Mitosis: Cell Division

• Cell division is fundamental for reproduction, growth, and repair in living organisms.
• Mitosis and meiosis are the two main types of cell division.
• Genetics explores heredity and variation in living organisms via genes

Mitosis

• Produces two genetically identical daughter cells from a single parent cell.
• Think of growth and repair.
• Involves one round of cell division.
• Functions of mitosis are growth, repair, and asexual reproduction.
• Remember: "My Two" identical cells.
• The process is divided into distinct phases: prophase, metaphase, anaphase, and telophase (PMAT).
  • Prophase: Chromosomes condense and become visible, nuclear envelope breaks down, spindle fibers emerge from the centrosomes.
  • Metaphase: Chromosomes line up along the metaphase plate (equator of the cell), spindle fibers attach to the centromeres of each chromosome.
  • Anaphase: Sister chromatids are separated and move to opposite poles of the cell, pulled by the spindle fibers.
  • Telophase: Chromosomes arrive at the poles and decondense, nuclear envelope reforms, spindle fibers disappear.
• Cytokinesis usually occurs concurrently with telophase, dividing the cytoplasm and forming two separate cells.

Meiosis

• Produces four genetically different daughter cells (gametes) from a single parent cell.
• Involves two rounds of cell division: meiosis I and meiosis II.
• Think, "Me, four!" diverse cells.
• Meiosis functions in sexual reproduction, creating genetic diversity.

Meiosis I

• Separates homologous chromosomes.
  • Prophase I: Chromosomes condense, nuclear envelope breaks down, homologous chromosomes pair up to form tetrads, crossing over occurs (exchange of genetic material between homologous chromosomes).
  • Metaphase I: Tetrads line up along the metaphase plate, spindle fibers attach to the centromeres of homologous chromosomes.
  • Anaphase I: Homologous chromosomes are separated and move to opposite poles of the cell, sister chromatids remain attached.
  • Telophase I: Chromosomes arrive at the poles, nuclear envelope may reform, cell divides (cytokinesis) into two cells.

Meiosis II

• Separates sister chromatids
  • Prophase II: Chromosomes condense, nuclear envelope breaks down, spindle fibers emerge.
  • Metaphase II: Chromosomes line up along the metaphase plate, spindle fibers attach to the centromeres of sister chromatids.
  • Anaphase II: Sister chromatids are separated and move to opposite poles of the cell.
  • Telophase II: Chromosomes arrive at the poles and decondense, nuclear envelope reforms, cell divides (cytokinesis) into two cells.
• Results of meiosis are four haploid daughter cells, each genetically distinct.

Genetic Variation

• Generated during meiosis through crossing over (in prophase I) and independent assortment of chromosomes (in metaphase I).
• Crossing over involves the exchange of genetic material between homologous chromosomes during prophase I of meiosis, where genes switch spots and creates new allele combinations.
• Independent assortment refers to the random orientation of homologous chromosome pairs during metaphase I of meiosis, resulting in different combinations of chromosomes in the daughter cells, which is an independent, random chromosome arrangement.
• Random fertilization further increases genetic variation; any sperm can fuse with any egg.

Punnett Squares

• A diagram used to predict offspring genotypes and phenotypes from a genetic cross, predicting genetic outcomes and progeny
• Based on Mendelian genetics principles.

Basic Concepts

• Genes: Units of heredity that carry information for specific traits, serving as genetic blueprints.
• Alleles: Different versions of a gene, which are alternative gene forms (e.g., A or a).
• Genotype: Genetic makeup of an individual, which is the genetic type (e.g., AA, Aa, aa).
• Phenotype: Observable characteristics of an individual, which are physical traits (e.g., tall, short).
• Homozygous: Having two identical alleles for a gene; "Homo" means "same" (e.g., AA or aa).
• Heterozygous: Having two different alleles for a gene (e.g., Aa).
• Dominant allele: An allele that masks the expression of the recessive allele in a heterozygous individual.
• Recessive allele: An allele whose expression is masked by the dominant allele in a heterozygous individual.

Monohybrid Cross

• Involves one gene with two alleles.
• Example: Pea plant height, where 'T' is the dominant allele for tallness and 't' is the recessive allele for shortness.
• Steps to Construct a Punnett Square:
  • Determine the genotypes of the parents (e.g., a heterozygous cross: Aa x Aa).
  • Write the alleles of one parent across the top of the square and the alleles of the other parent down the side.
  • Fill in the boxes by combining the alleles from each parent.
  • Analyze the resulting genotypes and phenotypes of the offspring.
• Example Outcome (Aa x Aa):
  • Genotypes: AA, Aa, Aa, aa
  • Genotypic ratio: 1 AA : 2 Aa : 1 aa
  • Phenotypic ratio: 3 tall : 1 short (assuming complete dominance)

Dihybrid Cross

• Involves two genes, each with two alleles.
• Example: Pea plant seed color (Y = yellow, y = green) and seed shape (R = round, r = wrinkled).
• Steps to Construct a Punnett Square:
  • Determine the genotypes of the parents (e.g., a heterozygous cross: YyRr x YyRr).
  • Determine the possible gametes each parent can produce (using the FOIL method if necessary). For YyRr, the gametes are YR, Yr, yR, yr.
  • Write the gametes of one parent across the top of the square and the gametes of the other parent down the side (4x4 square).
  • Fill in the boxes by combining the gametes from each parent.
  • Analyze the resulting genotypes and phenotypes of the offspring.
• Example Outcome (YyRr x YyRr):
  • Phenotypic ratio: 9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled (assuming independent assortment and complete dominance).

Beyond Simple Mendelian Genetics

• Incomplete Dominance: Heterozygous individuals show an intermediate phenotype (e.g., red flower x white flower = pink flower).
• Codominance: Both alleles are expressed in the heterozygous individual (e.g., blood type AB).
• Multiple Alleles: More than two alleles exist for a gene in the population (e.g., human ABO blood types).
• Sex-linked Genes: Genes located on the sex chromosomes (typically the X chromosome), which can result in different inheritance patterns in males and females (e.g., hemophilia).

Applications

• Genetic Counseling: Punnett squares are used to predict the probability of inheriting genetic disorders.
• Agriculture: Predicting traits in crop plants and livestock.
• Research: Understanding gene interactions and inheritance patterns.

Mitosis is a type of cell division that produces two identical daughter cells for growth and repair. It goes through prophase, metaphase, anaphase, and telophase (PMAT). During mitosis, chromosomes condense, align, separate, and new nuclear envelopes form.