Cell Division: Purpose and Binary Fission

Cell Division Purposes

• Allows organism growth by increasing cell numbers
• Repairs tissues by replacing damaged or dead cells
• Asexual reproduction in single-celled organisms
• Maintains tissue function by producing new cells

Binary Fission Steps

• DNA Replication: Single circular DNA chromosome is copied
• Chromosome Segregation: Two DNA molecules move to opposite sides of the cell
• Cell Elongation: Cell size increases, further separating DNA molecules
• Formation of Septum: A division plate (septum) forms in the cell's center
• Cell Division: Cell splits into two identical daughter cells, each with a DNA copy

Binary Fission

• Undergone by bacteria, archaea, and some protozoa
• Asexual reproduction: One parent produces identical offspring

Chromatin vs. Chromosomes

• Chromatin: Loose, thread-like DNA in non-dividing cells
• Chromosomes: Tightly coiled, condensed chromatin in dividing cells
• Chromatin found in interphase, chromosomes in mitosis or meiosis

Diploid and Haploid

• Diploid (2n): Two complete chromosome sets (one from each parent) in somatic cells
• Haploid (n): One complete chromosome set in gametes (sperm and egg cells)
• Human diploid number = 46, haploid number = 23

Sister Chromatids

• Two identical copies of a single chromosome after DNA replication
• Held together by a centromere and separate during cell division

Centromere

• Region where sister chromatids join
• Spindle fibers attach here during cell division

Cell Cycle Steps

• G1 (Gap 1): Cell grows and performs normal functions
• S (Synthesis): DNA replication occurs
• G2 (Gap 2): Cell grows further and prepares for division
• M (Mitosis): Nucleus and cytoplasm divide into two daughter cells

Cell Cycle Control

• Loss of control can cause cancer: Cells divide uncontrollably, forming tumors

Mitosis

• Results in two daughter cells with same chromosome number as the parent cell (diploid)
• Daughter cells are genetically identical to the mother cell (assuming no mutations)

Mitosis Phases

• Prophase: Chromosomes condense, nuclear membrane breaks down, spindle fibers form
• Metaphase: Chromosomes align at the cell's equator
• Anaphase: Sister chromatids are pulled apart to opposite sides
• Telophase: Nuclear membranes reform, cell begins to split

Mitosis: Plants vs. Animals

• Plant cells: Cell plate forms to divide cytoplasm
• Animal cells: Cleavage furrow forms to pinch cell into two

Cytoplasm in Daughter Cells

• Contents are generally identical
• Organelle distribution may be uneven during cytokinesis

Meiosis

• Cell division that reduces chromosome number by half
• Produces four non-identical (gametes) daughter cells for sexual reproduction

Meiosis Divisions

• Two nuclear divisions (meiosis I and meiosis II)
• Results in four daughter cells

Gametes

• Reproductive cells
• Sperm in males, eggs in females
• Carry half the organism's chromosome number (haploid)

Homologous Chromosomes

• Chromosome pairs, one from each parent
• Similar in shape, size, and genetic content
• May carry different versions (alleles) of genes

Bivalent Pair and Tetrad

• Bivalent Pair: Two homologous chromosomes paired during meiosis I.
• Tetrad: A structure formed by the bivalent pair during prophase I of meiosis, consisting of four chromatids.

Meiosis Steps

• Meiosis I:
  • Prophase I: Chromosomes condense, homologous chromosomes pair up (bivalents), crossing-over occurs.
  • Metaphase I: Bivalents align at the metaphase plate.
  • Anaphase I: Homologous chromosomes are separated to opposite poles.
  • Telophase I: Nuclear membranes form, resulting in two haploid cells.
• Meiosis II:
  • Prophase II: Chromosomes condense in both haploid cells.
  • Metaphase II: Chromosomes align at the metaphase plate in both cells.
  • Anaphase II: Sister chromatids are separated.
  • Telophase II: Nuclear membranes form, resulting in four haploid daughter cells.

Tetrads

• Form during prophase I of meiosis
• Enable crossing over: Exchange of genetic material between homologous chromosomes, introduces genetic variation

Metaphase I

• Bivalents (paired homologous chromosomes) align at the metaphase plate

Anaphase I

• Homologous chromosomes (not chromatids) are pulled to opposite poles

Telophase I

• Chromosomes are haploid
• Each chromosome consists of two chromatids

Meiosis II vs. Mitosis

• Similarity: Sister chromatid separation
• Differences:
  • Meiosis II: Occurs in haploid cells, results in four non-identical daughter cells
  • Mitosis: Occurs in diploid cells, results in two identical daughter cells

Meiosis Cell Genetics

• Cells are not genetically identical to the mother cell or each other
• Due to crossing over and random chromosome assortment

Spermatogenesis and Oogenesis

• Spermatogenesis: Occurs in testes, diploid germ cells undergo meiosis to form four haploid sperm cells
• Oogenesis: Occurs in ovaries, diploid germ cell undergoes meiosis to produce one large egg and three polar bodies

Meiosis Importance

• Reduces chromosome number by half
• Ensures proper offspring chromosome number
• Increases genetic variation through crossing over and independent assortment

Genetic Variability in Meiosis

• Crossing over: Exchange of genetic material between homologous chromosomes
• Independent assortment: Random chromosome distribution to gametes

