Rule for Independent Events (Rolling "Boxcars" / Two 66s):     P(Boxcars)=16×16=136P(\text{Boxcars}) = \frac{1}{6} \times \frac{1}{6} = \frac{1}{36}
    • Rolling Five Dice simultaneously for Yahtzee (all 11s on first roll):     P(Yahtzee)=16×16×16×16×16=(16)5=17776P(\text{Yahtzee}) = \frac{1}{6} \times \frac{1}{6} \times \frac{1}{6} \times \frac{1}{6} \times \frac{1}{6} = \left(\frac{1}{6}\right)^5 = \frac{1}{7776}

  • Fundamental Rules of Card Deck Probability:

    • Standard Card Deck Specifications: 5252 total cards, 22 colors (Red, Black), 44 suits (Hearts, Diamonds, Spades, Clubs), 1313 cards per suit (11 through 1010, Jack, Queen, King).
    • Single Event Probabilities (with fraction reduction):
    • Probability of drawing a Red card: 2652=12\frac{26}{52} = \frac{1}{2}
    • Probability of drawing a Heart suit card: 1352=14\frac{13}{52} = \frac{1}{4}
    • Probability of drawing an Ace: 452=113\frac{4}{52} = \frac{1}{13}
    • Suicide Jack: A specific card depiction where the jack holds a sword/knife through its head. There are 22 suicide jacks in a deck.
    • Probability of drawing a Suicide Jack: 252=126\frac{2}{52} = \frac{1}{26}

  • Multi-Event Compound Probability Calculation (Independent Draws with Replacement):

    • To calculate the probability of sequentially drawing a Red card, replacing it, drawing a Heart, replacing it, drawing an Ace, replacing it, and drawing a Suicide Jack:     P=12×14×113×126=12704P = \frac{1}{2} \times \frac{1}{4} \times \frac{1}{13} \times \frac{1}{26} = \frac{1}{2704}

  • Independence of Historical Events (Roulette Wheel Principle):

    • Independent Trial Principle: Previous sequential outcomes have zero impact on future independent outcomes.
    • Example: On a fair roulette wheel with equal Red and Black spaces, if the marble lands on Black for 1010 consecutive spins, the probability of landing on Black on the 1111th spin remains exactly 50%50\% (12\frac{1}{2}). Displaying past outcomes on tracking boards does not alter statistical odds.