yay awesome

How do cells divide to give rise to identical cells?

Through mitosis, where a diploid somatic cell divides once to produce two genetically identical diploid daughter cells. DNA is replicated in S phase, and chromosomes are evenly segregated during mitosis.

Compare the roles of mitosis, meiosis, and fertilization in the human life cycle.

• Mitosis: Growth, repair, asexual reproduction (produces identical diploid cells).

• Meiosis: Produces haploid gametes (sperm/egg) with genetic variation.

  • Mitosis = Maintenance (clones).

  • Meiosis = Mix (diversity).

• Fertilization: Combines gametes to restore diploidy, creating genetically unique offspring.

What are the steps of DNA replication and key enzymes involved?

1. Helicase unwinds DNA.

2. Primase synthesizes RNA primers.

3. DNA polymerase adds nucleotides (leading/lagging strands).

4. Ligase seals Okazaki fragments.

---

Goal: Copy DNA accurately before cell division (S phase of interphase).

1. Initiation

• Helicase unwinds the double helix at the origin of replication, creating a replication fork.

• Single-Strand Binding Proteins (SSBs) stabilize the unwound strands.

• Topoisomerase relieves twisting tension ahead of the fork by cutting and rejoining DNA.

2. Elongation

• Primase synthesizes a short RNA primer (provides a starting point).

• DNA Polymerase III adds nucleotides to the 3’ end of the primer:

  • Leading strand: Synthesized continuously toward the fork.

  • Lagging strand: Synthesized in Okazaki fragments (away from fork).

• DNA Polymerase I replaces RNA primers with DNA.

3. Termination

• Ligase seals gaps between Okazaki fragments.

• Replication ends when forks meet or at telomeres (ends of chromosomes).

---

Key Enzymes & Mnemonics

Enzyme	Function	Memory Trick
Helicase	Unzips DNA	"Helicase = Helicopter" (unwinds)
Primase	Makes RNA primer	"Prime-time starter"
DNA Pol III	Main builder (adds nucleotides)	"Pol III = Pro builder"
DNA Pol I	Removes primers, fills gaps	"Pol I = Primer cleaner"
Ligase	Glues fragments	"Ligase = Ligation (stitching)"
Topoisomerase	Prevents supercoiling	"Topo = Top (stops knots)"

Mnemonic for Order:
"Helpful People Take Polished Little Items"

1. Helicase

2. Primase

3. Topoisomerase

4. Pol III/Pol I

5. Ligase

---

Visual Analogy:

Imagine building a train track:

• Helicase opens the tracks.

• Primase lays the first brick (primer).

• Pol III lays most tracks (leading/lagging strands).

• Pol I replaces temporary bricks (primers) with permanent ones.

• Ligase welds the tracks together.

Diagram the eukaryotic cell cycle, mitosis, and meiosis.

• Cell Cycle: G1 → S (DNA replication) → G2 → M (mitosis/meiosis) → Cytokinesis.

• Mitosis: Prophase → Metaphase → Anaphase → Telophase.

• Meiosis: Two divisions (Meiosis I: homologous pairs separate; Meiosis II: sister chromatids separate).

How do cancer cells differ from normal cells?

• Uncontrolled division (ignore checkpoints).

• Avoid apoptosis.

• Metastasize (spread).

• Genetic instability.

Define apoptosis and its functions.

Programmed cell death; removes damaged/unneeded cells; prevents cancer.

How is genetic information passed between generations?

Via gametes (haploid) formed by meiosis; fertilization restores diploidy.

Compare sexual vs. asexual reproduction.

• Asexual: Identical offspring (fast, no mate).

• Sexual: Genetic variation (slower, requires mate).

Distinguish autosomes, sex chromosomes, homologous chromosomes, and sister chromatids.

• Autosomes: Non-sex chromosomes (1-22 in humans).

  • Number: Humans have 22 pairs of autosomes (44 total) + 1 pair of sex chromosomes (XX or XY).

  • Function: Control most traits (e.g., hair color, height, metabolism).

  • Inheritance: Always inherited in pairs—one from each parent.

