Genetics Memory Tricks | Biology

🧬 Genetics

Memory Tricks

Proven mnemonics — fast to learn, hard to forget.

DNA Structure

A pairs with T, G pairs with C — "AT the GC"

Adenine-Thymine · Guanine-Cytosine — Chargaff's Rules

In DNA, base pairing is always A-T (2 hydrogen bonds) and G-C (3 hydrogen bonds). "AT the GC" — AT the Golf Course. Purine (A,G) always pairs with Pyrimidine (T,C). In RNA, Uracil replaces Thymine — so A pairs with U.

📖 Full Lesson →Difficulty: Beginner

Purines (double ring)

Adenine and Guanine — remember PuRine = Pure As Gold (A and G). Larger molecules with two fused rings.

Pyrimidines (single ring)

Thymine, Cytosine, Uracil — CUT the Py (C, U, T are pyrimidines). Smaller molecules with one ring.

Why 3 H-bonds for G-C?

More hydrogen bonds = stronger pairing. G-C rich regions of DNA are harder to denature — important for PCR and melting temperature calculations.

RNA difference

RNA uses Uracil instead of Thymine, and is single-stranded. A still pairs with U in RNA (2 H-bonds). mRNA, tRNA, and rRNA all follow this rule.

Mendelian Genetics

Dominant covers Recessive — "Big D dominates little d"

Uppercase = dominant allele · Lowercase = recessive allele

In Mendelian genetics, uppercase letters represent dominant alleles and lowercase represent recessive. A dominant allele (D) masks the recessive (d) when both are present. Only when an organism is homozygous recessive (dd) will the recessive trait show. Heterozygous (Dd) = dominant phenotype.

📖 Full Lesson →Difficulty: Beginner

Homozygous dominant (DD)

Two dominant alleles — expresses dominant trait. Cannot carry recessive trait to offspring.

Heterozygous (Dd)

One of each — expresses dominant phenotype but carries recessive allele. Also called a "carrier."

Homozygous recessive (dd)

Two recessive alleles — only way to express the recessive phenotype. Both parents must carry at least one recessive allele.

Punnett square ratio

Dd × Dd cross gives 1 DD : 2 Dd : 1 dd genotype ratio and 3 dominant : 1 recessive phenotype ratio.

DNA Replication

PESA — Primers, Enzymes, Strands, Assembly

Primase → Helicase → Polymerase → Ligase — replication order

DNA replication steps: Helicase unwinds the double helix, Primase lays down RNA primers, DNA Polymerase adds new nucleotides (5'→3' only), and Ligase seals the gaps between Okazaki fragments on the lagging strand. Replication is semi-conservative — each new DNA molecule has one old and one new strand.

📖 Full Lesson →Difficulty: Intermediate

Leading vs lagging strand

Leading strand: synthesized continuously toward replication fork. Lagging strand: synthesized in Okazaki fragments away from fork. Both made 5'→3'.

Semi-conservative replication

Each daughter DNA has one original parental strand and one new strand. Proven by Meselson-Stahl experiment using heavy nitrogen (¹⁵N).

DNA Polymerase III

Main replication enzyme in prokaryotes. Adds nucleotides 5'→3', has proofreading ability (3'→5' exonuclease). Cannot start a new strand — needs a primer.

Telomeres

Repetitive sequences at chromosome ends that shorten with each replication. Telomerase enzyme rebuilds them in stem cells and cancer cells.

Protein Synthesis

DNA → RNA → Protein: "Don't Rely on Promises"

Transcription (DNA→mRNA) then Translation (mRNA→Protein)

The central dogma: DNA is transcribed into mRNA, which is translated into protein. "Don't Rely on Promises" = DNA → RNA → Protein. Transcription happens in the nucleus; translation happens at ribosomes in the cytoplasm. Each codon (3 bases) codes for one amino acid.