Chromosome Theory of Inheritance

• Genes are located on chromosomes
• Chromosome behavior during meiosis explains inheritance patterns

Karyotype and Autosomes

• Karyotype: A picture of an individual's complete set of chromosomes, arranged by size, shape, and number.
• Autosome: Non-sex chromosome, humans have 22 pairs

Sex Chromosomes and Mutation

• Sex Chromosomes: X and Y chromosomes
• Males: XY, Females: XX
• Mutation: Change in DNA sequence

Chromosome Mutations

• Structural: Deletions, duplications, inversions, translocations
• Numerical: Monosomy and trisomy

Monosomy and Trisomy

• Monosomy: Missing one chromosome from a pair
• Trisomy: Extra chromosome in a pair
• Caused by nondisjunction: Chromosomes fail to separate correctly during meiosis

Common Autosomal Abnormality

• Down syndrome: Trisomy 21 (three copies of chromosome 21)

Child with Down Syndrome Characteristics

• Intellectual disability
• Distinct physical features (flat face, almond-shaped eyes)
• Increased risk of medical conditions

Down Syndrome Risk

• Advanced maternal age increases the incidence

Sex Chromosome Abnormalities

• Turner’s syndrome: One X chromosome, symptoms include short stature, infertility
• Metafemale: More than two X chromosomes, symptoms may include developmental delays and infertility
• Klinefelter’s syndrome: One X and two Y chromosomes, symptoms include infertility, learning difficulties
• Jacob’s syndrome: Two X and one Y chromosome, symptoms include tall stature, acne, possible learning disabilities

Chromosome Structure Changes

• Caused by DNA replication errors or chromosome breakage/rejoining

Chromosome Mutations

• Inversion: A chromosome segment is reversed or flipped
• Translocation: A segment of one chromosome breaks off and attaches to a different chromosome
• Deletion: A portion of a chromosome Lost or missing
• Duplication: A portion of a chromosome copied which results in multiple copies of a segment

Blending Theory of Inheritance

• Proposed offspring inherit a mixture of traits from both parents, creating intermediate phenotypes
• Is not true

Gregor Mendel

• Austrian monk, father of modern genetics
• Studied pea plants in the mid-1800s

Mendel's Laws

• Law of Segregation: Each individual has two alleles per gene, which separate during gamete formation.
• Law of Independent Assortment: Alleles for different traits segregate independently during gamete formation.

Alleles, Dominant, and Recessive

• Alleles: Different forms of a gene at a specific locus
• Dominant Alleles: Expressed in the phenotype with one copy present
• Recessive Alleles: Only expressed when two copies are present

Gene Locus and Homozygous/Heterozygous

• Gene locus: Specific gene location on a chromosome
• Homozygous: Two identical alleles (AA or aa)
• Heterozygous: Two different alleles (Aa)

Genotype and Phenotype

• Genotype: Genetic makeup (Aa, AA, aa)
• Phenotype: Physical expression of traits

Punnett Square and Testcross

• Punnett square: Predicts genetic outcomes of a cross
• Testcross: Determines genotype of dominant phenotype by crossing with homozygous recessive

Testcross Ratios

• Homozygous dominant: All offspring show the dominant phenotype
• Heterozygote: 1:1 phenotype ratio (50% dominant, 50% recessive)

Probability of Independent Events

• Calculated by multiplying individual event probabilities

Mendel Crosses

• Monohybrid Cross (Tt x Tt): Phenotype ratio is 3 dominant phenotype: 1 recessive phenotype
• Dihybrid Cross (TtGg x TtGg): Phenotype ratio is typically 9:3:3:1

Pedigree Charts

• Family tree showing trait inheritance across generations
• Used to trace specific traits or genetic disorders within a family

Autosomal Recessive Disorders

• Require two copies of the defective allele to be expressed
• Carriers have one normal and one defective allele, show no symptoms
• Examples: Cystic fibrosis, sickle cell anemia, Tay-Sachs disease

Autosomal Dominant Disorders

• Require only one copy of the defective allele to be expressed
• Affected individual has a 50% chance of passing the allele to offspring
• Examples: Huntington's disease, Marfan syndrome, achondroplasia

Incomplete Dominance, Multiple Alleles and Human Blood Type

• Incomplete dominance: Heterozygote phenotype is intermediate between homozygous parents, are still discrete
• Multiple alleles: More than two alleles for a gene in a population
• Human blood type: A, B, and O alleles exist.
• Type A: AA or AO, Type B: BB or BO, Type AB: AB, Type O: OO

Codominance, Polygenic Inheritance, and Human Traits

• Codominance: Both alleles are fully expressed, blood type AB is an example.
• Polygenic inheritance: Traits controlled by multiple genes, each with a small effect.
• Human polygenic traits: Height, skin color, eye color
• Human polygenic disorders: Asthma, diabetes, heart disease

Pleiotropy and Environment

• Pleiotropy: One gene influences multiple, seemingly unrelated traits.
• Marfan syndrome: Gene affects connective tissue
• Environment: Influences gene expression, affects phenotype.
• Sun exposure darkens skin color.

X-linked Inheritance

• Genes located on the X chromosome
• Males are more likely to express recessive disorders
• Examples: Color blindness, hemophilia, Duchenne muscular dystrophy