• Sex chromosomes: X/Y (determine sex).

• Homologs: Chromosome pairs (one from each parent).

• Sister chromatids: Identical copies (from replication).

Role of homologous chromosomes in sexual reproduction?

Allow genetic recombination (crossing over) and independent assortment, increasing variation.

---

Genetic Recombination (Crossing Over)

• When: Prophase I of meiosis.

• What Happens:

  • Homologs physically swap segments at chiasmata.

  • Creates new allele combinations on chromatids.

• Why It Matters:

  • Produces genetically unique gametes.

  • Explains why siblings share ~50% DNA but aren’t identical.

Example:
If one homolog has alleles A-B-C and the other has a-b-c, crossing over can yield A-b-C or a-B-c combinations.

---

2. Independent Assortment

• When: Metaphase I of meiosis.

• What Happens:

  • Homologous pairs line up randomly at the cell’s equator.

  • Maternal/paternal chromosomes sort independently into gametes.

• Why It Matters:

  • Generates 2^n possible gamete combinations (n = haploid number).

  • In humans (n=23), this allows ~8.4 million gamete variations before crossing over.
    

Example:
A cell with just 2 chromosome pairs can produce gametes with:

• Maternal 1 + Maternal 2

• Maternal 1 + Paternal 2

• Paternal 1 + Maternal 2

• Paternal 1 + Paternal 2

---

3. Ensuring Diploidy in Offspring

• What Happens:

  • Each parent contributes one homolog per chromosome via gametes.

  • Fertilization restores the diploid (2n) state in the zygote.

• Why It Matters:

  • Maintains stable chromosome numbers across generations.

  • Allows for heterozygosity (e.g., Aa), increasing genetic diversity.
    

Example:
A gene for eye color might have allele B (brown) on one homolog and b (blue) on the other. Offspring inherit one from each parent.

---

Key Contrast: Homologs vs. Sister Chromatids

Feature	Homologous Chromosomes	Sister Chromatids
Origin	One from mom, one from dad	Identical copies (from DNA replication)
Genetic Content	Same genes, different alleles	Identical alleles
Separate During	Meiosis I	Meiosis II or mitosis

---

Why This Matters for Evolution

• Homologs enable mixing of parental traits, driving adaptation.

• Errors in homolog separation (e.g., nondisjunction) cause disorders like Down syndrome.
  

Analogy:
Homologs are like two editions of a recipe book—similar content but with tweaked instructions (alleles). Sexual reproduction shuffles these "editions" to create new versions in offspring.

Explain meiosis, gamete formation, and fertilization in sexual reproduction.

• Meiosis: Reduces chromosome number (diploid → haploid).

• Gametes: Sperm/egg (haploid).

• Fertilization: Combines gametes (restores diploidy)

---

1. Diploid organism (2n) → meiosis → haploid gametes (n).

2. Gametes fuse (fertilization) → zygote (2n) → mitosis → new organism.
   

Key Points:

• Meiosis generates diversity; fertilization combines it.

• Gamete asymmetry (sperm vs. egg) reflects reproductive strategies.
  

Example:

• Your genome is a unique mix of your parents’ DNA, shuffled by meiosis and randomized by fertilization.

Haploid vs. diploid cells?

• Haploid (n): One chromosome set (gametes).

• Diploid (2n): Two sets (somatic cells).

Role of somatic vs. germ cells in sexual reproduction?

• Somatic: Body cells (mitosis).

• Germ: Produce gametes (meiosis).

---

Somatic Cells

• What they are: All body cells except reproductive cells (e.g., skin, muscle, neurons).

• Role in reproduction:

  • No direct role in passing DNA to offspring.

  • Maintain the organism’s body (support survival and reproduction indirectly).

• Division: Undergo mitosis to grow/repair tissues (produces identical diploid cells).
  

Example: A liver cell divides to replace damaged tissue, but its DNA never enters offspring.

Germ Cells

• What they are: Cells dedicated to reproduction (sperm in males, eggs in females).