📖 Full Lesson →Difficulty: Intermediate

Transcription

RNA Polymerase reads the template strand 3'→5' and produces mRNA 5'→3'. Promoter region signals start; terminator signals stop. Occurs in nucleus.

mRNA processing

In eukaryotes: 5' cap and poly-A tail added; introns spliced out, exons joined. Mature mRNA exits nucleus through nuclear pores.

Translation

Ribosome reads mRNA codons; tRNA anticodons bring matching amino acids. Start codon = AUG (methionine). Stop codons = UAA, UAG, UGA (U Are Awful, U Are Gone, U Go Away).

Genetic code

64 possible codons (4³) code for 20 amino acids — the code is redundant (degenerate). Most amino acids have multiple codons. The code is nearly universal across all life.

Mutations

Point mutations: Silent, Missense, Nonsense — "Students Miss Nothing"

Silent = same AA · Missense = different AA · Nonsense = stop codon

Three types of point mutations by effect: Silent mutations change a codon but code for the same amino acid (due to redundancy). Missense mutations change one amino acid for another. Nonsense mutations create a premature stop codon, truncating the protein. Frameshift mutations (insertions/deletions) are typically most damaging.

📖 Full Lesson →Difficulty: Intermediate

Frameshift mutations

Insertions or deletions that shift the reading frame — all downstream codons change. Usually catastrophic to protein function. Sickle cell is NOT a frameshift — it's a missense point mutation.

Sickle cell anemia

Classic missense mutation — single base change (A→T) converts glutamic acid to valine in hemoglobin β chain. Causes RBCs to sickle under low oxygen. Heterozygotes have malaria resistance.

Mutagens

Chemical mutagens (base analogs, alkylating agents), radiation (UV causes thymine dimers, X-rays cause breaks), and biological mutagens (transposons, viruses).

Proofreading and repair

DNA polymerase proofreads during replication. Mismatch repair, base excision repair, and NER (nucleotide excision repair) fix errors after replication. XP (xeroderma pigmentosum) = defective NER.

Inheritance Patterns

X-linked recessive: "Daughters carry, Sons suffer"

X-linked traits affect males more — they only have one X chromosome

X-linked recessive conditions (hemophilia, color blindness, Duchenne MD (muscular dystrophy)) affect males (XY) far more than females (XX) because males only have one X — if it carries the recessive allele, the trait is expressed. Females need two copies to be affected. Carrier females (X^A X^a) pass the gene to 50% of sons.

📖 Full Lesson →Difficulty: Intermediate

Autosomal dominant

One copy sufficient to express trait. Affects every generation. Huntington's, Marfan syndrome, achondroplasia. Affected parent has 50% chance of passing to each child.

Autosomal recessive

Two copies needed. Can skip generations. Cystic fibrosis, PKU (phenylketonuria — an inherited metabolic disorder), sickle cell. Carrier parents (Aa × Aa) have 25% affected, 50% carrier, 25% unaffected offspring.

Codominance vs incomplete dominance

Codominance: both alleles expressed equally (AB blood type). Incomplete dominance: blend of traits (red + white = pink snapdragons). Neither allele fully dominates.

Mitochondrial inheritance

mtDNA inherited only from mother — all offspring of affected mother are affected. Leber's hereditary optic neuropathy. Passed through egg, not sperm.

Meiosis

PMAT twice — "People Meet And Talk, People Meet And Talk"

Prophase · Metaphase · Anaphase · Telophase × 2 (Meiosis I and II)

Meiosis has two divisions: Meiosis I separates homologous chromosomes (reduces chromosome number by half), and Meiosis II separates sister chromatids (like mitosis). Result: 4 haploid cells (n) from 1 diploid cell (2n). Key event in Prophase I = crossing over (genetic recombination), which increases genetic diversity.

📖 Full Lesson →Difficulty: Intermediate

Meiosis I vs Mitosis

Key difference: in Meiosis I, homologous chromosomes pair up (synapsis) and cross over. Homologs separate, not sister chromatids. Result is 2 haploid cells, each with duplicated chromosomes.

Crossing over

Occurs at chiasmata during Prophase I. Homologous chromosomes exchange segments — creates new allele combinations. Greater distance between genes = more crossing over = higher recombination frequency.

