Cellular Reproduction and the Cell Cycle

Types of Reproduction

Reproduction is the biological process by which new individual organisms are produced. There are two main types:

• Asexual Reproduction: Involves a single parent and produces genetically identical offspring (clones). Common in unicellular organisms and some multicellular organisms (e.g., hydra, some plants).

• Sexual Reproduction: Involves two parents and the fusion of gametes (egg and sperm), resulting in genetically diverse offspring.

Key Features: Asexual reproduction is rapid and efficient but lacks genetic diversity. Sexual reproduction increases genetic variation, which is important for evolution.

The Cell Cycle

The cell cycle is the ordered sequence of events that a cell goes through from its formation to its own division.

• Interphase: The cell grows and DNA is replicated. Subdivided into G1 (growth), S (DNA synthesis), and G2 (preparation for division).

• Mitotic (M) Phase: Includes mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).

Mitosis

Mitosis is the process by which a eukaryotic cell separates its duplicated chromosomes into two identical sets.

• Purpose: Growth, repair, and asexual reproduction.

• Phases: Prophase, Metaphase, Anaphase, Telophase.

• Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Comparison: In animal cells, cytokinesis occurs via cleavage furrow; in plant cells, a cell plate forms due to the rigid cell wall.

Regulation of the Cell Cycle

• Checkpoints: Control points (G1, G2, M) where the cell assesses whether to proceed with division.

• Factors Affecting Division: Growth factors, cell size, DNA integrity, and environmental conditions.

Cancer: Results from uncontrolled cell division due to failures in cell cycle control mechanisms.

Meiosis and Sexual Life Cycles

Human Chromosome Numbers

• Diploid (2n): 46 chromosomes in humans (two sets of 23).

• Haploid (n): 23 chromosomes in human gametes (egg or sperm).

Homologous Chromosomes: Chromosome pairs (one from each parent) with the same genes but possibly different alleles.

Purpose and Process of Meiosis

Meiosis reduces the chromosome number by half, producing four genetically unique haploid cells (gametes).

• Stages: Meiosis I (homologous chromosomes separate) and Meiosis II (sister chromatids separate).

• Major Accomplishments: At the end of Meiosis I, cells are haploid with duplicated chromosomes; at the end of Meiosis II, four haploid cells with unduplicated chromosomes are produced.

Genetic Variation in Meiosis

• Crossing Over: Exchange of genetic material between homologous chromosomes during Prophase I.

• Independent Assortment: Random orientation of homologous pairs during Metaphase I.

These processes introduce genetic diversity in sexually reproducing populations.

Errors in Meiosis

• Nondisjunction: Failure of chromosomes to separate properly during Meiosis I or II, leading to abnormal chromosome numbers (e.g., trisomy 21 in Down syndrome).

• Chromosomal Mutations:

  • Deletion: Loss of a chromosome segment.

  • Duplication: Repetition of a chromosome segment.

  • Inversion: Reversal of a chromosome segment.

  • Translocation: Movement of a segment to a nonhomologous chromosome.

Patterns of Inheritance

Mendelian Genetics

• Law of Segregation: Each individual has two alleles for each gene, which separate during gamete formation.

• Law of Independent Assortment: Genes for different traits assort independently during gamete formation.

These laws are explained by the movement of chromosomes during meiosis and fertilization.

Key Genetic Terms

• Allele: Alternative form of a gene.

• Dominant Allele: Expressed in the phenotype when present.

• Recessive Allele: Masked in the presence of a dominant allele.

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

• Phenotype: Observable traits (e.g., tall, short).

• Homozygous: Two identical alleles (AA or aa).

• Heterozygous: Two different alleles (Aa).

Punnett Squares and Genetic Crosses

• Punnett Square: Tool to predict offspring genotypes and phenotypes from parental crosses.

• Monohybrid Cross: Involves one gene; typical ratio is 3:1 for dominant:recessive in F2 generation.

• Dihybrid Cross: Involves two genes; typical ratio is 9:3:3:1 in F2 generation.

• Testcross: Cross between an individual of unknown genotype and a homozygous recessive to determine genotype.

Patterns of Inheritance

• Autosomal Dominant: Trait appears in every generation; only one allele needed for expression.

• Autosomal Recessive: Trait can skip generations; two alleles needed for expression.

• X-linked Inheritance: Genes located on the X chromosome; males are more often affected.

Variations to Mendel's Laws

• Incomplete Dominance: Heterozygote shows intermediate phenotype (e.g., pink flowers from red and white parents).

• Codominance: Both alleles are fully expressed (e.g., AB blood type).

• Multiple Alleles: More than two alleles exist for a gene (e.g., ABO blood types: IA, IB, i).

• Polygenic Inheritance: Multiple genes affect a single trait (e.g., skin color, height).

• Pleiotropy: One gene affects multiple traits (e.g., sickle cell disease).

• Linked Genes: Genes located close together on the same chromosome tend to be inherited together; crossing over can separate them.

Pedigrees and Genetic Disorders

• Pedigree: Diagram showing inheritance patterns in families; used to determine genotypes and predict genetic disorders.

• Genetic Disorders: Can be autosomal dominant, autosomal recessive, or X-linked; examples include cystic fibrosis, sickle cell anemia, and hemophilia.

Sample Table: Comparison of Mitosis and Meiosis

Sample Equation: Probability in a Monohybrid Cross

The probability of producing a homozygous recessive offspring (aa) from two heterozygous parents (Aa x Aa):

Example: ABO Blood Types

• Three alleles: IA, IB, i

• Possible genotypes: IAIA, IAi, IBIB, IBi, IAIB, ii

• Phenotypes: Type A, Type B, Type AB, Type O

Additional info: This guide expands on the study guide prompts by providing definitions, examples, and a comparison table for mitosis and meiosis, as well as a sample probability equation and an example of multiple alleles in blood types.