• Role in reproduction:

  • Produce haploid gametes via meiosis (ensures genetic diversity).

  • Pass genetic material to the next generation.

• Special features:

  • Undergo recombination (crossing over) during meiosis.

  • Unique in meiosis: Reduce chromosome number (diploid → haploid).
    

Example: Ovaries and testes contain germ cells that develop into eggs/sperm.

---

Key Differences

Feature	Somatic Cells	Germ Cells
Ploidy	Diploid (2n)	Become haploid (n) via meiosis
Division	Mitosis	Meiosis
Genetic Variation	None (clones of parent cell)	High (crossing over, random assortment)
Role in Reproduction	Indirect (support organism)	Direct (create offspring)

---

Why It Matters

• Germ cells are the only way DNA is transmitted to offspring.

• Mutations in germ cells affect future generations; somatic mutations do not.

• Cancer connection: Most cancers arise from somatic cells; germ cell cancers are rare but can impact fertility.

Analogy:

• Somatic cells = "Staff" maintaining a business (body).

• Germ cells = "Founders" passing the business (genes) to heirs (offspring).

Three ways meiosis generates genetic variability?

1. Crossing over (prophase I).

   1. Homologous chromosomes exchange segments at chiasmata (cross-shaped junctions).

      1. Creates new allele combinations on chromosomes.

      2. Produces recombinant chromatids (neither identical to parent chromosomes).

2. Random orientation (metaphase I).

   1. Homologous chromosome pairs line up randomly at the cell’s equator.

   2. Each pair aligns independently of other pairs.

      1. Produces 2^n possible gamete combinations (where n = haploid chromosome number).

      2. Humans (n=23): 8.4 million possible gamete combinations from this alone.

         1. A cell with 2 chromosome types can produce gametes with:

            1. Maternal 1 + Maternal 2

            2. Maternal 1 + Paternal 2

            3. Paternal 1 + Maternal 2

            4. Paternal 1 + Paternal 2

3. Random fertilization (sperm + egg combinations).

   1. Any sperm (of ~8.4 million possible) can fuse with any egg (of ~8.4 million possible).

      1. Results in ~70 trillion (2²³ × 2²³) possible zygotes in humans.

---

Mechanism	Stage	Effect on Diversity
Crossing Over	Prophase I	Mixes alleles within chromosomes.
Random Orientation	Metaphase I	Shuffles whole chromosomes.
Random Fertilization	Fusion of gametes	Combines two unique gametes.

Key Point: These mechanisms ensure that no two offspring (except clones/identical twins) are genetically identical.

Need an analogy? Think of:

• Crossing Over = Swapping chapters between two recipe books.

• Random Orientation = Randomly picking one book from each of 23 pairs.

• Random Fertilization = Combining two randomly selected recipe collections.

Compare crossing over, random orientation, and random fertilization.

• Crossing over: Exchanges DNA between homologs.

• Random orientation: Homologs align randomly (metaphase I).

• Random fertilization: Any sperm + egg combination.

Define gene, allele, locus, chromosome.

• Gene: DNA sequence for a trait.

• Allele: Variant of a gene.

• Locus: Gene’s location on chromosome.

• Chromosome: DNA + protein structure.

Dominant vs. recessive alleles?

• Dominant: Masks recessive (expressed if present).

• Recessive: Only expressed if homozygous.

Genotype vs. phenotype? Homozygous vs. heterozygous?

• Genotype: Genetic makeup (e.g., AA).

• Phenotype: Physical expression (e.g., tall).

• Homozygous: Same alleles (AA/aa).

• Heterozygous: Different alleles (Aa).

Use Punnett squares for Mendelian (1 gene: AA x aa) and non-Mendelian (incomplete/codominance) inheritance.

• Mendelian: Dominant/recessive (3:1 ratio).

• Non-Mendelian:

  • Incomplete dominance: Blending (pink flowers).

  • Codominance: Both expressed (A/B blood).

Codominance in blood types? Possible genotypes/phenotypes?

• Genotypes: IAIA, IAi (A); IBIB, IBi (B); IAIB (AB); ii (O).