Independent assortment

During Metaphase I, homolog pairs align randomly — either chromosome of each pair can go to either pole. Produces 2^n combinations (2^23 = 8 million for humans). Explains Mendel's second law.

Nondisjunction

Failure of chromosomes to separate properly during meiosis. Produces aneuploid gametes. Trisomy 21 (Down syndrome) results from nondisjunction of chromosome 21 during meiosis I.

Gene Regulation

Lac operon: No lactose = repressor ON = genes OFF

Negative control — repressor blocks transcription when lactose absent

The lac operon in E. coli is the classic gene regulation example. When lactose is absent, the repressor protein binds the operator and blocks RNA polymerase — genes are OFF. When lactose is present, allolactose binds the repressor, inactivating it — genes turn ON. The cell only makes lactose-digesting enzymes when lactose is available.

📖 Full Lesson →Difficulty: Advanced

Operon structure

Promoter → Operator → Structural genes (lacZ, lacY, lacA). Regulator gene produces repressor protein separately. RNA polymerase binds promoter; repressor blocks operator.

Catabolite repression (CAP)

When glucose is present, cAMP levels drop → CAP cannot bind → reduced transcription even if lactose present. Cell prefers glucose. Double control: repressor (negative) + CAP (positive).

Trp operon (repressible)

Opposite of lac operon. When tryptophan is abundant, it acts as corepressor — activates repressor → genes OFF. When tryptophan is scarce, repressor inactive → genes ON. Makes tryptophan when needed.

Eukaryotic regulation

More complex: transcription factors, enhancers, silencers, chromatin remodeling, DNA methylation, histone acetylation. Gene expression can be regulated at transcription, processing, transport, translation, and protein levels.

Biotechnology

PCR = "Photocopier for DNA" — denature, anneal, extend

Polymerase Chain Reaction — amplifies specific DNA sequences exponentially

PCR copies a specific DNA segment millions of times. Three steps cycle repeatedly: Denaturation (heat to 94°C separates strands), Annealing (cool to ~55°C — primers attach), Extension (heat to 72°C — Taq polymerase extends). Each cycle doubles the DNA — 30 cycles = 2³⁰ = over 1 billion copies from a single template.

📖 Full Lesson →Difficulty: Advanced

Why Taq polymerase?

Isolated from Thermus aquaticus — a bacterium living in hot springs. Heat-stable, survives the 94°C denaturation step. Regular DNA polymerase would denature and stop working.

Gel electrophoresis

Separates DNA fragments by size. Smaller fragments migrate farther through agarose gel toward positive electrode. DNA ladder provides size reference. Used to visualize PCR products.

Restriction enzymes

Cut DNA at specific palindromic sequences. EcoRI cuts at GAATTC. Produce sticky ends that can be joined to other DNA cut with the same enzyme. Foundation of recombinant DNA technology.

CRISPR-Cas9

Guide RNA directs Cas9 protein to specific DNA sequence. Cas9 cuts both strands. Cell repairs by NHEJ (Non-Homologous End Joining — error-prone, disables gene) or HDR (Homology-Directed Repair — can insert new sequence). Revolutionary gene editing tool.

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What does this stand for?

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Essential Amino Acids

Any Help In Learning These Little Molecules Proves Truly Valuable — Arginine, Histidine, Isoleucine, Leucine, Threonine, Lysine, Methionine, Phenylalanine, Tryptophan, Valine

10 essential amino acids the body cannot synthesize

Essential amino acids must come from diet because the body cannot synthesize them. The 10 essential amino acids: Arginine (conditionally essential), Histidine, Isoleucine, Leucine, Threonine, Lysine, Methionine, Phenylalanine, Tryptophan, Valine. Complete proteins (meat, eggs, dairy, soy) contain all essential amino acids. Incomplete proteins (most plants) lack one or more.

Difficulty: Intermediate

Why can't we make them?

