Mitosis and Meiosis

Mitosis vs. Meiosis

• Mitosis produces diploid cells (2n=46) for tissue repair
• Meiosis produces haploid gametes (n=23) for sexual reproduction
• Meiosis involves two cell divisions and crossing-over
• Mitosis and meiosis are both preceded by interphase
• Mitosis and meiosis both include chromosome condensation, spindle formation, and cytokinesis

Purposes of Mitosis vs. Meiosis

• Mitosis functions in growth, tissue repair, and asexual reproduction
• Meiosis enhances genetic variation in sexual reproduction

Human Chromosomes

• Humans have 46 total chromosomes, or 23 pairs
• 22 pairs are autosomes
• 1 pair is sex chromosomes

Autosomes, Somatic Cells, and Gametes

• Autosomes: 22 pairs of non-sex chromosomes
• Somatic cells: diploid body cells (e.g., skin cells)
• Gametes: 1 pair of reproductive cells (eggs and sperm)

Diploid vs. Haploid Cells

• Somatic cells (diploid 2n) contain two sets of chromosomes
• Gametes (haploid n) contain one set of chromosomes

Homologous Chromosomes vs. Sister Chromatids

• Homologous chromosomes are chromosome pairs (one from each parent) with the same genes in the same order but different alleles
• Sister chromatids are genetically identical copies of a single chromosome, connected at the centromere

Chromatin

• Chromatin is an uncondensed DNA-protein complex during interphase

Cell Cycle Phases

• Interphase (90% of cycle)
• G1: major growth state

S Phase

• DNA replication occurs (46 chromosomes -> 92 chromatids)
• Centrosomes replicate

G2 Phase

• Preparation for cell division
• Centrosomes move to opposite poles

M Phase

• Mitosis (cell division)
• Cytokinesis

DNA Condensation

• DNA is condensed into chromosomes during cell division (meiosis and mitosis)-prophase
• DNA is in the form of chromatin during interphase (G1, S, G2) for transcription and replication

Mitosis Phases

• Prophase: chromosomes condense, nuclear envelope breaks down, spindle fibers form
• Metaphase: chromosomes align at the metaphase plate
• Anaphase: sister chromatids separate and move to opposite poles
• Telophase: chromosomes de-condense, nuclear envelopes reform
• Cytokinesis: cytoplasm divides, forming two genetically identical 2n daughter cells

Meiosis I

• P: homologous chromosomes pair up, crossing over happens
• M: homologous pairs align at the metaphase plate
• A: homologous chromosomes separate
• T: two haploid cells form, each with duplicated chromosomes

Meiosis II

• P: chromosomes condense
• M: chromosomes align at the metaphase plate (random order)
• A: sister chromatids separate
• T: four haploid 1n daughter cells (gametes) form

Crossing Over

• Crossing over (recombination) occurs during prophase 1 of meiosis
• Homologous chromosomes pair up and exchange genetic material

Genetic Variation in Meiosis

• Meiosis contributes to genetic variation through crossing over
• Meiosis contributes to genetic variation through independent assortment
• Meiosis contributes to genetic variation through random fertilization (different egg and sperm fertilize)

DNA Sharing Among Relatives

• Parent: 50%
• Siblings: 50% (may vary due to recombination)
• Grandparents: 25%

Gregor Mendel's Pea Plants

• Easy to grow
• Distinct traits
• Controlled mating

Mendel's Monohybrid Crosses

• F2 Ratio: 3:1 (3 tall; 1 short), indicating dominance and recessiveness

Mendel's Law of Segregation

• Alleles segregate during gamete formation
• Each gamete carries only one allele for each gene

Genetic Terms

• Gene: DNA segment for a trait
• Allele: gene variant (T/t)
• Genotype: genetic makeup (TT/tt)
• Phenotype: physical trait (tall/short)
• Dominant: allele that masks the recessive allele
• Recessive: allele that is masked by the dominant allele
• Homozygous: two identical alleles for a gene
• Heterozygous: two different alleles for a gene

Testcross

• Cross unknown (T?) with homozygous recessive (tt)
• If offspring show recessive trait, the parent was Tt (heterozygous)

Mendel's Dihybrid Crosses

• F2 ratio: 9:3:3:1

Mendel's Law of Independent Assortment

• Alleles for different genes separate independently

Linked Genes

• Genes located close together on the same chromosome
• They do not assort independently and are inherited together

Product Law of Probabilities

• The probability of two independent events occurring together is the product of their individual probabilities
• P(A and B)= P(A)*P(B)

Autosomal Recessive Inheritance

• Requires two copies (aa) to be affected
• Carrier: Aa

Autosomal Dominant Inheritance

• Only one copy (Aa/AA) needed to be considered affected
• There is no carrier

Chi-Square Analysis

• Tests if observed data matches expected ratios

Chi-Square Analysis Formula

• x2 = Σ(Οί – Εί)2/Ei

P-Value

• P<0.05: significant deviation/difference (null hypothesis rejected)

Wild-Type Allele

• The most common allele (+) subscript

Source of New Alleles

• Mutation is the source

Loss-of-Function Mutation

• Reduced/no protein

Gain-of-Function Mutation

• Enhanced/new function

Silent Mutation

• No effect

Incomplete Dominance

• Either allele is completely dominant or recessive (blended genotype: red+white flowers=pink)

Codominance

• Both alleles are fully expressed (AB blood type)

Blood Groups

• MN: codominant
• ABO: A/B codominant, O recessive
• Rh: Rh+ dominant, Rh- recessive

Lethal Allele

• Causes death during the embryonic period if homozygous (TT=lethal)

Epistasis

• When one gene influences (affects or masks) the expression of another
• Dominant epistasis (12:3:1 ratio)

Recessive epistasis

• Recessive epistasis (9:4:3 ratio)

Complementation Analysis

• If two mutants crossed -> wild type -> mutations in different genes
• If two mutants crossed -> mutant -> mutations happen in the same gene

Pleiotropy

• When a single gene has multiple phenotypic effects
• Example: Marfan syndrome affects connective tissues in many body parts

X-Linked Recessive Disorders

• Males are more affected (XY only one X chromosome -> no “backup” X)

Sex-Limited Inheritance

• A certain phenotype limited to only one sex (e.g., milk production)

Sex-Influenced Inheritance

• More expressed in one sex (e.g., baldness)

Penetrance

• % showing that phenotype

Expressivity

• Range/degree/severity of expression of that phenotype

Environmental Influence

• Temperature-sensitive gene affecting fur color in Siamese cats (more black pigments in winter)

Genetic Anticipation

• The severity of a genetic disorder increases over generations due to trinucleotide repeat expansions

Imprinting

• Gene expression depends on which parent it was inherited from
• Example: PWS-paternal inherited gene, AS-maternal inherited gene

Epigenetics and DNA Methylation

• Heritable changes in gene expression without altering DNA sequences
• DNA methylation influences gene activity and silences genes

Organelle Heredity

• Traits inherited through organelles (mitochondria/chloroplasts)

Maternal Effect

• Phenotype determined/affected by the mother's genotype (gene products presented in the egg)

Heteroplasmy

• Mixed organelle DNA (mitochondria with mutant + normal DNA)