• Phenotypes: A, B, AB, O.

Mendel’s Law of Segregation vs. Independent Assortment?

• Segregation: Alleles separate in meiosis (gametes get one).

• Independent Assortment: Genes on different chromosomes sort independently.

How do linked genes violate Mendel’s laws?

Genes close on same chromosome are inherited together (reduced recombination).

How do pleiotropy and epistasis affect phenotype?

• Pleiotropy: One gene affects multiple traits.

• Epistasis: One gene masks another (e.g., coat color in labs).

Why are X-linked recessive traits more common in males?

Males (XY) lack a second X to compensate for recessive alleles (e.g., colorblindness).

Why is one X inactivated in female cells?

X-inactivation (Barr body) balances gene expression between XX females and XY males.

How do environment and polygenic traits influence phenotype?

• Environment: Can alter gene expression (e.g., sun darkens skin).

• Polygenic traits: Multiple genes contribute (e.g., height, skin color).

What is a Barr body, and how does it function in female cells?

• Definition: A Barr body is an inactivated X chromosome in female somatic cells, appearing as a dense, dark-staining structure near the nuclear envelope.

• Purpose: Ensures dosage compensation (equal X-linked gene expression between XX females and XY males).

• Key Points:

  • Females are functional mosaics for X-linked genes (some cells express maternal X, others paternal X).

  • Randomly chosen in each cell early in development → mosaic pattern (e.g., calico cat fur).

  • Inactivation prevents "double dosing" of X-linked proteins.

Example:

• Calico cats: Orange/black patchy fur results from random X inactivation (orange/black alleles are X-linked).

What is pleiotropy?

When one gene affects multiple, unrelated traits.
Example: Marfan syndrome (one gene causes long limbs, heart defects, and vision problems).

Pleiotropy = A single remote control messing up your TV, lights, and AC.

What is epistasis?

When one gene masks or modifies the effect of another gene.
Example: Labrador coat color—the "E" gene determines if pigment is deposited (even if the "B" gene codes for black/brown).

Epistasis = A parent overriding a kid’s choice of clothes.

What is a polygenic trait?

• A trait controlled by multiple genes (often dozens/hundreds), each contributing a small effect.

• Results in continuous variation (a range of phenotypes, not distinct categories).
  

Examples:

• Human height

• Skin color

• Intelligence

• Risk of diabetes/heart disease

---

Key Features:

1. Not Mendelian (no simple dominant/recessive pattern).

2. The environment often influences the outcome (e.g., nutrition affects height).

3. Bell curve distribution (most people are average, few extremes).

Like many tiny dials adjusting a single outcome (e.g., mixing paint colors to get endless shades).

What is Mendel's Law of Segregation?

• Each organism has two alleles for a gene (one from each parent).

• During gamete formation (meiosis), the alleles separate—each gamete gets only one.

• Explains why offspring inherit one allele from each parent.

Example:

• Parent with Aa can pass on A or a (not both).

What is Mendel's Law of Independent Assortment?

• Genes for different traits (on different chromosomes) are inherited independently of each other.

• Applies only to genes not linked (far apart on the same chromosome).

Example:

• Seed color (yellow/green) and shape (round/wrinkled) are inherited separately.

Mendelian Inheritance (Two Genes)

Example: Pea plant seed color (Yellow Y = dominant, green y = recessive) and shape (Round R = dominant, wrinkled r = recessive)
Cross: Double heterozygotes (YyRr × YyRr)

	YR	Yr	yR	yr
YR	YYRR	YYRr	YyRR	YyRr
Yr	YYRr	YYrr	YyRr	Yyrr
yR	YyRR	YyRr	yyRR	yyRr
yr	YyRr	Yyrr	yyRr	yyrr

Result:

• Phenotypic ratio: 9 Yellow/Round : 3 Yellow/Wrinkled : 3 Green/Round : 1 Green/Wrinkled

Non-Mendelian: Incomplete Dominance

Example: Snapdragon flowers (Red RR, White WW, Pink RW)
Cross: Pink × Pink (RW × RW)