Humans lack specific enzymes needed to synthesize the carbon skeletons of essential amino acids. These biosynthetic pathways were lost during evolution — easier to obtain from diet. Non-essential amino acids can be synthesized from intermediates of glycolysis and Krebs cycle.

Protein complementation

Vegetarians combine incomplete proteins to get all essential amino acids. Examples: rice + beans (rice lacks lysine, beans lack methionine — together complete), peanut butter + bread. Don't need to eat at same meal — just same day.

🎓 Common Exam Questions

Q: Explain Mendelian inheritance using a Dd x Dd cross. What are the genotypic and phenotypic ratios?

A: In a monohybrid cross between two heterozygotes (Dd x Dd): genotypic ratio = 1 DD : 2 Dd : 1 dd (1:2:1). Phenotypic ratio = 3 dominant : 1 recessive (3:1), because both DD and Dd express the dominant phenotype. This demonstrates Mendel's Law of Segregation — alleles separate during gamete formation so each gamete carries only one allele. For a dihybrid cross (two traits): 9:3:3:1 phenotypic ratio, demonstrating independent assortment. Deviations from these ratios indicate linked genes, incomplete dominance, codominance, or other non-Mendelian inheritance.

Q: Describe the PESA steps of DNA replication.

A: PESA = Primers, Enzymes, Strands, Assembly. Primers: Primase synthesizes short RNA primers to provide a 3' OH start point (DNA polymerase cannot start de novo). Enzymes: Helicase unwinds the double helix; DNA polymerase III extends (5'→3' only); DNA polymerase I removes RNA primers; Ligase seals nicks. Strands: Leading strand — synthesized continuously toward the replication fork. Lagging strand — synthesized in Okazaki fragments away from the fork (then joined by ligase). Assembly: result is two identical daughter DNA molecules, each with one parental strand and one new strand (semiconservative replication — proven by Meselson-Stahl experiment, 1958).

Q: What is CRISPR-Cas9 and how does it work? What are NHEJ and HDR?

A: CRISPR-Cas9 is a gene editing system adapted from bacterial immune defense. A guide RNA (gRNA — ~20 nucleotides) complementary to the target sequence directs the Cas9 endonuclease protein to cut both DNA strands at the target location. The cell then repairs the break by one of two pathways: NHEJ (Non-Homologous End Joining) — error-prone, often introduces insertions or deletions that disrupt the gene (used for knockouts). HDR (Homology-Directed Repair) — uses a provided DNA template to introduce precise edits (used for corrections). Applications: disease gene correction, cancer research, agricultural improvement. Nobel Prize 2020 to Doudna and Charpentier.

Q: Explain X-linked inheritance — why does it affect males more, and what is a carrier female?

A: X-linked genes are on the X chromosome. Males (XY) have only one X — if it carries a recessive disease allele, the disease is expressed (hemizygous). Females (XX) need two copies of the recessive allele to be affected — much less likely. Carrier female (X^A X^a): carries one normal (X^A) and one disease (X^a) allele; usually unaffected but passes the gene to 50% of sons (who will be affected) and 50% of daughters (who will be carriers). Classic X-linked recessive conditions: hemophilia A, color blindness, Duchenne muscular dystrophy (DMD), fragile X syndrome. X-linked dominant conditions: affected males pass to all daughters but no sons.

Q: How is gene expression regulated in eukaryotes — from DNA to protein?

A: Gene expression is regulated at multiple levels: Transcriptional control (most common): transcription factors bind enhancers/promoters to activate or repress RNA polymerase; chromatin remodeling (histone acetylation opens chromatin, methylation closes it); DNA methylation (CpG methylation = gene silencing). RNA processing: alternative splicing of pre-mRNA produces different proteins from same gene; mRNA stability regulated by 5' cap, 3' poly-A tail, and miRNA binding. Translational control: ribosome availability, miRNAs block translation, mRNA secondary structure. Post-translational: protein folding, modification (phosphorylation, glycosylation), degradation (ubiquitin-proteasome system). This multi-level regulation allows ~20,000 genes to produce far more functional diversity.

Memory tricks for DNA, heredity & mutations

Memory Tricks