	R	W
R	RR	RW
W	RW	WW

Result:

• Phenotypic ratio: 1 Red : 2 Pink : 1 White

Non-Mendelian: Codominance

Example: Human blood type (A and B codominant, O recessive)
Cross: Type A (heterozygous AO) × Type B (heterozygous BO)

	A	O
B	AB	BO
O	AO	OO

Result:

• Phenotypic ratio: 1 AB : 1 A : 1 B : 1 O

Meiosis: Creating Haploid Gametes

• Purpose: Reduces chromosome number by half (diploid → haploid) to prevent doubling in offspring.
  

• Key Stages:

  • Meiosis I: Homologous chromosomes separate.

    • Prophase I: Crossing over occurs (genetic recombination).

    • Metaphase I: Random orientation of homologous pairs.

    • Anaphase I: Homologs pulled apart (sister chromatids stay together).

  • Meiosis II: Sister chromatids separate (like mitosis).

• Result: 4 genetically unique haploid cells (sperm or egg).
  

Why it matters: Ensures genetic diversity via:

• Crossing over (new allele combinations).

• Random orientation (independent assortment).

Gamete Formation (Spermatogenesis vs. Oogenesis)

Feature	Spermatogenesis (Sperm)	Oogenesis (Egg)
Location	Testes	Ovaries
Output	4 functional sperm per meiosis	1 egg + 3 polar bodies (discarded)
Timing	Continuous from puberty	Starts in fetus, pauses until ovulation
Cytokinesis	Equal division	Unequal (egg retains most cytoplasm)

Key Adaptations:

• Sperm: Streamlined, motile, packed with mitochondria for energy.

• Egg: Large, nutrient-rich (yolk), blocks polyspermy after fertilization.

Fertilization: Restoring Diploidy

• Process:

  1. Sperm penetrates egg (via acrosomal enzymes).

  2. Cortical reaction: Egg releases enzymes to harden zona pellucida (blocks other sperm).

  3. Haploid nuclei fuse (sperm + egg pronuclei) → zygote (2n).

• Genetic Impact: Combines DNA from two parents, creating novel genotypes.
  

Why it matters:

• Random fertilization: Any sperm (of ~8 million) × any egg (of ~8 million) → ~70 trillion possible zygotes.

• Ensures variation: Even siblings share ~50% DNA (but differ in recombination events).

Mitosis vs. Meiosis: Key Differences

Feature	Mitosis	Meiosis
Purpose	Growth, repair, asexual repro.	Sexual reproduction (gametes)
Divisions	1 division → 2 diploid cells	2 divisions → 4 haploid cells
Genetic Output	Identical to parent cell	Genetically unique
Stages	PMAT (Prophase → Telophase)	PMAT I + PMAT II
Crossing Over?	No	Yes (Prophase I)

Mitosis Stages: Easy Mnemonic

Remember the order with "PMAT" (like a "People Meet And Talk" party):

1. Prophase:

   • Chromosomes condense.

   • Nuclear envelope breaks down.

   • Spindle fibers form.

2. Metaphase:

   • Chromosomes line up single-file at the cell’s equator.

3. Anaphase:

   • Sister chromatids pull apart to opposite poles.

4. Telophase:

   • Chromosomes de-condense.

   • Nuclear envelopes reform.

   • Cytokinesis splits the cell into two.

Visual Trick: Imagine a dance party:

• Prepare (Prophase) → Mingle in middle (Metaphase) → Apart (Anaphase) → Time to leave (Telophase).

Meiosis Stages (Two Rounds of PMAT)

Meiosis I: Separates homologous chromosomes.

• Prophase I: Crossing over!

• Metaphase I: Homologous pairs line up.

• Anaphase I: Homologs separate (sisters stay together).

• Telophase I: Two haploid cells form.
  

Meiosis II: Separates sister chromatids (like mitosis).

• Prophase II → Telophase II: Ends with 4 unique haploid gametes.
  

Mnemonic: "Meet Someone New" (Meiosis = Mixing DNA).